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1
Siu, Billy Chin Pang. "Condensation heat transfer in microchannel /." View abstract or full-text, 2004. http://library.ust.hk/cgi/db/thesis.pl?MECH%202004%20SIU.
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Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2004.Includes bibliographical references (leaves 43-46). Also available in electronic version. Access restricted to campus users.
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Lee, Man. "Forced convection heat transfer in integrated microchannel heat sinks /." View abstract or full-text, 2006. http://library.ust.hk/cgi/db/thesis.pl?MECH%202006%20LEE.
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Turgay, Metin Bilgehan. "Effect Of Surface Roughness In Microchannels On Heat Transfer." Master's thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/12610253/index.pdf.
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In this study, effect of surface roughness on convective heat transfer and fluid flow in two dimensional parallel plate microchannels is analyzed by numerically. For this purpose, single-phase, developing, laminar fluid flow at steady state and
in the slip flow regime is considered. The continuity, momentum, and energy equations for Newtonian fluids are solved numerically for constant wall temperature boundary condition. Slip velocity and temperature jump at wall boundaries are imposed to observe the rarefaction effect. Effect of axial
conduction inside the fluid and viscous dissipation also considered separately. Roughness elements on the surfaces are simulated by triangular geometrical obstructions. Then, the effect of these roughness elements on the velocity field and Nusselt number are compared to the results obtained from the analyses of flows in microchannels with smooth surfaces. It is found that increasing surface roughness reduces the heat transfer at continuum conditions. However in slip flow regime, increase in Nusselt number with increasing roughness height is observed. Moreover, this increase is found to be more obvious at low rarefied flows. It is also found that presence of axial conduction and viscous dissipation has increasing effect on heat transfer in smooth and rough channels.
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Al-Waaly, Ahmed. "The effect of heat transfer on temperature measurement and its applications to study microchannel heat sinks." Thesis, University of Glasgow, 2015. http://theses.gla.ac.uk/6781/.
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Analytical, numerical and experimental analyses have been performed to investigate the effects of thermocouple wire electrical insulation on the temperature measurement of a reference surface. Two diameters of K-type thermocouple, 80μm and 200μm, with different exposed stripped wire lengths (0 mm, 5mm, 10mm, 15mm and 20mm) were used to measure various surface temperatures (4oC, 8oC, 15oC, 25oC and 35oC). Measurements were made when the thermocouple probe is in direct contact with the surface and the wires are extended vertically and exposed to natural convection from outside environment. Experimental results confirmed that the thermal effect from the electrical insulation on temperature measurement was within -0.5oC and therefore it can be neglected. Moreover, the experimental results agree well with those obtained by both the analytical and numerical methods and further confirm that the diameter of the thermocouple has an impact on the temperature measurement. Analytical results of the thermocouple wire with insulation confirm that there is no specific value for the critical radius and the rate of heat flux around the thermocouple wire continuously increases with the wire radius even when this is larger than the critical radius. Experimental and numerical analyses have been performed to investigate the heating impact of using thermocouples for the temperature measurement of small volumes of cold water. Two sizes of K-type thermocouple have been used: 80μm and 315μm to measure the temperature of the cold water inside a small chamber while the thermocouple wires were extended vertically in the outside environment. For this study, the chamber temperature was adjusted to 4oC. The results show that the heating effect of the thermocouple decreases for the greater depth measurements and this effect is eliminated when the thermocouple junction is close to the chamber bottom surface. The increase in the thermal resistance between the bottom surface and the thermocouple junction raises the heating effect of the thermocouple impact. Moreover, the exposed length of thermocouple wires to the environment has no effect over a specific length where the wire end temperature is equal to that of the environment. Experimental and numerical analyses have been carried out to study the effect of using subchannels in heat sink to minimise the effect of hotspots generated on a chip circuit. Two devices of heat sink – with and without subchannels – were fabricated in order to investigate this effect. The first device was manufactured with a normal parallel channel while the second one was designed to extract more heat by dividing the main channels above the hotspot into two subchannels. A hotspot heat flux (16.7×104 [W/m2]) was applied at the centre of the channels while a uniform heat flux (4.45×104 [W/m2]) was applied at upstream and downstream of the channels. Five mass flow rates have generated under gravity force to investigate the performance of devices under different operating conditions. The results showed the maximum surface temperature was reduced by 4oC the temperature uniformity was improved. Moreover, thermal resistance was reduced by 25% but the pumping power was increased as a result of the presence of the subchannels.
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Ojada, Ejiro Stephen. "Analysis of mass transfer by jet impingement and study of heat transfer in a trapezoidal microchannel." [Tampa, Fla] : University of South Florida, 2009. http://purl.fcla.edu/usf/dc/et/SFE0003297.
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Determan, Matthew D. "Experimental and Analytical Investigation of Ammonia-Water Desorption in Microchannel Geometries." Thesis, Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7149.
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An experimental and analytical study of a microchannel ammonia-water desorber was conducted in this study. The desorber consists of 5 passes of 16 tube rows each with 27, 1.575 mm outside diameter x 140 mm long tubes per row for a total of 2160 tubes. The desorber is an extremely compact 178 mm x 178 mm x 0.508 m tall component, and is capable of transferring the required heat load (~17.5 kW) for a representative residential heat pump system. Experimental results indicate that the heat duty ranged from 5.37 kW to 17.46 kW and the overall heat transfer coefficient ranges from 388 to 617 W/m2-K. The analytical model predicts temperature, concentration and mass flow rate profiles through the desorber, as well as the effective wetted area of the heat transfer surface. Heat and mass transfer correlations as well as locally measured variations in the heating fluid temperature are used to predict the effective wetted area. The average wetted area of the heat and mass exchanger ranged from 0.25 to 0.69 over the range of conditions tested in this study. Local mass transfer results indicate that water vapor is absorbed into the solution in the upper stages of the desorber leading to higher concentration ammonia vapor and therefore reducing the rectifier cooling capacity required. These experimentally validated results indicate that the microchannel geometry is well suited for use as a desorber. Previous experimental and analytical research has demonstrated the performance of this microchannel geometry as an absorber. Together, these studies show that this compact geometry is suitable for all components in an absorption heat pump, which would enable the increased use of absorption technology in the small capacity heat pump market.
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Rastan, Hamidreza. "Investigation of the heat transfer of enhanced additively manufactured minichannel heat exchangers." Thesis, KTH, Skolan för industriell teknik och management (ITM), 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-264278.
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Mini-/microchannel components have received attention over the past few decades owing to their compactness and superior thermal performance. Microchannel heat sinks are typically manufactured through traditional manufacturing practices (milling and sawing, electrodischarge machining, and water jet cutting) by changing their components to work in microscale environments or microfabrication techniques (etching and lost wax molding), which have emerged from the semiconductor industry. An extrusion process is used to produce multiport minichannel-based heat exchangers (HXs). However, geometric manufacturing limitations can be considered as drawbacks for all of these techniques. For example, a complex out-of-plane geometry is extremely difficult to fabricate, if not impossible. Such imposed design constraints can be eliminated using additive manufacturing (AM), generally known as three-dimensional (3D) printing. AM is a new and growing technique that has received attention in recent years. The inherent design freedom that it provides to the designer can result in sophisticated geometries that are impossible to produce by traditional technologies and all for the redesign and optimization of existing models. The work presented in this thesis aims to investigate the thermal performance of enhanced minichannel HXs manufactured via metal 3D printing both numerically and experimentally. Rectangular winglet vortex generators (VGs) have been chosen as the thermal enhancement method embedded inside the flat tube. COMSOL Multiphysics, a commercial software package using a finite element method (FEM), has been used as a numerical tool. The influence of the geometric VG parameters on the heat transfer and flow friction characteristics was studied by solving a 3D conjugate heat transfer and laminar flow. The ranges of studied parameters utilized in simulation section were obtained from our previous interaction with various AM technologies including direct metal laser sintering (DMLS) and electron-beam melting (EBM). For the simulation setup, distilled water was chosen as the working fluid with temperaturedependent thermal properties. The minichannel HX was assumed to be made of AlSi10Mg with a hydraulic diameter of 2.86 mm. The minichannel was heated by a constant heat flux of 5 Wcm−2 , and the Reynolds number was varied from 230 to 950. A sensitivity analysis showed that the angle of attack, VG height, VG length, and longitudinal pitch have notable effects on the heat transfer and flow friction characteristics. In contrast, the VG thickness and the distance from the sidewalls do not have a significant influence on the HX performance over the studied range. On the basis of the simulation results, four different prototypes including a smooth channel as a reference were manufactured with AlSi10Mg via DMLS technology owing to the better surface roughness and greater design uniformity. A test rig was developed to test the prototypes. Owing to the experimental facility and working fluid (distilled water), the experiment was categorized as either a simultaneously developing flow or a hydrodynamically developed but thermally developing flow. The Reynolds number ranged from 175 to 1370, and the HX was tested with two different heat fluxes of 1.5 kWm−2 and 3 kWm−2 . The experimental results for the smooth channel were compared to widely accepted correlations in the literature. It was found that 79% of the experimental data were within a range of ±10% of the values from existing correlations developed for the thermal entry length. However, a formula developed for the simultaneously developing flow overpredicted the Nusselt number. Furthermore, the results for the enhanced channels showed that embedding VGs can considerably boost the thermal performance up to three times within the parameters of the printed parts. Finally, the thermal performance of the 3D-printed channel showed that AM is a promising solution for the development of minichannel HXs. The generation of 3D vortices caused by the presence of VGs ii can notably boost the thermal performance, thereby reducing the HX size for a given heat duty.
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Gokaltun, Seckin. "Lattice Boltzmann Method for Flow and Heat Transfer in Microgeometries." FIU Digital Commons, 2008. http://digitalcommons.fiu.edu/etd/64.
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Recent technological developments have made it possible to design various microdevices where fluid flow and heat transfer are involved. For the proper design of such systems, the governing physics needs to be investigated. Due to the difficulty to study complex geometries in micro scales using experimental techniques, computational tools are developed to analyze and simulate flow and heat transfer in microgeometries. However, conventional numerical methods using the Navier-Stokes equations fail to predict some aspects of microflows such as nonlinear pressure distribution, increase mass flow rate, slip flow and temperature jump at the solid boundaries. This necessitates the development of new computational methods which depend on the kinetic theory that are both accurate and computationally efficient. In this study, lattice Boltzmann method (LBM) was used to investigate the flow and heat transfer in micro sized geometries. The LBM depends on the Boltzmann equation which is valid in the whole rarefaction regime that can be observed in micro flows. Results were obtained for isothermal channel flows at Knudsen numbers higher than 0.01 at different pressure ratios. LBM solutions for micro-Couette and micro-Poiseuille flow were found to be in good agreement with the analytical solutions valid in the slip flow regime (0.01 < Kn < 0.1) and direct simulation Monte Carlo solutions that are valid in the transition regime (0.1 < Kn < 10) for pressure distribution and velocity field. The isothermal LBM was further extended to simulate flows including heat transfer. The method was first validated for continuum channel flows with and without constrictions by comparing the thermal LBM results against accurate solutions obtained from analytical equations and finite element method. Finally, the capability of thermal LBM was improved by adding the effect of rarefaction and the method was used to analyze the behavior of gas flow in microchannels. The major finding of this research is that, the newly developed particle-based method described here can be used as an alternative numerical tool in order to study non-continuum effects observed in micro-electro-mechanical-systems (MEMS).
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Bakaraju, Omkareshwar Rao. "Heat Transfer in Electroosmotic Flow of Power-Law Fluids in Micro-Channel." Cleveland State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=csu1263337731.
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Cetin, Barbaros. "Analysis Of Single Phase Convective Heat Transfer In Microtubes And Microchannels." Master's thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12605820/index.pdf.
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Heat transfer analysis of two-dimensional, incompressible, constant property, hydrodynamically developed, thermally developing, single phase laminar flow in microtubes and microchannels between parallel plates with negligible axial conduction is performed for constant wall temperature and constant wall heat flux thermal boundary conditions for slip flow regime. Fully developed velocity profile is determined analytically, and energy equation is solved by using finite difference method for both of the geometries. The rarefaction effect which is important for flow in low pressures or flow in microchannels is imposed to the boundary conditions of the momentum and energy equations. The viscous dissipation term which is important for high speed flows or flows in long pipelines is included in the energy equation. The effects of rarefaction and viscous heating on temperature profile and local Nusselt number are discussed. The results of the numerical method are verified with the well-known analytical results of the flow in macrochannels (i.e. Kn =0, Br =0) and with the available analytical results of flow in microchannels for simplified cases. The results show significant deviations from the flow in macrochannels.
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Bard, Ari. "Modeling and Predicting Heat Transfer Coefficients for Flow Boiling in Microchannels." Case Western Reserve University School of Graduate Studies / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=case1619091352188123.
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Martínez, Ballester Santiago. "NUMERICAL MODEL FOR MICROCHANNEL CONDENSERS AND GAS COOLERS WITH AN IMPROVED AIR-SIDE APPROACH." Doctoral thesis, Universitat Politècnica de València, 2012. http://hdl.handle.net/10251/17453.
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La presente tesis se ha llevado a cabo en el Instituto de Ingeniería Energética de la Universitat Politècnica de València y durante una estancia en el National Institute of Standards and Technology (NIST). El objetivo principal de la tesis es desarrollar un modelo de alta precisión para intercambiadores de calor de microcanales (MCHX), que tiene que ser útil, en términos de coste computacional, para tareas de diseño.
En la opinión del autor, existen algunos inconvenientes cuando los modelos existentes se aplican a algunos diseños recientes de intercambiador de calor, tales como MCHXs, bien de tubos en serpentín o en paralelo. Por lo tanto, la primera etapa de la tesis identifica los fenómenos que tienen el mayor impacto en la precisión de un modelo para MCHX. Adicionalmente, se evaluó el grado de cumplimiento de varias simplificaciones y enfoques clásicos. Con este fin, se desarrolló el modelo de alta precisión Fin2D como una herramienta para llevar a cabo la investigación mencionada.
El modelo Fin2D es una herramienta útil para analizar los fenómenos que tienen lugar, sin embargo requiere un gran coste computacional, y por tanto no es útil para trabajos de diseño. Es por ello que en base a los conocimientos adquiridos con el modelo Fin2D, se ha desarrollado un nuevo modelo, el Fin1Dx3. Este modelo tan sólo tiene en cuenta los fenómenos más importantes, reteniendo casi la misma precisión que Fin2D, pero con una reducción en el tiempo de cálculo de un orden de magnitud. Se introduce una novedosa discretización y un esquema numérico único para el modelado de la transferencia de calor del lado del aire. Este nuevo enfoque permite modelar los fenómenos existentes de forma consistente con mayor precisión y con mucho menos simplificaciones que los modelos actuales de la literatura. Por otra parte, se logra un coste razonable de cálculo para el objetivo fijado. La tesis incluye la validación experimental de este modelo tanto para un condensador y un enfriador de gas.
Con eMartínez Ballester, S. (2012). NUMERICAL MODEL FOR MICROCHANNEL CONDENSERS AND GAS COOLERS WITH AN IMPROVED AIR-SIDE APPROACH [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/17453
Palancia
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Stocks, Marc Darren. "Geometric optimisation of heat transfer in channels using Newtonian and non-Newtonian fluids." Diss., University of Pretoria, 2012. http://hdl.handle.net/2263/33348.
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The continual advance in manufacturing processes has resulted in significantly more compact, high performance, devices. Consequently, heat extraction has become the limiting factor, and of primary concern. Therefore, a substantial amount of research has been done regarding high efficiency micro heat exchangers, employing novel working fluids.
This dissertation numerically investigated the thermal behaviour of microchannel elements cooled by Newtonian and non-Newtonian fluids, with the objective of maximising thermal conductance subject to constraints. This was done, firstly, for a two-dimensional simple microchannel, and secondly, for a three-dimensional complex microchannel. A numerical model was used to solve the governing equations relating to the flow and temperature fields for both cases. The geometric configuration of each cooling channel was optimised for Newtonian and non-Newtonian fluids, at a fixed inlet velocity and heat transfer rate. In addition, the effect of porosity on thermal conductance was investigated.
Geometric optimisation was employed to the simple and complex microchannels, whereby an optimal geometric ratio (height versus length) was found to maximise thermal conductance. Moreover, analysis indicated that the bifurcation point of the complex microchannel could be manipulated to achieve a higher thermal conductance.
In both cases, it was found that the non-Newtonian fluid characteristics resulted in a significant variation in thermal conductance as inlet velocity was increased. The ii
characteristics of a dilatant fluid greatly reduced thermal conductance on account of shear-thickening on the boundary surface. In contrast, a pseudoplastic fluid showed increased thermal conductance.
A comparison of the simple and complex microchannel showed an improved thermal conductance resulting from greater flow access to the conductive area, achieved by the complex microchannel.
Therefore, it could be concluded that a complex microchannel, in combination with a pseudoplastic working fluid, substantially increased the thermal conductance and efficiency, as opposed to a conventional methodology.Dissertation (MEng)--University of Pretoria, 2012.
gm2014
Mechanical and Aeronautical Engineering
unrestricted
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Fronk, Brian Matthew. "Coupled heat and mass transfer during condensation of high-temperature-glide zeotropic mixtures in small diameter channels." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52265.
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Zeotropic mixtures exhibit a temperature glide between the dew and bubble points during condensation. This glide has the potential to increase system efficiency when matched to the thermal sink in power generation, chemical processing, and heating and cooling systems. To understand the coupled heat and mass transfer mechanisms during phase change of high-glide zeotropic mixtures, a comprehensive investigation of the condensation of ammonia and ammonia/water mixtures in small diameter channels was performed. Condensation heat transfer and pressure drop experiments were conducted with ammonia and ammonia/water mixtures. Experiments on ammonia were conducted for varying tube diameters (0.98 < D < 2.16 mm), mass fluxes (75 < G < 225 kg m⁻² s⁻¹) and saturation conditions (30 < Tsat < 60°C). Zeotropic ammonia/water experiments were conducted for multiple tube diameters (0.98 < D < 2.16 mm), mass fluxes (50 < G < 200 kgm⁻² s⁻¹) and bulk ammonia mass fraction (xbulk = 0.8, 0.9, and > 0.96). An experimental methodology and data analysis procedure for evaluating the local condensation heat duty (for incremental ∆q), condensation transfer coefficient (for pure ammonia), apparent heat transfer coefficient (for zeotropic ammonia/water mixtures), and frictional pressure gradient with low uncertainties was developed. A new heat transfer model for condensation of ammonia in mini/microchannels was developed. Using the insights derived from the pure ammonia work, an improved zeotropic condenser design method for high-temperature-glide mixtures in small diameter channels, based on the non-equilibrium film theory, was introduced. The key features of the improved model were the consideration of annular and non-annular flow effects on liquid film transport, including condensate and vapor sensible cooling contributions, and accounting for mini/microchannel effects through the new liquid film correlation. By understanding the behavior of these mixtures in microchannel geometries, highly efficient, compact thermal conversion devices can be developed.
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Broderick, Spencer L. "Thermally Developing Electro-Osmotic Convection in Circular Microchannels." BYU ScholarsArchive, 2004. https://scholarsarchive.byu.edu/etd/232.
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Thermally developing, electro-osmotically generated flow has been analyzed for a circular microtube under imposed constant wall temperature (CWT) and constant wall heat flux (CHF) boundary conditions. Established by a voltage potential gradient along the length of the microtube, the hydrodynamics of such a flow dictate either a slug flow velocity profile (under conditions of large tube radius-to-Debye length ratio, a/lambda_d) or a family of electro-osmotic flow (EOF) velocity profiles that depend on a/lambda_d. The imposed voltage gradient results in Joule heating in the fluid with an associated volumetric source of energy. For this scenario coupled with a slug flow velocity profile, the analytical solution for the fluid temperature development has been determined for both thermal boundary conditions. The local Nusselt number for the CHF boundary condition is shown to reduce to the classical slug flow thermal development for imposed constant wall heat flux, and is independent of Joule heating source magnitude. For the CWT boundary condition, a local minimum in the streamwise variation in local Nusselt number for moderate positive dimensionless inlet temperature is predicted. For negative dimensionless inlet temperature, which arises if the fluid entrance temperature is below the tube wall temperature, the fluid is initially heated, then cooled, resulting in a singularity in the local Nusselt number at the axial location of the heating/cooling transition. The thermal development length is considerably larger than for traditional pressure-driven flow heat transfer, and is a function of the magnitudes of Peclet number and dimensionless inlet temperature. For the EOF velocity profile scenario, numerical techniques were used to predict the fluid temperature development for both wall boundary conditions by utilizing a finite control volume approach. In addition to Joule heating as an energy source, viscous dissipation is also considered. The results predict that for decreasing a/lambda_d, the local Nusselt number decreases for all axial positions and the thermal development shortens for both wall boundary conditions. Viscous dissipation has significant effect only at intermediate values of a/lambda_d. Results predict local Nusselt numbers to increase for a CWT boundary condition and to decrease for an imposed constant wall heat flux with increasing viscous dissipation.
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Korniliou, Sofia. "Experimental study on local heat transfer coefficients and the effect of aspect ratio on flow boiling in a microchannel." Thesis, University of Edinburgh, 2018. http://hdl.handle.net/1842/31080.
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Flow boiling in integrated microchannel systems is a cooling technology that has received significant attention in recent years as an effective option for high heat flux microelectronic devices as it provides high heat transfer and small variations in surface temperature. However, there are still a number of issues to be addressed before this technology is used for commercial applications. Amongst the issues that require further investigation are the two-phase heat transfer enhancement mechanisms, the effect of channel geometry on heat transfer characteristics, two-phase flow instabilities, critical heat flux and interfacial liquid-vapour heat transfer in the vicinity of the wall. This work is an experimental study on two-phase flow boiling in multi- and single-rectangular microchannels. Experimental research was performed on the effect of the channel aspect ratio and hydraulic diameter, particularly for parallel multi-microchannel systems in order to provide design guidelines. Flow boiling experiments were performed using deionised water in silicon microchannel heat sinks with width-to-depth aspect ratios (a) from 0.33 to 3 and hydraulic diameters from 50 μm to 150 μm. The effect of aspect ratio on two-phase flow boiling local heat transfer coefficient and two-phase pressure drop was investigated as well as the two-phase heat transfer coefficients trends with mass flux for the constant heat fluxes of 151 kW m-2, 183 kW m- 2, 271 kW m-2 and 363 kW m-2. Wall temperature measurements were obtained from five integrated thin nickel film temperature sensors. An integrated thin aluminium heater enabled uniform heating with a small thermal resistance between the heater and the channels. The microfabricated temperature sensors were used with simultaneous high-speed imaging and pressure measurements in order to obtain a better insight related to temperature and pressure fluctuations caused by two-phase flow instabilities under uniform heating in parallel microchannels. The results demonstrated that the aspect ratio of the microchannels affects flow boiling heat transfer coefficients. However, there is not clear trend of the aspect ratio on the heat transfer coefficient. Pressure drop was found to increase with increasing aspect ratio. Wide microchannels but not very shallow, with a = 1.5 and Dh = 120 μm, have shown good heat transfer performance, by producing modest two-phase pressure drop of maximum 200 mbar for the highest heat flux and heat transfer coefficients of 200 kW m-2 during two-phase flow boiling conditions. For the high aspect ratio, values of 2 and 3 two-phase flow boiling heat transfer coefficients were measured to be lower compared to aspect ratio of 1.5. Microchannels with aspect ratios higher than 1.5 produced severe wall temperature fluctuations for high heat fluxes that periodically reached extreme wall temperature values in excess of 250 ˚C. The consequences of these severe wall temperature and pressure fluctuations at high aspect ratios of 2 and 3 resulted in non-uniform flow distribution and temporal dryout. Abrupt increase in two-phase pressure drop occurred for a > 1.5. The effect of the inlet subcooling was found to be significant on both heat transfer coefficient and pressure drop. Furthermore, the effects of bubble growth on flow instabilities and heat transfer coefficients have been investigated. Although the thin film nickel sensors provide the advantage of much faster response time and smaller thermal resistance compared to classic thermocouples, they do not allow for full two-dimensional wall temperature mapping of the heated surface. An advanced experimental method was devised in order to produce accurate two-dimensional heat transfer coefficient data as a function of time. Infrared (IR) thermography was synchronised with simultaneous high-speed imaging and pressure measurements from integrated miniature pressure sensors inside the microchannel, in order to produce two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient maps across the full domain of a polydimethylsiloxane (PDMS) microchannel. The microchannel was characterised by a high aspect ratio (a = 22) and a hydraulic diameter of 192 μm. The PDMS microchannel was bonded on a transparent indium tin oxide (ITO) thin film coated glass. The transparent thin film ITO heater allowed the recording of high quality synchronised high - speed images of the liquid-vapour distribution. This work presents a better insight into the two-phase heat transfer coefficient spatial variation during flow instabilities with two-dimensional heat transfer coefficient plots as a function of time during the cycles of liquid-vapour alternations for different mass flux and heat flux conditions. High spatial and temporal resolution wall temperature measurements and pressure data were obtained for a range of mass fluxes from 7.37 to 298 kg m-2 s-1 and heat fluxes from 13.64 to 179.2 kW m-2 using FC-72 as a dielectric liquid. 3D plots of spatially averaged two-phase heat transfer coefficients at the inlet, middle and outlet of the microchannel are presented with time. The optical images were correlated, with simultaneous thermal images. The results demonstrate that bubble growth in microchannels differs from macroscale channels and the confinement effects influence the local two-phase heat transfer coefficient distribution. Bubble nucleation and axial growth as well as wetting and rewetting in the channel were found to significantly affect the local heat transfer physical mechanisms. Bubble level heat transfer coefficient measurements are important as previous researchers have experimentally investigated local temperature and high speed visualisation in bubbles during pool boiling conditions and not flow boiling. The effect of the confined bubble axial growth to the two-phase heat transfer coefficient distribution at the channel entrance was investigated at low mass fluxes and low heat fluxes. The 3D plots of the 2D two-phase heat transfer coefficient with time across the microchannel domain were correlated with liquid-vapour dynamics and liquid film thinning from the contrast of the optical images, which caused suspected dryout. The 3D plots of heat transfer coefficients with time provided fine details of local variations during bubble nucleation, confinement, elongated bubble, slug flow and annular flow patterns. The correlation between the synchronised high-resolution thermal and optical images assisted in a better understanding of the heat transfer mechanisms and critical heat flux during two-phase flow boiling in microchannels.
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Mlcak, Justin Dale. "Simulation of three-dimensional laminar flow and heat transfer in an array of parallel microchannels." [College Station, Tex. : Texas A&M University, 2007. http://hdl.handle.net/1969.1/ETD-TAMU-1671.
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Wang, Wei-Wen William. "Condensation and single-phase heat transfer coefficient and flow regime visualization in microchannel tubes for HFC-134A /." The Ohio State University, 1999. http://rave.ohiolink.edu/etdc/view?acc_num=osu1488192119266647.
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Bustamante, John Gabriel. "Falling-film evaporation over horizontal rectangular tubes." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52296.
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The present study is the first investigation of falling-film evaporation over horizontal rectangular tubes. This geometry is representative of the external profile
of microchannel tubes. Incorporating these designs into shell-and-tube heat exchangers has the potential to provide compact, high-performance components for a wide
range of applications. This fluid flow was investigated experimentally, targeting three areas: measurements of heat transfer coefficients, quantification of flow
characteristics, and the performance of flow distributors. Falling-film evaporation experiments were conducted using water on a rectangular test section with
dimensions of 203 × 1.42 × 27.4 mm (length × width × height), measuring heat transfer coefficients over a range of saturation temperatures, test section spacings,
heat fluxes, and film Reynolds numbers. This was supported by a flow visualization study that quantified droplet and wave parameters using image analysis of high
speed videos. Finally, the performance of eight liquid distributors, which are used to establish falling-film flows, was quantified and the size of the generated
droplets and jets was measured. Three models were developed to predict the flow regime, wetted tube area, and heat transfer coefficient. The flow regime model is
based on a thermodynamic analysis, while the wetted tube area is found with a hydrodynamic model based on idealized flow assumptions. Finally, the heat transfer
model relies on a relationship with the classic Nusselt (1916) film theory. Each of these models demonstrated good agreement with the experimental data, as well as
trends in the literature. The increased understanding of falling-film evaporation gained in this study will enable the accurate design of shell-and-tube heat
exchangers with microchannel tubes.
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Hu, Xinqun. "Design of a microchannel reactor for gas phase heterogeneous reactions : enhanced mass and heat transfer for process intensification." Thesis, University of Sheffield, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.246984.
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Gozukara, Arif Cem. "Analysis Of Single Phase Convective Heat Transfer In Microchannels With Variable Thermal Conductivity And Variable Viscosity." Master's thesis, METU, 2010. http://etd.lib.metu.edu.tr/upload/2/12611549/index.pdf.
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In this study simultaneously developing single phase, laminar and incompressible flow in a micro gap between parallel plates is numerically analyzed by including the effect of variation in thermal conductivity and viscosity with temperature. Variable property solutions for continuity, momentum and energy equations are performed in a coupled manner, for air as a Newtonian fluid. In these analyses the rarefaction effect, which is important for the slip flow regime, is taken into
account by imposing slip velocity and temperature jump boundary conditions to the wall boundaries. Mainly, the influence of viscous dissipation, axial conduction, geometric parameters and rarefaction on the property variation effect
is aimed to be discussed in detail. Therefore, the effects of variable thermal conductivity and viscosity are investigated simultaneously with the effects of rarefaction, geometric parameters, viscous dissipation and axial conduction. The
difference between constant and variable solutions in terms of heat transfer characteristics is related to the effects of viscous dissipation axial conduction and rarefaction. According to results, property variation is substantially effective in the entrance region where temperature and velocity gradients are high. On the other hand, property variation effects are not significant for fully developed air
flows in microchannel.
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Goktolga, Mustafa Ugur. "Simulation Of Conjugate Heat Transfer Problems Using Least Squares Finite Element Method." Master's thesis, METU, 2012. http://etd.lib.metu.edu.tr/upload/12614787/index.pdf.
Full textAbstract:
In this thesis study, a least-squares finite element method (LSFEM) based conjugate heat transfer solver was developed. In the mentioned solver, fluid flow and heat transfer computations were performed separately. This means that the calculated velocity values in the flow calculation part were exported to the heat transfer part to be used in the convective part of the energy equation. Incompressible Navier-Stokes equations were used in the flow simulations. In conjugate heat transfer computations, it is required to calculate the heat transfer in both flow field and solid region. In this study, conjugate behavior was accomplished in a fully coupled manner, i.e., energy equation for fluid and solid regions was solved simultaneously and no boundary conditions were defined on the fluid-solid interface. To assure that the developed solver works properly, lid driven cavity flow, backward facing step flow and thermally driven cavity flow problems were simulated in three dimensions and the findings compared well with the available data from the literature. Couette flow and thermally driven cavity flow with conjugate heat transfer in two dimensions were modeled to further validate the solver. Finally, a microchannel conjugate heat transfer problem was simulated. In the flow solution part of the microchannel problem, conservation of mass was not achieved. This problem was expected since the LSFEM has problems related to mass conservation especially in high aspect ratio channels. In order to overcome the mentioned problem, weight of continuity equation was increased by multiplying it with a constant. Weighting worked for the microchannel problem and the mass conservation issue was resolved. Obtained results for microchannel heat transfer problem were in good agreement in general with the previous experimental and numerical works.
In the first computations with the solverquadrilateral and triangular elements for two dimensional problems, hexagonal and tetrahedron elements for three dimensional problems were tried. However, since only the quadrilateral and hexagonal elements gave satisfactory results, they were used in all the above mentioned simulations.
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Martin, Callizo Claudi. "Flow Boiling Heat Transfer in Single Vertical Channels of Small Diameter." Doctoral thesis, KTH, Tillämpad termodynamik och kylteknik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-25797.
Full textAbstract:
Microchannel heat exchangers present many advantages, such as reduced size, high thermal efficiency and low fluid inventory; and are increasingly being used for heat transfer in a wide variety of applications including heat pumps, automotive air conditioners and for cooling of electronics.However, the fundamentals of fluid flow and heat transfer in microscalegeometries are not yet fully understood. The aim of this thesis is to contribute to a better understanding of the underlying physical phenomena in single-phase and specially flow boiling heat transfer of refrigerants in small channels. For this purpose, well-characterized heat transfer experiments have been performed in uniformly heated, single, circular, vertical channels ranging from 0.64 to 1.70 mm in diameter and using R-134a, R-22 and R-245fa as working fluids. Furthermore, flow visualization tests have been carried out to clarify the relation between the two-phase flow behavior and the boiling heat transfer characteristics. Single-phase flow experiments with subcooled liquid refrigerant have confirmed that conventional macroscale theory on single-phase flow and heat transfer is valid for circular channels as small as 640μm in diameter. Through high-speed flow boiling visualization of R-134a under non adiabatic conditions seven flow patterns have been observed: isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow, and mist flow. Two-phase flow pattern observations are presented in the form of flow pattern maps. Annular-type flow patterns are dominant for vapor qualities above 0.2. Onset of nucleate boiling and subcooled flow boiling heat transfer of R-134a has been investigated. The wall superheat needed to initiate boiling was found as large as 18 ºC. The experimental heat transfer coefficients have been compared to predictions from subcooled flow boiling correlationsav ailable in the literature showing poor agreement. Saturated flow boiling heat transfer experiments have been performed with the 640 μm diameter test section. The heat transfer coefficient has been found to increase with heat flux and system pressure and not to change with vapor quality or mass flux when the quality is less than ∼0.5. For vapor qualities above this value, the heat transfer coefficient decreases with vapor quality. This deterioration of the heat transfer coefficient is believed to be caused by the occurrence of intermittent dryout in this vapor quality range. The experimental database, consisting of 1027 data points, has been compared against predictions from correlations available in the literature. The best results are obtained with the correlations by Liu and Winterton (1991) and by Bertsch et al. (2009). However, better design tools to correctly predict the flow boiling heat transfer coefficient in small geometries need to be developed. Dryout incipience and critical heat flux (CHF) have been investigated in detail. CHF data is compared to existing macro and microscale correlations. The comparison shows best agreement with the classical Katto and Ohno (1984) correlation, developed for conventional large tubes.QC 20101101
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Shinde, Pradeep R. "Investigation of Low Reynolds Number Flow and Heat Transfer of Louvered Surfaces." FIU Digital Commons, 2016. http://digitalcommons.fiu.edu/etd/3038.
Full textAbstract:
This study focuses on the investigation of flow behavior at low Reynolds numbers by the experimental and numerical performance testing of micro-channel heat exchangers. An experimental study of the heat transfers and pressure drop of compact heat exchangers with louvered fins and flat tubes was conducted within a low air-side Reynolds number range of 20 < ReLp < 225. Using an existing low-speed wind tunnel, 26 sample heat exchangers of corrugated louver fin type, were tested. New correlations for Colburn j and Fanning friction f factor have been developed in terms of non-dimensional parameters. Within the investigated parameter ranges, it seems that both the j and f factors are better represented by two correlations in two flow regimes (one for ReLp = 20 – 80 and one for ReLp = 80 – 200) than a single regime correlation in the power-law format. The results support the conclusion that airflow and heat transfer at very low Reynolds numbers behaves differently from that at higher Reynolds numbers. The effect of the geometrical parameters on the heat exchanger performance was investigated.
The numerical investigation was conducted for further understanding of the flow behavior at the range of experimentally tested Reynolds number. Ten different heat exchanger geometries with varied geometrical parameters obtained for the experimental studies were considered for the numerical investigation. The variations in the louver angle were the basis of the selection. The heat transfer and pressure drop performance was numerically investigated and the effect of the geometrical parameters was evaluated. Numerical results were compared against the experimental results. From the comparison, it is found that the current numerical viscous laminar models do not reflect experimentally observed transitional two regime flow behavior from fin directed to the louver directed at very low Reynolds number ranging from 20 to 200.
The flow distribution through the fin and the louver region was quantified in terms of flow efficiency. The flow regime change was observed at very low Reynolds number similar to the experimental observations. However, the effect of two regime flow change does not reflect on the thermal hydraulic performance of numerical models. New correlations for the flow efficiency � have developed in terms of non-dimensional parameters.
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Cowley, Adam M. "Hydrodynamic and Thermal Effects of Sub-critical Heating on Superhydrophobic Surfaces and Microchannels." BYU ScholarsArchive, 2017. https://scholarsarchive.byu.edu/etd/6572.
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This dissertation focuses on the effects of heating on superhydrophobic (SHPo) surfaces. The work is divided into two main categories: heat transfer without mass transfer and heat transfer in conjunction with mass transfer. Numerical methods are used to explore the prior while experimental methods are utilized for the latter. The numerical work explores convective heat transfer in SHPo parallel plate microchannels and is separated into two stand-alone chapters that have been published archivally. The first considers surfaces with a rib/cavity structure and the second considers surfaces patterned with a square lattice of square posts. Laminar, fully developed, steady flow with constant fluid properties is considered where the tops of the ribs and posts are maintained at a constant heat flux boundary condition and the gas/liquid interfaces are assumed to be adiabatic. For both surface configurations the overall convective heat transfer is reduced. Results are presented in the form of average Nusselt number as well as apparent temperature jump length (thermal slip length). The heat transfer reduction is magnified by increasing cavity fraction, decreasing Peclet number, and decreasing channel size relative to the micro-structure spacing. Axial fluid conduction is found to be substantial at high Peclet numbers where it is classically neglected. The parameter regimes where prior analytical works found in the literature are valid are delineated. The experimental work is divided into two stand-alone chapters with one considering channel flow and the other a pool scenario. The channel work considers high aspect ratio microchannels with one heated SHPo wall. If water saturated with dissolved air is used, the air-filled cavities of SHPo surfaces act as nucleation sites for mass transfer. As the water heats it becomes supersaturated and air can effervesce onto the SHPo surface forming bubbles that align to the underlying micro-structure if the cavities are comprised of closed cells. The large bubbles increase drag in the channel and reduce heat transfer. Once the bubbles grow large enough, they are expelled from the channel and the nucleation and growth cycle begins again. The pool work considers submerged, heated SHPo surfaces such that the nucleation behavior can be explored in the absence of forced fluid flow. The surface is maintained at a constant temperature and a range of temperatures (40 - 90 °C) are explored. Similar nucleation behavior to that of the microchannels is observed, however, the bubbles are not expelled. Natural convection coefficients are computed. The surfaces with the greatest amount of nucleation show a significant reduction in convection coefficient, relative to a smooth hydrophilic surface, due to the insulating bubble layer.
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Celik, Sitki Berat. "Analysis Of Single Phase Fluid Flow And Heat Transfer In Slip Flow Regime By Parallel Implementation Of Lattice Boltzmann Method On Gpus." Master's thesis, METU, 2012. http://etd.lib.metu.edu.tr/upload/12614943/index.pdf.
Full textAbstract:
In this thesis work fluid flow and heat transfer in two-dimensional microchannels are studied numerically. A computer code based on Lattice Boltzmann Method (LBM) is developed for this purpose. The code is written using MATLAB and Jacket software and has the important feature of being able to run parallel on Graphics Processing Units (GPUs). The code is used to simulate flow and heat transfer inside micro and macro channels. Obtained velocity profiles and Nusselt numbers are compared with the Navier-Stokes based analytical and numerical results available in the literature and good matches are observed. Slip velocity and temperature jump boundary conditions are used for the micro channel simulations with Knudsen number values covering the slip flow regime. Speed of the parallel version of the developed code running on GPUs is compared with that of the serial one running on CPU and for large enough meshes more than 14 times speedup is observed.
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Owhaib, Wahib. "Experimental Heat Transfer, pressure drop, and Flow Visualization of R-134a in Vertical Mini/Micro Tubes." Doctoral thesis, KTH, Tillämpad termodynamik och kylteknik, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4299.
Full textAbstract:
For the application of minichannel heat exchangers, it is necessary to have accurate design tools for predicting heat transfer and pressure drop. Until recently, this type of heat exchangers was not well studied, and in the scientific literature there were large discrepancies between results reported by different investigators. The present thesis aims to add to the knowledge of the fundamentals of single- and two-phase flow heat transfer and pressure drop in narrow channels, thereby aiding in the development of this new, interesting technology with the possibility of decreasing the size of electronics through better cooling, and of increasing the energy efficiency of thermal processes and thermodynamic cycles through enhanced heat transfer. A comprehensive experimental single-phase flow and saturated flow boiling heat transfer and pressure drop study has been carried out on vertical stainless steel tubes with inner diameters of 1.700, 1.224 and 0.826 mm, using R-134a as the test fluid. The heat transfer and pressure drop results were compared both to conventional correlations developed for larger diameter channels and to correlations developed specifically for microscale geometries. Contrary to many previous investigations, this study has shown that the test data agree well with single-phase heat transfer and friction factor correlations known to be accurate for larger channels, thus expanding their ranges to cover mini/microchannel geometries. The main part of the study concerns saturated flow boiling heat transfer and pressure drop. Tests with the same stainless steel tubes showed that the heat transfer is strongly dependent on heat flux, but only weakly dependent on mass flux and vapor fraction (up to the location of dryout). This behavior is usually taken to indicate a dominant influence of nucleate boiling, and indicates that the boiling mechanism is strongly related to that in nucleate boiling. The test data for boiling heat transfer was compared to several correlations from the literature, both for macro- and mini-channels. A new correlation for saturated flow boiling heat transfer of refrigerant R-134a correlation was obtained based on the present experimental data. This correlation predicts the presented data with a mean absolute deviation of 8%. The frictional pressure drop results were compared to both macro- and mini channel correlations available from the literature. The correlation suggested by Qu and Mudawar (2003) gave the best prediction to the frictional two-phase pressure drop within the studied ranges. A unique visualization study of saturated flow boiling characteristics in a vertical 1.332 mm inner diameter quartz tube, coated with a transparent heater has also been conducted. The complete evaporation process in a heated circular mini-channel has been studied visually in detail using high speed CCD camera. The study revealed the developments of the flow patterns and the behavior from bubble nucleation to the dry out of the liquid film. The bubble departure frequency, diameter, growth rate, and velocity were determined by analyzing the images. Finally, a flow pattern map for boiling flow in microchannels has been developed based on the test data.QC 20100812
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Nascimento, Francisco Júlio do. "Estudo teórico-experimental da transferência de calor e da perda de pressão em um dissipador de calor baseado em microcanais." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/18/18147/tde-14082012-113947/.
Full textAbstract:
A presente dissertação trata de um estudo teórico-experimental sobre escoamento monofásico e bifásico em um dissipador de calor baseado em microcanais. Este tipo de dissipador de calor tem sido usado para a intensificação da troca de calor em sistemas compactos e de alto desempenho. A intensificação da troca de calor promovida pelo escoamento em microcanais é acompanhada de um incremento na perda de pressão, portanto o estudo destes dois parâmetros é essencial para o entendimento dos fenômenos relacionados e fundamental para o desenvolvimento de ferramentas de projeto para dissipadores de calor baseados em microcanais. Inicialmente, um levantamento bibliográfico extenso sobre a ebulição convectiva em microcanais de reduzido diâmetro foi realizado. Este estudo da literatura trata de critérios de transição entre micro- e macro-escala, padrões de escoamento, métodos de previsão do coeficiente de transferência de calor e perda de pressão. Atenção específica foi dada a estudos de dissipadores de calor baseados em microcanais. Com base nesta análise da literatura, uma bancada experimental foi confeccionada para que dados experimentais de transferência de calor e perda de pressão pudessem ser levantados a partir de um dissipador de calor de microcanais. O dissipador de calor fabricado para este estudo é constituído de 50 microcanais retangulares dispostos paralelamente com 15 mm de comprimento, 100 µm de largura, 500 µm de profundidade e espaçados entre si de 200 µm. Experimentos foram executados para o R134a, velocidades mássicas de 400 a 1500 kg/m²s, título de vapor máximo de 0,35 e fluxos de calor de até 310 kW/m². Como conclusão deste trabalho observa-se perda de pressão elevada em relação aos valores fornecidos pelos métodos de previsão da literatura e um coeficiente de transferência de calor próximo ao estimado pelo modelo de três zonas proposto por Thome et al. (2004).This study presents a theoretical and experimental investigation on single and two-phase flows in a microchannel based heat sink. Multi-microchannel heat sinks are able of dissipating extremely high heat fluxes under confined conditions. Such characteristics have attracted the attention of academia and industry and actually several studies are being carried out in order to evaluate and optimize such devices. Initially, an extensive investigation of the literature concerning convective boiling in micro-scale channels was performed. This literature review covers transitional criteria between micro- and macro-scale flow boiling, two phase flow patterns, heat transfer coefficient and pressure drop during convective boiling. Special attention was given to studies concerning microchannels based heat sinks. Based on this investigation, an experimental facility was built for performing heat transfer and pressure drop measurements during single-phase flow and flow boiling in microchannel based heat sinks. For this study, a microchannel based heat sink was also manufactured. The heat sink contains 50 rectangular parallel microchannels, 15 mm long, 100 µm wide by 500 µm deep and separated by 200 µm walls. Experiments were performed for R134a, mass velocity of 400-1500 kg/m²s, maximum vapor quality of 0,35 and heat fluxes up to 310 kW/m². The database obtained in the present study was compared against pressure drop and heat transfer coefficient prediction methods from the literature. It was found that no one method is accurate in predicting heat sink pressure drop while heat transfer coefficient results were accurately predicted by the 3-zone model proposed by Thome et al. (2004).
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Leão, Hugo Leonardo Souza Lara. "Análise experimental dos efeitos do fluido e da orientação do escoamento no desempenho de dissipadores de calor baseados na ebulição convectiva em microcanais." Universidade de São Paulo, 2014. http://www.teses.usp.br/teses/disponiveis/18/18147/tde-19082014-102054/.
Full textAbstract:
A pesquisa realizada envolveu a avaliação experimental dos efeitos do fluido e da orientação do escoamento no desempenho de um dissipador de calor baseado na ebulição convectiva em microcanais. Estes dissipadores de calor são usados como uma nova aplicação para a refrigeração dos novos dispositivos eletrônicos que geram altas taxas de calor. Efetuou-se inicialmente uma extensa pesquisa bibliográfica sobre o escoamento monofásico e a ebulição convectiva em microcanais e em multi-microcanais através da qual levantou-se os principais métodos de previsão do coeficiente de transferência de calor e da perda de pressão. Então, utilizando o aparato experimental desenvolvido durante o mestrado de Do Nascimento (2012) avaliou-se a transferência de calor e perda de pressão de um dissipador de calor baseado em multi-microcanais paralelos. O dissipador de calor avaliado possui 50 microcanais retangulares dispostos paralelamente com 15 mm de comprimento, 100 µm de largura, 500 µm de altura e espaçados de 200 µm. Ensaios experimentais foram executados para o R245fa, fluido de baixa pressão utilizado em ciclos frigoríficos de baixa pressão, e R407C, fluido de alta pressão usado para conforto térmico, temperatura de saturação de 25 e 31°C, velocidades mássicas de 400 a 1500 kg/m²s, graus de subresfriamento do líquido de 5, 10 e 15°C, título de vapor máximo de até 0,38, fluxos de calor de até 350 kW/m², e para 3 orientações diferentes do dissipador de calor, horizontal, vertical com os canais alinhados horizontalmente e vertical com escoamento ascendente. Os resultados obtidos foram parametricamente analisados e comparados com métodos da literatura. Coeficientes de transferência de calor médios de até 35 kW/m² °C foram obtidos. Resultados adquiridos para o R245fa e R407C foram inferiores aos levantados por Do Nascimento (2012) para o R134a utilizando o mesmo dissipador. O fluido R407C apresentou frequências e amplitudes de oscilações inferiores aos fluidos R134a e R245fa. Nenhum método para o coeficiente de transferência de calor e perda de pressão proporcionou previsões satisfatórias dos dados experimentais. O modelo Homogêneo com viscosidade da mistura bifásica dada por Cicchitti et al. (1960) apresentou as melhores previsões da perda de pressão, já para o coeficiente de transferência de calor, os métodos de Bertsch et al. (2009) e Liu e Winterton (1991) apresentaram as melhores previsões. O dissipador com sua base posicionada horizontalmente fornece coeficientes de transferência de calor superiores enquanto sua base na vertical e escoamento ascendente verificam-se perdas de pressão inferiores. Imagens do escoamento bifásico foram obtidas com uma câmera de alta velocidade e analisadas.This study presents an experimental investigation on the effect of the fluid and the footprint orientation on the performance of a heat spreader based on flow boiling inside micro-scale channels. This heat spreader is used in an electronics cooling application with high-power density. Initially an extensive investigation of the literature concerning single-phase and two-phase flow inside a single microchannels and multi-microchannels was performed. In this literature review the leading predictive methods for heat transfer coefficient and pressure drop are described. The experimental study was carried out in the apparatus developed by Do Nascimento (2012). The heat sink evaluated in the present study is comprised of fifty parallel rectangular microchannels with cross-sectional dimensions of 100 µm width and of 500 µm depth, and total length of 15 mm. The fins between consecutive microchannels are 200 µm thick. Experimental tests were performed for R245fa, low-pressure fluid used in low pressure refrigeration cycles, and R407C, high-pressure fluid used for heat comfort, saturation temperature of 25 and 31°C, mass velocities from 400 to 1500 kg/m² s, degrees of subcooling of the liquid of 5, 10 and 15°C, outlet vapor quality up to 0.38, heat fluxes up to 350 kW/m², and for the following footprint heat sink orientations: horizontal, vertical with the microchannels aligned horizontally and vertical with upward flow. The results were parametrically analyzed and compared again the predictive methods from literature. Average heat transfer coefficients up to 35 kW/m² °C were obtained. The results for R134a from Do Nascimento (2012) for the same heat sink presented heat transfer coefficients higher than R245fa and R407C. The fluid R407C presented oscillation of the temperature due to thermal instability effects with lower frequency and amplitude lower than R134a, and R245fa. None predictive method provided satisfactory heat transfer coefficient and pressure drop predictions of the experimental data. The Homogeneous model with the viscosity given by Cicchitti et al. (1960) provided the best pressure drop prediction while the heat transfer coefficient was best predicted by Bertsch et al. (2009) and Liu and Winterton (1991). The horizontal orientation of the footprint provided the highest heat transfer coefficients while the vertical footprint orientation with upward flow the lowest pressure drops. Images of the two-phase flow were obtained with a high-speed camera and analyzed.
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Tibiriçá, Cristiano Bigonha. "Estudo teórico-experimental da transferência de calor e do fluxo crítico durante a ebulição convectiva no interior de microcanais." Universidade de São Paulo, 2011. http://www.teses.usp.br/teses/disponiveis/18/18147/tde-22092011-161901/.
Full textAbstract:
A pesquisa realizada tratou do estudo da transferência de calor e do fluxo crítico durante a ebulição convectiva no interior de canais de diâmetro reduzidos a partir de dados levantados em bancadas experimentais construídas para esta finalidade. Extensa pesquisa bibliográfica foi efetuada e os principais métodos disponíveis para previsão de coeficiente de transferência de calor, fluxo crítico e mapas de escoamento foram levantados. Os resultados obtidos foram parametricamente analisados e comparados com os métodos da literatura. Pela primeira vez para microcanais, resultados experimentais foram levantados por um mesmo autor em laboratórios distintos buscando verificar a tendência e comportamentos. Tal comparação tem sua importância destacada em face das elevadas discrepâncias observadas na literatura quando resultados de autores distintos, obtidos em condições similares, são comparados. Os resultados levantados foram utilizados na elaboração de modelos que consideram os padrões de escoamento observados em microcanais. A incorporação dos padrões permitiu o desenvolvimento de modelos mecanísticos para coeficiente de transferência de calor, fluxo crítico e critérios para a caracterização da transição entre macro e microcanais baseados na formação do padrão de escoamento estratificado e na simetria do filme líquido no escoamento anular.This research comprises an experimental and theoretical study on flow boiling heat transfer and critical heat flux inside small diameter tubes based on data obtained in experimental facilities specially designed for this purpose. A broad literature review was carried out and the main methods to predict the heat transfer coefficient, critical heat flux and flow patterns were pointed out. The experimental results were parametrically analyzed and compared against the predictive methods from literature. For the first time, microchannels experimental results obtained by an unique researcher in distinct laboratories were compared and a reasonable agreement was observed. The importance of such a comparison is high-lighted for flow boiling inside microchannels due to the high discrepancies ob-served when results from independent laboratories obtained under similar experimental conditions are compared. Moreover, the experimental results obtained in the present study were used to develop correlations and models for the heat transfer coefficient and heat flux that takes into account the flow patterns observed in microchannels. The heat transfer coefficient and critical heat flux models were developed based on mechanistic approach. In addition, criteria to characterize macro to microchannel transition were proposed based in the occurrence of the stratified flow pattern and the liquid film symmetry under annular flow conditions.
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Mala, Gh Mohiuddin. "Heat transfer and fluid flow in microchannels." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape8/PQDD_0005/NQ39562.pdf.
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Moore, Bryce Kirk. "Gas-liquid flows in adsorbent microchannels." Thesis, Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/47519.
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A study of two the sequential displacement of gas and liquid phases in microchannels for eventual application in temperature swing adsorption (TSA) methane purification systems was performed. A model for bulk fluid displacement in 200 m channels was developed and validated using data from an air-water flow visualization study performed on glass microchannel test sections with a hydraulic diameter of 203 m. High-speed video recording was used to observe displacement samples at two separate channel locations for both the displacement of gas by liquid and liquid by gas, and for driving pressure gradients ranging from 19 to 450 kPa m-1. Interface velocities, void fractions, and film thicknesses were determined using image analysis software for each of the 63 sample videos obtained.
Coupled 2-D heat and mass transfer models were developed to simulate a TSA gas separation process in which impurities in the gas supply were removed through adsorption into adsorbent coated microchannel walls. These models were used to evaluate the impact of residual liquid films on system mass transfer during the adsorption process. It was determined that for a TSA methane purification system to be effective, it is necessary to purge liquid from the adsorbent channel. This intermediate purge phase will benefit the mass transfer performance of the adsorption system by removing significant amounts of residual liquid from the channel and by causing the onset of rivulet flow in the channel. The existence of the remaining dry wall area, which is characteristic of the rivulet flow regime, improves system mass transfer performance in the presence of residual liquid.
The commercial viability of microchannel TSA gas separation systems depends strongly on the ability to mitigate the presence and effects of residual liquid in the adsorbent channels. While the use of liquid heat transfer fluids in the microchannel structure provides rapid heating and cooling of the adsorbent mass, the management of residual liquid remains a significant hurdle. In addition, such systems will require reliable prevention of interaction between the adsorbent and the liquid heat transfer fluid, whether through the development and fabrication of highly selective polymer matrix materials or the use of non-interacting large-molecule liquid heat transfer fluids. If these hurdles can be successfully addressed, microchannel TSA systems may have the potential to become a competitive technology in large-scale gas separation.
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Su, Qian. "Experimental investigation of condensation heat transfer in microchannels." Thesis, Queen Mary, University of London, 2007. http://qmro.qmul.ac.uk/xmlui/handle/123456789/1588.
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The thesis describes experiments aimed at measurement of heat-transfer coefficients for condensation in a multi micro channel tube. Experiments were performed with steam and R113, fluids chosen to cover a wide range of thermophysical properties, in particular, surface tension which plays an important role during condensation in small, non-circular channels. The aluminum extruded condenser tube used had cooled length 748 mm and 13 parallel channels each with height 1.38 mm and width 1.41 mm. The upper and lower outer surfaces were cooled separately by water in counter flow in channels above and below the test tube. The mass flow rates in the two channels were adjusted to be the same. Coolant temperatures were measured at 17 positions along each of the coolant channels as well as at inlet and exit. An accurate direct measurement of the overall inlet-to-outlet coolant temperature difference was also measured directly with a 10 junction thermopile for each of the two coolant streams with junctions downstream of mixers. Temperatures of the condenser tube wall were measured at 10 positions on each of the upper and lower surfaces using embedded thermocouples. Temperatures and pressures of the vapour were measured in chambers at the inlet and outlet of vapour stream. Pressures were also measured in the condenser channels just upstream and just downstream of the cooled section. Data have been obtained for cases where the vapour was saturated (for both steam and R113) at inlet. Runs were made for complete and incomplete condensation within the tube. Earlier investigations are critically reviewed and seen to exhibit wide scatter and disagreement. For reasons which will become clear in the thesis, the present results cannot, unfortunately, be claimed to have superior accuracy and generally fall within the ranges of earlier data. A new and innovative test section has been designed and will be used in forthcoming experiments.
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Barber, Jacqueline Claire. "Hydrodynamics, heat transfer and flow boiling instabilities in microchannels." Thesis, University of Edinburgh, 2010. http://hdl.handle.net/1842/4000.
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Boiling in microchannels is a very efficient mode of heat transfer with high heat and mass transfer coefficients achieved. Less pumping power is required for two-phase flows than for single-phase liquid flows to achieve a given heat removal. Applications include electronics cooling such as cooling microchips in laptop computers, and process intensification with compact evaporators and heat exchangers. Evaporation of the liquid meniscus is the main contributor to the high heat fluxes achieved due to phase change at thin liquid films in a microchannel. The microscale hydrodynamic motion at the meniscus and the flow boiling heat transfer mechanisms in microchannels are not fully understood and are very different from those in macroscale flows. Flow instability phenomena are noted as the bubble diameter approaches the channel diameter. These instabilities need to be well understood and predicted due to their adverse effects on the heat transfer. A fundamental approach to the study of two-phase flow boiling in microchannels has been carried out. Simultaneous visualisation and hydrodynamic measurements were carried out investigating flow boiling instabilities in microchannels using two different working fluids (n-Pentane and FC-72). Rectangular, borosilicate microchannels of hydraulic diameter range 700-800 μm were used. The novel heating method, via electrical resistance through a transparent, metallic deposit on the microchannel walls, has enabled simultaneous heating and visualisation to be achieved. Images and video sequences have been recorded with both a high-speed camera and an IR camera. Bubble dynamics, bubble confinement and elongated bubble growth have been shown and correlated to the temporal pressure fluctuations. Both periodic and nonperiodic instabilities have been observed during flow boiling in the microchannel. Analysis of the IR images in conjunction with pressure drop readings, have allowed the correlation of the microchannel pressure drop to the wall temperature profile, during flow instabilities. Bubble size is an important parameter when understanding boiling characteristics and the dynamic bubble phenomena. In this thesis it has been demonstrated that the flow passage geometry and microchannel confinement effects have a significant impact on boiling, bubble generation and bubble growth during flow boiling in microchannels.
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Irwansyah, Ridho [Verfasser], Christian J. [Akademischer Betreuer] Kähler, Christian J. [Gutachter] Kähler, and Christian [Gutachter] Cierpka. "On the experimental investigation of the laminar convective heat transfer of Al₂O₃-water nanofluids in a microchannel / Ridho Irwansyah ; Gutachter: Christian J. Kähler, Christian Cierpka ; Akademischer Betreuer: Christian J. Kähler ; Universität der Bundeswehr München, Fakultät für Luft- und Raumfahrttechnik." Neubiberg : Universitätsbibliothek der Universität der Bundeswehr München, 2018. http://d-nb.info/1169088996/34.
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Agarwal, Akhil. "Heat Transfer and Pressure Drop During Condensation of Refrigerants in Microchannels." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/14129.
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Two-phase flow, boiling, and condensation in microchannels have received considerable
attention in the recent past due to the growing interest in the high heat
fluxes made possible by these channels. This dissertation presents a study on
the condensation of refrigerant R134a in small hydraulic diameter (100 < Dh
< 160 mm) channels. A novel technique is used for the measurement
of local condensation heat transfer coefficients in small quality increments,
which has typically been found to be difficult due to the low heat transfer
rates at the small flow rates in these microchannels. This method is used to
accurately determine pressure drop and heat transfer coefficients for mass
fluxes between 300 and 800 kg/m2-s and quality 0 < x <
1 at four different saturation temperatures between 30 and 60oC. The
results obtained from this study capture the effect of variations in mass flux,
quality, saturation temperature, hydraulic diameter, and channel aspect ratio
on the observed pressure drop and heat transfer coefficients. Based on the available
flow regime maps, it was assumed that either the intermittent or annular flow
regimes prevail in these channels for the flow conditions under consideration.
Internally consistent pressure drop and heat transfer models are proposed
taking into account the effect of mass flux, quality, saturation temperature, hydraulic
diameter, and channel aspect ratio. The proposed models predict 95% and 94% of
the pressure drop and heat transfer data within ±25%, respectively. Both
pressure drop and heat transfer coefficient increase with a decrease in
hydraulic diameter, increase in channel aspect ratio and decrease in saturation
temperature. A new non-dimensional parameter termed Annular Flow Factor is also
introduced to quantify the predominance of intermittent or annular flow in the
channels as the geometric parameters and operating conditions change. This
study leads to a comprehensive understanding of condensation in microchannels
for use in high-flux heat transfer applications.
37
Kuan, Wai Keat. "Experimental study of flow boiling heat transfer and critical heat flux in microchannels /." Link to online version, 2006. https://ritdml.rit.edu/dspace/handle/1850/1887.
Full text
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Injeti, Phaninder. "Numerical simulation of steady state and transient heat transfer in microchannels." [Tampa, Fla.] : University of South Florida, 2007. http://purl.fcla.edu/usf/dc/et/SFE0002157.
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Tu, Kuan-Hsu, and 凃冠旭. "Condensation Heat Transfer Enhancementin Microchannel." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/61905943294136803719.
Full textAbstract:
碩士國立臺灣大學
機械工程學研究所
101
The microchannel evaporator with two-phase heat transfer achieves a high heat transfer coefficient, and low working fluid demands; therefore, is considered to have high potential. Thus, the use of microchannel condenser that uses two-phase heat transfer is seen as a cooling component with very high potential. By making it a porous structure that creates a 3D porous network, resulting in advantages such as large evaporation area, high capillary force, and high permeability, the heat transfer performance of the microchannel condenser is expected to increase significantly. This study used copper to manufacture both the flat-plate microchannel condenser and porous microchannel condenser, with 30 microchannels of width and depth 500μm×155μm; Using water as working fluid, with mass flux range of 65~95 kg/m2 s, for heat transfer performance test. This study first investigates the effect of copper powder size and the structure’s base thickness of a porous microchannel condenser on heat transfer performance, then compares its heat transfer performance, pressure drop, and flow patterns to those of flat-plate microchannel condenser. Comparing experimental results for heat transfer performance to heat transfer correlation of the conventional channel showed that the MAE is still large. With regard to pressure drop, compare with the correlation of microchannel developed recently, it correlated with our result, indicating a certain degree of reliability. For flat-plate microchannel condenser, from flow visualizations of flows, 5 most common types of flow in condensation process can be seen clearly: droplet flow, annular flow, injection flow, slug flow, and bubbly flow. Heat transfer coefficient and pressure drop is positive correlative with increasing mass flux. When mass flux increases, the flow velocity increases, and the liquid-vapor interface shear stress increases, resulting in thinning of the liquid film, and the annular flow region increases; the heat transfer performance was thus enhanced accordingly. Heat transfer coefficient is 23~79kw/m2k. The overall pressure drop was also enhanced due to increased flow rate of the working fluid and elongation of the two-phase region. For porous microchannel condenser, manufacturing parameters such as the base thickness range of 150~300μm and copper powder diameter range of 1~150μm was investigated. Experimental results showed that highest heat transfer coefficient was achieve with base thickness of 150μm and powder diameter of 88μm; heat transfer coefficient is 43~161kw/m2k, on average, the heat transfer coefficient of porous microchannel condenser was increased by 110% compared to that of flat-plate microchannel condenser. The absorption of condensed water build up by a porous microchannel structure allows for the thinning of the condensed liquid film and the annular flow region is more extended, therefore its heat transfer performance is better than that of a regular flat-plate microchannel condenser. Concerning pressure drop, the overall pressure drop is greater than that for flat-plate microchannel condenser, with a greatest enhancement value of 15kpa. To summarize this study, porous microchannel effectively enhance the heat transfer performance of condenser, It is highly potential for the high-power thermal management application.
40
Huang, Yu Hsiang, and 黃友相. "Laminar Heat Transfer in Microchannel." Thesis, 1999. http://ndltd.ncl.edu.tw/handle/33228770672319143894.
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41
Li, Po-Yen, and 李柏諺. "Condensation Heat Transfer Enhancement by Porous Microchannel." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/28395483904510377861.
Full textAbstract:
碩士國立臺灣大學
機械工程學研究所
100
The microchannel evaporator with two phase heat transfer is considered to be one of the most potential cooling techniques because of its high heat flux, good temperature uniformity and the lesser requirement for coolant flow rate. In recent years, the heat dissipation rate of the high tech products has increased day by day. The traditional single-phase heat exchanger could not efficiently cool down in a limited area, so the microchannel condenser with two phase heat transfer is regarded as a high potential cooling component in the future. The central purpose of the present research is to enhance the condensation heat transfer by utilizing the two pore size distributions of a biporous surface structure. This surface is sintered from the mixture of dendritic copper powders and the pore former, Na2CO3, which formed the different size pores in the microchannel. By changing the volumetric ratio of pore former, it was able to alter the porosity and the numbers of larger pores, further increasing the heat transfer coefficient. During condensation, vapor could go through the larger pores. The smaller pores could absorb the liquid and help to reduce the liquid film thickness. It decreased the heat resistant and increased the heat transfer coefficient. First, a plane surface microchannel system was built as a compared base. The test section of the 30 channels where the width and the depth is 500μm and 155μm, respectively. The test section was made by oxygen-free copper. Water steam is using as working fluid. In the experiment of the plane surface microchannel, the heat transfer coefficient and the pressure drop werw positively related to the increasing mass flux. When increasing the mass flux, the velocity of working fluid becomes faster due to the increasement of wall shear stress. Therefore, it caused the thickness of the liquid film much thinner, decreased the heat resistance and also increased the heat transfer coefficient. Compared with the heat transfer correlation of the conventional channel, the result showed the MAE is quite large. That means there is much room to make progress on the heat transfer correlation of the microchannel. With regard to the pressure drop, comparing with the correlation of microchannel in recent years, it considerably correlated with the results. That shows the result is reliable. For experiment of the biporous surface microchannel, the parameter with copper powder is 61~70 μm diameter and volumetric ratio of Na2CO3 is 30%. Comparing with the plane surface microchannel, the results showed that the heat transfer coefficient is enhanced to 72.9% on average when increased the pressure drop to 28.6% on average. The main reasons of enhancing the heat transfer are high water absorbing capacity and good ability for reducing liquid thickness.
42
Liu, Bing-Han, and 劉秉翰. "Heat Transfer Enhancement in Porous Microchannel Evaporator." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/25368331614143280375.
Full textAbstract:
博士國立臺灣大學
機械工程學研究所
99
The microchannels evaporator, which possesses the advantages of high heat transfer coefficient, good temperature uniformity, and small requirement for coolant flow rates, is considered as a potential cooling technology. In recent years, the raising of heat dissipation rate in electrical products becomes an important issue. The heat-transfer enhanced microchannels are suitable for the applications. The porous structure with a large number of nuclear sites as well as the re-entrance cavities is expected to enhance the heat transfer performance in a microstructure. In the present study, porous microchannel evaporators are designed and manufactured. The effects of powder size, thickness of structure, and pore size distribution upon the heat transfer performance are investigated. The comparisons of heat transfer characteristics, pressure drop, pressure instability, and heat transfer enhanced effects between the plane and the porous microchannel evaporator are made. The flow boiling experiments were conducted with a plane and a porous microchannel evaporator using R-134a as coolant. Both microchannels had 62 channels (225μm in width; and 660μm in depth) on copper substrates with one square inch in area. For the plane microchannel evaporators, the results showed that the nucleation boiling and the force convection boiling mechanisms both appeared in microchannels. When the quality in the microchannels was smaller than 0.4, the heat transfer coefficient mainly increased with increasing heat flux and did not vary with the mass flow rate or the quality. This region (quality was under 0.4) was dominated by the nucleation boiling mechanism. On the other hand, when the quality was larger than 0.4, the heat transfer coefficient increased with a increasing mass flux. This region (quality was over 0.4) was dominated by the force convection boiling. The experiment results were substituted into the correlations in which the surface tension force was taken into consideration. The predictions showed a good agreement with experimental data. The critical heat flux (CHF) increased with increasing flow rates. A CHF correlation that incorporates the surface tension force showed an excellent accuracy for the experimental data. Pressure drop were raised by increasing flow rates and heat fluxes. The separation model incorporating surface tension force had a good agreement. The pressure drop oscillation suggested that the presence of instability inside the plane microchannels as well as the maximum amplitude of oscillation were found near the onset of nucleation (ONB). For the porous microchannels evaporator, the experimental results depicted that the heat transfer coefficient reached a peak value at low quality and decreased with a increasing quality. However, the heat transfer coefficient did not vary with the mass flow rate. This was apparently different from the plane microchannels. The heat-transfer behavior dominated by the mass fluxes belongs to the force convection boiling mechanism. In contrast of the plane microchannel evaporator, the heat transfer coefficient in the porous microchannels evaporator had an enhancement of 5 times in average. The CHF in porous microchannel evaporator increased with increasing mass fluxes and did not enhanced significantly. Furthermore, the trend of pressure drop in porous microchannel was similar in the plane microchannels. The pressure drop was higher than plane microchannels; however, the maximum pressure drop was not over 50%. The amplitude of average pressure drop oscillation near the high heat flux as well as ONB was 1/6 and 1/2 smaller than in the plane microchannels. This result presented that the porous microchannels evaporators provided a stable boiling behavior when the nucleation began. The porous microchannel evaporators were sintered under the following parameters: the powder diameter dp ranged from 1~100μm, thickness of porous structure δ ranged from 150~375μm, and δ/dp ranged from 2~20, respectively. The investigation on the effect of particle size dp as well as thickness δ indicated that the ratio of the thickness to the particle size δ/dp had a significance in the heat transfer performance. This ratio must be properly chosen in order to reach a better heat transfer performance. The better ratio of δ/dp was between 8~12 in our work. Moreever, the pore size distribution dominated the heat transfer behavior. Smaller pore size with a higher heat transfer capacity. The bi-porous structure was better than the mono-porous structure in about 2 times. To conclude the present study, the porous microchannel evaporator is highly potential for the industrial applications.
43
Chiu, Yi-Shan, and 邱義善. "Investigation of Heat Transfer on Microchannel Evaporator." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/24616800940770022083.
Full textAbstract:
碩士國立臺灣大學
機械工程學研究所
95
Microchannles provide a large heat transfer surface area per unit flow volume. Phase change in microchannel evaporator makes it desirable for three reasons：(1) high critical heat flux (2) high heat transfer coefficient (3) low coolant flow rate. Therefore, they are well suited for high heat flux removal and high temperature uniformity cooling applications. Present research successfully established a reliable microchannel evaporator experimental system to investigate heat transfer behavior in microchannels and enhance heat transfer performance by sintered porous structure. The working liquid used is refrigerant R-134a, operating pressure is 8 bar, and mass flux ranges from 222 to 464kg/m2s. The microchannel evaporator was fabricated from oxygen-free copper, and top platform was cut to form 62 parallel rectangular 225μm×660μm microchannels. The top platform of porous microchannel evaporator was sintered to form 62 parallel rectangular 210μm×660μm microchannels. The thickness of porous surface structure is 96μm, and the porosity is 54%. The average particle size is 30μm. The results reveal that flow boiling pressure drop is primarily affected by mass velocity and heat flux, which increases with increasing mass velocity and heat flux. The predictability of separated flow model is much better than homogeneous equilibrium model on flow boiling pressure drop in microchannel, and the lowest MAE is 10.6%. Flow boiling in microchannel can be classified either as boiling-dominated region or convection-dominated region. In boiling-dominated region, the heat transfer coefficient increases with increasing heat flux. In convection-dominated region, the heat transfer coefficient decreases with decreasing vapor quality. These two region are separated by the peak value of heat transfer coefficient, and this separation will change if mass velocity differs. The heat transfer data closely match with some previous correlations, and the lowest MAE is 13.6%. The critical heat flux is primarily affected by mass velocity, which increases with increasing mass velocity. The CHF data also closely match with some previous correlations, and the lowest MAE is 2.6%. As for heat transfer enhancement, in the same volume flow rate 167 ml/min, the heat transfer coefficient and CHF of porous microchannel evaporator is enhanced by 2-3.8 times and 19-23% respectively.
44
Li, Wei-ping, and 李偉平. "Heat Transfer Analysis and Channel Designs of Microchannel Heat Sinks." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/96896024583791424982.
Full textAbstract:
碩士國立臺南大學
綠色能源科技研究所碩士班
100
In this thesis, the numerical analysis is performed to examine the possible methods to enhance the heat transfer performance of microchannel heat sink (MCHS) by computational fluid dynamics software. The effects of geometric parameters and nanofluids are discussed in details for the enhancement of heat transfer performance in MCHs. For the design of two-layer MCHS, the effects of geometric parameters such as channel number, channel width ratio, channel aspect ratio, and pumping power on the temperature distribution and thermal resistance are discussed in details. For the design of tapered MCHS, the effects of tapered ratios of height and width the thermal performance are discussed. For the nanofluids, the double-layered MCHS with different particle volume fractions, particle sizes, and pumping powers are presented. Predictions show that the heat transfer performance of the two-layered MCHS can be improved for a system. For the triple-layered MCHS, higher performance is found for a system with lower aspect ratio of the middle layer when the aspect ratio of the bottom layer is fixed. As for the tapered MCHS, the tapered channel in MCHS would affect the flow field and pressure drop. The pressure drop increases with both the tapered ratios in height and width. For fixed pumping power, the effects of tapered channel in width on the thermal performance. With fixed pumping power, the nanofluid MCHSs with lower base fluid viscosity have a more effective heat transfer enhancement, relatively to those of pure fluid MCHSs. Besides, the predicted showed that best thermal performance of MCHS is found for a nanofluid with 1% particle volume fraction.
45
Tunc, Gokturk. "Convective heat transfer in microchannel gaseous slip flow." Thesis, 2002. http://hdl.handle.net/1911/18142.
Full textAbstract:
A new set of slip boundary conditions is developed to be used beyond the slip flow-early transition by using more accurate representation of the velocity and temperature gradients at the wall. The new model agrees well with the results from the solution of the Boltzman equation.
The effect of rarefaction on steady-state heat transfer in microchannels in the slip flow regime is investigated by the integral transform technique with the implementation of the first order slip boundary conditions. Uniform temperature and/or uniform heat flux boundary conditions are considered for flow between two parallel plates, in circular and rectangular channels and annular sections. Thermal entrance length is solved as well as the fully developed region. Transient effects are obtained by performing the analysis for a cylindrical pipe with a sudden wall temperature change. Two characteristics of rarefaction namely the velocity slip and the temperature jump have opposite effects on heat transfer. It is found that the Nusselt number decreases with increasing rarefaction. Viscous heat dissipation is also included in the analyses and the change in the heat transfer due to this effect is clarified. Viscous heating may increase or decrease the heat transfer coefficient depending on the direction of the external heat transfer.
46
Chen, Kuei-Yen, and 陳奎延. "Wettability Effect on Microchannel Condenser Heat Transfer Enhancement." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/18284612827192116300.
Full textAbstract:
碩士國立臺灣大學
機械工程學研究所
100
In recent years, the microchannel evaporator with two phase heat transfer, due to its highly heat flux and little requirement for coolant flow rate, is considered as one of potential cooling techniques. When the traditional single phase heat exchanger cannot efficiently cool in a limited area, collocating the microchannel condenser with two phase heat transfer is regarded as a potential cooling component. Because of the highly developed technology, the heat dissipation rate raise in many products. Therefore the microchannel condenser with enhanced heat transfer is more applicale. The hydrophobic surface has wide contact angle, and worse wettbility. In the process of condensation it will form dropwise condensation. The heat transfer coefficient is increased dramatically in the large scale condenser within hydrophobic surface. We assume that it will show the same result in the microchannel condenser. Thus, this research design and manufacture the hydrophobic and hydrophilic microchannel condenser, compare to the heat transfer coefficient and pressure drop with uncoated microchannels condenser. The test section has 30 channels 500μm in width and 155μm in depth using water as working fluid. Using layer-by-layer (LbL) assembly method manufacture the hydrophobic and hydrophilic structure the same geometric dimensions microchannel condenser. In the experiment of the uncoated microchannel, the heat transfer coefficient and pressure drop is positive correlative with the increasing mass flux. Because increasing the mass flux, the velocity becomes faster, along with increasing the wall shear stress. This will make the thickness of the liquid film become much thinner, and reduce the heat resistant, further increase the heat transfer coefficient. Compared with the heat transfer correlation of the conventional channel, the result shows the MAE is still large. Currently, there is still much room to make progress on the heat transfer correlation of the microchannels. With regard to the pressure drop, compare with the correlation of micrchannel developed recently, it correlated with our result and shows that our result is reliable. In coated surface, we use layer-by-layer (LbL) assembly method to change the contact angle between water and copper, to manufacture the hydrophobic and hydrophilic structure. The contact angle of the hydrophilic structure change from 87°to 43°. Also the contact angle of the hydrophobic structure rise up to 135°. Compared with the heat transfer coefficient of the uncoated surface, the hydrophobic microchannels can increase roughly 100% on average, with remarkable difference. The droplet cannot adhere on the hydrophobic surface to cause the dropwise condensation. The mechanism is different from the filmwise condensation. Therefore, the heat transfer coefficient is much higher. When the mass flux is small, the heat transfer coefficient doesn’t increase remarkably. Because the velocity is slower, the liquid film is easy to form. Therefore, the mechanism that dropwise condensation could increase the heat transfer coefficient cannot be observed within the small mass flux in this experiment. As increasing the mass flux, the velocity is so fast that is not easy to form the liquid film. This will make the heat transfer coefficient increase remarkably. For the pressure drop, the contact angle of the hydrophilic structure is smaller, and it can extend liquid, making the flow easier. In this experiment the pressure drop decrease about 40% on average. It shows that the hydrophilic structure can greatly improve the pressure drop for the microchannel condenser.
47
Chuang, Jason, and 莊志升. "Experimental Study of Heat Transfer in Nanofluid-cooled Microchannel Heat Sink." Thesis, 2005. http://ndltd.ncl.edu.tw/handle/25517146589492126235.
Full textAbstract:
碩士國立中興大學
機械工程學系
93
The major goal of this study is to investigate the microchannel heat sink performance using nanofluids for the coolant. Pure water, nanofluids with volume fraction of 0.204%, 0.25%, 0.294% and 0.4%.and Ethylene glycol- nanofluids with volume fraction of 0.208% are employed in this study. Under the fixed heating power, microchannel heat sink performance in terms of thermal resistance and overall Nusselt number are evaluated bead on the measured with temp variations along the heat sink base plate . The coolant flow rate employed in the rage of 10 to 20 ml/min. As comparedwith the pure water-cooled microchannel heat sink, theexperiment results show the nanofluid-cooled heat sink has better performance when the flow rate is low. At high coolant volume flow rate,nanofluid-cooled microchannel heat sink is worse than pure water–cooled one due to serious nanoparticle agglomeration and deposition. Suitable dispersion agent in nanofluid is required in the heat sink application in order to enhance the device performance.
48
Chou, Chi-Sheng, and 周記生. "Flow boiling Heat Transfer Enhancement in Porous Microchannel Evaporator." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/38925659236684849096.
Full textAbstract:
碩士國立臺灣大學
機械工程學研究所
100
The microchannel evaporator,which possesses the advantage of high heat transfer coefficient,good temperature uniformity,and small requirement for coolant flow rates,is considered as a potential cooling technology.The porous structure with a large number of nucleation site density as well as the reentrant grooves is to enhance the heat transfer performance in the microchannels evaporator. In present study,the flow boiling experiments were conducted with a plane and porous microchannels evaporator on one square inch copper substrates. Using water as working fluid,the mass flux from103~207 kg/m^2 s and the saturated pressure of 140kpa. Both microchannels have 62 channels(225μm in width；and 660μm in depth).The effects of powder size,thickness of structure upon heat transfer performance are investigated.The comparsions of heat transfer characteristics,pressure drop, pressure instability,and heat transfer enhanced effects between the plane and the porous microchannels evaporator are made.Finally,the comparisons of heat transfer performance,pressure drop,pressure instability between two different working fluid water and R-134a in microchannels. The experiment results were substituted into the heat transfer correlations in which the surface tension force was taken into consideration.The mean average error was16.5%. Pressure drop raised by increasing heat fluxes,but did not vary with increasing mass flux.The experiment results were substituted into the separation model incorporating surface tension force. The mean average error was 21.3%. The pressure drop oscillation suggested that the presence of instability inside plane microchannels as well as the maximum amplitude of oscillation were found near the onset of nucleation. The porous microchannel evaporators were sintered under the following parameters: the powder diameter dp ranged from 1~100μm, thickness of porous structure δ ranged from 225~375μm, and δ/dp ranged from 3~20, respectively. The investigation on the effect of particle size dp as well as thickness δ indicated that the ratio of the thickness to the particle size δ/dp had a significance in the heat transfer performance. This ratio must be properly chosen in order to reach a better heat transfer performance. The better ratio of δ/dp was between 3~4 in our work,withδ 225μm and dp 53μm.The average heat transfer coefficient enhanced about 3 times larger than the plane microchannels. For the porous microchannels evaporator,the heat transfer results different from the plane microchannels evaporator,heat transfer coefficient varied with varing mass flux.Pressure drop in porous microchannel evaporator was raised by increasing heat fluxes.The pressure drop was higher than plane microchannels；however,the maximum pressure drop was not over 50%. The maximum amplitude of oscillation was 66% lower than plane microchannels.This result presented that the porous microchannels evaporator provided a stable boiling behavior when nucleation began. For the porous microchannels： Working fluid water,the better ratio ofδ/dp was between 3~4；however, the better ratio ofδ/dp was between 8~12 when R-134a as working fluid.Surface tension force was probably the different choose between the better ratio ofδ/dp .The comparisons between two different working fluid water and R-134a in microchannels： The pressure results showed that water in the plane microchannels,its maximum amplitude of oscillation was larger than R-134a.The maximum amplitude of oscillation was obviously lower than the plane microchannels in two different working fluids. To conclude the present study, the porous microchannel evaporator is highly potential for the industrial applications
49
Hsiao, Pang Chi, and 蕭邦佶. "Analysis of Gaseous Flow and Heat Transfer in Microchannel." Thesis, 2001. http://ndltd.ncl.edu.tw/handle/16210586071884750049.
Full textAbstract:
碩士國立中興大學
機械工程學系
89
The major purpose of present study is to investigate the characteristics of flow and heat transfer in a microchannel. The fluid drived by difference pressure at inlet and outlet in laminar flow to investigate compressibility and rarefaction to case the effect of gas flow and heat transfer. In flow, the fluid is assumed to be fully-developed and constant temperature. The investigation shows that the pressure distribution and velocity distribution are the same as Arkilic et al.(1997). Furthermore, the present study employs conservation of mass viewpoint, get the relationship between mass flow rate, inlet/outlet pressure ratio, and the length of microchannel. In heat transfer, the fluid is assumed to be incompressible, the present study is Graetz problem extended to the effect of rarefaction at constant heat flux and constant wall temperature. The investigation shows that the length of thermal fully-developed is longer than traditional heat transfer expected, and Nusselt numbers are decrease with increasing Knudsen number. Furthermore, the study gains Nusselt numbers are function of length of microchannel, and higer Nusselt numbers distribution at higer inlet/outlet pressure ratio when consider effects of compressibility and rarefaction.
50
Lo, Chung-Yeu, and 羅仲禹. "DSMC of Gaseous Flow and Heat Transfer in Microchannel." Thesis, 2001. http://ndltd.ncl.edu.tw/handle/86644974844481757793.
Full textAbstract:
碩士國防大學中正理工學院
兵器系統工程研究所
89
The study of micro-channel flow is partly in response to the need for thermal control in the operation of MEMS in which the range of gas flow is from slip flow to transition regime. It will lead to incorrect results if we consider gas flow in micro-channel as continuum phenomena. In this paper, molecular approach DSMC has been used to study the flow and heat transfer characteristic of rarefied gas in micro-channel. In one-dimensional simulation, a constant acceleration body force is applied to the system and the flow is restricted in laminar and subsonic state. The simulation results show that the discrepancies of hydrodynamic prediction are widening as indicated from velocity and temperature profiles when the flow in continuum regime transfers to slip flow regime, then low transition regime by increasing Knudsen number（Kn）. The data predicted by VHS model differ quantitatively from HS model, but it exists qualitative consistency between them. The macroscopic flow phenomena could be related to and described by the microscopic molecular motion based on simulation results. Pressure-driven flows in micro-channel are simulated by varying inlet/exit pressure for a range of slip to transition regime flows. Both ambient and hot wall temperature cases are investigated. The simulation results of the former case show that the temperature in the flow field is lower than that of the channel wall. It is opposite to one-dimensional flow because of the difference of driving force. It is found that pressure distribution along the channel and streamwise velocity distribution in the transverse direction become more linear and flatter respectively with the increase of the Kn. In addition, the slip velocity increases along the streamwise direction. In hot surface case, the heat flux through the channel wall is more pronounced than in the cold surface case. The flow properties such as temperature, density and pressure are strongly dependent on Kn and heat transfer. The effect of heat transfer from hot wall increases the rarefaction of the flow field and the inlet influence at the same boundary condition. Additionally, the pressure ratio in the flow field is higher than that without heat transfer.