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Malan, A. G., e R. W. Lewis. "An artificial compressibility CBS method for modelling heat transfer and fluid flow in heterogeneous porous materials". International Journal for Numerical Methods in Engineering 87, n. 1-5 (11 febbraio 2011): 412–23. http://dx.doi.org/10.1002/nme.3125.
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Shajii, A., e J. P. Freidberg. "Theory of low Mach number compressible flow in a channel". Journal of Fluid Mechanics 313 (25 aprile 1996): 131–45. http://dx.doi.org/10.1017/s0022112096002157.
Testo completoAbstract (sommario):
The properties of a relatively uncommon regime of fluid dynamics, low Mach number compressible flow are investigated. This regime, which is characterized by an exceptionally large channel aspect ratio L/d ∼ 106 leads to highly subsonic flows in which friction dominates inertia. Even so, because of the large aspect ratio, finite pressure, temperature, and density gradients are required, implying that compressibility effects are also important. Analytical results are presented which show, somewhat unexpectedly, that for forced channel flow, steady-state solutions exist only below a critical value of heat input. Above this value the flow reverses against the direction of the applied pressure gradient causing fluid to leave both the inlet and outlet implying that the related concepts of a steady-state friction factor and heat transfer coefficient have no validity.
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Romano, V., U. Tammaro e P. Capuano. "A 2-D FEM thermal model to simulate water flow in a porous media: Campi Flegrei caldera case study". Nonlinear Processes in Geophysics 19, n. 3 (10 maggio 2012): 323–33. http://dx.doi.org/10.5194/npg-19-323-2012.
Testo completoAbstract (sommario):
Abstract. Volcanic and geothermal aspects both exist in many geologically young areas. In these areas the heat transfer process is of fundamental importance, so that the thermal and fluid-dynamic processes characterizing a viscous fluid in a porous medium are very important to understand the complex dynamics of the these areas. The Campi Flegrei caldera, located west of the city of Naples, within the central-southern sector of the large graben of Campanian plain, is a region where both volcanic and geothermal phenomena are present. The upper part of the geothermal system can be considered roughly as a succession of volcanic porous material (tuff) saturated by a mixture formed mainly by water and carbon dioxide. We have implemented a finite elements approach in transient conditions to simulate water flow in a 2-D porous medium to model the changes of temperature in the geothermal system due to magmatic fluid inflow, accounting for a transient phase, not considered in the analytical solutions and fluid compressibility. The thermal model is described by means of conductive/convective equations, in which we propose a thermal source represented by a parabolic shape function to better simulate an increase of temperature in the central part (magma chamber) of a box, simulating the Campi Flegrei caldera and using more recent evaluations, from literature, for the medium's parameters (specific heat capacity, density, thermal conductivity, permeability). A best-fit velocity for the permeant is evaluated by comparing the simulated temperatures with those measured in wells drilled by Agip (Italian Oil Agency) in the 1980s in the framework of geothermal exploration. A few tens of days are enough to reach the thermal steady state, showing the quick response of the system to heat injection. The increase in the pressure due to the heat transport is then used to compute ground deformation, in particular the vertical displacements characteristics of the Campi Flegrei caldera behaviour. The vertical displacements range from 1 cm to 10 cm in accordance with the mini uplift, characterizing the recent behaviour of the caldera. The time needed to move fluid particles from the bottom to the upper layer (years) is compatible with the timing of the mini uplift.
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Cortellessa, Gino, Fausto Arpino, Simona Di Fraia e Mauro Scungio. "Two-phase explicit CBS procedure for compressible viscous flow transport in porous materials". International Journal of Numerical Methods for Heat & Fluid Flow 28, n. 2 (5 febbraio 2018): 336–60. http://dx.doi.org/10.1108/hff-02-2017-0080.
Testo completoAbstract (sommario):
Purpose In this work, a new two-phase version of the finite element-based Artificial Compressibility (AC) Characteristic-Based Split (CBS) algorithm is developed and applied for the first time to heat and mass transfer phenomena in porous media with associated phase change. The purpose of this study is to provide an alternative for the theoretical analysis and numerical simulation of multiphase transport phenomena in porous media. Traditionally, the more complex Separate Flow Model was used in which the vapour and liquid phases were considered as distinct fluids and mathematically described by the conservation laws for each phase separately, resulting in a large number of governing equations. Design/methodology/approach Even though the adopted mathematical model presents analogies with the conventional multicomponent mixture flow model, it is characterized by a considerable reduction in the number of the differential equations for the primary variables. The fixed-grid numerical formulation can be applied to the resolution of general problems that may simultaneously include a superheated vapour region, a two-phase zone and a sub-cooled liquid region in a single physical domain with irregular and moving phase interfaces in between. The local thermal non-equilibrium model is introduced to consider the heat exchange between fluid and solid within the porous matrix. Findings The numerical model is verified considering the transport phenomena in a homogenous and isotropic porous medium in which water is injected from one side and heated from the other side, where it leaves the computational domain in a superheated vapour state. Dominant forces are represented by capillary interactions and two-phase heat conduction. The obtained results have been compared with the numerical data available in the scientific literature. Social implications The present algorithm provides a powerful routine tool for the numerical modelling of complex two-phase transport processes in porous media. Originality/value For the first time, the stabilized AC-CBS scheme is applied to the resolution of compressible viscous flow transport in porous materials with associated phase change. A properly stabilized matrix inversion-free procedure employs an adaptive local time step that allows acceleration of the solution process even in the presence of large source terms and low diffusion coefficients values (near the phase change point).
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Azadbakhti, Reza, Farzad Pourfattah, Abolfazl Ahmadi, Omid Ali Akbari e Davood Toghraie. "Eulerian–Eulerian multi-phase RPI modeling of turbulent forced convective of boiling flow inside the tube with porous medium". International Journal of Numerical Methods for Heat & Fluid Flow 30, n. 5 (17 luglio 2019): 2739–57. http://dx.doi.org/10.1108/hff-03-2019-0194.
Testo completoAbstract (sommario):
Purpose The purpose of this study is simulation the flow boiling inside a tube in the turbulent flow regime for investigating the effect of using a porous medium in the boiling procedure. Design/methodology/approach To ensure the accuracy of the obtained numerical results, the presented results have been compared with the experimental results, and proper coincidence has been achieved. In this study, the phase change phenomenon of boiling has been modeled by using the Eulerian–Eulerian multi-phase Rensselaer Polytechnic Institute (RPI) wall boiling model. Findings The obtained results indicate using a porous medium in boiling process is very effective in a way that by using a porous medium inside the tub, the location of changing the liquid to the vapor and the creation of bubbles, changes. By increasing the thermal conductivity of porous medium, the onset of phase changing postpones, which causes the enhancement of heat transfer from the wall to the fluid. Generally, it can be said that using a porous medium in boiling flows, especially in flow with high Reynolds numbers, has a positive effect on heat transfer enhancement. Also, the obtained results revealed that by increasing Reynolds number, the created vapor phase along the tube decreases and by increasing Reynolds number, the Nusselt number enhances. Originality/value In present research, by using the computational fluid dynamics, the effect of using a porous medium in the forced boiling of water flow inside a tube has been investigated. The fluid boiling inside the tube has been simulated by using the multi-phase Eulerian RPI wall boiling model, and the effect of thermal conductivity of a porous medium and the Reynolds number on the flow properties, heat transfer and boiling procedure have been investigated.
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Massarotti, Nicola, Michela Ciccolella, Gino Cortellessa e Alessandro Mauro. "New benchmark solutions for transient natural convection in partially porous annuli". International Journal of Numerical Methods for Heat & Fluid Flow 26, n. 3/4 (3 maggio 2016): 1187–225. http://dx.doi.org/10.1108/hff-11-2015-0464.
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Purpose – The purpose of this paper is to focus on the numerical analysis of transient free convection heat transfer in partially porous cylindrical domains. The authors analyze the dependence of velocity and temperature fields on the geometry, by analyzing transient flow behavior for different values of cavity aspect ratio and radii ratio; both inner and outer radius are assumed variable in order to not change the difference ro-ri. Moreover, several Darcy numbers have been considered. Design/methodology/approach – A dual time-stepping procedure based on the transient artificial compressibility version of the characteristic-based split algorithm has been adopted in order to solve the transient equations of the generalized model for heat and fluid flow through porous media. The present model has been validated against experimental data available in the scientific literature for two different problems, steady-state free convection in a porous annulus and transient natural convection in a porous cylinder, showing an excellent agreement. Findings – For vertically divided half porous cavities, with Rayleigh numbers equal to 3.4×106 for the 4:1 cavity and 3.4×105 for the 8:1 cavity, the numerical results show that transient oscillations tend to disappear in presence of cylindrical geometry, differently from what happens for rectangular one. The magnitude of this phenomenon increases with radii ratio; the porous layer also affects the stability of velocity and temperature fields, as oscillations tend to decrease in presence of a porous matrix with lower value of the Darcy number. Research limitations/implications – A proper analysis of partially porous annular cavities is fundamental for the correct estimation of Nusselt numbers, as the formulas provided for rectangular domains are not able to describe these problems. Practical implications – The proposed model represents a useful tool for the study of transient natural convection problems in porous and partially porous cylindrical and annular cavities, typical of many engineering applications. Moreover, a fully explicit scheme reduces the computational costs and ensures flexibility. Originality/value – This is the first time that a fully explicit finite element scheme is employed for the solution of transient natural convection in partially porous tall annular cavities.
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Qi, Xiaoni, e Yongqi Liu. "Heat Storage Performance of a Honeycomb Ceramic Monolith". Open Fuels & Energy Science Journal 7, n. 1 (31 dicembre 2014): 113–20. http://dx.doi.org/10.2174/1876973x01407010113.
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Honeycomb ceramic is the key component of the regenerative system. The three-dimensional numerical model has been established for thermal process in honeycomb regenerator. The numerical simulation was performed using FLUENT, a commercial computational fluid dynamics (CFD) code, to compare simulation results to the test data. The regenerative process of a honeycomb ceramic regenerator was simulated under different conditions. The results under different flow rates, different flowing time, different materials and different wall thickness were investigated. The work in this paper provides a theory basis and guide to the exploitation and appliance of HTAC system and the results of the numerical calculation can be used as the foundation of engineering design. The results may be utilized for design of porous media reactors and process optimization.
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Otomo, Yusuke, Edgar Santiago Galicia e Koji Enoki. "Enhancement of Subcooled Flow Boiling Heat Transfer with High Porosity Sintered Fiber Metal". Applied Sciences 11, n. 3 (29 gennaio 2021): 1237. http://dx.doi.org/10.3390/app11031237.
Testo completoAbstract (sommario):
We conducted experimental research using high-porosity sintered fiber attached on the surface, as a passive method to increase the heat flux for subcooled flow boiling. Two different porous thicknesses (1 and 0.5 mm) and one bare surface (0 mm) were compared under three different inlet subcooling temperatures (30, 50 and 70 K) and low mass flux (150–600 kg·m−2·s−1) using deionized water as the working fluid under atmospheric pressure. The test section was a rectangular channel, and the hydraulic diameter was 10 mm. The results showed that the heat flux on porous surfaces with a thickness of 1 and 0.5 mm increased by 60% and 40%, respectively, compared to bare surfaces at ΔTsat = 40 K at a subcooled temperature of 50 K and mass flux of 300 kg·m−2·s−1. An abrupt increase in the wall superheat was avoided, and critical heat flux (CHF) was not reached on the porous surfaces. The flow pattern and bubble were recorded with a high-speed camera, and the bubble dynamics are discussed.
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Xing, Z. B., Xingchao Han, Hanbing Ke, Q. G. Zhang, Zhiping Zhang, Huijin Xu e Fuqiang Wang. "Multi-phase lattice Boltzmann (LB) simulation for convective transport of nanofluids in porous structures with phase interactions". International Journal of Numerical Methods for Heat & Fluid Flow 31, n. 8 (22 marzo 2021): 2754–88. http://dx.doi.org/10.1108/hff-07-2020-0481.
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Purpose A combination of highly conductive porous media and nanofluids is an efficient way for improving thermal performance of relevant applications. For precisely predicting the flow and thermal transport of nanofluids in porous media, the purpose of this paper is to explore the inter-phase coupling numerical methods. Design/methodology/approach Based on the lattice Boltzmann (LB) method, this study combines the convective flow, non-equilibrium thermal transport and phase interactions of nanofluids in porous matrix and proposes a new multi-phase LB model. The micro-scale momentum and heat interactions are especially analyzed for nanoparticles, base fluid and solid matrix. A set of three-phase LB equations for the flow/thermal coupling of base fluid, nanoparticles and solid matrix is established. Findings Distributions of nanoparticles, velocities for nanoparticles and the base fluid, temperatures for three phases and interaction forces are analyzed in detail. Influences of parameters on the nanofluid convection in the porous matrix are examined. Thermal resistance of nanofluid convective transport in porous structures are comprehensively discussed with the models of multi-phases. Results show that the Rayleigh number and the Darcy number have significant influences on the convective characteristics. The result with the three-phase model is mildly larger than that with the local thermal non-equilibrium model. Originality/value This paper first creates the multi-phase theoretical model for the complex coupling process of nanofluids in porous structures, which is useful for researchers and technicians in fields of thermal science and computational fluid dynamics.
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MOHAMMADI, ALIASGHAR, e REGHAN J. HILL. "Dynamics of uncharged colloidal inclusions in polyelectrolyte hydrogels". Journal of Fluid Mechanics 669 (14 gennaio 2011): 298–327. http://dx.doi.org/10.1017/s0022112010005045.
Testo completoAbstract (sommario):
We calculate the dynamics of an uncharged colloidal sphere embedded in a quenched polyelectrolyte hydrogel to (i) an oscillatory (optical and magnetic) force, as adopted in classical micro-rheology, and (ii) an oscillatory electric field, as adopted in electrical micro-rheology and electro-acoustics. The hydrogel is modelled as a linearly elastic porous medium with the charge fixed to the skeleton and saturated with a Newtonian electrolyte; and the colloidal inclusion is modelled as a rigid, impenetrable sphere. The dynamic micro-rheological susceptibility, defined as the ratio of the particle displacement to the strength of an applied oscillatory force, depends on the fixed-charge density and ionic strength and is bounded by the limits for incompressible and uncharged, compressible skeletons. Nevertheless, the influences of fixed charge and ionic strength vanish at frequencies above the reciprocal draining time, where the polymer and the electrolyte hydrodynamically couple as a single incompressible phase. Generally, the effects of fixed charge and ionic strength are small compared with, for example, the influences of polymer slip at the particle surface. The electrical susceptibility, defined as the ratio of the particle displacement to the strength of an applied oscillatory electric field, is directly influenced by charge at all frequencies, irrespective of skeleton compressibility. At low frequencies, polymer charge modulates the driving (electro-osmotic) and restoring (electrostatically enhanced elastic) forces, whereas charge has no influence on the restoring force at high frequencies where dilational strain is suppressed by hydrodynamic coupling with the electrolyte. In striking contrast to charged inclusions in uncharged hydrogels (Wang & Hill, J. Fluid Mech., vol. 640, 2009, pp. 357–400), the electrical susceptibility at high frequencies is independent of electrolyte concentration. Rather, the dynamics primarily reflect the elastic modulus, charge and hydrodynamic permeability, with a relatively weak dependence on particle size. Interestingly, the dynamic mobility in the zero-momentum reference frame, which is central to the electro-acoustic response, is qualitatively different from the dynamic mobility in the skeleton-fixed reference frame. Finally, we propose a phenomenological harmonic-oscillator model to address – in an approximate manner – the dynamics of charged particles in charged hydrogels. This shows that particle dynamics at low frequencies are dominated by particle charge, whereas high-frequency dynamics are dominated by hydrogel charge.
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Das, S., Asgar Ali e R. N. Jana. "Insight into the dynamics of magneto-casson hybrid nanoliquid caused by a plate rotation". World Journal of Engineering 18, n. 1 (18 novembre 2020): 66–84. http://dx.doi.org/10.1108/wje-07-2020-0261.
Testo completoAbstract (sommario):
Purpose This paper aims to present the analytical investigation on an unsteady magneto-convective rotation of an electrically conducting non-Newtonian Casson hybrid nanoliquid past a vertical porous plate. The effects of thermal radiation, heat source/sink and hydrodynamic slip phenomenon are also taken into account. Ethylene glycol (EG) is adopted as a base Casson fluid. The Casson fluid model is accounted for to describe the rheological characteristics of non-Newtonian fluid. EG with copper and alumina nanoparticles is envisaged as a non-Newtonian Casson hybrid nanoliquid. The copper-alumina-ethylene glycol hybrid nanoliquid is considered as the regenerative coolant. Design/methodology/approach The perturbation method is implemented to develop the analytical solution of the modeled equations. Acquired solutions are used to calculate the shear stresses and the rate of heat transfer in terms of amplitudes and phase angles. Numerical results are figured out and tabled to inspect the physical insights of various emerging parameters on the pertinent flow characteristics. Findings This exploration discloses that the velocity profiles are strongly diminished by the slip parameter. Centrifugal and Coriolis forces caused by the plate rotation are found to significantly change the entire flow regime. The supplementation of nanoparticles is to lessen the amplitude of the heat transfer rate. A comparative study is carried out to understand the improvement of heat transfer characteristics of Casson hybrid nanoliquid and Casson nanoliquid. However, the Casson hybrid nanoliquid exhibits a lower rate of heat transfer than the usual Casson nanoliquid. Practical implications This proposed model would be pertinent in oceanography, meteorology, atmospheric science, power engineering, power and propulsion generation, solar energy transformation, thermoelectric and sensing material processing, tumbler in polymer manufacturing, etc. Motivated by such practical implications, the proposed study has been unfolded. Originality/value The novelty of this paper is to examine the simultaneous effects of the magnetic field, Coriolis force, suction/injection, slip condition and thermal radiation on non-Newtonian Casson hybrid nanoliquid flow past an oscillating vertical plate subject to periodically heating in a rotating frame of reference. A numerical comparison is also made with the existing published results under some limiting cases and it is found that the results are in good agreement with them. An in-depth review of the literature and the author’s best understanding find that such aspects of the problem have so far remained unexplored.
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Kubilay, Aytaç, Jonas Allegrini, Dominik Strebel, Yongling Zhao, Dominique Derome e Jan Carmeliet. "Advancement in Urban Climate Modelling at Local Scale: Urban Heat Island Mitigation and Building Cooling Demand". Atmosphere 11, n. 12 (4 dicembre 2020): 1313. http://dx.doi.org/10.3390/atmos11121313.
Testo completoAbstract (sommario):
As cities and their population are subjected to climate change and urban heat islands, it is paramount to have the means to understand the local urban climate and propose mitigation measures, especially at neighbourhood, local and building scales. A framework is presented, where the urban climate is studied by coupling a meteorological model to a building-resolved local urban climate model, and where an urban climate model is coupled to a building energy simulation model. The urban climate model allows for studies at local scale, combining modelling of wind and buoyancy with computational fluid dynamics, radiative exchange and heat and mass transport in porous materials including evaporative cooling at street canyon and neighbourhood scale. This coupled model takes into account the hygrothermal behaviour of porous materials and vegetation subjected to variations of wetting, sun, wind, humidity and temperature. The model is driven by climate predictions from a mesoscale meteorological model including urban parametrisation. Building energy demand, such as cooling demand during heat waves, can be evaluated. This integrated approach not only allows for the design of adapted buildings, but also urban environments that can mitigate the negative effects of future climate change and increased urban heat islands. Mitigation solutions for urban heat island effect and heat waves, including vegetation, evaporative cooling pavements and neighbourhood morphology, are assessed in terms of pedestrian comfort and building (cooling) energy consumption.
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Ferrari, Andrea, Aytac Kubilay, Dominique Derome e Jan Carmeliet. "Design of smart wetting of building materials as evaporative cooling measure for improving the urban climate during heat waves". E3S Web of Conferences 172 (2020): 03001. http://dx.doi.org/10.1051/e3sconf/202017203001.
Testo completoAbstract (sommario):
An urban microclimate model is used to design a smart wetting protocol for multilayer street pavements in order to maximize the evaporative cooling effect as a mitigation measure for thermal discomfort during heat waves. The microclimate model covers a computational fluid dynamics (CFD) model for solving the turbulent air, heat and moisture flow in the air domain of a street canyon. The CFD model is coupled to a model for heat and moisture transport in porous urban materials, to a radiative exchange model, determining the net solar and longwave radiation on each urban surface and to a wind driven rain model able to determine the wetting flux on each surface during a rain event. We first evaluate the evaporative cooling potential for different pavement systems during normal summer conditions after a long rain event during night in order to select an optimal pavement system. Then, we design a smart wetting protocol answering the questions ‘when’, ‘how much’ and ‘how long’ a pavement should be artificially wetted for having a maximum cooling effect. We found that a daily amount of 5mm wetting over 10 minutes in the morning, preferentially between 8:00 and 10:00 am, guarantees a maximal evaporative cooling for one day and night during a heat wave.
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Wang, Qiang, Shengli Tang, Sen Tian, Xiaojian Wei e Tiefeng Peng. "Molecular Simulations of Adsorption and Thermal Energy Storage of Mixed R1234ze/UIO-66 Nanoparticle Nanofluid". Journal of Nanomaterials 2019 (16 giugno 2019): 1–5. http://dx.doi.org/10.1155/2019/5154173.
Testo completoAbstract (sommario):
In the process of adsorption and separation of fluid molecules on the solid surface of porous nanomaterials, the mutual transformation of thermal energy and surface energy can improve the heat absorption and energy utilization efficiency of circulating working medium. In this study, the adsorption, thermal energy storage, and mean square displacement of the minimum energy adsorption configuration of R1234ze in UIO-66 were studied by molecular simulations, including molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. The results show that the thermal energy storage density of R1234ze/UIO-66 mixed working medium is significantly higher than that of pure working medium in the temperature range of 20 K-140 K. However, the increase rate of thermal energy storage density decreases significantly as temperature rises, and the mean square displacement and diffusion coefficient increase with increasing temperature.
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Abdulkarim, Ali H., Ali Ates, Kemal Altinisik e Eyüb Canli. "Internal flow analysis of a porous burner via CFD". International Journal of Numerical Methods for Heat & Fluid Flow 29, n. 8 (5 agosto 2019): 2666–83. http://dx.doi.org/10.1108/hff-07-2018-0413.
Testo completoAbstract (sommario):
Purpose This study aims to introduce a metal porous burner design. Literature is surveyed in a comprehensive manner to relate the current design with ongoing research. A demonstrative computational fluid dynamics (CFD) analysis is presented with projected flow conditions by means of a common commercial CFD code and turbulence model to show the flow-related features of the proposed burner. The porous metal burner has a novel design, and it is not commercially available. Design/methodology/approach Based on the field experience about porous burners, a metal, cylindrical, two-staged, homogenous porous burner was designed. Literature was surveyed to lay out research aspects for the porous burners and porous media. Three dimensional solid computer model of the burner was created. The flow domain was extracted from the solid model to use in CFD analysis. A commercial computational fluid dynamics code was utilized to analyze the flow domain. Projected flow conditions for the burner were applied to the CFD code. Results were evaluated in terms of homogenous flow distribution at the outer surface and flow mixing. Quantitative results are gathered and are presented in the present report by means of contour maps. Findings There aren’t any flow sourced anomalies in the flow domain which would cause an inefficient combustion for the application. An accumulation of gas is evident around the top flange of the burner leading to higher static pressure. Generally, very low pressure drop throughout the proposed burner geometry is found which is regarded as an advantage for burners. About 0.63 Pa static pressure increase is realized on the flange surface due to the accumulation of the gas. The passage between inner and outer volumes has a high impact on the total pressure and leads to about 0.5 Pa pressure drop. About 0.03 J/kg turbulent kinetic energy can be viewed as the highest amount. Together with the increase in total enthalpy, total amount of energy drawn from the flow is 0.05 J/kg. More than half of it spent through turbulence and remaining is dissipated as heat. Outflow from burner surface can be regarded homogenous though the top part has slightly higher outflow. This can be changed by gradually increasing pore sizes toward inlet direction. Research limitations/implications Combustion via a porous medium is a complex phenomenon since it involves multiple phases, combustion chemistry, complex pore geometries and fast transient responses. Therefore, experimentation is used mostly. To do a precise computational analysis, strong computational power, parallelizing, elaborate solid modeling, very fine meshes and small time steps and multiple models are required. Practical implications Findings in the present work imply that a homogenous gas outflow can be attained through the burner surfaces while very small pressure drop occurs leading to less pumping power requirement which is regarded as an advantage. Flow mixing is realizable since turbulent kinetic energy is distinguished at the interface surface between inner and outer volumes. The porous metal matrix burner offers fluid mixing and therefore better combustion efficiency. The proposed dimensions are found appropriate for real-world application. Originality/value Conducted analysis is for a novel burner design. There are opportunities both for scientific and commercial fields.
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Ip, Ralph W. L., e Elvis Iok Cheong Wan. "New Use Heat Transfer Theories for the Design of Heat Setting Machines for Precise Post-Treatment of Dyed Fabrics". Defect and Diffusion Forum 312-315 (aprile 2011): 748–51. http://dx.doi.org/10.4028/www.scientific.net/ddf.312-315.748.
Testo completoAbstract (sommario):
Fabrics are needed further treatment after dyeing to restore their original mechanical properties by suitable drying/shrinkage process because of wetted and elongated fabrics cannot be used for clothes making. Heating up the dyed fabrics at suitable temperature can restore their original shapes and geometries by releasing the internal stress introduced by dyeing process. Thus, heat setting is a commonly used post-treatment process to stabilize fabric geometrical dimensions and prevent further shrinkage. Hot air jet impingement [1] and moist heat are conventional drying methods for different applications. Despite the well establishments of these drying technologies, most of the applications are for materials like clay and paper, and few on the study of textile materials. In fact that most of the developed heat setting machines used in textile industry are only designed by empirical models and lack of theoretical bases. This situation will obstruct further improvement of the drying technology. In this paper, a theoretical basis heat transfer model is developed for a precise description of a heated air flowing process for heat setting machine design. In the machine design, a better airflow circulation strategy for an efficient drying is addressed. Equations for heat and mass transfer in moist porous materials and theories on thermo- and fluid-dynamics are used to support the machine design. Outcomes from the research are to develop a heat transfer model that provides more precise and effective calculation for heat setting machine design that unavailable from the developed machine prototypes.
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Zhang, Dan, Yanhong Wei, Xiaohong Zhan, Jie Chen, Hao Li e Yuhua Wang. "Numerical simulation of keyhole behaviors and droplet transfer in laser-MIG hybrid welding of Invar alloy". International Journal of Numerical Methods for Heat & Fluid Flow 28, n. 9 (3 settembre 2018): 1974–93. http://dx.doi.org/10.1108/hff-07-2017-0266.
Testo completoAbstract (sommario):
Purpose This paper aims to describe a three-dimensional mathematical and numerical model based on finite volume method to simulate the fluid dynamics in weld pool, droplet transfer and keyhole behaviors in the laser-MIG hybrid welding process of Fe36Ni Invar alloy. Design/methodology/approach Double-ellipsoidal heat source model and adaptive Gauss rotary body heat source model were used to describe electric arc and laser beam heat source, respectively. Besides, recoil pressure, electromagnetic force, Marangoni force, buoyancy as well as liquid material flow through a porous medium and the heat, mass, momentum transfer because of droplets were taken into consideration in the computational model. Findings The results of computer simulation, including temperature field in welded plate and velocity field in the fusion zone were presented in this article on the basis of the solution of mass, momentum and energy conservation equations. The correctness of elaborated models was validated by experimental results and this proposed model exhibited close correspondence with the experimental results with respect to weld geometry. Originality/value It lays foundation for understanding the physical phenomena accompanying hybrid welding and optimizing the process parameters for laser-MIG hybrid welding of Invar alloy.
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Fulpagare, Yogesh, Yogendra Joshi e Atul Bhargav. "Rack level transient CFD modeling of data center". International Journal of Numerical Methods for Heat & Fluid Flow 28, n. 2 (5 febbraio 2018): 381–94. http://dx.doi.org/10.1108/hff-10-2016-0426.
Testo completoAbstract (sommario):
Purpose The paper aims to capture the rack-level thermal dynamics in data center. It proposes the rack-level response experiments as well as transient Computational Fluid Dynamics (CFD) analysis to characterize the local thermal environment of the system. Design/methodology/approach A single sever simulator rack and its two neighboring racks with its cold and hot aisle containment have been modeled with known cold air supply temperature and flow rate for transient CFD analysis. The heat load was kept constant initially and varied case-to-case basis, which includes capturing the rack-level response with respect to changes in input. However, the response experiments on simulator rack were performed for 14 h by variation of server heat loads as step and ramp input. Findings The paper provides the detailed transient CFD analysis of data center racks. The local cold air flow rates and temperature at the vicinity of the racks showed significant effect due to changes in input. It was concluded that the rack-level dynamics impacts the thermal environment of data center and hence cannot be ignored. Research limitations/implications The high computing devices and faster internet demands have led to major thermal management concerns for data center operators. To tackle this issue, capturing the system thermal dynamics is imperative. However, the system-level CFD analysis is computationally expensive. Therefore, this paper deals with the rack-level transient CFD study using commercial tool STAR CCM+. Practical implications This paper includes the modeling of the servers as a porous media as well as the multigrid method to enhance the computational speed. The successful implementation of this approach validated through experiments. This would help to establish a base for research in any type of data center. Originality/value This paper provides the porous media approach to model servers and multigrid method to enhance the computational speed. At the same time, the thought of characterizing the local dynamics at the vicinity of data center racks is unique.
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Hossain, M., M. Acar e W. Malalasekera. "Modelling of the Through-air Bonding Process". Journal of Engineered Fibers and Fabrics 4, n. 2 (giugno 2009): 155892500900400. http://dx.doi.org/10.1177/155892500900400202.
Testo completoAbstract (sommario):
A computational fluid dynamics (CFD) modelling of the through-air bonding process of nonwoven fabric production is reported in this article. In the through-air process, hot air is passed through the fibrous web to heat and melt polymer fibers. Molten polymer subsequently flows to the point of contact between any two fibers to produce a bond. Two different modelling strategies are adapted to produce a comprehensive understanding of the through-air bonding process. In macroscale modelling, a CFD model is developed treating the whole web as a porous media in order to investigate the effect of process parameters. Results reveal that the time required to heat and melt the fibers decreases with the increasing porosity of the web and the velocity of hot air. The CFD modelling technique is then used to analyze the bonding process at a more fundamental level by considering the bonding of individual fibers at microscale. The effects of the fiber diameter, bonding temperature and contact angle between two fibers on the bonding time are investigated. Results show that the time required to bond fibers is weakly related to bonding temperature and fiber diameter. Fiber orientation angle, on the other hand, has significant effect on the progression of bond formation.
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Kieraś, Seweryn, Marek Jakubowski e Krzysztof Nadolny. "Simulation Studies on Centrifugal MQL-CCA Method of Applying Coolant during Internal Cylindrical Grinding Process". Materials 13, n. 11 (31 maggio 2020): 2506. http://dx.doi.org/10.3390/ma13112506.
Testo completoAbstract (sommario):
This paper describes simulation studies regarding the application of the centrifugal minimum quantity lubrication (MQL) method simultaneously with the delivery of a compressed cooled air (CCA) stream in the internal cylindrical grinding process. The idea of a new hybrid cooling and lubrication method connecting centrifugal (through a grinding wheel) lubrication by MQL with a CCA stream is described. The methodology of computational fluid dynamics (CFD) simulation studies, as well as the results of numerical simulations, are presented in detail. The aim of the simulations was to determine the most favourable geometrical and kinematic parameters of the system in the context of air-oil aerosol and CCA flow, as well as heat exchange. In the simulation, the variables were the grinding arbor geometrical parameters, the angle of CCA supply line outlets, and the grinding wheel and workpiece peripheral speed. As a result of the simulation studies, the most favourable geometrical parameters were designated, determining the orientation of the ends of the two CCA supply line outlets before and after the grinding zone, the number of openings in the drilled-out grinding arbor, and the influence of the grinding speed on the parameters of the coolant flow and temperature of objects in the grinding zone. In addition, the results of simulation tests made it possible to visualise the velocity vectors of the two-phase coolant flow in a complex system of air-oil aerosol delivery centrifugally through an open structure of a very fast rotating porous layer (grinding wheel), with an additional supply of CCA using an external cold air gun (CAG).
21
Fischer, Michael. "First-Principles Study of AlPO4-H3, a Hydrated Aluminophosphate Zeotype Containing Two Different Types of Adsorbed Water Molecules". Molecules 24, n. 5 (6 marzo 2019): 922. http://dx.doi.org/10.3390/molecules24050922.
Testo completoAbstract (sommario):
Porous aluminophosphate zeotypes (AlPOs) are promising materials for heat transformation applications using water as a working fluid. Two “types” of adsorbed water molecules can be distinguished in hydrated AlPOs: Water molecules adsorbed in the direct proximity of framework aluminium atoms form bonds to these Al atoms, with the coordination number of Al increasing from four to five or six. The remaining water molecules that are adsorbed in other parts of the accessible pore space are not strongly bonded to any framework atom, they interact with their environment exclusively through hydrogen bonds. The APC-type small-pore aluminophosphate AlPO4-H3 contains both types of H2O molecules. In the present work, this prototypical hydrated AlPO is studied using dispersion-corrected density functional theory (DFT) calculations. After validating the computations against experimental crystal structure and Raman spectroscopy data, three interrelated aspects are addressed: First, calculations for various partially hydrated models are used to establish that such partially hydrated phases are not thermodynamically stable, as the interaction with the adsorbed water molecules is distinctly weaker than in fully hydrated AlPO4-H3. Second, IR and Raman spectra are computed and compared to those of the dehydrated analogue AlPO4-C, leading to the identification of a few “fingerprint” modes that could be used as indicators for the presence of Al-coordinated water molecules. Finally, DFT-based molecular dynamics calculations are employed to study the dynamics of the adsorbed water molecules. All in all, this in-depth computational study of AlPO4-H3 contributes to the fundamental understanding of hydrated AlPOs, and should therefore provide valuable information for future computational and experimental studies of these systems.
22
Doumbia, E. Moustapha, David Janke, Qianying Yi, Guoqiang Zhang, Thomas Amon, Martin Kriegel e Sabrina Hempel. "On Finding the Right Sampling Line Height through a Parametric Study of Gas Dispersion in a NVB". Applied Sciences 11, n. 10 (17 maggio 2021): 4560. http://dx.doi.org/10.3390/app11104560.
Testo completoAbstract (sommario):
The tracer gas method is one of the common ways to evaluate the air exchange rate in a naturally ventilated barn. One crucial condition for the accuracy of the method is that both considered gases (pollutant and tracer) are perfectly mixed at the points where the measurements are done. In the present study, by means of computational fluids dynamics (CFD), the mixing ratio NH3/CO2 is evaluated inside a barn in order to assess under which flow conditions the common height recommendation guidelines for sampling points (sampling line and sampling net) of the tracer gas method are most valuable. Our CFD model considered a barn with a rectangular layout and four animal-occupied zones modeled as a porous medium representing pressure drop and heat entry from lying and standing cows. We studied three inflow angles and six combinations of air inlet wind speed and temperatures gradients covering the three types of convection, i.e., natural, mixed, and forced. Our results showed that few cases corresponded to a nearly perfect gas mixing ratio at the currently common recommendation of at least a 3 m measurement height, while the best height in fact lied between 1.5 m and 2.5 m for most cases.
23
Duggirala, Ravi K., Christopher J. Roy, S. M. Saeidi, Jay M. Khodadadi, Don R. Cahela e Bruce J. Tatarchuk. "Pressure Drop Predictions in Microfibrous Materials Using Computational Fluid Dynamics". Journal of Fluids Engineering 130, n. 7 (25 giugno 2008). http://dx.doi.org/10.1115/1.2948363.
Testo completoAbstract (sommario):
Three-dimensional computational fluid dynamics simulations are performed for the flow of air through microfibrous materials for void fractions of 0.41 and 0.47 and face velocities ranging between 0.04ms and 1.29m∕s. The microfibrous materials consist of activated carbon powder with diameters of 137×10−6m entrapped in a matrix of cylindrical fibers with diameters of 8×10−6m. These sintered microfibrous materials are a new class of patented materials with properties that are advantageous compared to traditional packed beds or monoliths. Microfibrous materials have demonstrated enhanced heat and mass transfer compared to packed beds of particles of similar dimensions. In this paper, the simulations are used to predict the pressure drop per unit length through the materials and to analyze the details of the flow that are difficult to interrogate experimentally. Various geometric approximations are employed in order to allow the simulations to be performed in an efficient manner. The Knudsen number, defined as the ratio of the mean free path between molecular collisions to the fiber diameter, is 0.011; thus, velocity-slip boundary conditions are employed and shown to have only a minor effect on the pressure drop predictions. Significant effort is made to estimate numerical errors associated with the discretization process, and these errors are shown to be negligible (less than 3%). The computational predictions for pressure drop are compared to available experimental data as well as to two theory-based correlations: Ergun’s equation and the porous media permeability equation. The agreement between the simulations and the experiments is within 30% and is reasonable considering the significant geometric approximations employed. The errors in the simulations and correlations with respect to experimental data exhibit the same trend with face velocity for both void fractions. This consistent trend suggests the presence of experimental bias errors that correlate with the face velocity. The simulations generally underpredict the experimental pressure drop for the low void fraction case and overpredict the experimental pressure drop for the high void fraction case.
24
Catton, Ivan. "Conjugate Heat Transfer Within a Heterogeneous Hierarchical Structure". Journal of Heat Transfer 133, n. 10 (11 agosto 2011). http://dx.doi.org/10.1115/1.4003576.
Testo completoAbstract (sommario):
Optimization of heat exchangers (HE), compact heat exchangers (CHE) and microheat exchangers, by design of their basic structures is the focus of this work. Consistant models are developed to describe transport phenomena in a porous medium that take into account the scales and other characteristics of the medium morphology. Equation sets allowing for turbulence and two temperature or two concentration diffusion are obtained for nonisotropic porous media with interface exchange. The equations differ from known equations and were developed using a rigorous averaging technique, hierarchical modeling methodology, and fully turbulent models with Reynolds stresses and fluxes in the space of every pore. The transport equations are shown to have additional integral and differential terms. The description of the structural morphology determines the importance of these terms and the range of application of the closure schemes. A natural way to transfer from transport equations in a porous media with integral terms to differential equations with coefficients that could be experimentally or numerically evaluated and determined is described. The relationship between computational fluid dynamics, experiment and closure needed for the volume averaged equations is discussed. Mathematical models for modeling momentum and heat transport based on well established averaging theorems are developed. Use of a “porous media” length scale is shown to be very beneficial in collapsing complex data onto a single curve yielding simple heat transfer and friction factor correlations. The general transport equations developed for a single phase fluid in a heat exchange medium have many more integral and differential terms than the homogenized or classical continuum mechanics equations. Once these terms are dealt with by closure, the resulting equation set is relatively simple and their solution is obtained using simple numerical methods quickly enough for multiple parameter optimization using design of experiment or genetic algorithms. Current efforts to significantly improve the performance of an HE for electronic cooling, a two temperature problem, and of a finned tube heat exchanger, a three temperature problem, are described.
25
Shi, Junxiang, e Xingjian Xue. "Bifunctionally Graded Electrode Supported SOFC Modeling and Computational Thermal Fluid Analysis for Experimental Design". Journal of Fuel Cell Science and Technology 8, n. 1 (1 novembre 2010). http://dx.doi.org/10.1115/1.4002141.
Testo completoAbstract (sommario):
A comprehensive 3D computational fluid dynamics (CFD) model is developed for a bi-electrode supported cell (BSC) solid oxide fuel cell (SOFC). The model includes complicated transport phenomena of mass/heat transfer, charge (electron and ion) migration, and electrochemical reactions. The uniqueness of the modeling study is that functionally graded porous electrode property is taken into account, including not only linear but also nonlinear porosity distributions. The model is validated using experimental data from open literature. Numerical results indicate that BSC performance is strongly dependent on both operating conditions and porous microstructure distributions of electrodes. Using the proposed fuel/gas feeding design, the uniform hydrogen distribution within the porous anode is achieved; the oxygen distribution within the cathode is dependent on porous microstructure distributions as well as pressure loss conditions. Simulation results also show that fairly uniform temperature distribution can be obtained with the proposed fuel/gas feeding design. This modeling work can provide a pre-experimental analysis and guide experimental designs for BSC test.
26
Qiu, Bo, e Jun Li. "Numerical Investigations on the Heat Transfer Behavior of Brush Seals Using Combined Computational Fluid Dynamics and Finite Element Method". Journal of Heat Transfer 135, n. 12 (27 settembre 2013). http://dx.doi.org/10.1115/1.4024556.
Testo completoAbstract (sommario):
Brush seals have been applied in more and more challenging high-temperature locations. The high speed bristle-rotor friction causes a considerable heat generation which accelerates the bristles wear. The frictional heat generation at bristle-rotor interface becomes another major concern in brush seal applications. This study presented detailed investigations on the heat transfer characteristics and contact mechanics of brush seals using a combined computational fluid dynamics (CFD) and finite element method (FEM) brush seal model. The CFD model of brush seal for mass and heat transfer employed Reynolds-averaged Navier–Stokes (RANS) solutions coupled with non-Darcian porous medium approach. The nonlinear contact model of brush seal was established using FEM with considerations of internal frictions (bristle to rotor, bristle to backing plate, and bristle to bristle) and aerodynamic loads on bristles. The numerical method involved iterations between CFD and FEM models to better evaluate the heat transfer behaviors of the brush seal with consideration of bristle deflections. The frictional heat generation was calculated from the product of bristle-rotor frictional force and sliding velocity. The bristle deflections and temperature distributions of the brush seal were predicted at various operational conditions using the iterative CFD and FEM brush seal model. The effects of pressure differential and rotational speed on the contact behavior, temperature distribution and bristle maximum temperature of brush seals were numerically investigated using the developed approach. The detailed pressure contours and streamline distributions of the brush seal were also illustrated.
27
DeGroot, Christopher T., Derek Gateman e Anthony G. Straatman. "The Effect of Thermal Contact Resistance at Porous-Solid Interfaces in Finned Metal Foam Heat Sinks". Journal of Electronic Packaging 132, n. 4 (24 novembre 2010). http://dx.doi.org/10.1115/1.4002724.
Testo completoAbstract (sommario):
A numerical study on the effect of thermal contact resistance and its impact on the performance of finned aluminum foam heat sinks has been conducted. Calculations are based on the solution of the volume-averaged mass, momentum, and energy equations under conditions of local thermal nonequilibrium using a finite-volume-based computational fluid dynamics code for conjugate fluid/porous/solid domains. Numerical results have been obtained for a wide range of contact resistances at the porous-solid interfaces, up to the limit of an effectively infinite resistance. As the contact resistance is increased to such high levels, the heat transfer is found to asymptote as conduction into the solid constituent of the foam is completely blocked. Even without conduction into the solid, a convective enhancement is obtained due to the presence of the foam material. It is reasoned that this is due to the thinning of the momentum boundary layers as a result of the presence of the porous material, which acts as a momentum sink. As a result of the thinner boundary layers, the flow speed near the finned surfaces and base is increased, which serves to increase the rate of convection from these surfaces. It is also found that for most reasonable interface materials, such as thermal epoxies, the impact of thermal contact resistance on the heat transfer performance in comparison to that for an ideal bond is small.
28
Mallikarjuna, B., J. Srinivas, G. Gopi Krishna, O. Anwar Bég e Ali Kadir. "Spectral Numerical Study of Entropy Generation in Magneto-Convective Viscoelastic Biofluid Flow Through Poro-Elastic Media With Thermal Radiation and Buoyancy Effects". Journal of Thermal Science and Engineering Applications 14, n. 1 (15 giugno 2021). http://dx.doi.org/10.1115/1.4050935.
Testo completoAbstract (sommario):
Abstract Electromagnetic high-temperature therapy is popular in medical engineering treatments for various diseases including tissue damage ablation repair, hyperthermia, and oncological illness diagnosis. The simulation of transport phenomena in such applications requires multi-physical models featuring magnetohydrodynamics, biorheology, heat transfer, and deformable porous media. Motivated by investigating the fluid dynamics and thermodynamic optimization of such processes, in the present article, a mathematical model is developed to study the combined influence of thermal buoyancy, magnetic field and thermal radiation on the entropy generation, and momentum and heat transfer characteristics in electrically conducting viscoelastic biofluid flow through a vertical deformable porous medium. It is assumed that heat is generated within the fluid by both viscous and Darcy (porous matrix) dissipations. The governing equations for fluid velocity, solid displacement, and temperature are formulated. The boundary value problem is normalized with appropriate transformations. The nondimensional biofluid velocity, solid displacement, and temperature equations with appropriate boundary conditions are solved computationally using a spectral method. Verification of accuracy is conducted via monitoring residuals of the solutions. The effects of various parameters on flow velocity, solid displacement, temperature, and entropy generation are depicted graphically and discussed. Increasing magnetic field and drag parameters are found to reduce the field velocity, solid displacement, temperature, and entropy production. Entropy production is enhanced with an increase in buoyancy parameter and volume fraction of the fluid. The novelty of the work is the simultaneous inclusion of multiple thermophysical phenomena, and the consideration of thermodynamic optimization in coupled thermal/fluid/elastic media. The computations provide an insight into multiphysical transport in electromagnetic radiative tissue ablation therapy and a good benchmark for more advanced simulations.
29
Vadi, Roozbeh, e Kamran Sepanloo. "Numerical Investigation of Regular and Hybrid Nanofluids Application as the Working Fluids on Thermal Performance of TPCT". Journal of Thermal Science and Engineering Applications 11, n. 4 (5 luglio 2019). http://dx.doi.org/10.1115/1.4043967.
Testo completoAbstract (sommario):
Two-phase closed thermosyphon (TPCT) is a cost-effective heat transfer device with high thermal efficiency owing to extensive interphase heat and mass transfer. Thus, TPCT has found many industrial applications. Proper selection of the working fluid could further improve efficiency of TPCT, and nanofluids with superior thermal properties are suitable choices. Numerical simulation of boiling and condensation, natural circulation, and hybrid nanofluid modeling in a closed space is a notable challenge and current study is devoted to this subject. In this study, a novel methodology for incorporating the effects of compressibility and thermal expansion into all thermophysical properties of both phases is developed and programmed into a validated computational fluid dynamics (CFD) code. Distilled water, a regular nanofluid, Al2O3/water, and a hybrid nanofluid, TiSiO4/water are selected as the working fluids. Experimental data for wall thermal profile are employed to validate the numerical simulation. Then, overall thermal resistance is evaluated in terms of nanoparticles concentration and input power variations. Results indicate that the numerical methodology developed in this study could evaluate the optimum state of TPCT in an efficient and accurate manner and the optimum state for regular and hybrid nanofluid demonstrates 48% and 54% improvement over distilled water, respectively. Furthermore, a subtle relation between the thermal resistance and the height to which fluid column rises in TPCT has been discerned and quantified, which is used as a supplement to the conventional qualitative method of reasoning to justify the somewhat controversial behaviors of nanofluid application in TPCT.
30
Alfieri, Fabio, Manish K. Tiwari, Igor Zinovik, Dimos Poulikakos, Thomas Brunschwiler e Bruno Michel. "3D Integrated Water Cooling of a Composite Multilayer Stack of Chips". Journal of Heat Transfer 132, n. 12 (22 settembre 2010). http://dx.doi.org/10.1115/1.4002287.
Testo completoAbstract (sommario):
New generation supercomputers with three dimensional stacked chip architectures pose a major challenge with respect to the removal of dissipated heat, which can reach currently as high as 250 W/cm2 in multilayer chip stacks of less than 0.3 cm3 volume. Interlayer integrated water cooling is a very promising approach for such high heat flux removal due to much larger thermal capacity and conductivity of water compared with air, the traditional cooling fluid. In the current work, a multiscale conjugate heat transfer model is developed for integrated water cooling of chip layers and validated with experimental measurements on an especially designed thermal test vehicle that simulates a four tier chip stack with a footprint of 1 cm2. The cooling heat transfer structure, which consists of microchannels with cylindrical pin-fins, is conceived in such a way that it can be directly integrated with the device layout in multilayer chips. Every composite layer is cooled by water flow in microchannels (height of 100 μm), which are arranged in two port water inlet-outlet configuration. The total power removed in the stack is 390 W at a temperature gradient budget of 60 K from liquid inlet to maximal junction temperature, corresponding to about 1.3 kW/cm3 volumetric heat flow. The computational cost and complexity of detailed computational fluid dynamics (CFD) modeling of heat transfer in stacked chips with integrated cooling can be prohibitive. Therefore, the heat transfer structure is modeled using a porous medium approach, where the model parameters of heat transfer and hydrodynamic resistance are derived from averaging the results of the detailed 3D-CFD simulations of a single streamwise row of fins. The modeling results indicate that an isotropic porous medium model does not accurately predict the measured temperature fields. The variation of material properties due to temperature gradients is found to be large; therefore, variable properties are used in the model. It is also shown that the modeling of the heat transfer in the cooling sublayers requires the implementation of a porous medium approach with a local thermal nonequilibrium, as well as orthotropic heat conduction and hydrodynamic resistance. The improved model reproduces the temperatures measured in the stack within 10%. The model is used to predict the behavior of multilayer stacks mimicking the change of heat fluxes resulting from variations in the computational load of the chips during their operation.
31
Zhou, Feng, e Ivan Catton. "A Numerical Investigation of Turbulent Flow and Heat Transfer in Rectangular Channels With Elliptic Scale-Roughened Walls". Journal of Heat Transfer 135, n. 8 (27 giugno 2013). http://dx.doi.org/10.1115/1.4024278.
Testo completoAbstract (sommario):
In the present paper, rectangular channels with six types of elliptic scale-roughened walls for heat transfer enhancement are numerically studied. Heat transfer and fluid flow characteristics for sixteen different scale-roughened models (with the scale height varying in the range from 1 mm to 2.5 mm) are numerically predicted using commercial computational fluid dynamics (CFD) code, Ansys cfx. The turbulent model employed is the k–ω based shear–stress transport (SST) model with automatic wall function treatment. In the performance evaluation, we use a “universal” porous media length scale based on volume averaging theory (VAT) to define the Reynolds number, Nusselt number, and friction factor. It is found that heat transfer performance is most favorable when the elliptic scales are oriented with their long axis perpendicular to the flow direction, while the scales elongated in the flow direction have lower Nusselt numbers and pressure drops compared with the circular scale-roughened channels. Results indicate that the scale-shaped roughness strongly spins the flow in the spanwise direction, which disrupts the near-wall boundary layers continuously and enhances the bulk flow mixing. With the flow marching in a more intense spiral pattern, a 40% improvement of heat transfer enhancement over the circular scale-roughened channels is observed.
32
Narayana, M., A. A. Khidir, P. Sibanda e P. V. S. N. Murthy. "Soret Effect on the Natural Convection From a Vertical Plate in a Thermally Stratified Porous Medium Saturated With Non-Newtonian Liquid". Journal of Heat Transfer 135, n. 3 (8 febbraio 2013). http://dx.doi.org/10.1115/1.4007880.
Testo completoAbstract (sommario):
The paper highlights the application of a recent seminumerical successive linearization method (SLM) in solving highly coupled, nonlinear boundary value problem. The method is presented in detail by solving the problem of free convection flow due to a vertical plate embedded in a non-Darcy thermally stratified porous medium saturated with a non-Newtonian power-law liquid. Thermal-diffusion (Soret) and variable viscosity effects are taken into consideration. The Ostwald–de Waele power-law model is used to characterize the non-Newtonian behavior of the fluid. The governing partial differential equations are transformed into a system of ordinary differential equations and solved by SLM. The accuracy of the SLM has been tested by comparing the results with those obtained using the shooting technique. The effect of various physical parameters such as power-law index, Soret number, variable viscosity parameter, and thermal stratification parameter on the dynamics of the fluid is analyzed through computed results. Heat and mass transfer coefficients are also shown graphically for different values of the parameters.
33
Kubilay, Aytaç, Andrea Ferrari, Dominique Derome e Jan Carmeliet. "Smart wetting of permeable pavements as an evaporative-cooling measure for improving the urban climate during heat waves". Journal of Building Physics, 4 novembre 2020, 174425912096858. http://dx.doi.org/10.1177/1744259120968586.
Testo completoAbstract (sommario):
An urban microclimate model is used to design a smart wetting protocol for multilayer street pavements in order to maximize the evaporative cooling effect as a mitigation measure for thermal discomfort during heat waves. The microclimate model is built upon a computational fluid dynamics (CFD) model for solving the turbulent air, heat and moisture flow in the air domain of a street canyon. The CFD model is coupled to a model for heat and moisture transport in porous urban materials and to a radiative exchange model, determining the net solar and thermal radiation on each urban surface. A two-layer pavement system, previously optimized for maximal evaporative cooling applying the principles of capillary pumping and capillary break, is considered to design a smart wetting protocol answering the questions “when,” “how much,” and “how long” a pavement should be artificially wetted. It was found for the current optimized pavement solutions that a daily amount of 6 mm wetting over 10 min in the morning, preferentially between 8:00 and 10:00, guarantees a maximal evaporative cooling for 24 h during a heat wave.
34
S., Shashi Prabha Gogate, Bharathi M. C. e Ramesh B. Kudenatti. "Linear Stability on the Local Thermal Nonequilibrium Model of Mixed Convection Boundary Layer Flow over a Moving Wedge in a Porous Medium: Viscous Dissipation and Radiation Effects". Journal of Heat Transfer 143, n. 4 (23 febbraio 2021). http://dx.doi.org/10.1115/1.4049514.
Testo completoAbstract (sommario):
Abstract This paper studies the local thermal nonequilibrium (LTNE) model for two-dimensional mixed convection boundary-layer flow over a wedge, which is embedded in a porous medium in the presence of radiation and viscous dissipation. It is considered that the temperature of the fluid and solid phases is not identical; hence, we require two energy equations: one for each phase. The motion of the mainstream and wedge is approximated by the power of distance from the leading boundary layer. The flow and heat transfer in the LTNE phase is governed by the coupled partial differential equations, which are then reduced to nonlinear ordinary differential equations via suitable similarity transformations. Numerical simulations show that when the interphase rate of heat transfer is large, the system attains the local thermal equilibrium (LTE) state and so is for porosity scaled conductivity. When LTNE is strong, the fluid phase reacts faster to the mainstream temperature than the corresponding solid phase. The state of LTE rather depends on radiation and viscous dissipation of the model. Further, numerical solutions successfully predicted the upper and lower branch solutions when the velocity ratio is varied. To assess which of these solutions is practically realizable, an asymptotic analysis on unsteady perturbations for a large time leading to linear stability needs to be performed. This shows that the upper branch solutions are always stable and practically realizable. The physical dynamics behind these results are discussed in detail.
35
Zhang, Li-Zhi. "Flow Maldistribution and Performance Deteriorations in Membrane-Based Heat and Mass Exchangers". Journal of Heat Transfer 131, n. 11 (26 agosto 2009). http://dx.doi.org/10.1115/1.3154832.
Testo completoAbstract (sommario):
Heat mass exchangers are crucial for the prevention of epidemic respiratory diseases such as H1N1 (swine flu). The flow maldistribution affects their performance seriously. The flow maldistribution and the consequent performance deteriorations in heat and mass exchangers are investigated. The focus is on moisture effectiveness deteriorations. As a first step, a computational fluid dynamics (CFD) code is used to calculate the flow distribution, by treating the plate-fin core as a porous medium. Then a coupled heat and moisture transfer model between the two air flows in the plate-fin channels is set up with slug flow assumption in the channels. Using the CFD predicted core face flow distribution data, the sensible heat and moisture exchange effectiveness and the performance deterioration factors are calculated with finite difference scheme. The results indicate that under current core to whole exchanger pressure drop ratio, when the channel pitch is below 2.0 mm, the flow distribution is quite homogeneous and the sensible and latent performance deteriorations due to flow maldistribution can be neglected. However, when the channel pitch is larger than 2 mm, the maldistribution is quite large and a 10–15% thermal deterioration factor and a 20–25% latent deterioration factor could be found. Mass transfer deteriorates much more than heat transfer does due to larger mass transfer resistance through membranes.
36
Biswas, Nirmalendu, Aparesh Datta, Nirmal K. Manna, Dipak Kumar Mandal e Rama Subba Reddy Gorla. "Thermo-bioconvection of oxytactic microorganisms in porous media in the presence of magnetic field". International Journal of Numerical Methods for Heat & Fluid Flow ahead-of-print, ahead-of-print (23 novembre 2020). http://dx.doi.org/10.1108/hff-07-2020-0410.
Testo completoAbstract (sommario):
Purpose This study aims to explore magnetohydrodynamic (MHD) thermo-bioconvection of oxytactic microorganisms in multi-physical directions addressing thermal gradient, lid motion, porous substance and magnetic field collectively using a typical differentially heated two-sided lid-driven cavity. The consequences of a range of pertinent parameters on the flow structure, temperature, oxygen isoconcentration and microorganisms’ isoconcentration are examined and explained in great detail. Design/methodology/approach Two-dimensional governing equations in a two-sided lid-driven porous cavity heated differentially and packed with oxytactic microorganisms under the influence of the magnetic field are solved numerically using the finite volume method-based computational fluid dynamics code. The evolved flow physics is analyzed assuming a steady laminar incompressible Newtonian flow within the validity of the Boussinesq approximation. The transport of oxytactic microorganisms is formulated by augmenting the continuum model. Findings The mechanisms involved with MHD-mixed thermo-bioconvection could have potential benefits for industrial exploitation. The distributions of fluid flow, temperature, oxygen and motile microorganisms are markedly modified with the change of convection regime. Both speed and direction of the translating walls significantly influence the concentration of the motile microorganisms. The concentration of oxygen and motile microorganisms is found to be higher at the upper portion of the cavity. The overall patterns of the fluid flow, temperature and the oxygen and microorganism distributions are markedly affected by the increase of magnetic field strength. Research limitations/implications The concept of the present study could be extended to other areas of bioconvection in the presence of gravity, light or chemical attraction. Practical implications The findings of the present study could be used to multi-physical applications like biomicrosystems, pollutant dispersion in aquifers, chemical catalytic converters, geothermal energy usage, petroleum oil reservoirs, enhanced oil recovery, fuel cells, thermal energy storage and others. Originality/value The MHD-mixed thermo-bioconvection of oxytactic microorganisms is investigated under different parametric conditions. The effect of pertinent parameters on the heat and mass transfers are examined using the Nusselt number and Sherwood number.
37
Miccoli, Claudio, Alessandro Turchi, Pierre Schrooyen, Domenic D’Ambrosio e Thierry Magin. "Detailed Modeling of Cork-Phenolic Ablators in Preparation for the Post-flight Analysis of the QARMAN Re-entry CubeSat". Aerotecnica Missili & Spazio, 28 giugno 2021. http://dx.doi.org/10.1007/s42496-021-00084-4.
Testo completoAbstract (sommario):
AbstractThis work deals with the analysis of the cork P50, an ablative thermal protection material (TPM) used for the heat shield of the qarman Re-entry CubeSat. Developed for the European Space Agency (ESA) at the von Karman Institute (VKI) for Fluid Dynamics, qarman is a scientific demonstrator for Aerothermodynamic Research. The ability to model and predict the atypical behavior of the new cork-based materials is considered a critical research topic. Therefore, this work is motivated by the need to develop a numerical model able to respond to this demand, in preparation to the post-flight analysis of qarman. This study is focused on the main thermal response phenomena of the cork P50: pyrolysis and swelling. Pyrolysis was analyzed by means of the multi-physics Computational Fluid Dynamics (CFD) code argo, developed at Cenaero. Based on a unified flow-material solver, the Volume Averaged Navier–Stokes (VANS) equations were numerically solved to describe the interaction between a multi-species high enthalpy flow and a reactive porous medium, by means of a high-order Discontinuous Galerkin Method (DGM). Specifically, an accurate method to compute the pyrolysis production rate was implemented. The modeling of swelling was the most ambitious task, requiring the development of a physical model accounting for this phenomenon, for the purpose of a future implementation within argo. A 1D model was proposed, mainly based on an a priori assumption on the swelling velocity and the resolution of a nonlinear advection equation, by means of a Finite Difference Method (FDM). Once developed, the model was successfully tested through a matlab code, showing that the approach is promising and thus opening the way to further developments.
38
Hasan, Mainul, e Latifa Begum. "Industrial Direct Chill Slab Caster of Tin Bronze (C903) Using a Porous Filter in the Hot-Top". Journal of Thermal Science and Engineering Applications 10, n. 2 (29 agosto 2017). http://dx.doi.org/10.1115/1.4037196.
Testo completoAbstract (sommario):
A 3D computational fluid dynamics (CFD) modeling study has been carried out for the tin bronze (C903) slab of industrial size in a vertical direct chill caster. The melt is delivered from the top across the entire cross section of the caster. An insulated hot-top is considered above the 80-mm mold to control the melt level in the mold. A porous filter is considered in the hot-top region of the mold to arrest the incoming inclusions and homogenize the flow into the mold. The melt flow through the porous filter is modeled on the basis of the Brinkmann–Forchheimer-extended non-Darcy model. Results are obtained for four casting speeds varying from 40 to 100 mm/min. The metal–mold contact region, as well as the convective heat transfer coefficient at the mold wall, is also varied. In addition to the above, the Darcy number for the porous media is also changed. All parametric studies are performed for a fixed inlet melt superheat of 62 °C. The results are presented pictorially in the form of temperature and velocity fields. The sump depth, mushy region thickness, solid shell thickness (ST) at the exit of the mold, and axial temperature profiles are also presented and correlated with the casting speed through regression analysis.
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Saurabh, Sandeep Kumar e D. S. Murthy. "Insights Into Thermal Transactions of a Novel Rotating Packed Bed". Journal of Thermal Science and Engineering Applications 14, n. 1 (11 giugno 2021). http://dx.doi.org/10.1115/1.4050836.
Testo completoAbstract (sommario):
Abstract The field of the rotating packed bed (RPB) and its applications in the transfer processes are multidisciplinary in nature. The achievement of significant volume reduction by employing the RPB in distillation towers has been fairly established in the mass transfer domain. Nevertheless, the prospect of RPB in the heat transfer domain still remains dormant. The current work addresses this very issue by exploring the characteristics of thermal transactions across the novel RPB device. This study succinctly presents the related aspects with multi-phase flow of participating fluids in the counter-current direction across the porous, rotating packed bed structure. However, the simultaneous involvement of these multivariate intrinsic attributes makes the understanding of the transfer phenomenon quite complex upon viewing from the experimental perspective alone. Hence, the computational fluid dynamics (CFD) tool has been used for the assimilation of the physical understanding of the thermal transactions along with the effects of operating parameters. The thermal contours, main effect, and surface plots for heat transfer rate directly contribute toward a better appreciation of the thermal transaction mechanism and could be employed for suitable volume reduction in heat exchanger devices.
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Yuan, Jinliang, Guogang Yang e Bengt Sunden. "Simulation of Surface Reactions and Multiscale Transport Processes in a Composite Anode Domain Relevant for Solid Oxide Fuel Cells". Journal of Fuel Cell Science and Technology 10, n. 2 (21 marzo 2013). http://dx.doi.org/10.1115/1.4023540.
Testo completoAbstract (sommario):
There are various transport phenomena (gas-phase species, heat, and momentum) occurring at different length scales in anode-supported solid oxide fuel cells (SOFCs), which are strongly affected by catalytic surface reactions at active triple-phase boundaries (TPBs) between the void space (for gas), Ni (catalysts for electrons), and YSZ (an electrolyte material for ions). To understand the multiscale chemical-reacting transport processes in the cell, a three-dimensional numerical calculation approach (the computational fluid dynamics (CFD) method) is further developed and applied for a composite domain including a porous anode, fuel gas flow channel, and solid interconnect. By calculating the rate of microscopic surface-reactions involving the surface-phase species, the gas-phase species/heat generation and consumption related to the internal reforming reactions have been identified and implemented. The applied microscopic model for the internal reforming reactions describes the adsorption and desorption reactions of six gas-phase species and surface reactions of 12 surface-adsorbed species. The predicted results are presented and analyzed in terms of the gas-phase species and temperature distributions and compared with those predicted by employing the global reaction scheme for the internal reforming reactions.
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Wernet, Mark P., Nicholas J. Georgiadis e Randy J. Locke. "Raman temperature and density measurements in supersonic jets". Experiments in Fluids 62, n. 3 (marzo 2021). http://dx.doi.org/10.1007/s00348-021-03162-2.
Testo completoAbstract (sommario):
AbstractPrediction of flow-field properties in supersonic jets using computational fluid dynamics (CFD) code predictions has become routine; however, obtaining accurate solutions becomes more challenging when there is a significant temperature difference between the jet core and the ambient air and/or compressibility effects are significant. Benchmark sets of flow field property data are required in order to assess current CFD capabilities and develop better modeling approaches for these turbulent flow fields where accurate calculation of temperatures and turbulent heat flux is important. Particle Image Velocimetry, spontaneous rotational Raman scattering spectroscopy, and Background-Oriented Schlieren (BOS) have been previously used to acquire measurements of the mean and root-mean-square (rms) velocities, the mean and rms gas temperatures, and density gradients in subsonic jet flows and film cooling flows. In this work, the ability to measure density is added to the list of measurands available using the acquired Raman spectra. The suite of measurement techniques are now applied to supersonic jet flows. The computation of the local gas pressure in the potential core of an over-expanded jet is demonstrated using the Raman measured gas temperature and density. Additionally, a unique density feature in temperature matched, perfectly expanded jet flow shear layers identified using BOS was verified using the Raman measurement technique. These non-intrusive flow measurements are compared against RANS predictions of the supersonic jet flow properties as a means of assessing their prediction accuracy. Graphic abstract
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Cai, Shouyin, Sen Tian, Yiyu Lu, Guangjin Wang, Yu Pu e Kang Peng. "Molecular Simulations of Adsorption and Energy Storage of R1234yf, R1234ze(z), R134a, R32, and their Mixtures in M-MOF-74 (M = Mg, Ni) Nanoparticles". Scientific Reports 10, n. 1 (29 aprile 2020). http://dx.doi.org/10.1038/s41598-020-64187-x.
Testo completoAbstract (sommario):
Abstract The refrigerant circulation heat can be enhanced through the mutual transformation between thermal energy and surface energy during the adsorption and separation process of fluid molecules in porous materials. In this paper, the adsorption and energy storage of R1234ze(z), R1234yf, R32 and R134a, as well as their mixed refrigerants in Mg-MOF-74 and Ni-MOF-74 nanoparticles were investigated by means of molecular dynamics simulations and grand canonical Monte Carlo simulations. The results suggested that, in the case of pure refrigerant adsorption, the adsorption quantities of R32 and R134a in MOFs were higher than those of R1234yf and R1234ze(z). However, in the case of saturation adsorption, the desorption heat of R32 was lower than that of R1234yf and R1234ze(z). The addition of MOF-74 nanoparticles (NPs) could enhance the energy storage capacity of the pure refrigerant; besides, R1234yf and R1234ze(z) nanofluids had superior enhancement effect to that of R32 nanofluid. In mixed refrigerant adsorption, the adsorption quantities of R1234ze(z) and R1234yf were lower than those of R32 and R134a; with the increase in temperature, the adsorption of R1234ze(z) and R1234yf showed a gradually increasing trend, while that of R32 was gradually decreased.
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"Les Modtles Asymptotiques de la MBcanique des Fluides I, II. By R. KH. ZEYTOUNIAN. Springer. Vol. I, 1986. 260 pp. DM 38; Vol. 11, 1987. 315 pp. DM 53. Dynamics of Fluids in Hierarchical Porous Media. Edited by J. H. CUSHMAN. Academic, 1990. 505 pp. £48. The Mathematical Theory of Non-uniform Gases. By S. CHAPMAN and T. G. COWLING. Cambridge University Press, 1990. 423 pp. £19.50 or $32.50. Theory of Macroscopic Systems. By C. OUWERKERK. Springer, 1991. 245 pp. DM 48. BASIC Fluid Mechanics. By J. J. SHARP. Butterworths, 1988. 139 pp. £9.95. BASIC Hydrodynamics. By A. C. THOMSON. Butterworths, 1987. 179 pp. £9.95. BASIC Heat Transfer. By D. H. BACON. Butterworths, 1989. 172 pp. £12.95." Journal of Fluid Mechanics 231 (ottobre 1991): 691–93. http://dx.doi.org/10.1017/s0022112091223563.
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