bluuebunny/crossref_metadata_2025 huggingface dataset:
Micro-Heat-Sinks for Space Applications
Authors: Marengo M. , Zhdanov S. , Chignoli L. , Cossali G. E.
Published: December 2008

During the space missions, the problems related to the thermal conditioning of devices, to the personnel comfort and to the thermo-mechanical stresses are known and important. Furthermore for a space mission certain priorities are stressed, such as the small dimension and the lightness of thermal equipments. Due to the presence of high temperature gradients, which straightforwardly implies significant heating/cooling powers, these characteristics are sometimes difficult to obtain. The decreasing of the satellites payloads in terms of mass and volume has brought to the necessity of a further development of traditional space technologies, such as heat pipes and radiators. A promising technology is the fabrication of micro-heat-sinks for active and passive thermal control systems suitable for the space environment, which is always an important workshop for future progresses. In fact, miniaturized heat sinks will have a terrestrial large industrial diffusion for electronic component cooling, in propulsion and in the power production for satellites, spacecrafts and airplanes, in various biomedical applications and in cloth conditioning in harsh environmental conditions. The present paper intends to introduce the reader to the standard space requirements, to present some new prospective and experiments to present some new prospective and experimental results and to discuss the use of thermal MEMS for micro- and nano-satellites.
Boiling and Two-Phase Flow in Channels With Extremely Small Dimensions: Review of Japanese Research
Authors: Ohta Haruhiko
Published: December 2008

Researches concerning micro actuators utilizing vapor-liquid interfacial phenomena are extensively investigated to develop thermal devices applied to micro machines. On the other hand, the application of two-phase flow is useful for the removal of waste heat from the semiconductor chips with highly increased heat generation density to be integrated in notebook PCs. In the present paper, the latest Japanese research on boiling and two-phase flow in mini channels is reviewed covering those for the fundamental phenomena and practical applications. Boiling in a narrow channel between parallel plates is an ideal system for the development of the high-performance heat exchangers with extremely small sizes. The promising approaches to increasing the critical heat flux are introduced those by the present author to compensate the disadvantage inherent in this system.
Fluid Dynamics and Transfer Processes in Bended Micro Channels
Authors: Kockmann Norbert , Engler Michael , Haller Daniel , Woias Peter
Published: December 2008

The understanding of the flow processes in microchannels and micro mixers is essential for the design of micro fluidic devices like micro reactors or analytical equipment. We have performed a systematic numerical CFD-study of mixing and mass transfer in sharp 90° bends and heat transfer in T-joints to obtain a detailed insight into the flow patterns and corresponding transfer processes in a wide range of Reynolds numbers. With increasing flow velocity the straight laminar flow starts to form symmetrical vortices in the bend, at the entrance of the mixing channel, and in T-joints. The vortices enhance the transport processes like heat and mass transfer in the channels significantly. The influence of the geometry and the flow conditions is shown by an analytical estimation of the relevant forces. The appearance of convective transport processes is used for the definition of microflows which are controlled by viscous forces and diffusive transfer processes.
Spectral Intensity Mapping and Analysis of Dyed Microflows
Authors: Curran K. , Davies M.
Published: December 2008

The objective of this work is to develop simple reliable software to observe and quantify diffusion phenomena in microfluidic devices. One of the great advantages of microfluidic technology is that it permits the flow and diffusion of multiple streams in a single channel. The accurate control of a diffusion-based process has applications in bio-analytical chemistry, production of organic compounds and combinatorial chemistry. This method has been discussed in the literature as Laminar Fluid Diffusion Interface technology. It is heavily dependant on the controlled and reproducible introduction of several fluids into one channel and enables the design of separation and detection systems based on laminar fluid diffusion interfaces. A method of analyzing and interpreting the diffusion behavior of multiple microflows using the MATLAB programming language as an image analysis tool is presented here. This paper considers two dimensional brightfield and time series images but the method can be applied to other forms, including fluorescent and three-dimensional images. The approach taken relies on the fact that a digital image stores its colour information in signal channels. The information contained in the channels depends on the colour method being used to define the image. Software-based spectral filtering is performed, yielding three dimensional intensity maps of dyed and clear microflows. These maps can be used to monitor diffusion behavior in a number of different areas simultaneously. Spectral noise reduction techniques are also incorporated without significant reduction in original data quality. The technique is used to determine the aqueous diffusion coefficient of the dye Green S by processing digital images taken at set time intervals of two seconds in a stopped-flow experiment. The approach is applied to microflows in straight, two-dimensional serpentine and three-dimensional serpentine channel configurations.
Analyses of Liquid Flow in Micro-Conduits
Authors: Koo Junemo , Kleinstreuer Clement
Published: December 2008

Experimental observations of liquid microchannel flow are reviewed and results of computer experiments concerning channel entrance, wall slip, non-Newtonian fluid, surface roughness, viscous dissipation and flow instability effects on the friction factor are discussed Specifically, based on numerical friction factor analyses, the entrance effect should be taken into account for any microfluidic system. It is a function of channel length, aspect ratio and the Reynolds number. Non-Newtonian fluid flow effects are expected to be important for polymeric liquids and dense particle suspension flows. The wall-slip effect is negligible for liquid flows. For relatively low Reynolds numbers, i.e., Re > 1,200, onset to instabilities may have to be considered because of possible geometric non-uniformities, including a contraction and/or bend at the microchannel inlet as well as substantial surface roughness. Significant roughness effects, described with a new porous medium layer (PML) model, are a function of the Darcy number, the Reynolds number and cross-sectional configurations. This model was applied to micro-scale liquid flows in straight channels, tubes and rotating cylinders, and validated with experimental data sets. In contrast to published models, PML model simulations yield both an increase and decrease of the friction factor depending on the Darcy number. Viscous dissipation in microchannels is a strong function of the channel aspect ratio, Reynolds number, Eckert number, Prandtl number, and conduit hydraulic diameter. Specifically, viscous dissipation effects are quite important for fluids with low specific heat capacities and high viscosities, even for very low Reynolds numbers, i.e., ReD < 1. The viscous dissipation effect was found to decrease as the fluid temperature increases. As the aspect ratio deviates from unity, the viscous dissipation effect increases. It was found that ignoring the viscous dissipation effect could ultimately affect friction factor measurements for flows in micro-conduits. This could imply a significant amount of viscous heat generation and, for example, may diminish a projected micro-heat-exchanger performance.
Artificial-Moving-Wall Condition in Simulation of Micro Couette Flow
Authors: Liu HsinChih Frank
Published: December 2008

A boundary condition, Artificial-Moving-Wall (AMW) condition, is developed with DSMC method to simulate the partial domain of micro Couette flow that contains at least one interface condition. Partial domain exists in the cases of multiple numerical algorithms applied on one full (complete) domain to deal with the variation of Knudsen number in microflows; in Couette flow, partial domain can be formed by either one wall condition and one interface condition or two interface conditions. As opposed to free stream condition applied on the interface of a full (complete) domain that directly injects incoming particle through interface, AMW condition utilizes a virtual solid surface as a medium to transfer the interface information to the simulated partial domain for improving the convergence rate and speed up the computational time. The formulation of AMW condition and the concepts of compensative term and compensated boundary/interface information in artificial boundary condition will be addressed in this paper. The full domain and three different partial domain cases in micro Couette flow were computed in this research, and the physical results including velocity, temperature, and density are shown to demonstrate the effectiveness of AMW condition.
An Experimental Study of Water Flow in Smooth and Rough Rectangular Micro-Channels
Authors: Baviere R. , Ayela F. , Le Person S. , Favre-Marinet M.
Published: December 2008

This paper presents experimental results concerning water flow in smooth and rough rectangular micro-channels. It is part of a work intended to test the classical fluid mechanics laws when the characteristic length scale of inner liquid flows falls below 500μm. The method consists in determining experimental friction coefficients as a function of the Reynolds number. This implies simultaneous measurements of pressure drop and flow rates in microstructures. The two experimental apparatus used in this study enabled us to explore a wide range of length scales (7μm to 300μm) and of Reynolds number (0.01 to 8,000). Classical machining technologies were used to make micro-channels of various heights down to a scale of 100μm. Smaller silicon-Pyrex micro-channels were also made by means of silicon-based micro technologies. In these structures, friction coefficients have been measured locally with Cu-Ni strain gauges. For every height tested, both smooth and rough walls were successively used. When compared to macro-scale correlation the results demonstrate that i) In the smooth case, friction is correctly predicted by the Navier-Stokes equations with the classical kinematic boundary conditions, ii) For 200μm high channels, visualizations show transition to turbulence at Reynolds number of about 3,000. The presence of roughness elements did not significantly influence this result and iii) Roughness considerably increases the friction coefficient in the laminar regime. However, the Poiseuille number remains independent of the Reynolds number.
Effect of Entrance Condition on Frictional Losses and Transition to Turbulence in Minichannel Flows
Authors: Campbell Levi A. , Kandlikar Satish
Published: December 2008

In studying the fluid flow and heat transfer in microchannels and minichannels, various claims have been made regarding transition at Reynolds numbers significantly below 2300. As a first step in identifying the reasons for such reports on early transition, the effect of entrance geometry on the pressure drop and transition to turbulence was studied in a conventional channel of 19 mm inside diameter (Kandlikar and Campbell [1]). As a second step, the effect of entrance condition on pressure drop and transition to turbulence is studied in small channels with diameters of 1.067 mm and 0.457 mm. The two entrance conditions employed for both channels are re-entrant and smooth. The experimental results show the effect of entrance condition on local friction factor, transition Reynolds number, and Hagenbach’s factor.
Hydrodynamic Behaviour and Influence of Channel Wall Roughness and Hydrophobicity in Microchannels
Authors: Celata G. P. , Cumo M. , McPhail S. , Zummo G.
Published: December 2008

The sometimes contradictory results available for fluid flow in micropipes show that much is yet to be verified in micro fluid dynamics. In this study the influence of channel wall roughness and of channel wall roughness and of channel wall hydrophobicity on adiabatic flow in circular microchannels is investigated, varying in diameter from 70 μm to 326 μm. The hydrodynamic behaviour of water in smooth tubes down to 30 μm inner diameter (ID) is also ascertained. Within the current experimental accuracy it is found that the classical Hagen-Poiseuille law for friction factor vs. Reynolds number is respected for all diameters measured and Re > 300. With degassed water, no effect of slip flow conditions due to hydrophobic channel walls even at 70 μm ID was noted, which might suggest that the slip flow phenomenon is associated with local desorption of dissolved gases on the hydrophobic surface, as reported elsewhere in the literature. For roughened glass channels, an increase in friction factor above 64/Re was observed only at the smallest diameter measured, 126 μm. For all experiments, no anticipated transition to turbulent flow was observed (2000 < Retr < 3000).
Turbulent Statistics of Rarefield Flows in Microchannel Flow and Heat Transfer
Authors: Li G. X. , Tao W. Q. , Li Z. Y. , Yu B.
Published: December 2008

Direct numerical simulation has been carried out to investigate the effect of weak rarefaction on turbulent gas flow and heat transfer characteristics in mirochannel. The Reynolds number based on the friction velocity and the channel half width is 150. Grid number is 64×128×64. Fractional time step method is employed for the unsteady Navier-Stokes equations, and the governing equations are discretized with Finite Difference Method. Statistical quantities such as turbulent intensity, Reynolds shear stress, turbulent heat flux and temperature variance are obtained under various Knudsen number from 0 to 0.05. The results show that rarefaction can influence the turbulent flow and heat transfer statistics. The streamwise mean velocity and temperature increase with increase of Kn number. In the near wall region rarefaction can increase the turbulent intensities and temperature variance. The effect of rarefaction on Reynolds shear stress and wall-normal heat flux are presented. The instantaneous velocity fluctuations in the vicinity of the wall are visualized and the influence of Kn number on the flow structure is discussed.
Compressibility Effects in Micro Synthetic Jets
Authors: Timchenko Victoria , Reizes John , Leonardi Eddie
Published: December 2008

Effects of including compressibility in the numerical modeling of flows produced by and in synthetic jet actuators — consisting of an oscillating diaphragm in a cavity with a small circular orifice in the face opposite the diaphragm — has been studied for axisymmetric configurations. Numerical results obtained on the assumption of incompressible and compressible flows with orifice diameters of the 20 and 40 μm and with an orifice length of 50 μm are compared. There are significant differences between compressible and incompressible flows for the 20 μm orifice, in that the jet velocity is greatly reduced when compressible flow is assumed, whereas the differences are much smaller in the 40 μm case. For both orifices the pressure rise upstream of the orifice is smaller when the fluid is compressible. It follows that results obtained on the assumption of incompressible flow cannot be extrapolated for micro-synthetic jet actuators handling compressible fluids.
Heat Transfer Characteristics of Gaseous Flow in a Micro-Tube
Authors: Asako Yutaka
Published: December 2008

Two-dimensional compressible momentum and energy equations are solved numerically to obtain the heat transfer characteristics of gaseous flow in a micro-tube with isothermal wall. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The stagnation temperature is fixed at 300 K and the computations were done for the wall temperatures of 305 K, 310 K and 350 K. The bulk temperature based on the static temperature is compared with that of the incompressible flow in a conventional sized tube. The static bulk temperature of the gaseous flow in a micro-tube decreases approaching to the outlet due to the energy conversion into the kinetic energy, when flow is fast. The total temperatures are also compared witht he bulk temperature of the incompressible flow in a conventional sized tube. The total temperature is slightly higher than the bulk temperature of the incompressible flow. This is due to the additional heat transfer near the outlet. A correlation for the prediction of the heat transfer rate of the gaseous flow in the micro tube is proposed.
Control of Separated Flow Past Backward-Facing Step in Microchannel
Authors: Baysal O. , Erbas N. , Koklu M.
Published: December 2008

A key concern for micro device design is its power consumption. When such a device involves microflows, actively controlling the flow losses often reduces the power requirements. In the present study, a micro synthetic jet is proposed as a flow control device. The method used is an automated design optimization methodology coupled with computational fluid dynamics. Microflows in the Knudsen range of 10−3 to 10−1 are modeled using a Navier-Stokes solver but with slip velocity and temperature jump boundary conditions derived for micro-sized geometries. First, an uncontrolled flow past a backward facing step in a channel is computed. Then, a synthetic jet actuator is placed downstream of the step where the separation occurs. A large number of test cases have been analyzed. It has been observed that the reattachment point of the separated flow and the flow dissipation are quite sensitive to the location and the geometry of the synthetic jet as well as the parameters of the oscillating membrane. The best flow control, defined as the largest decrease in dissipation, is obtained when the actuator cavity width and the membrane oscillation amplitude are increased simultaneously.
Experimental Study on Gas Cooling Heat Transfer for Supercritical CO2 in Microchannels
Authors: Kuang G. , Ohadi M. M. , Zhao Y.
Published: December 2008

The conventional refrigerants have considerable ozone depleting effect (CFC/HCFC) and global warming impact (HFC). Carbon Dioxide (CO2) is being investigated as an alternative refrigerant for vapor compression systems. In addition to its environmental benefits, Carbon Dioxide offers certain attractive thermal characteristics such as small surface tension, small liquid viscosity and large refrigerant capacity. Furthermore, when used with micro channels CO2 heat exchangers provide additional advantage of high compaction, low weight/low volume, while yielding excellent thermal performance. The objective of the present work was to study the heat transfer and pressure drop characteristics of supercritical CO2 gas cooling process in microchannels. A 10 ports microchannels tube with ID of 0.79mm was tested for the pressure range of 8 to 10MPa and mass flux range of 300 to 1200 kg/m2 s. As expected, mass flux has a significant influence both on the supercritical heat transfer and pressure drop coefficients. Pseudo-critical temperature (temperature at which the specific heat has maximum value for the given pressure) is found to play an important role in the CO2 heat exchanging process as well. Conventional forced convection heat transfer correlations fail to accurately predict the heat transfer coefficients of supercritical CO2 with deviations as much as 50% from experimental data, especially near pseudo-critical temperature. As the gas cooling pressure increases, the pressure drop decreases, which is due to the lower viscosity & higher density. Employing average specific heat along the entire tube length, a semi-empirical correlation was developed to predict the supercritical gas cooling process of CO2 in microchannels, within an error of 20%.
Effect of Compressibility on Heat Transfer in Microchannels
Authors: Turner Stephen E. , Asako Yutaka , Faghri Mohammad
Published: December 2008

In this paper an experimental investigation of convective heat transfer is presented for laminar gas flow through a microchannel. A test stand was setup to impose thermal boundary conditions of constant temperature gradient along the microchannel length. Additionally, thin film temperature sensors were developed and used to directly measure the microchannel surface temperature. Heat transfer experiments were conducted in the laminar flow regime with the outlet Ma between 0.10 and 0.42. The experimental measurements of inlet and outlet gas temperature and the microchannel wall temperature were used to validate a 2D numerical model for gaseous flow in microchannel. The model was then used to determine bcal values of Ma, Re, and Nu. The numerical results show that after the entrance region, Nu approaches 8.23, the fully developed value of Nu for incompressible flow for constant wall heat flux if Nu is defined based on (Tw-Taw) and plotted as a function of the new dimensionless axial length, X* = (x/2H)(Ma2)/(RePr).
Modelling of Two Phase Fluid Flow in Small Circular and Rectangular Channels
Authors: Onbasioglu S. U.
Published: December 2008

The two phase flow of air-water mixture in circular and rectangular channels with hydraulic diameters of 100 μm to 1 mm has been modeled via computational tools. Two phase flow patterns obtained from the computed results have been compared to those of the ones reported in literature and a reasonable agreement has been obtained. After the validation of the computational procedure, keeping the dimensionless numbers relevant to the small hydraulic diameters constant, the size of the channel has been changed to understand the transition to the flow regime in which the “conventional” governing equations fail. Gas and liquid superficial velocities range from 0.083 m/s to 70 m/s and from 0.043 m/s to 5.67 m/s, respectively.
Correlations of the Holdup and the Frictional Pressure Drop in Air-Water Two-Phase Flow in a Flat Capillary Rectangular Channel
Authors: Ide Hideo , Fukano Tohru
Published: December 2008

Both vertical upward and horizontal gas-liquid two-phase flows in a flat capillary rectangular channel were studied to clarify the flow phenomena, the holdup and the frictional pressure drop. The dimension of the channel used was 9.9 mm × 1.1 mm. The orientations of the channel were with the wide side vertical and the wide side horizontal. The differences between the flow characteristics in such orientations were investigated. New correlations of holdup and frictional pressure drop for flat capillary channels are proposed, in which the effect of aspect ratio has been taken into consideration.
Effect of Channel Diameter and Liquid Property on Void Fraction in Adiabatic Two-Phase Flow Through Microchannels
Authors: Kawaji Masahiro , Kawahara Akimaro , Chung Peter M.-Y. , Sadatomi Michio , Okayama Kazuya
Published: December 2008

Adiabatic two-phase flow experiment have been conducted to investigate the effects of channel diameter and liquid property on void fraction in horizontal microchannels. Water/nitrogen gas and ethanol-water/nitrogen gas mixtures were pumped through circular microchannels of 50, 75, 100 and 251 μm diameter. The concentration of ethanol in water was varied to change the surface tension and liquid viscosity. The void fraction data were obtained by an image analysis technique and correlated as a function of homogeneous void fraction. The void fraction data in channels with a diameter between 50 and 100 μm conformed well to Kawahara et al.’s (2002) correlation, but the data for a 251 μm diameter channel agreed with the Armand correlation (1946) suitable for minichannels. There was no significant effect of liquid properties on void fraction for all the channel sizes. Thus, these results suggest that the boundary between microchannels and minichannels would lie between 100 and 251 μm, and not be sensitive to the fluid property.
Subchannel Analysis on Two-Phase Flow Characteristics Through Narrow Gaps in a Tight-Lattice Rod Bundle
Authors: Tamai Hidesada , Ohnuki Akira , Akimoto Hajime
Published: December 2008

Evaluation of a critical heat flux is one of the most important issues for design of an advanced water-cooled reactor core. Since it becomes difficult to perform full-scale experiments due to a larger scale of the advanced reactor cores, an analytical approach has been widely noticed in the core design. To predict the critical heat flux in high accuracy, it is required to correctly understand a horizontal distribution of a two-phase flow in the rod bundles. In this study, the two-phase flow characteristics through narrow gaps in the tight-lattice 37-rod bundle experiment at JAERI were investigated using the subchannel analysis code, NASCA. At the center of the bundle, liquid flowed toward the periphery due to the diversion cross-flow at the elevation where boiling started and the turbulent mixing and the void drift were not influential as they can be neglected. On the periphery of the bundle, the flow mixings due to the diversion cross flow, turbulent mixing and void drift were almost the same order. Gas flowed in the same way with the liquid phase due to the diversion cross-flow, and the turbulent mixing and the void drift moved the gas in the opposite way of the liquid phase migration. An amount of the diversion cross-flow for the liquid phase increased in proportion to the square of the mass velocity. The characteristics of cross flow were almost the same in the different local power peaking and in the different gap widths in the present model.
Diffusion Modeling in a Microchannel for Separation of Species
Authors: Rajesh P. K. , Ponnambalam P. , Ramakrishnan N. , Prakasan K.
Published: December 2008

Recently there is an increased interest in the design of microfluidic devices for research in biotechnological studies, applied to sample detection and analysis of species. When fluids are confined to small volumes, mixing results almost entirely by diffusion due to low velocities of flow in microchannels. As a result, it is possible to design microfluidic systems in which dissimilar fluids flow along side each other over long distances without significant mixing. The H-filter is a microfluidic device used for the extraction of molecular analytes from liquids containing interfering particles. The principle behind H filter is that small molecules will diffuse quickly from a sample stream to the buffer stream while very large molecules and particles will remain indefinitely in the sample stream because of their much larger size and much decreased diffusion rate. Because the Reynolds number in most microfluidic channels is generally kept well below 1, no turbulent mixing of fluids occurs. The only means by which solvents, solutes and suspended particles move in a direction transverse to the direction of flow is by diffusion. Differences in diffusion coefficients can be used to separate molecules of large particles over time. The time spent in flowing in a channel is proportional to the length of the channel. Before carrying out experiments, it is worthwhile to simulate the diffusion process in a microfluidic device for various properties of species and channel geometry. This paper attempts to model the diffusion process in an H-filter for typical species using CFD-ACE+, a software for solving problems in fluid dynamics with multi-physics capabilities. A module of CFD ACE+, called user-scalar that allows the user to define scalar quantities and boundary conditions for this scalar is used in the simulation. As seen from the studies, the diffusivities of species A and B in the buffer influence their diffusion. Optimization of geometry for a given species can be done with this method and separation can be achieved. The results from such a study will be useful for the design optimization and fabrication of such devices.
A Relation Between Two-Phase Pressure Drop and Flow Distribution in a Compact Heat Exchanger Header
Authors: Kim Jong-Soo , Lee Ki-Taek , Kim Jae-Hong , Ha Soo-Jung , Im Yong-Bin
Published: December 2008

In this paper an experimental study was performed for relation between two-phase pressure drop and flow distribution in compact heat exchanger using small diameter tubes. We performed the experimental study in non-heating mode. A test section was consisted of the horizontal header (circular tube: φ 5 mm × 80 mm) and 10 upward circular channels (φ 1.5 mm × 850 mm) using acrylic tube. Three different types of tube insertion depth were tested for the mass flux and inlet quality ranging from of 50–200 kg/m2s and 0.1–0.3, respectively. Air and water were used as the test fluids. Two-phase pressure drop of each channel and three type of distribution header was measured. As whole, single-phase and two-phase, pressure drop in rear channel is found to be lower than that in front channel. In conclusion, we can claim that principle of distribution is almost same pressure drop in each channel. Comparing pressure drop in branch tube with correlation equation, it was found that in single-phase flow, experimental value was 10% lower than Hagen-Poiseuille, Blasius equation (Eq. 40) in two-phase flow.
Two-Phase Flow Analysis in Multi-Channel
Authors: Ha Man Yeong , Kim Cheol Hwan , Jung Yong Won , Jeong Giho , Heo Seong Geun
Published: December 2008

In the present study, we have carried out the experimental and numerical studies for the single- and two- phase flow characteristics and the corresponding pressure drop in the single- and multi-channels. We used the finite volume method to solve the mass and momentum conservation equations. The volume of fluid model is used to predict the two-phase flow in the channel. The calculated results for the single- and two-phase flow are partly compared with the present experimental data, showing relatively good agreement between them. The numerical scheme used in this study predicts well characteristics of single- and two-phase flow in a multi-channel. Thus we expect that system performances could be improved by obtaining the optimal conditions from the present calculation.
Liquid Diffusion Measurement in Micro/Mini Channels From Full-Field Digital Phase Measurement Interferometry (PMI)
Authors: Garvey Julie , Newport David , Dalton Tara
Published: December 2008

This paper considers division of amplitude interferometry as a means to extract fluid information from micro-systems. Initially the phase measurement technique is analysed and the measurement limitations of mixing measurement are assessed. Accurate phase measurements are then made of the concentration in a 3 dimensional channel flow. A mini sized channel with tow fluid flows at Reynolds numbers of 0.848 and 0.0848 is numerically analysed. The same channel is experimentally tested and the results for the mixing concentration gradients in channel flow are compared with those obtained numerically. The requirement for experimental measurement for accurate measurement of binary liquid diffusion is observed by the variation between experimental and numerical results. The diffusion coefficient measurement verifies PMI as a means of mixture measurement, or more broadly as a phase measurement technique for small-scale, or micro scale, fluidic analysis. PMI’s potential is finally discussed as a measurement technique for concentration, and hence fluidic analysis of micro channel mixing.
Evaporation in Microchannels: Influence of the Channel Diameter on Heat Transfer
Authors: Dupont V. , Thome J. R.
Published: December 2008

This paper discusses the effect of diameter on both flow boiling heat transfer and transition from macro to microchannel evaporation. A recently proposed three-zone flow boiling model based on evaporation of elongated bubbles in microchannels is briefly described and used for the present analysis. In the microscale range, the model predicts an increase in the two-phase heat transfer coefficient with a decrease of d for low values of the vapor quality and a decrease of h for larger values of x. This behavior is explained by the influence of the liquid film thickness, deposited periodically behind passing liquid slugs.
Feasibility Study on the Prototype Microchannel Evaporator for the Residential Air-Conditioning Application
Authors: Cho Honggi , Cho Keumnam , Koo Hyoungmo , Kil Seong-Ho , Kim Jeung-Hoon
Published: December 2008

The present study was aimed to test the feasibility of the prototype microchannel evaporator for the residential air-conditioning application using R-22 refrigerant under wet condition. Eight prototype evaporators were manufactured and tested using a psychrometric calorimeter. Each evaporator consisted of two or three parallel flow heat exchangers connected with return pipes. The parallel flow heat exchanger had 41 parallel microchannel tubes that brazed to the inlet and outlet headers. The tube had 8 rectangular ports with the hydraulic diameter of 1.3mm. The louvered fin had louver angle of 27°, louver pitch of 1.4mm and flow depth of 18.8mm. The cooling capacities of the different test evaporators were severely changed as both mass flow rate and inlet quality were increased due to the flow mal-distribution in the evaporator. The cooling capacity was increased as the vertical inclination angle of the evaporator increased. The condensate under wet condition was also measured. The flow area ratio of the evaporator affected the most seriously among the test parameters on the cooling capacity. Pressure drops on both refrigerant and air sides for the best prototype evaporator were 28.6kPa and 2.5mmAq, respectively.
Experimental Study of Flow Patterns, Pressure Drop and Flow Instabilities in Parallel Rectangular Minichannels
Authors: Balasubramanian Prabhu , Kandlikar Satish G.
Published: December 2008

The use of phase change heat transfer in parallel minichannels and microchannels is one of the solutions proposed for cooling high heat flux systems. The increase in pressure drop in a two phase system is one of the problems, that need to be studied in detail before proceeding to any design phase. The pressure drop fluctuations in a network of parallel channels connected by a common head need to be addressed for stable operation of flow boiling systems. The current work focuses on studying the pressure-drop fluctuations and flow instabilities in a set of six parallel rectangular minichannels, each with 333 μm hydraulic diameter. Demonized and degassed water was used for all the experiments. Pressure fluctuations are recorded and signal analysis is performed to find the dominant frequencies and their amplitudes. These pressure fluctuations are then mapped to their corresponding flow patterns observed using a high speed camera. The results help us to relate pressure fluctuations to different flow characteristics, and their effect on flow instability.
High-Speed Imaging of Flow in Microchannel Array Water Evaporators
Authors: Henning Torsten , Brandner Juergen J. , Schubert Klaus
Published: December 2008

The flow of water and steam in an electrically powered micro heat exchanger consisting of an array of 68 wavy microchannels in parallel, each 200 μm wide, 100 μm deep and approximately 40mm long, was investigated by high speed videography. Semi-quantitative, time and space resolved information about the void fraction was extracted by pixel intensity calculations. Fluctuations at sub-audio frequencies (pulsation) were always visible in the microchannel array. Pulsation on the outlet went into a minimum with increasing heating power, accompanied by a shift in the lateral distribution of channels filled with more steam or water, respectively. Almost complete evaporation could be achieved with this microstructure at a water mass flow rate of 86 kg m−2 s−1. Differences between the test device and typical devices for applications in thermal and process engineering are identified.
Visualization of Low Heat and Mass Flux Boiling in a Small Metal Pipe Using Neutron Radiography
Authors: Oseen-Senda K. M. , Lundell F. , Hillenbach A. , Pauchet J.
Published: December 2008

High speed visualization of boiling pentane in a circular steel tube (Di = 1.2 mm, Do = 2 mm) has been performed at the Neutrograph instrument at the Institut Laue-Langevin in Grenoble, France. The heat and mass flux were both very low and appropriate for cooling of PEM fuel cells. The spatial resolution of the images is approximately 0.15 mm and the maximum frequency is 154 Hz. In the images, the liquid-vapor differentiation is clearly visible. Time resolved measurements of the outer pipe wall temperature, synchronized with the images, show that at low mass flow rates, the pipe wall is high above the saturation temperature and the pipe filled with vapor and liquid slugs. At higher flow rates, the wall is superheated when filled with liquid and at saturation temperature during boiling when exposed to a liquid-vapor mixture. An irregular switching between these two states was observed. The superheated wall is shown to be consistent with superheated liquid in the pipe prior to boiling. Unfortunately the strong γ-radiation produced by the neutrons has a substantial effect on the onset of boiling, which is why comparisons with non-irradiated systems might be difficult.
Effect of Gravitational Orientation on Flow Boiling of Water in 1054 × 197 µm Parallel Minichannels
Authors: Kandlikar Satish G. , Balasubramanian Prabhu
Published: December 2008

Microchannels and minichannels are being considered for high heat flux applications under microgravity environment in space missions. An experimental study is undertaken to determine the effect of gravitational orientation on flow boiling characteristics of water in a set of six parallel minichannels, each 1054 μm wide by 197 μm deep and 63.5 mm long with a hydraulic diameter of 333 μm. Three orientations — horizontal, vertical downflow and vertical upflow — are investigated under identical operating conditions of heat and mass fluxes. High-speed images are obtained to reveal the detailed two-phase flow structure and liquid-vapor interactions. The experimental data and high speed flow visualization indicate that compared to the horizontal case, the flow becomes less chaotic for vertical upflow, while the reversed flow becomes more pronounced in vertical downflow case. The resulting in increase in the back-flow is responsible for channel-to-channel flow maldistribution and heat transfer degradation. From the heat transfer data it is concluded that the performance of the tested channels under microgravity environment will be similar to the horizontal flow case.
Flow Boiling in Minichannels of Copper, Brass, and Aluminum Round Tubes
Authors: Bang K. H. , Choo W. H.
Published: December 2008

The past work on flow boiling heat transfer in minichannels ranging one to three millimeters of hydraulic diameter has indicated that the local heat transfer coefficients are largely independent of mass flux and vapor quality, but mainly a function of wall heat flux. The present work is a revisit of flow boiling in minichannels by conducting experiment using 1.67 mm inner diameter tubes of three different materials; aluminum, brass, and copper, to investigate an effect of the tube inner surface conditions with the focus on an effect on nucleate boiling. Tests were conducted for R-22, a fixed mass flux of 600 kg/m2s, 5∼30 kW/m2 of wall heat flux, 0.0∼0.9 of local vapor quality. The present experimental data confirmed that the flow boiling heat transfer coefficient in a minichannel varies only by heat flux, independent of mass flux and vapor quality. The effect of tube material was found small for the tubes used in the present work. The present data were well predicted by the correlation proposed by Tran et al. (1996).
Numerical Simulation of Growth of a Vapor Bubble During Flow Boiling of Water in a Microchannel
Authors: Mukherjee Abhijit , Kandlikar Satish G.
Published: December 2008

The present study is performed to numerically analyze growth of a vapor bubble during flow of water in a microchannel. The complete Navier-Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid vapor interface is captured using the level set technique. The microchannel is 200 microns in square cross-section and the bubble is placed at the center of the channel with superheated liquid around it. The results show steady initial bubble growth followed by a rapid axial expansion after the bubble fills the channel with a thin liquid film around it. The bubble then rapidly turns into a plug and fills up the entire channel. A trapped liquid layer is observed between the bubble and the channel as the plug elongates. The bubble growth rate increased with the incoming liquid superheat and formation of vapor patch at the walls is found to be dependent on the bubble growth rate. The upstream interface of the bubble is found to exhibit both forward and reverse movement during bubble growth. Results show little effect of gravity on the bubble growth under the specified conditions. The bubble growth features obtained from numerical results are found to be qualitatively similar to experimental observations.
Microbubbles Emission Flow Boiling in a Microchannel and Minichannel
Authors: Tange Manabu , Yuasa Maki , Takagi Shu , Shoji Masahiro
Published: December 2008

This paper reports the results of experimental investigations on microbubbles emission boiling (MEB) in a microchannel and minichannel. MEB is a boiling phenomenon under high subcool and high heat flux condition. To understand the mechanisms and processes of MEB, pool boiling on a thin wire under subcooled condition was firstly performed. Under various subcooling condition, the experiments of subcool boiling were performed. From photographic observation, distributions of bubble diameters and their dependence on subcooling were investigated. To achieve better understanding of the process of MEB and to find the incipient subcooling of MEB, a sound of ebullition was recorded and analyzed. It was found that 30 to 40 K are the incipient subcooling of MEB on the thin wire. In the experiment of MEB in microchannel and minichannel, the special attention was paid to longitudinal transition of boiling behavior, since bulk liquid temperature increases in short length in case of subcooled flow boiling. Boiling curves for various channel height were obtained from experimental data, and pressure drop through whole channel was measured. In a minichannel, annular flow flushed out the whole channel from the downstream to the upstream and the pressure drop fluctuates, while in a microchannel, flow pattern was divided into two regions: bubbly flow in upstream and annular flow in downstream.
Conjugated Heat Transfer at Flow Condensation in Minchannel With Longitudinal Fins
Authors: Kabov O. A. , Marchuk I. V. , Legros J. C.
Published: December 2008

A mathematical model for vapor condensation in cylindrical longitudinally finned minichannel has been proposed taking into account all temperature non-uniformity. The model describes annular and rivulet flow regimes until complete grooves flooding. It is shown that when the model does not account for the wall heat conductivity the maximal calculated enhancement of heat transfer is predicted for the “sharp” trapezoidal fins. The most enhanced heat transfer in the case of non-isothermal fins has been obtained for the curvilinear fins of expanded Adamek’s parametric family. When reducing the heat conductivity the curvilinear fins become still more effective in comparison with the trapezoidal fins. The wall non-isothermity is a factor, which cannot be neglected when modeling condensation in a minichannels with finned surfaces.
Alternating Condensation Flow Patterns in Microchannels
Authors: Wu H. Y. , Cheng Ping , Siu Billy Chin-Pang , Lee Yi-Kuen
Published: December 2008

A simultaneous visualization and measurement experiment was carried out to investigate condensation flow patterns of steam flowing through an array of parallel microchannels with a hydraulic diameter of 82.8 μm and a length of 30mm in a <100> silicon wafer. These microchannels were covered with a thin transparent pyrex glass from the top that enabled the visualization of flow patterns in the test section. The degassed and deionized water steam flowing in the microchannels was cooled by cooling water of 8°C at the bottom of the wafer. Experiments were performed for different inlet pressures while the outlet pressure was maintained at a value of 105Pa (the atmospheric pressure). When the inlet pressure was decreased to the value of 1.45×105Pa and the corresponding mass flux was decreased to 23.6g/cm2s, a succession of droplet/injection/slug-bubble flow was observed in the microchannels. Under this condition, the upstream, midstream, and downstream of the microchannels were occupied by the droplet flow, injection flow, and slug-bubble flow, respectively. This concurring droplet/injection/slug-bubble flow appeared periodically in the microchannels that caused large fluctuations of wall temperatures and fluid temperatures with respect to time. The droplet/injection/slug-bubble flow pattern in microchannels has never been reported in the literature.
Film Condensation in Horizontal Triangular Section Microchannels: A Theoretical Model
Authors: Wang Huasheng , Rose John W.
Published: December 2008

The paper presents a theoretical model to predict film condensation heat transfer from a vapor flowing in a horizontal tube with equilateral triangular section minichannels or microchannels. The model is based on fundamental analysis which assumes laminar condensate flow on the channel walls and takes account of surface tension, vapor shear stress and gravity. The case considered here is where the channel wall temperature is uniform and the vapor is saturated at inlet. Sample numerical results are given for the channel size (side of triangle) of 1.0 mm and for refrigerant R134a. The general behaviour of the condensate flow pattern (spanwise and streamwise profiles of the condensate film), as well as streamwise variation in quality and local mean (over section perimeter) heat-transfer coefficient, are qualitatively in accord with expectations on physical grounds.
Integrated Micro-Channel Cooling in Industrial Applications
Authors: Nicole C. C. S. , Dekker R. , Aubry A. , Pijnenburg R.
Published: December 2008

Experiments and simulations have been performed in order to assess the feasibility of integrated single phase forced convection in silicon micro-channels for the cooling of electronics. A silicon micro-channel device has been fabricated with channel size of 100 by 300 μm. Cooling has been achieved with a heater dissipating up to 370 W (750 W/cm2) with a flow rate of 0.1 1/min. In this case the maximum junction temperature was 130°C. This paper presents characteristics of such a cooling device as well as its description and fabrication. Experimental results are shown and compared with simulations. A description of a rough optimization of the channels size is given followed by comments describing the main advantages and drawbacks regarding industrial feasibility.
Optimization of Microchannel Geometry for Direct Chip Cooling Using Single Phase Heat Transfer
Authors: Upadhye Harshal R. , Kandlikar Satish G.
Published: December 2008

Direct cooling of an electronic chip of 25mm × 25mm in size is analyzed as a function of channel geometry for single-phase flow of water through small hydraulic diameters. Fully developed laminar flow is considered with both constant wall temperature and constant channel wall heat flux boundary conditions. The effect of channel dimensions on the pressure drop, the outlet temperature of the cooling fluid and the heat transfer rate are presented. The results indicate that a narrow and deep channel results in improved heat transfer performance for a given pressure drop constraint.