bluuebunny/crossref_metadata_2025 huggingface dataset:
Entrance, Viscous Dissipation and Temperature Dependent Viscosity Effects on the Laminar Forced Convection in Straight Microchannels With Uniform Wall Heat Flux
Authors: Nonino C. , Savino S. , Del Giudice S.
Published: June 2009

A parametric investigation is carried out on the effects of viscous dissipation and temperature dependent viscosity in simultaneously developing laminar flows of liquids in straight microchannels of constant cross-sections. Reference is made to fluid heating conditions with a uniform heat flux imposed on the walls of the microchannels. Three different cross-sectional geometries are considered, chosen among those usually adopted for microchannels (rectangular, trapezoidal and hexagonal). Viscosity is assumed to vary with temperature according to an exponential relation, while the other fluid properties are held constant. A finite element procedure is employed for the solution of the parabolized momentum and energy equations. Due to the high value of the ratio between the total length and the hydraulic diameter in microchannels, such an approach is very advantageous with respect to the one based on the steady-state solution of the elliptic form of the governing equations in a three-dimensional domain corresponding to the whole duct. Computed axial distributions of the local Nusselt number and of the apparent Fanning friction factor are presented. Numerical results confirm that, in the laminar forced convection in the entrance region of straight microchannels, the effects of viscous dissipation and temperature dependent viscosity cannot be neglected in a wide range of operative conditions.
Enhancing Two-Phase Flow Mixing and Mass Transfer in Microchannel With Surface Features
Authors: Qiu Dongming , Tonkovich Anna Lee , Fanelli Maddalena , Fitzgerald Sean , Marco Jenn , Stevenson Rick , Lamont Mike , Burton Christy , Lerou Jan , Silva Laura
Published: June 2009

Slug or plug flow is generally considered as major flow pattern in microchannels in gas-liquid two-phase flow. A new microchannel design has enabled experimental interfacial surface area density exceeding 10,000 m2/m3 based on the two-phase volume in bubbly flow, and mass transfer coefficients exceeding 10sec−1. Numerical simulations as well as experiments are presented with the new microchannel design. The velocity components of secondary flow induced by specially designed angled microgrooves break the gas phase into small bubbles, where otherwise much larger gas pockets/slugs would dominate in flat or smooth wall microchannels. As such, mixing of the two phases and mass transfer are greatly enhanced as a results of increased interfacial surface area density and reduced average mass transfer distance. The Volume-Of-Fluid (VOF) method is used in the numerical computations for different surface feature patterns, gas and liquid flow rates, liquid viscosity and surface tension. In the experiments, nitrogen, carbon dioxide and water are used as the two phase media. The two-phase superficial velocity in the channel is in the range 0.45–2.75 m/s. The results show that the two-phase flow in the microchannel with the angled microgrooves leads to enhanced mass transfer relative to the flat microchannel. Higher flow rates and higher liquid viscosity lead to smaller gas bubbles and in turn enhanced mixing. Opportunities for additional improvement exist with increasing flow rates and optimized processing conditions.
Modelling of Impinging Flow and Heat Transfer in a Confined Narrow Gap
Authors: Zu Y. Q. , Yan Y. Y.
Published: June 2009

In this paper, the flow and heat transfer characteristics of a circular air jet vertically impinging on a flat plate near to the nozzle (H/d = 1∼6, where H is the nozzle-to-target spacing, d the diameter of the jet) are numerical analyzed using the CFD code FLUENT 6.1.18. The relative performance of seven versions of turbulent models, including the standard k–ε model, the renormalization group k–ε model, the realizable k-ε model, the standard k–ω model, the Shear-Stress Transport (SST) k–ω model, the Reynolds Stress (RS) model and the Large Eddy Simulation (LES), for the prediction of this type of flow and heat transfer is investigated by comparing the numerical results with available benchmark experimental data. It is found that Shear-Stress Transport k–ω model and Large Eddy Simulation time-variant model can give better predictions of fluid flow and heat transfer properties; especially, the SST k–ω model is recommended as the best compromise between the computational cost and accuracy. Using SST k-ω model, the effects of jet Reynolds number (Re), jet plate length-to-jet diameter ratio (L/d), target spacing-to-jet diameter ratio (H/d) and jet plate width-to-jet diameter ratio (W/d) on local Nusselt number (Nu) of the target plate are examined. A correlation for the stagnation Nu is presented.
A Numerical Study of Quasi-Nucleate Boiling in Mini- and Micro Channels
Authors: Zu Y. Q. , Yan Y. Y.
Published: June 2009

In this paper, a numerical study of quasi-nucleate boiling in horizontal mini- and micro-channels is carried out. The numerical simulation which is based on the multiphase model of volume of fluid (VOF) method available in commercial CFD software (Fluent 6.1.18) aims to study the corresponding flow behaviors of nucleate bubbles generated from the tube walls in mini/micro channels so as to understand the surface dominated effect of micro/mini channels on nucleate bubbles and heat transfer. In the present simulation, water enters a 2D channels with a uniform velocity of 0.2m/s. In order to examine the effects of channels size on bubbles flow and heat transfer, numerical simulations for three types of rectangular channels which have the same length of 30mm but different channel height respectively at 5mm, 0.5mm and 0.2mm are carried out. To have the nucleate effect or a quasi-nucleation condition, bubbles are generated approximately by injecting a small vapour bubble timely through an inlet located on the heated wall as the nucleate centre; meanwhile a transient vapour mass flow, based on the boiling parameters, is applied to approximate the bubble growth. While, the boiling parameters used in the simulation, such as bubble frequency, departure diameter and so on, are obtained by the calculations of correlations available from previous experimental studies.
Non-Continuum Effects in Micro-Channel Turbulent Gas Flow
Authors: Lewandowski T. , Jebauer S. , Czerwinska J. , Doerffer P.
Published: June 2009

Motivation of this work has its origin in the boundary layer control for aeronautics and turbomachinery. For that purpose boundary layer can be modified by perforated plates with holes (small channels) of specific sizes. The questions which rise in such configuration are related to the existence of optimal size of the holes (channels) and how micro-scale effects influence behavior of the global flow. This paper concentrates on the issue of the micro-scale channel effects. It is shown, that mass flow rate is influenced by slip effects and also by global behavior of the flow. Hence, these two phenomena are balanced and the micro-channel configuration is in most cases less efficient for boundary layer control than adequate macrochannel setup. Therefore it is proved that for specific condition an optimal hole (channel) size configuration exists. The numerical investigation presented here has shown various interesting micro-scale effects for laminar and turbulent micro-channel flow, which can improve understanding of gas flow behavior in micro-channels.
Slip Flow Structures in Confined Geometries
Authors: Jebauer Steffen , Czerwinska Justyna
Published: June 2009

This paper presents various flow structures related to velocity slip and temperature jump in very low Reynolds number gas flow. The structures differ significantly from the ones observed in continuum regime for laminar flow, especially if the geometry has complex structure, which is very often the case in microfluidic devices. We are modelling the flow as a continuum Navier-Stokes gas flow with additional velocity slip and temperature jump boundary conditions for curved surfaces for slip flows with Knudsen numbers Kn < 0.1. For complex channel geometries with obstacles and curved walls vortex patterns are observed that are related to the thermal stress slip flow. This type of flow is induced only when non-uniform temperature distributions inside flow domains are present. The investigated geometries consist of one or more cylinder walls with diameters of up to a few 100 μm placed inside of confined microchannels, with all setups being two-dimensional. In gaseous microdevices the resulting complex flow patterns can be utilised to enhance mixing or heat transfer.
FALCO: Fast Linear Corrector for Modelling DNA-Laden Flows
Authors: Benke M. , Shapiro E. , Drikakis D.
Published: June 2009

The paper concerns the development of a numerical algorithm for improving the efficiency of computational fluid dynamics simulations of transport of biomolecules in microchannels at low number densities. For this problem, the continuum approach based on the concentration field model becomes invalid, whereas time scales involved make purely molecular simulations prohibitively computationally expensive. In this context, meta-models based on coupled solution of fluid flow equations and equations of motion for a simplified mechanical model of biomolecules provide a viable alternative. Meta-models often rely on particle-corrector algorithms, which impose length constraints on the mechanical DNA model. Particle-corrector algorithms are not sufficiently robust, thus resulting in slow convergence. A new geometrical particle corrector algorithm — called FALCO — is proposed in this paper, which significantly improves computational efficiency in comparison with the widely used SHAKE algorithm. It is shown that the new corrector can be related to the SHAKE algorithm by an appropriate choice of Lagrangian multipliers. Validation of the new particle corrector against a simple analytic solution is performed and the improved convergence is demonstrated for a macromolecule motion in a micro-cavity. This work has been supported in part by the European Commission under the 6th Framework Program (Project: DINAMICS, NMP4-CT-2007-026804).
Flow Rate Limits in Surface Tension Dominated Open Channel Flows: An Efficient Design Tool
Authors: Klatte Joerg , Weislogel Mark M. , Dreyer Michael
Published: June 2009

In the present study a forced liquid flow through an open capillary channel is investigated. Due to the interplay between inertial and viscous forces the pressure along the flow path decreases causing the free surface to bend inwards the open channel. Because the curvature of the free surface is in turn dependent on the channel pressure a maximum flow rate is achieved beyond which the free surface collapses and gas ingestion occurs. This critical flow rate depends intricately on the channel geometry and liquid properties. A new numerical method has been developed to predict the pressure, free surface curvature, and velocity parameters for such open capillary channel flows with constant flow rates. The solver is able to calculate the critical flow rate for various channel geometries and can be used as an efficient design tool for open channel flows in capillary systems. As a demonstration, the method is validated herein via parabolic flight and drop tower experiments.
Coupled FVM-DSMC Simulation of Micro-Nozzle With Unstructured-Grid
Authors: Sun Zhixin , Li Zengyao , He Yaling , Tao Wenquan
Published: June 2009

The flow field and temperature distributions of free molecular micro-electro-thermal resist jet (FMMR) were studied resorting to DSMC-FVM coupled method. Direct Simulation Monte Carlo (DSMC) method is the most useful tool to simulate the flow field of FMMR and unstructured grid is suitable for the flow simulation in a complicated region with tilted wall surface. DSMC code based on unstructured grid system was developed and then the result was compared with that of structured grid and analytical solution to validate the reliability of the developed code. The DSMC method was then used to simulate the fluid flow in the micro-nozzle (Kn>0.01) and the temperature distribution in the nozzle wall was obtained by the Finite Volume Method (FVM). The Dirichlet-Neumann method was used to couple the wall heat flux and temperature between flow field and solid area. The effect of different income pressure was studied in detail and the results showed that the temperature of solid area changed drastically at different income pressure, so the commonly-adopted method of pre-setting boundary temperature before simulation was unreasonable. The results showed that the influence of boundary layer decreased as the pressure increased.
A Molecular Dynamics Study on Obstructed Flow Around Single and Two Paratactic Circular Cylinders
Authors: Sun Jie , He Ya-Ling , Li Yin-Shi , Tao Wen-Quan
Published: June 2009

Obstructed flow around single and two paratactic circular cylinders were investigated with two-dimensional molecular dynamics simulation (MDS) methods in the view of discrete particles. The transient and time-averaged profiles of streamline and density were obtained in order to analyze the characteristics of the wake flow. For single cylinder case, different flow patterns, i.e. Stokes flow, steady vortices flow, periodic vortices-shedding flow with the Ka´rma´n vortex street and supersonic flow, were divided based on Reynolds number (Re), 4<Re<12, 12<Re<20, 20<Re<62 and Re>62 respectively. For two paratactic cylinders case, different flow patterns, namely periodic vortices-shedding flow, periodic vortices-shedding flow with gap-flow, bistable flow and synchronized vortex-shedding flow, were observed with different center-to-center pitch ratios (D*/d*), D*/d* = 1.0, 1.0<D*/d*<1.1, 1.1<D*/d*<1.8 and D*/d*>1.8 respectively. The results show qualitative or quantitative agreement with those obtained from experiments and other MDS and indicate that most macroscopic obstructed flow patterns still exist even in nanoscale.
Characterization of Micro-Fluidic Exhaust Sensors Through Fully Coupled Modeling of Multi-Component Gas Transport and Reaction
Authors: Go¨ll Stephan , Piesche Manfred
Published: June 2009

Transport and reaction of gas mixtures in porous media are common phenomena in many chemical engineering applications. One favored method of modeling the transport processes is to notionally substitute a uniform bundle of tortuous channels for the irregular porous structure. Then, accurate equations of motion for the gas flow and diffusion inside these small-sized channels can be used. This advantage comes at the cost of two additional parameters that enter into the equations, the tortuosity factor and the equivalent capillary diameter. In this work, an existing model for transient transport of multi-component gas mixtures has been expanded to comprise heterogeneous fluid domains and chemical reaction. It can be applied to fluid domains that partially or completely enclose porous regions. The potential of the present model is demonstrated by simulating the electro-chemically induced and transport-limited signal formation inside an exhaust gas sensor.
Addressing Concentration Spaces by Multi Injectors for Micro Fluid Segment Sequences
Authors: Gu¨nther P. Mike , Mo¨ller Frances , Henkel Thomas , Ko¨hler J. Michael
Published: June 2009

Micro fluid segments attract increasing interest due to the possibility of easy generation, manipulation and use for diagnostics, chemical analytics, synthesis, cell cultivation and chemical as well as biological screenings. The addressing of chemical composition is a very important precondition for the application of micro fluid segments in highly parallelized operations. Here, we report the addressing of larger concentration spaces by use of a double-twin injector made in micro lithographic glass/glass technology. Two-dimensional concentration distributions can be achieved by varying flow rate ratios during the formation of fluid segments. Two different dyes (Orange G and Crystal violet) have been used to generate the two dimensional concentration space. The composition of dye containing segments is monitored by in-line flow-through micro photometry.
Correlations and Theory for Microchannel Condensation
Authors: Su Qian , Yu Guang Xu , Wang Hua Sheng , Rose John W.
Published: June 2009

In recent years several correlations have been proposed for calculating the heat-transfer coefficient during condensation in circular and non-circular channels of typical dimension around 1 mm where surface tension effects are important and correlations for larger diameter channels are inappropriate. A wholly theoretical approach applicable to annular flow has also been proposed. The correlations are all based on data for R134a, while the theory is applicable to any fluid. In this paper comparisons are made between the correlations for R134a and ammonia; plots based on theory are also included. Fair agreement is seen between all calculation methods for R134a but wide differences are seen for ammonia indicating that the correlations, based only on one fluid, do not capture the fluid property dependence accurately.
Study of Evaporation in Capillary Tubes by Infrared Thermography and Ombroscopy Techniques
Authors: Chauvet Fabien , Duru Paul , Prat Marc
Published: June 2009

We study the evaporation of a volatile liquid in a capillary tube of square internal cross section (side length = 1mm) by combining an infrared thermography technique with ombroscopy visualizations. This makes it possible to obtain the temperature profile along the outer surface of the capillary together with the evolution of the bulk meniscus position within the tube. Evaporation in such a tube is significantly faster than in tubes of circular section owing to the liquid films developing along the tube internal corners under the effect of capillary forces as the bulk meniscus recedes inside the tube. When the tube is held horizontal the temperature minimum stays at the tube entrance and the evaporation rate reaches a stationary value. This is the signature of the liquid films, which transport the liquid up to the tube entrance, where the mass transfer is important. When the tube is vertical and open at the top, the temperature minimum stays at the entrance (top) for a while. In contrast with the horizontal case, however, the position of the temperature minimum changes when the bulk meniscus has sufficiently receded inside the tube. The rate of evaporation then decreases significantly. The different behaviour between the vertical case and the horizontal one is explained by the thinning of the corner films in the vertical tube entrance region under the conjugated effects of gravity and viscous forces. When the tube is held horizontal, the capillary effects are dominant and the film thickness remains essentially constant in the tube entrance region.
Three-Dimensional Analysis of Fluid Flow and Heat Transfer in Single- and Two-Layered Micro-Channel Heat Sinks
Authors: Levac M. L.-J. , Soliman H. M. , Ormiston S. J.
Published: June 2009

Micro-channel heat sinks are currently at the forefront of cooling technologies for computer chips where the input heat flux is projected to exceed 100 W/cm2 [1, 2]. The quest for better heat-sink designs has produced different ideas, one of which is the idea of using multi-layered micro-channel heat sinks [3, 4]. The objectives of the present investigation were to conduct a detailed numerical study of the hydrodynamic and thermal behavior of a two-layered micro-channel heat sink and to compare the performance of the two-layered heat sink with that of a single-layered sink under laminar flow conditions.
Micro Heat Changers and Surface-Micro-Coolers for High Heat Flux
Authors: Schygulla Ulrich , Brandner Ju¨rgen J. , Anurjew Eugen , Hansjosten Edgar , Schubert Klaus
Published: June 2009

This publication describes the development of a new microstructure to transfer high heat fluxes. With a simple mathematical model based on heat conduction theory for the heat transfer in a micro channel at laminar flow conditions it was deduced that for the transmission of high heat fluxes only the initial part at the beginning of the micro channels is of importance, i.e. the micro channels should be short. Based on this principle a micro structure was designed with a large number of short micro channels taken in parallel. With this newly developed microstructure a prototype of a micro heat exchanger and a surface micro cooler was manufactured and tested. Using the prototype of the micro heat exchanger, manufactured of plastic, heat fluxes up to 500 W/cm2 were achieved at a pressure loss of 0.16 MPa and a mass flow of the water of 200 kg/h per passage. Due to the use of materials with a higher temperature resistance and higher stability like aluminum or ceramic, higher water throughputs and higher flow velocities could be realized in the micro channels. Thus it was possible to increase the heat flux up to approx. 800 W/cm2 at a pressure loss of approx. 0.35 MPa and a mass flow of 350 kg/h per passage. The important focus of investigation of the surface micro cooler was set on the examination of the surface temperatures for different heat fluxes and different velocities of the water in the micro channels. The experimental results of these surface micro coolers are summarized to characteristic maps. With this characteristic maps it is possible to determine whether a micro surface cooler can be used for a specific application.
Effects of Nanoparticles Deposition on Pool Boiling Heat Transfer Coefficient
Authors: Phan H. T. , Caney N. , Marty P. , Colasson S. , Gavillet J. , Marechal A.
Published: June 2009

Objectives In pool boiling heat transfer conditions: • To characterize nanoparticle deposition. • To understand the effects of surface coating by nanoparticles deposition.
Surface Roughness Effects on Heat Transfer in Microscale Single Phase Flow: A Critical Review
Authors: Young Perry L. , Kandlikar Satish G.
Published: June 2009

There has been increasing interest in research regarding microscale transport phenomena over the past decade. The increased surface area to volume ratio of a microchannel presents enhanced heat transfer characteristics when compared to conventional channels. For this reason, there has been heightened interest in the use of microchannels to meet the high heat dissipation demands of electronics. The fundamental understanding of microscale transport phenomena is an increasingly important area of research, and one area where such understanding is lacking is the effects of surface roughness on transport phenomena. There is very little published literature discussing the effects of surface roughness on the heat transfer characteristics of microchannels, and what literature exists exhibits discrepancies between experimental results. This paper serves as a critical review of literature from 2000 to the present, both experimental and theoretical, involving surface roughness effects on heat transfer in microscale transport phenomena.
Heat Transfer on Walls in Molecular Dynamics Simulations: Modelling With Vibrating Reflective Walls
Authors: van den Akker E. A. T. , Frijns A. J. H. , van Steenhoven A. A. , Hilbers P. A. J.
Published: June 2009

In simulations of micro channel cooling, the heat exchange from fluid to channel wall is an important aspect. Hence the heat exchange should be included in the model. Although numerically very expensive, it can be done by using a molecular wall. Numerically cheap implementations of a wall are the reflective wall and the thermal wall, and the combination of both, the diffusive-specular wall. In this paper we introduce the concept of a vibrating reflective wall as a boundary condition for molecular dynamics simulations. It is shown that the heat transfer with the vibrating reflective wall is the same as with a molecular wall, and that computation time is reduced greatly. As a competitive model, the diffusive-specular boundary condition is analyzed; it is shown that a good choice of parameters can give similar results in the same computation time, but the choice of parameters is not known a priori, therefore the vibrating reflective wall boundary condition is preferable.
Boiling Investigation on a Surface With Artificial and Natural Nucleons Sites
Authors: Kuzma-Kichta Yu. A. , Lavrikov A. , Afonin S. , Shustov M.
Published: June 2009

The water and Na2SO4 water solution boiling investigation had been carried in pool on the surfaces with artificial and natural nucleons sites under different pressures using high speed digital camera. The boiling of water was investigated at atmosphere pressure on a surface with a artificial nucleons site, which had a micro scale (cavity’s diameters – 100 and 200 μm and depth – 80 μm), in the Fridrich-Alexander University Erlangen-Nu¨rnberg, Germany in the Institute of Fluid Mechanics. The boiling of water and Na2SO4 water solution with concentration 20 g/l was investigated on a surface with natural nucleons sites in the pressure range 0.1 – 1 MPa in the Moscow Power-Engineering Institute (Technical University) on department of Thermal Physics. The “hand” video processing was used for the study’s results in case of the pool boiling on a surface with a artificial nucleon site. The data processing was realized for the study’s results using with a program “Bubble Detector”, which was specially developed for case of the pool boiling on a surface with the natural nucleon sites, and the “hand” processing was carried for video, which showed the reliability of the program “Bubble Detector”. First the distributions and dependences of basic boiling characteristics (frequency, departure diameter) were obtained at water pool boiling on a surface with a artificial cavity in the range of heat flux from 20 to 128 kW/m2 and the time dependence of vapor bubble’s grow. It was obtained, that departure diameters of vapor bubbles do not depend in case of boiling on a surface with a single cavity practically on heat flux. First the distributions and dependences of swimming velocities and equivalent diameters were obtained at water and Na2SO4 water solution with concentration 20 g/l pool boiling on a surface with natural nucleon sites in range of pressures from 0.1 to 1 MPa. The comparison of diameters’ and swimming velocities’ distributions of vapor bubbles was carried for under consideration conditions.
Numerical Analysis of Packed Bed Structure and the Flow Pressure Drop Using 3D Computer Tomography and Reconstruction Technology
Authors: Hu Yukun , Ding Jing , Yang Jianping , Yang Xiaoxi
Published: June 2009

It is known that flow pressure drop in porous media highly depends on the microstructure of solid matrix. Porous structure formed by packed particles, the effect of which on flow pressure drop behavior has been investigated by a series of numerical simulation methods. At the same time, the geometry information of the porous media for the simulations can be obtained by use of the 3D computer tomography and reconstruction technology. Otsu algorithm is introduced to handle the binary edges of the gray, while Amira for generation computing grid. The simulations are in a quite reasonable agreement with the available experimental results, and some further features are also explored. The elongation and the contraction of fluid elements are essential factors for the pressure loss in porous media flow, and the surface friction of micro-channels and the inertial and viscous resistance of the porous media plays important roles in increasing the pressure drop. The value of predicted pressure drop is in a qualitative accordence with the experimental results obtained in previous investigation.
Effects of Roughness on Turbulent Flow in Microchannels and Minichannels
Authors: Brackbill Timothy P. , Kandlikar Satish G.
Published: June 2009

The effect of roughness ranging from smooth to 24% relative roughness on laminar flow has been examined in previous works by the authors. It was shown that using a constricted parameter, εFP, the laminar results were predicted well in the roughened channels ([1],[2],[3]). For the turbulent regime, Kandlikar et al. [1] proposed a modified Moody diagram by using the same set of constricted parameters, and using the modification of the Colebrook equation. A new roughness parameter εFP was shown to accurately portray the roughness effects encountered in laminar flow. In addition, a thorough look at defining surface roughness was given in Young et al. [4]. In this paper, the experimental study has been extended to cover the effects of different roughness features on pressure drop in turbulent flow and to verify the validity of the new parameter set in representing the resulting roughness effects. The range of relative roughness covered is from smooth to 10.38% relative roughness, with Reynolds numbers up to 15,000. It was found that using the same constricted parameters some unique characteristics were noted for turbulent flow over sawtooth roughness elements.
Numerical Investigation on Flow Pattern and Pressure Drop Characteristics of Slug Flow in a Micro Tube
Authors: He Qunwe , Kasagi Nobuhide
Published: June 2009

In the present study, numerical simulation of adiabatic air-water slug flow in a micro tube is carried out. The focus is laid upon the pressure drop characteristics and its modeling. The Phase-Field method is employed to capture the interface between the phases, while the surface tension force is represented by the chemical potential formulation. The numerical results agree fairly well with available experimental results in terms of bubble shape and flow pattern. Simulation is repeated under different conditions of pressure gradient, void fraction and bubble frequency. It is found that the total pressure drop of a slug flow can be decomposed into two parts, i.e., the frictional pressure drop associated with a liquid slug sandwiched by bubbles, and the pressure drop over a bubble itself. For the former, when the liquid slug is longer than one tube diameter, the cross-sectional velocity distribution resembles a Poiseuille flow profile, so that the corresponding pressure drop can be predicted by the theoretical solution of single-phase liquid flow, i.e., fReTP = 64. For the latter, if it is assumed that the surface tension force is strong enough to sustain a thin liquid film between the interface and the tube wall, the pressure drop in this region is negligible. The pressure drop over a bubble is solely dependent on the two-phase superficial Reynolds number ReTP, which can be correlated as: Δpbubb′ = 0.07 + 42.4 / ReTP. This correlation predicts well the two-phase pressure drop in the form of the two-phase multiplier correlation as a function of the Lockhart-Martinelli parameter.
Geometry Optimization of a Flow-Based Device to Maximize Selectin-Mediated Hematopoietic Stem Cell Enrichment
Authors: Charles Nichola , Liesveld Jane L. , King Michael R.
Published: June 2009

Rare cell enrichment techniques must selectively capture and successfully retain cells that exist at < 5% in a suspension. We developed a device to capture hematopoietic stem and progenitor cells (HSPCs) from adult bone marrow using immobilized adhesion molecules called selectins in the presence of a flow field. While we continue to optimize the immobilized protein surface and improve the selectivity of the device for HSPCs, it appears to be at the expense of cell recovery. To address this issue, we used experimental and computational methods to identify the hydrodynamic factors that contribute to cell capture, and present new designs for the device that could improve HSPC recovery without affecting the selectivity. We tested KG1a cell recovery using four configurations of tubing — straight cylindrical, helical, flattened and axially pinched, and two cell loading conditions: static and continuous oscillatory. We utilized COMSOL 3.4 finite element modeling software to investigate the path of particles introduced into these geometries, the fluid velocities and local wall shear stresses, and the predicted recovery efficiency based on the number of particles that contact and stick to the walls of the geometries. We found that, experimentally, the highest number of cells were recovered using static cell loading in a straight cylindrical tubing, but we predict that a corrugated or pinched geometry could further increase recovery under desirable flow conditions.
Parametric Study for Two-Dimensional Rectangular Composite Microchannel With Heat Flux Boundary Condition
Authors: Gari Abdullatif A.
Published: June 2009

Microchannels used for cooling computer Silicon chips with heat flux boundary condition. Composite microchannels were not studied thoroughly. This paper introduces a new material at the bottom of the silicon ship with different thermal conductivity to enhance the heat flow rate and thus the cooling performance. A geometric model was considered and a numerical code was established to calculate the heat flow rate and Nusselt number at different interfaces of the microchannel. Different parameters were studied: composite materials, different working fluid, cross-sectional area of the composite material, heat flux at the boundary, and the fluid bulk temperature. Results shows how the heat flow rate change with the change of different parameters.
Laser-Induced Melting and Phase Explosion in Liquid Metal Films
Authors: Klentzman Jill , Ajaev Vladimir S. , Willis David A.
Published: June 2009

We develop a mathematical model of liquid flow and phase change phenomena during fabrication of micro- and nanochannels by laser-induced melting and evaporation of thin metal films deposited on glass substrates. Channels of cross-sectional sizes between several hundred nanometers and a few micrometers can be manufactured by wet etching or contact photolithography after the desired pattern is created by the laser in the metal film. Interaction of the laser beam with the metal film is a complicated process, characterized by high temperature gradients. In this work we investigate the regime where phase explosion takes place in a small region of the metal film surrounded by a pool of molten metal. In the melt region, both evaporation from the surface and viscous flow induced by thermocapillary stresses take place; all these processes are incorporated into the model. Evolution of the surface of the molten film is investigated, and the impact of phase explosion on the flow is discussed.
Effect of Wetting on Adiabatic Gas-Liquid Two-Phase Flow in a Microchannel
Authors: Ide Hideo , Kimura Ryuji , Inoue Kazuki , Kawaji Masahiro
Published: June 2009

An optical measurement system and video camera were used to investigate gas-liquid two-phase flow characteristics in wetting and poorly wetting circular microchannels of 100 μm diameter. By examining the optical sensor signals from which void fraction and the lengths and velocities of gas slugs and liquid slugs were measured, the effects of wetting on the adiabatic two-phase flow characteristics of nitrogen gas and water were investigated. The data were obtained using a T-junction with the same internal diameter as the microchannel, but the T-junction itself was well wetting in both experiments. Besides the flat nose and tail of gas plugs/slugs at low gas and liquid flow rates, poorly wetting microchannel showed higher void fraction and friction pressure drop compared to the well-wetting microchannel. The poorly wetting microchannel also showed the presence of short and fast moving liquid slugs which were absent in well-wetting microchannel.
Visualizations of Viscoelastic Fluid Flow in Microchannels
Authors: Li F.-C. , Kinoshita H. , Oishi M. , Fujii T. , Oshima M.
Published: June 2009

Solutions of flexible high-molecular-weight polymers or some kinds of surfactant can be viscoelastic fluids. The elastic stress is induced in such viscoelastic fluids and grow nonlinearly with the flow rate and results in many special flow phenomena, including purely elastic instability in the viscoelastic fluid flow. The elastic flow instability can even result in a special kind of turbulent motion, the so-called elastic turbulence, which is a newly discovered flow phenomenon and arises at arbitrary small Reynolds number. In this study, we experimentally investigated the peculiar flow phenomena of viscoelastic fluids in several different microchannels with curvilinear geometry by visualization technique. The viscoelastic working fluids were aqueous solutions of surfactant, CTAC/NaSal (cetyltrimethyl ammonium chloride/Sodium Salysilate). CTAC solutions with weight concentration of 200 ppm (part per million) and 1000 ppm, respectively, at room temperature were tested. For comparison, water flow in the same microchannels was also visualized. The Reynolds numbers for all the microchannel flows were quite small (for solution flows, the Reynolds numbers were smaller than 1) and the flow should be definitely laminar for Newtonian fluid. It was found that the regular laminar flow patterns for low-Reynolds number Newtonian fluid flow in different microchannels were strongly deformed in solution flows: either asymmetrical flow structures or time-dependent vortical flow motions appeared. These phenomena were considered to be induced by the viscoelasticity of the CTAC solutions.
Electrokinetic Actuation of Low Conductivity Dielectric Liquids
Authors: Raghavan Rohan , Qin Jiaxing , Yeo Leslie Y. , Friend James R. , Takemura Kenjiro , Yokota Shinichi , Edamura Kazuya
Published: June 2009

There has been some amount of confusion over the origin of electrohydrodynamic phenomena responsible for the actuation of dielectric fluids in the presence of an electric field. Previous studies have accounted for the possibility of conduction pumping, ion drag pumping and induction pumping as driving mechanisms but have ignored the possibility of Maxwell (electric) pressure driven flow. Until recently, this mechanism has been poorly understood and as a result has often been overlooked. This paper demonstrates how a Maxwell pressure gradient can induce flow in dielectric liquids in the presence of a non-uniform field. We derive, from first principles using lubrication theory, an expression for the flow velocity which exhibits a quadratic dependence on the applied voltage and also proportionality to the ratio of the permittivity and viscosity. The theoretical predictions are supported by experimental results. Although we have examined the phenomenon for a particular class of dielectric liquids, it is believed that this mechanism could well be responsible for the actuation of other low conductivity dielectric fluids previously attributed to conduction or ion drag pumping. In any case, we discuss ways to identify the dominant mechanism by comparing the salient features for a given type of flow.
Transverse Jets in Confined Laminar Micro Crossflow
Authors: Kastner Johannes , Woias Peter
Published: June 2009

The orthogonal injection of jets into microchannels with laminar crossflow is a common microfluidic operation. It is used for mixing in analytical micro systems (μTAS), chemical micro reactors or emulsification. If parallelized on a large scale, it has potential for fuel premixing in turbo machinery. Furthermore, it can be used as a transistor-like element for flow control or even as a sensor for flow properties. The above mentioned applications rely on the unique flow field induced by jets in confined crossflow: the momentum injected perpendicularly to the crossflow direction is transferred into a counter rotating vortex pair. The jet trajectory is mainly determined by the ratio of momenta and viscosity, resulting in three basic flow modes. With low ratios, the jet is too slow and cannot lift off from the channel bottom (creep flow), while the jet impinges almost undeflected onto the opposite channel wall at large ratios (impingement flow). The third type, core flow, occurs for intermediate ratios, and is characterized by well-defined jet boundaries and a distinctive vortex pair occupying the core domain of the channel. This paper investigates the basic theory of laminar liquid/liquid jets in rectangular microchannels with a characteristic diameter of some hundred microns. The nozzles are of a circular shape with diameters between 50 μm and 150 μm. Two simple models relating geometry and flow properties with the jet trajectory are derived, with particular consideration for the dimensionless characteristics. These analytic predictions are compared to observations for liquid flow. CFD simulations are used to study the impact of the flow mode on heat transfer, as well as the role of the counter rotating vortex pair. Finally, confocal laser microscopy is used for an experimental visualization of the flow field.
Two-Phase Flow Pressure Drop of R-134A in a Vertical Circular Mini/Micro Channel
Authors: Owhaib Wahib , Marti´n-Callizo Claudi , Palm Bjo¨rn
Published: June 2009

This paper presents new experimental results on saturated flow boiling pressure drop in microchannels. The two-phase pressure drops were measured for vertical saturated boiling of R-134a in circular tubes with internal diameters of 1.700, 1.224, and 0.826 mm, and a uniformly heated length of 220 mm, the total flow length is 580 mm. The total pressure drops were obtained for a heat flux range of 10–30 kW/m2, a mass flux range of 100–400 kg/(m2s), and outlet vapor qualities up to 0.6. Experiments were conducted at the average saturation pressure of 8.62 bar. The experimental data was compared to predictions by correlations available in the literature both for microchannels and larger diameter tubes.
Flow Dynamic Behaviors and Characteristics of Two-Stage Viscous Pump in Microchannels
Authors: Lu Jianfeng , Ding Jing
Published: June 2009

The flow dynamic behaviors and characteristics of a two-stage viscous pump with two rotating cylinderes in microchannels were numerically studied by laminar model. At low Reynolds numbers, the dimensionless mass flow rate almost keeps constant for similar flow structure in the identical type of pump, and it also has little variation with different types of pump expect for the staggered pump with very small distance between different cylinders. As the Reynolds number increasing, the dimensionless mass flow rate drops because of the vortices evolution, and the distance between different cylinders significantly affects the flow dynamic process. For large distance, two steady vortices appear behind the cylinders, and the flow dynamics of aligned and staggered pump are very similar. For small distance, the flow rate of the staggered pump almost decreases with the distance rising, while the flow rate of the aligned pump has a maximum at a specific distance. Corresponding to the single-stage pump, the mass flow rate of two-stage pump is remarkably enhanced, and the enhanment ratio will increase as Reynolds number droping. Meanwhile, the driving power of two cylinders drops very slowly as the distance decreasing.
Numerical Investigation of Single Phase Heat Transfer Enhancement in a Microchannel With Grooved Surfaces
Authors: Baghernezhad Nemat , Abouali Omid
Published: June 2009

This paper presents an investigation for two types of the grooves (rectangular and arc shapes) fabricated in the microchannel surfaces which leads to enhancement in single phase cooling. The pressure drop and heat transfer characteristic of the single phase microchannel heat sink were investigated numerically for laminar flow. For this purpose the conjugate heat transfer problem involving simultaneous determination of the temperature field in both solid and liquid regions was solved numerically. The heat sink includes an array of rectangular microchannels with grooved surface structures in the side walls and floor of the channel. The effect of these grooves on the pressure drop, outlet temperature of cooling fluid and the heat transfer rate were analyzed. The result showed that using a microchannel with grooved surfaces has a noticeable effect and heat removal rate can be increased using this technique. Also the grooves with the arc shapes have a better performance compared with a rectangular shape groove.
Prediction of Gas Flow Through Short and Long 2-D Micro and Nano-Channels Using a Generalized Slip Model
Authors: Raisee M. , Vahedi N. , Rostamzadeh A.
Published: June 2009

This paper reports the outcome of our recent effort in prediction of gas slip flow through microchannels. Slip Nitrogen flow through short (L/h = 20) and long (L/h = 2500) microchannels is analyzed and discussed using spectral element method and the generalize slip model boundary condition of Karniadakis and Beskok [1]. The well-known curvature in pressure distribution, due to the compressible behavior of the gas flow, is observed. Comparison of numerical results (for density and pressure variation as well as the slip velocity distribution with the experimental and DSMC data as well as reported numerical results showed that the generalize slip model is able to produce reliable results for both short and long microchannels.
Flow Boiling of Refrigerants Inside a Single Circular Minichannel
Authors: Bortolin Stefano , Cavallini Alberto , Del Col Davide , Matkovic Marko , Rossetto Luisa
Published: June 2009

The present paper reports the heat transfer coefficients measured during flow boiling of HFC-32 and HFC-134a in a 0.96 mm diameter single circular channel. The test runs have been performed during vaporization at around 30°C saturation temperature, correspondent to 19.3 bar for R32 and 7.7 bar for R134a. As a peculiar characteristic of the present technique, the heat transfer coefficient is not measured by imposing the heat flux; instead, the boiling process is governed by controlling the inlet temperature of the heating secondary fluid. The quality of the inner surface of the test tube has been measured to check the influence of surface roughness on the heat transfer coefficient. The flow boiling data taken in the present test section is presented and discussed, with particular regard to the effect of heat flux, mass velocity, vapor quality and fluid properties.
Numerical Analysis of the Drag Forces Acting on the Stationary Impenetrable Nano-Tube
Authors: Meigounpoory M. R. , Rahi A. , Mirbozorgi A.
Published: June 2009

The motion of nano-particles is important in many technical areas, for example super hydrophobic spray for coating materials, nano-capsules for detecting cancer, and additive nano-particles in oil and fuel for automotive engines. In this paper computations are performed to determine the steady flow forces acting on the stationary nano-tube for Reynolds numbers in the range of 0.1<Re<40 and 0.01<Kn<0.1 with the slip boundary condition. A single dimensionless parameter, the so-called slip number (Tr), is defined to account for the slip at the cylinder’s boundary. Increasing slip causes to delay of flow separation around the nano-tube surface. Numerical results confirm that slip variations can not affect on the drag coefficient values at low Reynolds regime. At higher Reynolds number increasing slip causes to drag reduction of flow around the nano tube.
Numerical Investigation of the Drag and Lift Forces Acting on Impenetrable Rotating Spherical Nano-Particle
Authors: Meigounpoory M. R. , Rahi A. , Mirbozorgi A.
Published: June 2009

The drag and lift forces acting on a rotating impenetrable spherical suspended nano-particle in a homogeneous uniform flow are numerically studied by means of a three-dimensional numerical simulation with slip boundary condition. The effects of both the slip coefficient and rotational speed of the nanosphere on the drag and lift forces are investigated for Reynolds numbers in the range of 0.1 < Re < 100. Increase of rotation increases the drag and lift force exerted by flow at the surface of nano-sphere. By increasing slip coefficient the values of drag and lift coefficients decreases. At full slip condition, rotation of the nano-sphere has not significant effects on the drag and lift coefficient values moreover the lift coefficient of flow around the rotating spherical particle will be vanished. Present numerical results at no-slip condition are in good agreements with certain results of flow around of rotating sphere.