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Dissertations / Theses on the topic 'Microchannels'
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Adams, Thomas M. "Turbulent convection in microchannels." Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/19421.
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Martel, Joseph Maurice. "Particle Focusing in Microchannels." Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:11206.
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The ability to control the motion of particles and cells in microchannels has been a center of fascination since the advent of microfluidics. Entire fields have been created in order to accomplish separation, volume reduction and overall positioning of particles and cells within microfluidic devices in the fastest and most accurate manner possible. While most of these technologies rely on low Reynolds number operation, one technique entitled inertial focusing takes advantage of the inertia of the surrounding fluid and the interaction between a particle and the channel itself which cause the lateral migration of particles across streamlines to equilibrium positions within a flow. The major advantage of inertial microfluidics in biomedical and microfluidic applications is that it is inherently high throughput being dependent on inertia whereas most microfluidic concepts are dependent on low Reynolds number operation.<br>Engineering and Applied Sciences
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Gerson, Eleanor. "Promoting Endothelial Cell Growth within Microchannels - Modification of Polydimethylsiloxane and Microfabrication of Circular Microchannels." Thesis, Université d'Ottawa / University of Ottawa, 2018. http://hdl.handle.net/10393/37555.
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Polydimethylsiloxane (PDMS) microfluidic channels, fabricated using low cost and simple soft lithography methods, conventionally have rectangular cross-sections. Despite being often used for organs-on-a-chip and cardiovascular research, these devices do not mimic the circular cross-sections of blood vessels in the human body, creating potential inaccuracies in observed flow conditions and cell behaviours. The purpose of this thesis is to (i) compare and optimize fabrication techniques for microchannels with circular cross-sections, (ii) assess biocompatibility of different surface functionalization approaches for Human Umbilical Vein Endothelial Cell (HUVEC) adhesion and growth, (iii) culture HUVECs within circular microchannels to mimic blood vessel features, and (iv) compare gene expression of HUVECs cultured in 3D circular microchannels to those cultured on 2D surfaces. We show that wire molding is superior to the gas stream technique for producing circular cross-section microchannels with high aspect ratios, circularity, and channel geometry precision. Fibronectin (FN) and polydopamine (PD) surface coatings on PDMS, as well as alternative collagen substrates, were tested for biocompatibility with HUVECs in 2D cultures; fibronectin coated PDMS (PDMS-FN) substrates facilitated cell attachment, spreading and growth. We demonstrate the capability of growing HUVECs on the inner surface of circular PDMS microchannels created using the wire-mold method and treated with fibronectin. A syringe pump was used to induce shear stress on the HUVECs grown in circular microchannels. Relative to static growth conditions, longer cell culture growth periods were more feasible under flow and altered cell morphology was observed. Finally, Microarray analysis revealed significantly different gene expression profiles for HUVECs cultured within PDMS-FN circular cross-section microchannels as compared to HUVECs cultured on PDMS-FN in a 2D environment, thereby highlighting the critical importance of in vitro conditions for mimicking the in vivo reality.
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Kuravi, Sarada. "Numerical Study of Encapsulated Phase Change Material (EPCM) Slurry Flow in Microchannels." Doctoral diss., University of Central Florida, 2009. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/4093.
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Heat transfer and flow characteristics of phase change material slurry flow in microchannels with constant heat flux at the base were investigated. The phase change process was included in the energy equation using the effective specific heat method. A parametric study was conducted numerically by varying the base fluid type, particle concentration, particle size, channel dimensions, inlet temperature, base heat flux and melting range of PCM. The particle distribution inside the microchannels was simulated using the diffusive flux model and its effect on the overall thermal performance of microchannels was investigated. Experimental investigation was conducted in microchannels of 101 [micro]m width and 533 [micro]m height with water as base fluid and n-Octadecane as PCM to validate the key conclusions of the numerical model. Since the flow is not fully developed in case of microchannels (specifically manifold microchannels, which are the key focus of the present study), thermal performance is not as obtained in conventional channels where the length of the channel is large (compared to length of microchannels). It was found that the thermal conductivity of the base fluid plays an important role in determining the thermal performance of slurry. The effect of particle distribution can be neglected in the numerical model under some cases. The performance of slurry depends on the heat flux, purity of PCM, inlet temperature of the fluid, and base fluid thermal conductivity. Hence, there is an application dependent optimum condition of these parameters that is required to obtain the maximum thermal performance of PCM slurry flows in microchannels.<br>Ph.D.<br>Department of Mechanical, Materials and Aerospace Engineering;<br>Engineering and Computer Science<br>Mechanical Engineering PhD
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Dydek, EthelMae Victoria. "Electrochemistry and Electrokinetics in Microchannels." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/70401.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2012.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (p. 119-123).<br>The main body of this work considers the design and development of a microfluidic, continuous electrochemical sensor capable of measuring accurate potential differences. The key challenge in creating such a device is the implementation of a miniaturized reference electrode and salt bridge. The purpose of a salt bridge is to allow ionic conduction between the reference and working electrodes while maintaining a physical separation between the two systems. Macro reference electrode and salt bridge techniques are difficult to implement on a micro scale. Instead of attempting to conform one of these techniques to function in a micro system, new methods were developed that take advantage of the conditions in a continuous microfluidic device. In particular, laminar flow and slow relative diffusion times allow for a reference electrode that does not require a physical salt bridge. Ionic conduction is maintained between neighboring reference and analyte streams while slow mixing effectively separates the two systems. Several different device designs were investigated focusing on the prevention of reference electrode contamination. If the reference electrode is chemically contaminated it will no longer behave as expected and can not be used as a reference point. Contamination at the reference electrode was evaluated while varying flow rates and the geometry of the microfluidic device. Mathematical models were simulated in order to understand the mass transport in each device design. Based on these simulations, dimensionless groups were found that defined the dominant physics in each system. These dimensionless numbers were then validated experimentally and numerically over a range of device parameters. Subsequently, operation criteria were developed to ensure that the reference electrode remains stable and uncontaminated. By creating a stable reference electrode on chip, any homogeneous electrochemical system that was previously studied on the macro scale can now be studied continuously in a microfluidic device. A secondary portion of this work investigates the role of surface charge with respect to electrodynamics in a microchannel. As the surface area to volume ratio increases, the concentration of charge at a channel wall may begin to approach the electrolyte concentration in the bulk solution. This phenomenon is studied numerically, with and without convection, in particular as it relates to a possible mechanism for overlimiting current. Additionally, a potential de-ionization device is theorized based on this mechanism along with scaling arguments that can be used to aid device design.<br>by EthelMae Victoria Dydek.<br>Ph.D.
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Massenburg, Sorell S. "Clogging Mechanisms in Converging Microchannels." Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:26718735.
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Many technological and biomedical applications ranging from water filtration and oil extraction to arteriosclerosis and vein thrombosis rely upon the transport of solids in liquids. Particulate matter suspended in liquid flowing through channels that are often microscopic or millimeters in size which leads to clogging. This dissertation examines the clogging behavior of microscopic channels by microscopic particles suspended in liquid. We physically model clogging in microchannels by flowing microparticles through microfluidic channels. Unlike previous studies, we choose non-uniform microchannels; specifically, we study clogging in microchannels whose width narrows over the length of the channel. Converging channels are inspired by the pore size variations in real porous media like membrane filters and sandstone.
Initially we study the clogging behavior of microparticles in arrays of parallel microchannels as we vary the microchannel entrance (mouth) width and microchannel length. We measure the time until each channel clogs and we calculate the number of particles that pass prior to clogging. Contrary to expectation, we show that the number of particles passing through a pore increases exponentially with increasing mouth width but decreases linearly as the channel length increases. Changing the dimensions of the channels changes the particulate suspension’s flow rate which in turn changes the shear stresses that particles experience near the channel wall. When particles experience higher near-wall shear stress, the particles are less likely to adhere to channel walls and engender clogging. We confirm the effect of flow rate on channel clogging by demonstrating that the number of particles needed to clog a tapered channel increases as the pressure applied to the particulate suspension increases.
The connection between flow rate and clogging highlights the interplay between hydrodynamic forces and intermolecular forces that govern particle attachment and ultimately clogging. We further explore this relationship by modulating the interaction between the particle and channel wall in a single tapered channel. While observing single channels clogging, we also resolve individual particles gradually building up on channel walls and forming clogs. Interestingly, particles also cluster on upstream channel walls only to later detach and clog at the downstream constriction. At low pressures, the channel clogs when particles accumulate individually near the constriction. At high pressures, the channel clogs when particle clusters detach from channel walls upstream and flow into the constriction. Finally, we compare the clogging behavior of particles with long, electrosteric stabilizing molecules on the surface to the clogging behavior of particles with shorter electrostatic stabilizing molecules on the surface. We also compare the clogging behavior of both particle types in the presence of varying concentrations of a monovalent salt. We show that clogging is mitigated when Debye length is comparable to the length of the stabilizing molecule on the particle’s surface.<br>Engineering and Applied Sciences - Applied Physics
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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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GUTIERREZ, JOSE ANGEL FLORIAN. "VISCOELASTIC FLOW THROUGH MICROCHANNELS WITH CONSTRICTION." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2018. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=37214@1.
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PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO<br>CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO<br>Alguns projetos pilotos de injeção de polímeros em campos de produção de petróleo mostraram um incremento na recuperação de óleo, embora os mecanismos que governam a dinâmica do escoamento não são bem compreendidos. Recentes investigações experimentais mostraram que as propriedades viscoelásticas de soluções poliméricas podem alterar o comportamento do escoamento na escala de poros e reduzir a saturação residual de óleo. Para entender esses fenômenos em meios porosos, é importante estudar o escoamento de soluções viscoelásticas através das gargantas dos poros. Este trabalho apresenta um estudo experimental do escoamento de uma solução viscoelástica de PEO (0,1 porcento em peso de óxido de polietileno) de alto peso molecular escoando através de uma constrição, utilizado como modelo de uma geometria de garganta de poro de um meio poroso. Medições de queda de pressão e campos de velocidade do escoamento são obtidos utilizando a técnica de velocimetria por imagem de partículas (Micro-PIV). Experimentos com uma solução viscosa de glicerina (45 porcento em peso de glicerina em
água) de viscosidade similar à solução de PEO foram também realizados com a finalidade de estimar os efeitos elásticos da solução de PEO. O escoamento da solução de PEO exibiu uma queda de pressão extra (comportamento não linear) acima de uma condição crítica do escoamento, acima da qual os efeitos elásticos passam a ser preponderantes. Para toda a faixa de vazão explorada, os campos de velocidade da solução de glicerina mostraram um regime de escoamento Newtoniano, enquanto a solução de PEO apresenta instabilidade no escoamento a partir de um número de Weissenberg crítico, coincidindo com o aumento da queda de pressão. Esta instabilidade pode ser relacionada ao aumento da viscosidade extensional na entrada da garganta acima de uma determinada taxa de extensão. Os resultados obtidos indicam a variação do padrão do escoamento da solução polimérica de PEO devido à presença dos efeitos elásticos do polímero, e fornecem informações importantes sobre o comportamento das soluções poliméricas viscoelásticas em um meio poroso e que podem impactar sua utilização na recuperação melhorada de óleo.<br>Some pilot projects of polymer injection in oil fiel ds have shown an increase in oil recovery, although the mechanisms that govern the flow dynamics are still not well understood. Recent experimental investigations have shown that the viscoelastic properties of polymer solutions may change the pore-scale flow behavior and reduce the residual oil saturation. To understand these phenomena in porous media, it is important to understand viscoelastic flow behavior through the pores-throats. This work presents experimental study of the flow of a high molecular viscoelastic PEO solution (0.1 wt percent Polyethylene Oxide) flowing through a constricted capillary, used as model for a pore-throat geometry of a porous media. Pressure drop measurements are performed and velocity fields are obtained using the micro-particle image velocimetry (Micro-PIV) technique. Experiments with a viscous solution of glycerin (45 wt percent glycerin in water), of similar shear viscosity to the PEO solution were also performed in order to isolate the elastic effects of the PEO solution. The flow of the PEO solution exhibited an extra pressure drop (nonlinear behavior) above a critical flow condition beyond which the elastic forces become relevant. For the entire flow rate range explored, the velocity field of the glycerin solution showed a Newtonian flow regime, while the PEO solution shows instability in the flow above a critical Weissenberg number, coinciding with the onset of the extra pressure drop. This instability in the flow is associated with the high extensional viscosity near the constriction at high enough extension rates. The results show the changes in the flow pattern of the PEO polymer solution due to the presence of the elastic effects of polymer, and provide important information on how viscoelastic polymer solutions behave in a porous media and can impact their use in Enhanced Oil Recovery operations.
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Adhikari, Param C. "Computational Analysis of Mixing in Microchannels." Youngstown State University / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=ysu1370799440.
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Yu, Tak For. "Gas flow in microchannels with cavities /." View Abstract or Full-Text, 2003. http://library.ust.hk/cgi/db/thesis.pl?MECH%202003%20YUT.
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Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003.<br>Includes bibliographical references (leaves 59-63). Also available in electronic version. Access restricted to campus users.
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Leung, Sharon Shui Yee. "Heat transfer in microchannels : taylor flow." Thesis, The University of Sydney, 2012. http://hdl.handle.net/2123/17835.
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Beutel, Dan. "Modelling Advection and Diffusion in Microchannels." Scholarship @ Claremont, 2003. https://scholarship.claremont.edu/hmc_theses/140.
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This project will investigate mixing in microchannels. Specifically, the advection and diffusion of a passive scalar, using a split step Monte Carlo method. Numerically the implementation of this method is well understood. The current experimental geometry is a rectangular pipe with grooves on one wall. Mixing results with straight walls agree closely with experiment. The velocity field over grooves is also studied.
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Wang, Hengzi, and na. "Passive mixing in microchannels with geometric variations." Swinburne University of Technology, 2004. http://adt.lib.swin.edu.au./public/adt-VSWT20061013.162737.
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This research project was part of the microfluidic program in the CRC for Microtechnology,
Australia, during 2000 to 2003. The aim of this research was to investigate the feasibility of
applying geometric variations in a microchannel to create effects other than pure molecular
diffusion to enhance microfluidic mixing. Geometric variations included the shape of a
microchannel, as well as the various obstacle structures inside the microchannel.
Generally, before performing chemical or biological analysis, samples and reagents
need to be mixed together thoroughly. This is particularly important in miniaturized Total
Analysis Systems (�TAS), where mixing is critical for the detection stage. In scaling
down dimensions of micro-devices, diffusion becomes an efficient method for achieving
homogenous solutions when the characteristic length of the channels becomes sufficiently
small. In the case of pressure driven flow, it is necessary to use wider microchannels to
ensure fluids can be pumped through the channels and the volume of fluid can provide
sufficient signal intensity for detection. However, a relatively wide microchannel makes
mixing by virtue of pure molecular diffusion a very slow process in a confined volume of
a microfluidic device. Therefore, mixing is a challenge and improved methods need to be
found for microfluidic applications.
In this research, passive mixing using geometric variations in microchannels was studied
due to its advantages over active mixing in terms of simplicity and ease of fabrication.
Because of the nature of laminar flow in a microchannel, the geometric variations were designed
to improve lateral convection to increase cross-stream diffusion. Previous research
using this approach was limited, and a detailed research program using computational fluid
dynamic (CFD) solvers, various shapes, sizes and layouts of geometric structures was undertaken
for the first time. Experimental measurements, published experimental data and analytical predictions were used to validate the simulations for selected samples. Mixing
efficiency was evaluated by using mass fraction distributions. It was found that the overall
performance of a micromixer should include the pressure drop in a microdevice, therefore,
a mixing index criterion was formulated in this research to combine the effect of mixing
efficiency and pressure drop. The mixing index was used to determine optimum parameters
for enhanced mixing, as well as establish design guidelines for such devices.
Three types of geometric variations were researched. First, partitioning in channels
was used to divide fluids into mixing zones with different concentrations. Various designs
were investigated, and while these provided many potential solutions to achieving good
mixing, they were difficult to fabricate. Secondly, structures were used to create lateral
convection, or secondary flows. Most of the work in this category used obstacles to disrupt
the flow. It was found that symmetric layouts of obstacles in a channel had little effect
on mixing, whereas, asymmetric arrangements created lateral convection to enhance crossstream
diffusion and increase mixing. Finally, structures that could create complex 3D
advections were investigated. At high Reynolds numbers (Re = 50), 3D ramping or obstacles
generated strong lateral convection. Microchannels with 3D slanted grooves were
also investigated. Mixers with grooved surfaces generated helicity at low Reynolds numbers
(Re � 5) and provided a promising way to reduce the diffusion path in microchannels
by stretching and folding of fluid streams. Deeper grooves resulted in better mixing efficiency.
The 3D helical advection created by the patterned grooves in a microchannel was
studied by using particle tracing algorithms developed in this research to generate streaklines
and Poincare maps, which were used to evaluate the mixing performance. The results
illustrated that all the types of mixers could provide solutions to microfluidic mixing when
dimensional parameters were optimized.
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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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Rossi, D. "Adipic acid sonocrystallization in continuous flow microchannels." Thesis, University College London (University of London), 2017. http://discovery.ucl.ac.uk/1557884/.
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Crystallization is widely employed in the manufacture of pharmaceuticals during the intermediate and final stages of purification and separation. The process defines drug chemical purity and physical properties: crystal morphology, size distribution, habit and degree of perfection. Particulate pharmaceuticals are typically manufactured in conventional batch stirred tank crystallizers that are still inadequate with regard to process controllability and reproducibility of the final crystalline product. Variations in crystal characteristics are responsible for a wide range of pharmaceutical formulation problems, related for instance to bioavailability and the chemical and physical stability of drugs in their final dosage forms. This thesis explores the design of a novel crystallization approach which combines in an integrated unit continuous flow, microreactor technology, and ultrasound engineering. By exploiting the various benefits deriving from each technology, the thesis focuses on the experimental characterization of two different nucleation systems: a droplet-based system and a single-phase system. In the former, channel fouling is avoided using a carrier fluid to segment the crystallizing solution in droplets, thus avoiding the contact with the walls. In the latter channel blockage is prevented using larger channel geometries and employing higher flow rates. The flexibility of the developed setup also allows performing stochastic nucleation studies to estimate the nucleation kinetics under silent and sonicated conditions. The experiments reveal that very high nucleation rates, small crystal sizes, narrow size distributions and high crystal yields can be obtained with both setups when the crystallizing solution is exposed to high pressure field as compared to silent condition. It is concluded that transient cavitation of bubbles and its consequences are a significant mechanism for enhancing nucleation of crystals among several proposed in the literature. A preliminary study towards the development and design of a growth stage is finally performed. Flow pulsation is identified as a potential method to enhance radial mixing and narrow residence time distribution therefore achieving optimal conditions for uniform crystal growth. The results suggest that increasing values of Strouhal number as well as amplitude ratio improve axial dispersion. Helically coiled tubes are identified as potential structures to further improve fluid dynamic dispersion.
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Sherwood, J. M. "Blood velocity and viscosity in bifurcating microchannels." Thesis, University College London (University of London), 2013. http://discovery.ucl.ac.uk/1400720/.
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Blood is a complex fluid comprised of predominantly red blood cells (RBCs) suspended in a continuous, Newtonian phase, the plasma. Blood viscosity is highly dependent on the RBC concentration (haematocrit) and also displays shear thinning properties, as a result of RBC deformation and aggregation at high and low shear rates, respectively. However, these two phenomena also lead to uneven haematocrit distributions, which are exacerbated in microvascular bifurcations. In the present study, multifaceted experiments of human blood, perfused through bifurcating microchannels, are used to further elucidate the relationship between haematocrit, velocity and viscosity. A custom pressure based perfusion system was developed and was coupled with image acquisition for velocity measurement with μPIV and further processing. The acquired data was analysed in order to investigate the flow characteristics of human blood in two different idealised bifurcation geometries. The `cell-depleted layer' (CDL), a region of reduced haematocrit which occurs near the walls of the channel, and the continuous haematocrit distribution were experimentally measured. Analytical and numerical approaches were used to extract further information on the effect of flow rate, flow ratio and the presence of aggregation on microhaemodynamics. In the parent branch of the bifurcation, RBC aggregation was observed to increase the radial migration of RBCs away from the vessel wall. This enhanced the non-uniformity of the haematocrit downstream of the bifurcation and altered the relative velocity between the RBCs and the suspending medium. A skewed distribution of cells was observed downstream of the bifurcation, which resulted in skewed velocity profiles, which were also captured in the analytical and computational approaches. The geometry of the bifurcation was observed to influence the results and RBC aggregation quite significantly modified the haemodynamic characteristics even at high flow rates.
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Nema, Gaurav. "Flow regime transitions during condensation in microchannels." Thesis, Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/22592.
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Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2008.<br>Committee Chair: Garimella, Srinivas; Committee Member: Ghiaasiaan, Seyed Mostafa; Committee Member: Mistree, Farrokh.
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Kapisthalam, Vasudevan Shibi Vasisht. "Electrohydrodynamic instabilities in microchannels a computational study /." [Ames, Iowa : Iowa State University], 2009.
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Maiellaro, Kathryn A. "Microfabricated silicon microchannels for cell rheology study." [Gainesville, Fla.] : University of Florida, 2003. http://purl.fcla.edu/fcla/etd/UFE0001145.
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Jakaboski, Blake Elaine. "Forced Convection in Microchannels with Nanostructures on One Wall." Thesis, Georgia Institute of Technology, 2004. http://hdl.handle.net/1853/4984.
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New electronic devices are faster than ever before, incorporate a higher level of integration, and as a result, need to dissipate higher heat fluxes. Active cooling is the only possible method of thermal management for these devices. A new type of microchannel heat sink has been developed and evaluated in this study. The device consists of silicon microchannels on whose bottom surfaces multi-walled carbon nanotubes are grown. The objective of the study is to investigate the effect of carbon nanotubes on the heat transfer characteristics. The heat sink size is 15 mm by 15 mm by 0.675 mm. It contains two microchannel designs. One consists of eight channels of cross section 682 micrometers by 50 micrometers; the other has six channels of cross section 942 micrometers by 50 micrometers. The heat sink is incorporated in an open loop flow facility, with water as the coolant. Six different configurations are compared. Two have no nanotubes, two have closely spaced nanotubes, while the last two designs have widely spaced nanotubes. The tests utilize an infrared camera as well as thermocouples placed in the flow for characterization. The heat transfer characteristics are compared for the different cases.
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Ali, Rashid. "Phase Change Phenomena During Fluid Flow in Microchannels." Doctoral thesis, KTH, Tillämpad termodynamik och kylteknik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-26796.
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Phase change phenomena of a fluid flowing in a micro channel may be exploited to make the heat exchangers more compact and energy efficient. Compact heat exchangers offer several advantages such as light weight, low cost, energy efficiency, capability of removing high heat fluxes and charge reduction are a few to mention. Phase change phenomena in macro or conventional channels have been investigated since long but in case of micro channels, fewer studies of phase change have been conducted and underlying phenomena during two-phase flow in micro channels are not yet fully understood. It is clear from the literature that the two-phase flow models developed for conventional channels do not perform well when extrapolated to micro scale. In the current thesis, the experimental flow boiling results for micro channels are reported. Experiments were conducted in circular, stainless steel and quartz tubes in both horizontal and vertical orientations. The internal diameters of steel tubes tested were 1.70 mm, 1.224 mm and the diameter of quartz tube tested was 0.781 mm. The quartz tube was coated with a thin, electrically conductive, transparent layer of Indium-Tin-Oxide (ITO) making simultaneous heating and visualization possible. Test tubes were heated electrically using DC power supply. Two refrigerants R134a and R245fa were used as working fluids during the tests. Experiments were conducted at a wide variety of operating conditions. Flow visualization results obtained with quartz tube clearly showed the presence of confinement effects and consequently an early transition to annular flow for micro channels. Several flow pattern images were captured during flow boiling of R134a in quartz tube. Flow patterns recorded during the experiments were presented in the form of Reynolds number versus vapour quality and superficial liquid velocity versus superficial gas velocity plots. Experimental flow pattern maps so obtained were also compared with the other flow pattern maps available in the literature showing a poor agreement. Flow boiling heat transfer results for quartz and steel tubes indicate that the heat transfer coefficient increases with heat flux and system pressure but is independent on mass flux and vapour quality. Experimental flow boiling heat transfer coefficient results were compared with those obtained using different correlations from the literature. Heat transfer experiments with steel tubes were continued up to dryout condition and it was observed that dryout conditions always started close to the exit of the tube. The dryout heat flux increased with mass flux and decreased with exit vapour quality. The dryout data were compared with some well known CHF correlations available in the literature. Two-phase frictional pressure drop for the quartz tube was also obtained under different operating conditions. As expected, two-phase frictional pressure drop increased with mass flux and exit vapour quality.<br>QC 20101206
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Abadie, Thomas. "Hydrodynamics of gas-liquid Taylor flow in microchannels." Phd thesis, Toulouse, INPT, 2013. http://oatao.univ-toulouse.fr/11986/1/abadie.pdf.
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This thesis focuses on the hydrodynamics of gas-liquid Taylor flow (or slug flow) in microchannels. These flows, which are generally dominated by surface tension forces, have been investigated in rectangular channels of various cross-sectional aspect ratios by means of both experimental visualizations and numerical simulations. The first experimental part aims at characterizing the bubble generation process (bubble length and frequency of break-up) depending on the operating conditions, the fluid properties, as well as the junction where both fluids merge. Numerical simulations of fully developed Taylor flow have been carried out with the JADIM code. The computation of such surface tension dominated flows requires an accurate calculation of the surface tension force. Some limitations of the Volume of Fluid method have been highlighted and a Level Set method has been developed in order to improve the calculation of capillary effects. Both methods have been compared in detail in terms of spurious currents. 3D numerical simulations have been performed and the influence of the capillary number, as well as the effects of geometry have been highlighted. Inertial effects have been taken into account and their influence on the pressure drop has been shown to be non-negligible. Mixing in the liquid slug has also been studied.
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Whiteley, Joseph L. "Study of magneto-rheological fluid flow in microchannels /." abstract and full text PDF (free order & download UNR users only), 2007. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1448022.
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Thesis (M.S.)--University of Nevada, Reno, 2007.<br>"May, 2007." Includes bibliographical references (leaves 73-77). Online version available on the World Wide Web. Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2007]. 1 microfilm reel ; 35 mm.
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Laker, Travis S. "Transport of microscopic particles in microchannels and microbubbles." Diss., Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/15816.
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Kennedy, Jonathan Edward. "Onset of flow instability in uniformly-heated microchannels." Thesis, Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/17357.
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Ketheeswaranathan, Nishanthi. "Rehological study of non-Newtonian fluid through microchannels." Thesis, University of Leeds, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.540775.
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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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Cantu-Perez, A. "Modelling and experiments of microchannels incorporating microengineered structures." Thesis, University College London (University of London), 2011. http://discovery.ucl.ac.uk/1310147/.
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Microreaction technology was conceived, thanks to the advances on microfabrication by the semiconductor industry. The first applications of microchannels used for performing reactions date back to the early nineties. Since then, many conferences dedicated to this topic are held worldwide such as the International Microreaction Technology Conference (IMRET) or the International Conference on Microchannels and Minichannels. The small dimensions of the microchannels lead to very high heat and mass transfer rates, reactions are therefore performed very efficiently on these devices. However, the small dimensions of the channels lead to high pressure drops. In addition, microchannels are very susceptible to clogging. This thesis studies the effect of different microchannel configurations in terms of mixing, mass transfer, residence time distribution and reaction. The objective is to design microreactors which incorporate different structures which make them efficient in terms of heat/mass transfer, but do not have the issue of high pressure drop and channel blockage.
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Lee, Eon Soo. "Water transport in two-phase fuel cell microchannels /." May be available electronically:, 2007. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
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Guo, Zhenyi. "CFD Simulation of Annular Flow Boiling in Microchannels." Thesis, The University of Sydney, 2015. http://hdl.handle.net/2123/14428.
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Flow boiling in microchannels has received enormous interest over the past few decades because of its importance in the thermal management of micro-structured devices. Few of previously published studies focus specifically on microchannel annular flow boiling which is very important due to its prevalence in this system. This thesis provides understanding of the heat and mass transfer in microchannel annular flow boiling via the use of a computational fluid dynamics (CFD) approach. The commercial software ANSYS Fluent was chosen to perform the CFD simulations. A modified height function (HF) method was implemented into the default CSF model to improve the performance of surface tension modelling. Simulation of an inviscid parallel flow demonstrated successful prediction of the onset of Kelvin-Helmholtz (K-H) instability in close accord with the analytical criterion. Studies of imposed numerical perturbations in laminar annular air-water flow showed that viscosity does not affect the stability of interfacial waves but has large impacts on the growth rates. A phase change model was formulated using a kinetic-based model to calculate the interphase mass flux. An established numerical smoothing procedure was used to improve numerical stability. A detailed study of a laminar annular flow boiling was performed using water at 160 kPa in a 0.5 mm diameter channel with constant fluid mass flux G = 60 kg m−2 s−1 and inlet vapour quality x = 0.1. Interfacial waves were observed and flow recirculation and a localised increase of heat transfer coefficient occurred at the interfacial wave troughs, where the liquid film was very thin. A parametric analysis showed that boiling heat transfer coefficient increases with increasing mass flux, system pressure, vapour quality and heat flux but decreases with increasing tube diameter.
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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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Aor, Bruno. "Engineering microchannels for vascularization in bone tissue engineering." Thesis, Bordeaux, 2018. http://www.theses.fr/2018BORD0430/document.
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In vitro, la formation de structures de type tubulaire avec des cellules endothéliales de veine ombilicale humaine (HUVEC) a été étudiée en combinant la fonctionnalisation de la chimie des matériaux et le développement de la géométrie tridimensionnelle. Le polycarbonate (PC) a été utilisé comme modèle pour le développement de l'échafaud. Le film de polysaccharide naturel, basé sur un dépôt alternatif couche par couche (LbL) d’acide hyaluronique (HA) et de chitosane (CHI), a d’abord été appliqué sur une surface PC et caractérisé en termes de croissance d’épaisseur microscopie à balayage lascar (CLSM). Cette première fonctionnalisation se traduit par un revêtement complet de la couche PC. Une biofonctionnalisation supplémentaire avec un peptide adhésif (RGD) et deux peptides angiogénétiques (SVV et QK) a été étudiée, immobilisant ces peptides sur le groupe carboxylique de HA précédemment déposé, en utilisant la chimie bien connue du carbodiimide. La version marquée de chaque peptide a été utilisée pour caractériser l’immobilisation et la pénétration des peptides dans les couches de polyélectrolytes, aboutissant à une greffe réussie avec une pénétration complète dans toute l’épaisseur du LbL. Des tests in vitro ont été effectués à l'aide de cellules HUVEC pour évaluer leur efficacité d'adhésion et leur activité métabolique sur la LbL avec et sans immobilisation de peptides, ce qui a permis d'améliorer l'activité préliminaire lorsque des combinaisons de peptides sont utilisées. Enfin, les micro-canaux PC (μCh) ont été développés et caractérisés pour la première fois, et les autres expériences ont été réalisées sur un micromètre de 25 μm de largeur, fonctionnalisé avec une architecture (HA / CHI) 12,5 (PC-LbL) avec des peptides RGD et QK -RGD + QK) ou avec des peptides RGD et SVV (PC-RGD + SVV). Notre première expérience de tubulogénèse a montré de manière surprenante la formation de structures de type tubulaire déjà après 2h d'incubation en utilisant la combinaison double-peptides, mais uniquement avec PC-RGD + QK. Les tubes étaient également présents après 3 et 4 heures de culture. L'expérience de co-culture avec des péricytes humains dérivés du placenta (hPC-PL) montre comment la stabilisation des tubes a été améliorée après 3 et 4 heures également pour l'échantillon de PC-RGD + SVV. Globalement, notre matériel bio-fonctionnel avec les peptides PC-RGD + QK et PC-RGD + SVV permet la formation d'une structure de type tubulaire à la fois dans une expérience de monoculture et de co-culture<br>In vitro, tubular-like structures formation with human umbilical vein endothelial cells (HUVECs) was investigated by combining material chemistry functionalization and three-dimensional geometry development. Polycarbonate (PC) was used as a template for the development of the scaffold. Natural polysaccharide’s film based on alternate layer-by-layer (LbL) deposition of hyaluronic acid (HA) and chitosan (CHI), was first applied to PC surface and characterized in terms of thickness growth both, in dry conditions using ellipsometry, and confocal lascar scanning microscopy (CLSM). This first functionalization results in a complete coating of the PC layer. Further biofunctionalization with one adhesive peptide (RGD) and two angiogenetic peptides (SVV and QK) was investigated, immobilizing those peptides on the carboxylic group of HA previously deposited, using the well-known carbodiimide chemistry. The labeled version of each peptide was used to characterize the peptides’ immobilization and penetration into the polyelectrolytes layers, resulting in a successful grafting with complete penetration through the entire thickness of the LbL. In vitro tests were performed using HUVECs to assess their adhesion efficiency and their metabolic activity on the LbL with and without peptide immobilization, resulting in a preliminary improved activity when peptide-combinations is used. Finally, PC micro-channels (μCh) were first developed and characterized, and the rest of the experiments were performed on μCh of 25μm width, functionalized with (HA/CHI)12.5 architecture (PC-LbL) with RGD and QK peptides (PC-RGD+QK) or with RGD and SVV peptides (PC-RGD+SVV). Our first tubulogenesis experiment surprisingly showed the formation of tubular-like structures already after 2h of incubation using the double-peptides combination but only using PC-RGD+QK the tubes were present also after 3 and 4 hours of culture. The co-culture experiment with human pericytes derived from placenta (hPC-PL) demonstrates how the stabilization of the tubes was improved after 3 and 4 hours also for the PC-RGD+SVV sample. Globally our bio-functional material with PC-RGD+QK and PC-RGD+SVV peptides allow the formation of tubular-like structure in both mono and co-culture experiment
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Esmaeili, Pourfarhangi Kamyar. "Movie7: Simulation of EGF diffusion within the microchannels." Diss., Cancer Invasion; Cell Migration; Chemotaxis; Contact Guidance; Invadopodia; Mechanobiology, 2019. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/584758.
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Bioengineering;<br>Ph.D.;<br>Metastasis is the leading cause of death among cancer patients. The metastatic cascade, during which cancer cells from the primary tumor reach a distant organ and form multiple secondary tumors, consists of a series of events starting with cancer cells invasion through the surrounding tissue of the primary tumor. Invading cells may perform proteolytic degradation of the surrounding extracellular matrix (ECM) and directed migration in order to disseminate through the tissue. Both of the mentioned processes are profoundly affected by several parameters originating from the tumor microenvironment (extrinsic) and tumor cells themselves (intrinsic). However, due to the complexity of the invasion process and heterogeneity of the tumor tissue, the exact effect of many of these parameters are yet to be elucidated. ECM proteolysis is widely performed by cancer cells to facilitate the invasion process through the dense and highly cross-linked tumor tissue. It has been shown in vivo that the proteolytic activity of the cancer cells correlates with the cross-linking level of their surrounding ECM. Therefore, the first part of this thesis seeks to understand how ECM cross-linking regulates cancer cells proteolytic activity. This chapter first quantitatively characterizes the correlation between ECM cross-linking and the dynamics of cancer cells proteolytic activity and then identifies ß1-integrin subunit as a master regulator of this process. Once cancer cells degrade their immediate ECM, they directionally migrate through it. Bundles of aligned collagen fibers and gradients of soluble growth factors are two well-known cues of directed migration that are abundantly present in tumor tissues stimulating contact guidance and chemotaxis, respectively. While such cues direct the cells towards a specific direction, they are also known to stimulate cell cycle progression. Moreover, due to the complexity of the tumor tissue, cells may be exposed to both cues simultaneously, and this co-stimulation may happen in the same or different directions. Hence, in the next two chapters of this thesis, the effect of cell cycle progression and contact guidance-chemotaxis dual-cue environments on directional migration of invading cells are assessed. First, we show that cell cycle progression affects contact guidance and not random motility of the cells. Next, we show how exposure of cancer cells to contact guidance-chemotaxis dual-cue environments can improve distinctive aspects of cancer invasion depending on the spatial conformation of the two cues. In this dissertation, we strive to achieve the defined milestones by developing novel mathematical and experimental models of cancer invasion as well as utilizing fluorescent time-lapse microscopy and automated image and signal processing techniques. The results of this study improve our knowledge about the role of the studied extrinsic and intrinsic cues in cancer invasion.<br>Temple University--Theses
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Chayantrakom, Kittisak. "Mathematical modelling of particle-fluid flows in microchannels." Thesis, Curtin University, 2009. http://hdl.handle.net/20.500.11937/1115.
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Flows of fluids and solid particles through microchannels have a very wide range of applications in biological and medical science and engineering. Understanding the mechanism of microflows will help to improve the development of the devices and systems in those applications. The aim of this study is to develop a sophisticated simulation and analysis technique for the study of fluid-particle flow through microchannels. This work involves construction of mathematical models, development of analytical methods and numerical algorithms, and numerical investigation and analysis.The study consists of three parts. The first part of the research focuses on the transient flow of an incompressible Newtonian fluid through a micro-annual with a slip boundary. The flow of the fluid is governed by the continuity equation and the Navier-Stokes equations, and is driven by the pressure field with a timevarying pressure gradient. By using the Fourier series expansion in time and Bessel functions in space, an exact solution is derived for the velocity field. The velocity solution is then used to obtain the exact solutions for the flow rate and the stress field. Based on the exact solutions, the influence of the slip parameter on the flow behaviour is then investigated.The second part of the research focuses on the particle-fluid flow in microchannels. The transport of fluid in the vessel is governed by the continuity equation and the transient Navier-Stokes equations, while the motion of the particles is governed by Newton’s laws. The particle-wall and particle-particle interactions are modelled by the interacting forces, while the particle-fluid interaction is described by the fluid drag force. A numerical scheme based on the finite element method and the Arbitary Lagrangian-Eulerian method is developed to simulate the motion of the particles and the fluid flow in the vessels. The influence of boundary slip on the velocity field in the fluid is also investigated numerically.Based on the work in the second part, the third part of the research focuses onthe control of the movement of particles in the fluid by applying an external magneticfield to the system. Maxwell’s equations are used to model the magnetic fieldgenerated by the external magnetic source, and a finite element based numericalscheme is developed to solve the underlying boundary value problem for the magneticflux density generated. From the computed flux density and magnetic vectorpotential, the magnetic forces acting on the particles are determined. These magneticforces together with the drag force and the particle-particle interacting forcesdominate the behaviour of the particle motion. A numerical scheme, similar to thatfor the second part of the research, is then developed to study the fluid-particle flowin microchannels under magnetic forces, followed by a numerical investigation onthe influence of the magnetic forces on the particle flow behaviour.
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Chayantrakom, Kittisak. "Mathematical modelling of particle-fluid flows in microchannels." Curtin University of Technology, Department of Mathematics and Statistics, 2009. http://espace.library.curtin.edu.au:80/R/?func=dbin-jump-full&object_id=129230.
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Flows of fluids and solid particles through microchannels have a very wide range of applications in biological and medical science and engineering. Understanding the mechanism of microflows will help to improve the development of the devices and systems in those applications. The aim of this study is to develop a sophisticated simulation and analysis technique for the study of fluid-particle flow through microchannels. This work involves construction of mathematical models, development of analytical methods and numerical algorithms, and numerical investigation and analysis.<br>The study consists of three parts. The first part of the research focuses on the transient flow of an incompressible Newtonian fluid through a micro-annual with a slip boundary. The flow of the fluid is governed by the continuity equation and the Navier-Stokes equations, and is driven by the pressure field with a timevarying pressure gradient. By using the Fourier series expansion in time and Bessel functions in space, an exact solution is derived for the velocity field. The velocity solution is then used to obtain the exact solutions for the flow rate and the stress field. Based on the exact solutions, the influence of the slip parameter on the flow behaviour is then investigated.<br>The second part of the research focuses on the particle-fluid flow in microchannels. The transport of fluid in the vessel is governed by the continuity equation and the transient Navier-Stokes equations, while the motion of the particles is governed by Newton’s laws. The particle-wall and particle-particle interactions are modelled by the interacting forces, while the particle-fluid interaction is described by the fluid drag force. A numerical scheme based on the finite element method and the Arbitary Lagrangian-Eulerian method is developed to simulate the motion of the particles and the fluid flow in the vessels. The influence of boundary slip on the velocity field in the fluid is also investigated numerically.<br>Based on the work in the second part, the third part of the research focuses onthe control of the movement of particles in the fluid by applying an external magneticfield to the system. Maxwell’s equations are used to model the magnetic fieldgenerated by the external magnetic source, and a finite element based numericalscheme is developed to solve the underlying boundary value problem for the magneticflux density generated. From the computed flux density and magnetic vectorpotential, the magnetic forces acting on the particles are determined. These magneticforces together with the drag force and the particle-particle interacting forcesdominate the behaviour of the particle motion. A numerical scheme, similar to thatfor the second part of the research, is then developed to study the fluid-particle flowin microchannels under magnetic forces, followed by a numerical investigation onthe influence of the magnetic forces on the particle flow behaviour.
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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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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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Herescu, Alexandru. "Two-Phase Flow In Microchannels| Morphology And Interface Phenomena." Thesis, Michigan Technological University, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=3565323.
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<p> The existence and morphology, as well as the dynamics of micro-scale gas-liquid interfaces is investigated numerically and experimentally. These studies can be used to assess liquid management issues in microsystems such as PEMFC gas flow channels, and are meant to open new research perspectives in two-phase flow, particularly in film deposition on non-wetting surfaces. For example the critical plug volume data can be used to deliver desired length plugs, or to determine the plug formation frequency. The dynamics of gas-liquid interfaces, of interest for applications involving small passages (e.g. heat exchangers, phase separators and filtration systems), was investigated using high-speed microscopy - a method that also proved useful for the study of film deposition processes. </p><p> The existence limit for a liquid plug forming in a mixed wetting channel is determined by numerical simulations using Surface Evolver. The plug model simulate actual conditions in the gas flow channels of PEM fuel cells, the wetting of the gas diffusion layer (GDL) side of the channel being different from the wetting of the bipolar plate walls. The minimum plug volume, denoted as critical volume is computed for a series of GDL and bipolar plate wetting properties. Critical volume data is meant to assist in the water management of PEMFC, when corroborated with experimental data. The effect of cross section geometry is assessed by computing the critical volume in square and trapezoidal channels. Droplet simulations show that water can be passively removed from the GDL surface towards the bipolar plate if we take advantage on differing wetting properties between the two surfaces, to possibly avoid the gas transport blockage through the GDL. </p><p> High speed microscopy was employed in two-phase and film deposition experiments with water in round and square capillary tubes. Periodic interface destabilization was observed and the existence of compression waves in the gas phase is discussed by taking into consideration a naturally occurring convergent-divergent nozzle formed by the flowing liquid phase. The effect of channel geometry and wetting properties was investigated through two-phase water-air flow in square and round microchannels, having three static contact angles of 20, 80 and 105 degrees. Four different flow regimes are observed for a fixed flow rate, this being thought to be caused by the wetting behavior of liquid flowing in the corners as well as the liquid film stability. Film deposition experiments in wetting and non-wetting round microchannels show that a thicker film is deposited for wetting conditions departing from the ideal 0 degrees contact angle. A film thickness dependence with the contact angle theta as well as the Capillary number, in the form <i>h<sub>R</sub> ≈ Ca(<sup>2/3 </sup>)/cos(&thetas;)</i> is inferred from scaling arguments, for contact angles smaller than 36 degrees. Non-wetting film deposition experiments reveal that a film significantly thicker than the wetting Bretherton film is deposited. A hydraulic jump occurs if critical conditions are met, as given by a proposed nondimensional parameter similar to the Froude number. Film thickness correlations are also found by matching the measured and the proposed velocity derived in the shock theory. The surface wetting as well as the presence of the shock cause morphological changes in the Taylor bubble flow.</p>
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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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Cummins, Gerard Pio. "Fabrication of microchannels for use in micro-boiling experiments." Thesis, University of Edinburgh, 2011. http://hdl.handle.net/1842/5035.
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Increased power densities in VLSI chips have led to a need to develop cooling methods that can cope with the increased heat produced by such chips. Currently one of the more attractive methods to meet this goal is through the use of two phase flow of a fluid as changing phase of the material allows high heat transfer rates for a low temperature change. To bring this technology to commercialisation a greater understanding of the underlying physics involved at the microscale is required as there is much debate within literature as to what occurs during two phase flow heat transfer at these scales. The work conducted as part of this thesis is a step towards improving the understanding of the mechanisms involved with this process. This thesis describes the fabrication of a novel microchannel structure, which can be used to experimentally characterise two phase heat transfer as it occurs. The final process reported for these microchannels structures provides the basis of a technology for the fabrication of microchannels with increased sensor densities. Two types of microchannel devices have been fabricated for this project. The first device of these was an array of parallel microchannels formed by the reactive ion etching (RIE) of silicon, which was then bonded with Pyrex glass. These microchannels were simple in that sensors were not integrated for local measurement. However the production of these devices incorporated fabrication techniques such as anodic bonding and inductively coupled plasma RIE that were essential to the fabrication of more complex devices. The second device built was a single microchannel that contained an integrated heater and several temperature sensors. The use of wafer bonding enabled the device to take full advantage of both bulk and surface micromachining technology as the placement of the temperature sensors on the channel floor would not be possible with conventional bulk micromachining. The initial microchannel structures demonstrated that wafer bonding could be used to fabricate novel devices, but they highlighted the difficulty of achieving strong anodic bonds due to the presence of dielectric films throughout the fusion bonded wafer stack used in the channel fabrication. To improve the performance of the device the process was optimised through the use of insitu, non-destructive test structures. These structures enabled the uniformity and strength of the bonds to be optimised through visualisation over the whole wafer surface. The integrated sensors enabled temperature measurements to be taken along the channel with a sensitivity 3.60 ΩK-1 while the integrated heater has delivered a controllable and uniform heat flux of 264 kWm-2.
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Scott, Timothy Peter 1980. "Contraction/expansion flow of dilute elastic solutions in microchannels." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/17948.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.<br>Includes bibliographical references (p. 145-154).<br>An experimental study is conducted on the nature of extensional flows of mobile dilute polymer solutions in microchannel. By observing such fluids on the microscale it is possible to generate large strain rates ([approximately] 50,000 s⁻¹) that are greater than values which have been observed in macroscale contraction flows. Subsequently, large Deborah numbers (equivalent to those observed on the macro scale in high viscosity solutions) are generated for low viscosity solutions without the interplay of significant inertial effects. High quality microfluidic channels are fabricated using soft lithography techniques. Rheological behavior in these channels is dominated by an abrupt planar contraction, which generates extensional flow in the working fluids. Dilute viscoelastic aqueous solutions of polyethylene oxide are passed through 16:1 planar micro-contractions. Fluids exhibit substantial elastic behavior marked by elastic instabilities followed by subsequent lip vortices and eventually stable vortex growth. The onset of flow instabilities (De =50) and the nature of vortex growth are similar for PEO solutions at various concentrations. Differential pressure measurements indicate that substantial extensional thickening occurs at the onset of flow instabilities and indicate that planar extensional viscosities grow rapidly with increasing strain rates. Also apparent Trouton ratios are calculated indicating that extensional viscosities are two orders of magnitude larger than shear viscosities at high Deborah numbers.<br>by Timothy Peter Scott.<br>S.M.
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Renaghan, Liam Eamon. "On-Chip Isotropic Microchannels for Cooling Three Dimensional Microprocessors." Thesis, Virginia Tech, 2009. http://hdl.handle.net/10919/36404.
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This thesis reports the fabrication of three dimensionally independent on-chip microchannels using a CMOS-compatible single mask deep reactive ion etching (DRIE) process for cooling 3D ICs. Three dimensionally independent microchannels are fabricated by utilizing the RIE lag effect. This allows complex microchannel configurations to be fabricated using a single mask and single silicon etch step. Furthermore, the microchannels are sealed in one step by low temperature oxide deposition. The micro-fin channels heat transfer characteristics are similar to previously published channel designs by being capable of removing 185 W/cm2 before the junction temperatures active elements exceed 85°C.
To examine the heat transfer characteristics of this proposed on-chip cooler, different channel geometries were simulated using computational fluid dynamics. The channel designs were simulated using 20°C water at different flow rates to achieve a laminar flow regime with Reynolds numbers ranging from 200 to 500. The steady state simulations were performed using a heat flux of 100 W/cm2. Simulation results were verified using fabricated test chips. A micro-fin geometry showed to have the highest heat transfer capability and lowest simulated substrate temperatures. While operating with a Reynolds number of 400, a Nusselt number per input energy (Nu/Q) of 0.24 W-1 was achieved. The micro-fin geometry is also capable of cooling a substrate with a heat flux of 100W/cm2 to 45ºC with a Reynolds number of 525. These channels also have a lower thermal resistance compared to external heat sinks because there is no heat spreader or thermal interface material layer.<br>Master of Science
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Peng, Zhizi. "Modeling of Particle and Biological Cell Transport in Microchannels." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1324660368.
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Rapolu, Prakash. "Capillarity Effect on Two-phase Flow Resistance in Microchannels." University of Cincinnati / OhioLINK, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1204082574.
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Kuntaegowdanahalli, Sathyakumar S. "Inertial microfluidics for continuous particle separation in spiral microchannels." University of Cincinnati / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1242240249.
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Chatterjee, Arpita. "Size-Dependant Separation of Multiple Particles in Spiral Microchannels." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1312480517.
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Kuntaegowdanahalli, Sathyakumar Srinidhi. "Inertial microfluidics for continuous particle separation in spiral microchannels." Cincinnati, Ohio : University of Cincinnati, 2009. http://rave.ohiolink.edu/etdc/view.cgi?acc_num=ucin1242240249.
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Thesis (M.S.)--University of Cincinnati, 2009.<br>Advisor: Ian Papautsky. Title from electronic thesis title page (viewed Aug. 3, 2009). Includes abstract. Keywords: Passive separation; Dean flows; Inertial microfluidics; Cell separation. Includes bibliographical references.
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Jain, Alok. "A bioparticle separation technique through microchannels using sequentials pressure." Cincinnati, Ohio : University of Cincinnati, 2004. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=ucin1085759166.
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Shie, Shian-Reui, and 謝獻瑞. "Mixing Performance of Staggered Curved Microchannels and Y-Type Grooved Microchannels." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/85022700467095614163.
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碩士<br>國立屏東科技大學<br>生物機電工程系所<br>99<br>This paper examines the micro-fluid flow in the staggered curved Y-shaped with a groove with micro-flow as a result of structural design allows fluid to produce the phenomenon of disturbance of the flow field, flow with a staggered curved centrifugal force generated Dingen vortex flow field to the disturbance, more because of the bifurcation geometry with the combined fluid flow channel design and re-split, effectively reducing the diffusion distance between molecules. Confocal microscope using the fluorescent dye in the film within the flow channel of distribution sections and ink, and pH indicator fluid mixed case taken to do validation, to prove that the two shares of fluid flow due to the curved design of the high velocity produced under a strong centrifugal force, so that the two continue to exchange shares of the location of fluid, Bifurcation and combined with the flow of fluid geometric design also split and superimposed with each other, so that the two share more effective fluid mixing. Y-shaped microfluidic channel in a groove, used to analyze the structure of the ink result in disturbances of the fluid flow phenomena, micro-flow of fluid within the groove can move along the groove, the groove can tilt in one direction produces a very fluid the role of strong side-stream, the fluid will have access to the back wall of the phenomenon, with both sides to the tilt of the groove, the two sides to allow fluid to produce the side effect of the fluid flow is generated in the flow direction of two different vortex.
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Wasim, Khan Rehan. "Slip flow in microchannels." Thesis, 2013. http://ethesis.nitrkl.ac.in/5245/1/211ME3165.pdf.
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The assumption that a liquid adheres to a solid boundary ("no-slip" boundary condition) is one of the central tenets of the Navier-Stokes theory. However, there are situations wherein this assumption does not hold.One-dimensional single phase models for microchannel flow with slip flow boundary conditions have been presented here. The geometry of the problem and meshing of it have been made in GAMBIT. The simulation of steady incompressible fluid flow and heat transfer is done using ANSYS 13 FLUENT Software. Usually, the slip is assumed to depend on the shear stress at the wall.In the present work, the slip flow of liquid through a microchannel has been modeled using a slip length assumption instead of using conventional Maxwell’s slip flow model, which essentially utilizes the molecular mean free path concept. The hydrodynamics and thermal behaviour of a rectangular microchannel are studied here. The variation wall temperature, pressure drop in the channel and the friction factors calculated using ANSYS Fluent can well predict the experimental data. The effect of Re on the behaviour the channel are also studied. The models developed, following this approach, lend an insight into the physics of liquid flow through microchannels. Initially, in order to study the physics of flow inside the microchannel a simple analysis was done in a circular pipe.