bluuebunny/crossref_metadata_2025 huggingface dataset:
Variance Reduction in Particle Methods for Solving the Boltzmann Equation
Authors: Baker Lowell L. , Hadjiconstantinou Nicolas G.
Published: September 2008

We present a new particle scheme for solving the Boltzmann equation; this scheme incorporates a recently developed variance reduction technique discussed in [L. L. Baker and N. G. Hadjiconstantinou, Physics of Fluids, vol. 17, art. no 051703, 2005] which exhibits a significant computational efficiency advantage for low speed flows, compared to traditional particle methods. This paper describes how this variance reduction approach, achieved by simulating only the deviation from equilibrium, can be implemented as a particle simulation method. The new scheme is validated using time dependent shear flow calculations.
Study on Micro-Pump Using Boiling Bubbles
Authors: Koizumi Yasuo , Ohtake Hiroyasu
Published: September 2008

By making use of boiling and condensation in a micro channel, a micro pump was developed. The length and the diameter of the half-circle cross-section micro-channel which had two open tanks at both ends were 26 mm and 0.5 mm, respectively. A 0.5×0.5 mm patch electrically heated was located at the offset location from the center between both ends of the micro channel; at 8.5 mm from the one end and at 17 mm from the other end. The micro channel and the two open tanks were filled with distilled water. The heating patch was heated periodically to cause periodical formation of a boiling bubble and its condensation. By this procedure, flow from the short side (8.5 mm side) to the long side was created. The flow rate increased as the heating rate was increased. The average flow velocity and flow rate obtained were approximately ∼ 12 mm/s and ∼ 3 mm3/s, respectively. The velocity of a interface between the bubble and liquid during the condensation period was much faster than that during the boiling period. During the condensation period, the velocity of the interface at the short channel side (8.5 mm side) was faster than that at the long channel side (17 mm side). The equation of motion of liquid in the flow channel was solved to calculate the traveling of liquid in the flow channel. Predicted velocities agreed well with the experimental results. The velocity differences between the short side and the long side as well as between the boiling period and the condensing period were expressed well by the calculation. Liquid began to move from the stationary condition during both boiling and condensing periods. Liquid in the inlet side (short side) moves faster than that in the outlet side (long side) during the condensation period because of less inertia than that in the long side. Since the condensation was much faster than boiling, this effect is more prominent during the condensation period. By iterating this, the net flow from the short side to the long side was created.
High Performance and Sub-Ambient Silicon Microchannel Cooling
Authors: Colgan E. G. , Furman B. , Gaynes M. , LaBianca N. , Magerlein J. H. , Polastre R. , Bezama R. , Marston K. , Schmidt R.
Published: September 2008

High performance single-phase Si microchannel coolers have been designed and characterized in single chip modules in a laboratory environment using either water at 22°C or a fluorinated fluid at temperatures between 20 and −40°C as the coolant. Compared to our previous work, key performance improvements were achieved through reduced channel pitch (from 75 to 60 microns), thinned channel bases (from 425 to 200 microns of Si), improved thermal interface materials, and a thinned thermal test chip (from 725 to 400 microns of Si). With multiple heat exchanger zones and 60 micron pitch microchannels with a water flow rate of 1.25 lpm, an average unit thermal resistance of 15.9 C-mm2/W between the chip surface and the inlet cooling water was demonstrated for a Si microchannel cooler attached to a chip with Ag epoxy. Replacing the Ag epoxy layer with an In solder layer reduced the unit thermal resistance to 12.0 C-mm2/W. Using a fluorinated fluid with an inlet temperature of −30°C and 60 micron pitch microchannels with an Ag epoxy thermal interface layer, the average unit thermal resistance was 25.6 C-mm2/W. This fell to 22.6 C-mm2/W with an In thermal interface layer. Cooling >500 W/cm2 was demonstrated with water. Using a fluorinated fluid with an inlet temperature of −30°C, a chip with a power density of 270 W/cm2 was cooled to an average chip surface temperature of 35°C. Results using both water and a fluorinated fluid are presented for a range of Si microchannel designs with a channel pitch from 60 to 100 microns.
Microtube Gas Flows With Second-Order Slip Flow and Temperature Jump Boundary Conditions
Authors: Xiao Nian , Elsnab John , Ameel Tim
Published: September 2008

Second-order slip flow and temperature jump boundary conditions are applied to solve the momentum and energy equations in a microtube for an isoflux thermal boundary condition. The flow is assumed to be hydrodynamically fully developed, and the thermal field is either fully developed or developing from the tube entrance. In general, first-order boundary conditions are found to over predict the effects of slip and temperature jump, while the effect of the second-order terms is most significant at the upper limit of the slip regime. The second-order terms are found to provide a correction to the first-order approximation. For airflows, the maximum second-order correction to the Nusselt number is on the order of 50%. The second-order effect is also more significant in the entrance region of the tube. Nusselt numbers are found to increase relative to their no-slip values when temperature jump effects are small. In cases where slip and temperature jump effects are of the same order, or where temperature jump effects dominate, the Nusselt number decreases when compared to traditional no-slip conditions.
Two-Phase Pressure Drop and Boiling Heat Transfer in a Single Horizontal Microchannel
Authors: Huh Cheol , Kim Moo Hwan
Published: September 2008

With a single microchannel and a series of microheaters made with MEMS technique, two-phase pressure drop and local flow boiling heat transfer were investigated using deionized water in a single horizontal rectangular microchannel. The test microchannel has a hydraulic diameter of 100 μm and length of 40 mm. A real time observation of the flow patterns with simultaneous measurement are made possible. Tests are performed for mass fluxes of 90, 169, and 267 kg/m2s and heat fluxes of from 100 to 600 kW/m2. The experimental local flow boiling heat transfer coefficients and two-phase frictional pressure gradient are evaluated and the effects of heat flux, mass flux, and vapor qualities on flow boiling are studied. Both the evaluated experimental data are compared with existing correlations. The experimental heat transfer coefficients are nearly independent on mass flux and the vapor quality. Most of all correlations do not provide reliable heat transfer coefficients predictions with vapor quality and prediction accuracy. As for two-phase pressure drop, the measured pressure drop increases with the mass flux and heat flux. Most of all existing correlations of two-phase frictional pressure gradient do not predict the experimental data except some limited conditions.
The Effect of the Time Dependent Pressure Difference on Bubble Dynamics in Microchannels
Authors: Keikhaee Aidin , Rouhani Shahin , Saboohi Yadollah
Published: September 2008

The physical processes responsible for bubble formation in microchannels are not well understood and lack fundamental understanding. Experimental results are not exactly in agreement with each other and there are no definite theories to explain the possible effects of different parameters. Among different parameters the microchannel hydraulic diameter can affect the bubble formation mechanism in microchannels strongly. In this paper the effect of the time dependent pressure difference between inside and outside of the bubble on bubble dynamics in microchannels have been investigated. The source of this time dependency can be the emergence of bubble embryos which produces a density perturbation in liquid. Results show that bubble growth rate and maximum bubble diameter in microchannels are smaller than them in ordinary size channels. These results are in good agreement with some researchers’ observations, sometimes called fictitious boiling and bubble-extinction, in microchannels.
Condensation Heat Transfer of Soliman Flow Pattern in Mini-Channel
Authors: Chen Q. , Amano R. S.
Published: September 2008

The condensation heat transfer for R134a in the two kinds of in-tube three-dimensional (3-D) micro-fin tubes with different geometries is experimentally investigated. Based on the flow pattern observations, the flow patterns in the Soliman flow regime map are divided into two-flow regimes; one with the vapor-shear-dominant annular regime and the other with the gravitational-force-dominant stratified-wavy regime. In the annular regime, the heat transfer coefficients of the two kinds of in-tube 3-D micro-fin tubes decreases as the vapor quality decreases. The regressed condensation heat transfer correlation from the experimental data of the annular flow region is obtained. The dispersibility of the experimental data is inside the limits of ±25%. In the stratified-wavy regime, the average heat transfer coefficient of the two kinds of in-tube 3-D micro fin tubes increases as the mass flux increases and the number of micro fins in the 3-D micro-fin tube is not the controlling factor for performance of a condensation heat transfer. The regressed condensation heat transfer correlation of the stratified-wavy flow regime is experimentally obtained. The dispersibility of the experimental data is inside the limits of ±22%. Combined with the criteria of flow pattern transitions, the correlations can be used for the design of a condenser with 3-D micro fin tubes.
Experimental Study of Annular and Stratified Flows in Horizontal Micro-Fin Tubes
Authors: Chen Q. , Amano R. S.
Published: September 2008

In this paper, a new kind of evaporative heat transfer experiment for the cooling process of coolers/condensers is conducted. The design of the test coils is immersed in an air-water bubbling layer. The air-water two-phase flow passes through the tubes of the coils. Due to the motion of the air bubbles in the water, a thin water film forms on the surface of the tube. As the air bubbles pass by the tube this water film is evaporated into the air. The tubes of coil reject heat to the water film, and the evaporation of the water film rejects heat to the air bubble stream. This heat transfer mode significantly increases the heat transfer coefficient between tubes and air. The consumption of the power of a water pump can be decreased. Moreover, the airflow rate required is less than that of an air-cooled condenser. The pressure drop of air through air-water bubbling layer and the heat transfer between the tube and water are experimentally investigated in this paper. The results show that the factors affecting the pressure drop and the heat transfer coefficient involve the pore geometry of sieve plate, the height of the air-water bubbling layer, the air flow rate through the sieve plate and the heat flux of tubes. The heat transfer coefficient between tube and water is two times larger than that of falling film of water on the outer surface of tube.
Numerical Simulation of Thermo-Electro-Magnetic Performances in Supersonic Channel Flow
Authors: Xu Z. , Lee C. , Amano R. S.
Published: September 2008

A compressible magnetohydrodynamic (MHD) model composed of MHD Navier-Stokes (N-S) equations and magnetic induction equations is proposed in the present study for analyzing the magnetohydrodynamic characteristics in MHD generator and MHD accelerator channels of Magneto-Plasma-Chemical propulsion system [10∼12]. A splitting algorithm based on an alternative iteration is also developed for solving the two sets of equations [9]. As a test case, a supersonic MHD flow in a square duct was simulated. The numerical results are compared with the results computed by solving the classical N-S equations for the perfect gas flow, together with the results computed utilizing the degenerate MHD N-S equations for the same channel flow with constant applied magnetic field. The thermo-electro-magnetic performances of the test cases with constant and variable applied fields are then discussed.
Study of the Combined Contributions of Porous Coverings and Mixture Surface Energies in Enhancing Pool Boiling
Authors: Siqueiros Eric , Reyes Rene
Published: September 2008

Factors as the boiling fluid surface tension and the characteristics of the solid surface where the heat transfer takes place could be modulated for increasing the boiling heat flux. In this work was observed the increase in the boiling convective heat-transfer coefficient (h) from the participation of: (a) the use of a binary mixture at its critical micelle concentration (16% w/w ethanol-water); (b) the addition of the surfactant sodium-lauryl-sulfate (SLS) to this aqueous mixture; and (c) the use of a porous covering fabricated from stainless steel bands with void volume 0.25, pore diameter 0.8 mm and covering thickness 8 mm. The sequence of results allowed the calculation of the relative participation of these factors in h (and the related values of excess temperature), for power supply from 100 to 1000 W on the same heater cartridge for all the experiments. For boiling water on the bare heater, hmax bare heater = 8.27 W/cm2 K; for boiling water on the porous covering, hmax covering = 19.36 W/cm2 K; the boiling of the water-ethanol (16%) mixture on the porous covering produced hmax covering+cmc = 31.72 W/cm2 K; and the binary mixture with 100 ppm of SLS, hmax covering+cmc+surfactant = 38.07 W/cm2 K. Considering this value of hmax covering+cmc+surfactant as the sum of the contributions, the relative participation of the mechanical forces breaking the escaping bubbles through the covering is 29.13%; the surface energies associated to the formation of micelle structures 32.47%; and the surface energies from the surfactant 16.67%. Thus, the search of enhancing heat transfer should consider the boiling mixture composition as well as the porous covering design. A comparison of the results obtained with the covering developed in this work with some coverings developed in a previous work reveals that the geometry of the covering material could be the base for heat transfer enhancement.
Experimental and Numerical Investigation of Convection Heat Transfer of CO2 at Super-Critical Pressures in a Vertical Mini Tube
Authors: Jiang Peixue , Zhang Yu , Shi Runfu
Published: September 2008

Convection heat transfer of CO2 at supercritical pressures in a 0.27mm diameter vertical mini tube was investigated experimentally and numerically. The tests investigated the effects of inlet temperature, pressure, mass flow rate, heat flux, buoyancy and flow direction on the convection heat transfer. The experimental results indicate that for inlet Reynolds numbers exceeding 4000, the flow direction and buoyancy force have little influence on the local wall temperature, with no deterioration of the convection heat transfer observed in either flow direction. The convection heat transfer coefficient initially increases with increasing heat flux and then decreases with further increases in the heat flux for both upward and downward flows. These effects are due to the variation of the thermophysical properties, especially cp. For inlet Reynolds numbers less than 2900, the local wall temperature varies nonlinearly for both flow directions. The numerical results correspond well with the experimental data for inlet Reynolds numbers exceeding 4000 using several turbulence models, especially the Realizable k-ε turbulence model. However, for inlet Reynolds numbers less than 2900, none of the turbulence models could properly simulate the convection heat transfer at super-critical pressures with high heat fluxes.
Experimental and Numerical Investigations of Air Flow Characteristics Through Microporous Media
Authors: Xu Ruina , Jiang Peixue
Published: September 2008

The effect of particle diameter on the air flow characteristics in various micro-porous media test sections was studied experimentally and numerically. The test sections were made of bronze particles with average diameters of 200 μm, 125 μm, 90 μm and 40 μm. The experimentally measured friction factors in the porous media with average diameters of 200 μm and 125 μm agree well with the known correlation. However, the experimental values for the friction factors in the micro-porous media with 90 μm a and 40 μm average diameters are much less than the known correlation. Also, the differences between the experimental results and the known correlation increase with decreasing average particle diameter. Numerical simulations of the air flow in micro-porous media including rarefaction were performed using the CFD code FLUENT 6.1 to predict the pressure drop characteristics in the four test sections. The calculated friction factors for the non-slip flow regime in the micro-porous media agree well with the known correlation and the experimental data. The numerically predicted friction factors for the slip flow regime in the micro-porous media with 90 μm and 40 μm diameter particles were less than the known correlation and close to the experimental data. The results show that rarefaction effects occur in air flows in the micro-porous media with particle diameters less than 90 a and that numerical calculations with velocity slip on the boundary can simulate the slip flows in micro-porous media.
Experimental Analysis of Laminar-to-Turbulent Transition for Gas Flows in Smooth Microtubes
Authors: Geoffroy Sandrine , Colin Ste´phane , Lorenzini Marco , Morini Gian Luca
Published: September 2008

Reviews of the experimental results related to the laminar-to-turbulent transition in microchannels have appeared in the last years in the literature. These studies indicate that the transition from the laminar to the turbulent flow in micro-scale passages has been experienced at critical Reynolds numbers ranging from 70 up to 10000. In this work the characteristics of the laminar-to-turbulent transition for gases flowing through commercial Peek microtubes used for gas-chromatography columns are experimentally investigated. The microtubes tested are characterized by an internal diameter ranging between 300 μm and 100 μm with a L/Dh ratio in the range between 167 to 4000. Since the absolute roughness of the microtubes tested was very low (less than 0.05 μm) it is possible to uncouple the effects of the roughness on the laminar-to-turbulent transition. In the paper the effects of the gas compressibility and of the L/Dh ratio on the critical Reynolds number for which the laminar-to-turbulent transition takes place will be analyzed and discussed by comparing the experimental results with the other data published in the literature.
Flow and Stability of Rivulets on Heated Surfaces With Topography
Authors: Gambaryan-Roisman Tatiana , Stephan Peter
Published: September 2008

Surfaces with topography promote rivulet flow patterns, which are characterized by a high cumulative length of contact lines. This property is very advantageous for evaporators and cooling devices, since the local evaporation rate in the vicinity of contact lines (micro region evaporation) is extremely high. The liquid flow in rivulets is subject to different kinds of instabilities, including the long-wave falling film instability (or the kinematic-wave instability), the capillary instability and the thermocapillary instability. These instabilities may lead to the development of wavy flow patterns and to the rivulet rupture. We develop a model describing the hydrodynamics and heat transfer in flowing rivulets on surfaces with topography under the action of gravity, surface tension, and thermocapillarity. The contact line behavior is modeled using the disjoining pressure concept. The perfectly wetting case is described using the usual h−3 disjoining pressure. The partially wetting case is modeled using the integrated 6-12 Lennard-Jones potential. The developed model is used for investigating the effects of the surface topography, gravity, thermocapillarity and the contact line behavior on the rivulet stability. We show that the long-wave thermocapillary instability may lead to splitting of the rivulet into droplets or into several rivulets, depending on the Marangoni number and on the rivulet geometry. The kinematic-wave instability may be completely suppressed in the case of the rivulet flow in a groove.
Numerical Modeling of Gas Expansion From Microtubes
Authors: Chamberlin Ryan E. , Gatsonis Nikolaos A.
Published: September 2008

The expansion of argon from microtubes into hard vacuum has been extensively investigated using a three dimensional unstructured DSMC code. The simulation results for cases with an aspect ratio of 1.5 have been shown to compare well with theoretical formulations of free jet expansion. The discrepancies between the theoretical formulation and the DSMC results have been found to increase with increasing Knudsen number. DSMC simulations have been used to investigate the effects of Knudsen number, aspect ratio, Reynolds number and microtube scale on plume structure. The plume shape has been found to narrow with increasing Knudsen number, increasing aspect ratio and decreasing Reynolds number. The relative number density drop along the flow axis has been found to decrease with increasing Knudsen number and Reynolds number.
Transient Flow Dynamics in Optical Micro Well Involving Gas Bubbles
Authors: Johnson B. , Chen C. P. , Jenkins A. , Spearing S. , Monaco L. A. , Steele A. , Flores G.
Published: September 2008

The Lab-On-a-Chip Application Development (LOCAD) team at NASA’s Marshall Space Flight Center is utilizing Lab-On-a-Chip to support technology development specifically for Space Exploration. In this paper, we investigate the transient two-phase flow patterns in an optic well configuration with an entrapped bubble through numerical simulation. Specifically, the filling processes of a liquid inside an expanded chamber that has bubbles entrapped. Due to the back flow created by channel expansion, the entrapped bubbles tend to stay stationary at the immediate downstream of the expansion. Due to the huge difference between the gas and liquid densities, mass conservation issues associated with numerical diffusion need to be specially addressed. The results are presented in terms of the movement of the bubble through the optic well. Bubble removal strategies are developed that involve only pressure gradients across the optic well. Results show that for the bubble to be moved through the well, pressure pulsations must be utilized in order to create pressure gradients across the bubble itself.
Validation of a Numerical Model for the Prediction of the Pressure Distribution in PEMFC Flow Field Plates With a Serpentine Channel
Authors: Sun L. , Atiyeh H. , Oosthuizen P. H. , McAuley K. B. , Peppley B.
Published: September 2008

Much effort has been expended in the past few years upon development of numerical models to obtain the detailed flow, current and temperature distributions in Polymer Electrolyte Membrane Fuel Cell (PEMFC). Therefore, the need for model validation also has increased to gain confidence in the accuracy of the numerical results. In the present work, a numerical model has been developed to study the pressure distribution in the flow field plate (FFP) and gas diffusion layer (GDL) assembly on the cathode side of a PEM fuel cell. The flow field plate has serpentine channels and the porous gas diffusion layer is adjacent to the flow field plate, to deliver the air to the catalyst layer where the electrochemical reaction occurs. Flow crossover of air through the porous GDL under the land from one part of the channel to another can occur, and this flow crossover affects the total pressure drop between the channel inlet and outlet, and the pressure difference between adjacent channels. The flow here has been assumed to be three-dimensional, steady, incompressible, isothermal and single-phase. The flow through the porous GDL has been described using the Darcy model. The governing equations have been written in dimensionless form and solved by using the commercial CFD solver, FIDAP. In parallel, experimental work has been conducted at the Queen’s-RMC Fuel Cell Research Center (FCRC), Canada, for comparison with the numerical results. The cathode FFP has a single serpentine channel. Flow of dry air at 20 °C and at 60 °C has been used for measuring pressure differences at specific locations in the flow field plate. The effects of Reynolds number, based on the mean channel width and the mean velocity at the channel inlet (values between 100 and 1500) have been studied. Other parameters that were considered are the land:channel width ratio (2:1 and 1:1) and the permeability of the GDL (values between 1.0E−19m2 and 1.0E−10m2 used). Good agreement was obtained between the numerical and experimental pressure distributions along the serpentine channel.
Experimental and Numerical Study on Flow Heat Transfer in Microchannels
Authors: Liu Jiang-Tao , Tian Yong , Peng Xiao-Feng
Published: September 2008

An investigation was conducted on liquid flow and heat transfer in partially heated microchannels etched on the surface of silicon substrates. The rectangular cross-section channels had a hydraulic diameter of 100 microns. A series of experiments were carried out at different liquid flow rates and heating power. A numerical approach was made for single-phase liquid flow in the microchannels. A laminar flow model was employed in the simulation. Solid-fluid coupled thermal energy model was proposed to solve the conjugate heat transfer in the fluid and solid domains. The comparison of simulations with the experiment results shows good agreement. High cooling capacity was found in present study compared with the available data, indicating strong entry-length effect.
Entrance, Viscous Dissipation and Temperature Dependent Viscosity Effects in Microchannel Flows With Convective Boundary Conditions
Authors: Nonino C. , Del Giudice S. , Savino S.
Published: September 2008

The effects of viscous dissipation and temperature dependent viscosity in simultaneously developing laminar flows of liquids in straight microchannels of circular cross-section are studied with reference to convective boundary conditions. Viscosity is assumed to vary linearly with temperature, in order to allow a parametric investigation, while the other fluid properties are held constant. A finite element procedure, based on a projection algorithm, is employed for the step-by-step solution of the parabolized momentum and energy equations. Axial distributions of the local overall Nusselt number and of the apparent Fanning friction factor are presented with reference to both heating and cooling conditions for three different values of the Biot number. Examples of temperature profiles at different axial locations are also shown.
Validation of Models for Two-Phase Pressure Drop in Porous Media With Volumetric Heat Sources
Authors: Scha¨fer Patrick , Kulenovic Rudi
Published: September 2008

An experimental investigation on flow-boiling in inductively heated porous media (particle bed of oxidized stainless steel spheres, 3 mm resp. 6 mm diameter) has been carried out. As coolant demineralized water was used. The main focus of the experiments is to determine the dryout heat flux and the pressure drop of the two-phase flow under various conditions. In the boiling experiments the inflow rate of the coolant was varied from 0 mm/s (self driven flow with water supply from the waterpool above the porous bed, “counter-current-flow”) to 7.2 mm/s (forced inflow condition at the bottom of the porous bed, “co-current-flow”). The experimental results are used to validate dryout models applied in reactor safety. The model verifications show clearly that models without the explicit consideration of the interfacial drag can only be utilized for co-current flows and are unsuitable for counter-current flows. Against it models with the explicit consideration of the interfacial drag show at least qualitative agreement for both, counter-and co-current flows.
Optimal Design and Operation for a Droplet-Based PCR Chip
Authors: Mohr S. , Zhang Y.-H. , Macaskill A. , Day P. J. R. , Barber R. W. , Goddard N. J. , Emerson D. R. , Fielden P. R.
Published: September 2008

A two-temperature continuous-flow PCR polymer chip has been constructed that takes advantage of droplet technology to avoid sample contamination and adsorption at the surface. Samples contained in aqueous droplets are continuously moved by an oil carrier-fluid through various temperature zones, introducing the possibility of real-time quantitative PCR. The use of droplet technology also makes it possible to perform high throughput analyses of single cells allowing the study of populations of cells and facilitating a more comprehensive understanding of biological variance with relation to disease. In the present device, the thermal cycling time is significantly reduced and the PCR samples are exposed to more uniform temperatures. This paper investigates many of the factors affecting droplet-based PCR chip design, including specific heat capacity, density, flow rate, and thermal resistance. The study focuses particularly on the fluid and substrate temperature distribution within the PCR chip and the droplet residence times in critical temperature zones. The results show that, in general, the carrier-fluid should have a low thermal mass to ensure minimal heating and cooling times. It was found that the predicted temperature distribution in the chip arises from a subtle thermal interaction between the substrate and the carrier-fluid. The simulations demonstrate that the flow rate strongly affects the carrier-fluid’s temperature field. Above a critical flow rate, the carrier-fluid fails to achieve the temperatures required for DNA amplification. Moreover, the thermal resistance of the different layers is shown to have a major impact on the temperature profile in the channel.
Experimental Characterization of Micro Mixers Using Microscale-Laser Induced Fluorescence and Particle Image Velocimetry
Authors: Hoffmann Marko , Schlu¨ter Michael , Ra¨biger Norbert
Published: September 2008

Microreactors are basic components of microfluidic systems for chemical and biochemical applications and the large area-to-volume ratio of micro-reactors enables a higher yield and selectivity than conventionally designed processes. To take advantage of the full potential of this ambitious technology, a fundamental understanding of the transport processes on the relevant time and length scales is necessary. Besides the approach of using commercial CFD programs for numerical flow visualization, the microscale fluid flow visualization is an important tool for acquiring localized flow information within these microreactors. To get a deeper insight the mixing characteristic of different T-shaped micro mixers with rectangular cross sections (dimensions: 100–400 micron) has been investigated by means of the non-invasive measurement techniques micro-Laser induced fluorescence (micro-LIF) and micro-Particle Image Velocimetry (micro-PIV). The analysis of the concentration fields proves that with a higher Re a stretching and thinning of liquid lamellae (vortex generation) occurs, causing an enlarged interfacial surface area and consequently leading to a better mixing performance by diffusion. The analysis of the velocity fields shows further the existence of a three dimensional flow in the entrance region of the mixing channel of a T-shaped micro mixer.
Experimental Investigation of Heat Transfer and Pressure Drop in Porous Metal Foams
Authors: Reutter Oliver , Smirnova Elena , Sauerhering Jo¨rg , Angel Stefanie , Fend Thomas , Pitz-Paal Robert
Published: September 2008

Metal foams made by the SlipReactionFoamSintering (SRFS)-process are investigated. In these foams the pores are produced by a reaction between iron and a weak acid. The generated hydrogen forms pores in a metal powder slurry. These pores remain in the foam after sintering. Also secondary pores are found in these foams because of the sintering of the metal powder slurry. The metal powder base of the foams is Inconel 625 and Hastelloy B. Foam samples with a variety of different porosities of the two metals in the range of about 62% to 87% are used as well as samples made out of sintered metal powder which were not foamed with porosities of around 50%. The motivation for this study is to use these foams as combustion chamber walls in gas fired power plants. By using porous walls effusion cooling can be applied to keep the wall temperatures low. Air is used as a fluid to study the flow characteristics of these samples. Their pressure drop with air at room temperature is measured in the range of velocities of up to around 1 m/s. The parameters characterizing the foams are obtained using the Darcy-Forchheimer equations resulting in the permeability and the inertial coefficients. The dependency on the porosity is discussed. The volumetric heat transfer is measured for the foams by a transient method based on an air flow with a sinusoidal temperature wave, which is attenuated by the sample. The obtained volumetric heat transfer coefficients are discussed and transferred to Nu-Re correlations. Correlations between the heat transfer coefficients and the pressure drop coefficients are made.
Temperature Dependency of the Effective Thermal Conductivity of Nickel Based Metal Foams
Authors: Sauerhering Joerg , Reutter Oliver , Fend Thomas , Angel Stefanie , Pitz-Paal Robert
Published: September 2008

This article presents experimental results of the thermal conductivity of sintered metal foams, which were manufactured by the Slip Reaction Foam Sintering (SRFS) process. For the determination of the thermal conductivity, the Transient Plane Source Technique, also known as Hot Disk, was employed. The thermal conductivity of cellular solids differs from that of their corresponding dense material. Therefore, the various pore size level effects contributing to the thermal conductivity are accounted for by introducing an effective thermal conductivity λeff. The thermal conductivity of the strut material, a sintered packed bed, was determined up to 700°C and compared to similar materials. The thermal diffusivity could also be determined by the Laser-Flash method and compared to the Hot Disk values. For the foams, λeff was determined for a total porosity of 0.85 up to 700°C. In this article, a dependency between the porosity and λeff can be shown. The linear rise of λeff up to 400°C can be due to the increase of the thermal conductivity of the solid phase. The measurements are validated by comparison of the received specific heat with values received by thermogravimetry measurements. The general applicability of the measurement method to heterogeneous materials such as metal foams is discussed and an outlook about further investigations is given.
Local Heat Transfer Coefficient During Condensation in a 0.8 mm Diameter Pipe
Authors: Cavallini Alberto , Del Col Davide , Matkovic Marko , Rossetto Luisa
Published: September 2008

The first preliminary tests carried on a new experimental rig for measurement of the local heat transfer coefficient inside a circular 0.8 mm diameter minichannel are presented in this paper. The heat transfer coefficient is measured during condensation of R134a and is obtained from the measurement of the heat flux and the direct gauge of the saturation and wall temperatures. The heat flux is derived from the water temperature profile along the channel, in order to get local values for the heat transfer coefficient. The test section has been designed so as to reduce thermal disturbances and experimental uncertainty. A brief insight into the design and the construction of the test rig is reported in the paper. The apparatus has been designed for experimental tests both in condensation and vaporization, in a wide range of operating conditions and for a wide selection of refrigerants.
Frictional Pressure Drop in Single-Phase Flow in Narrow Channels
Authors: Celata G. P. , Chmiel K. , Kulenovic R. , Marti´n-Callizo C. , McPhail S. , Mertz R. , Owhaib W. , Palm B. , Sobierska E. , Zummo G.
Published: September 2008

An evaluation of frictional pressure drop in micro-conduits was carried out, taking into consideration such quantities as channel diameter, shape and aspect ratio, inclination, working fluid and heat input. Experiments took place at three different institutions and results were confronted with classical theory, using a simulation model where necessary. Excellent agreement was verified between the experimental friction factor data and the Hagen-Poiseuille and Blasius equations for all conditions and diameters. The difference in behaviour between circular and rectangular channels is minimal within the aspect ratios considered, but in the case of large temperature variation the effect of viscosity change must not be disregarded. For inclined channels, the frictional contribution to the total pressure drop exceeds the gravitational part only after a given value of the mass flux. This value increases for larger diameter and fluid density.
Heat Transfer Mechanism and Flow Pattern During Flow Boiling of Water in a Vertical Narrow Channel: Experimental Results
Authors: Sobierska Ewelina , Kulenovic Rudi , Mertz Rainer
Published: September 2008

Experimental investigations on flow boiling phenomena in a vertical narrow rectangular microchannel with the hydraulic diameter dh = 0.48 mm were carried out. The experiments were performed under fluid-inlet subcooling conditions with deionised and degassed water for different mass fluxes. Investigations on pressure drop and heat transfer during single-and two-phase flow have been carried out. Moreover, flow visualisation of the two-phase flow patterns along the channel was performed using a digital high-speed video camera. The present work outlines local heat transfer coefficients for three mass fluxes (200, 700 and 1500 kg/m2s) and heat fluxes (30–110, 35–150 and 65–200 kW/m2, respectively) during two-phase flow. The fluid temperature at the inlet was about 50 °C what corresponds to inlet subcooling, depending on flow pressure conditions, from 34 °C to 57 °C. The visual observations were used to obtain a better insight about the heat transfer mechanism.
Steady Liquid Flow and Liquid-Vapor Interface Shapes in Different Groove Structures in Micro Heat Pipes
Authors: Markos Mulugeta , Ajaev Vladimir , Homsy G. M.
Published: September 2008

The paper presents a lubrication-type model of liquid flow and heat transfer in different groove structures in micro heat pipes under negligible gravity and small capillary number. In the adiabatic region the flow rate can be controlled by changing the shape of the cross-section. We have computed the flow rate as a function of geometric parameters. Determination of the vapor-liquid interface shape in the grooves requires coupling of fluid flow and heat transfer. We examined the shape of the interface when thermocapillary effect is insignificant and then consider the effect of thermocapilarity. Practical applications of our results are discussed in relation to the design and optimization of micro heat pipes.
Improvement of Micro-PIV Resolution by Selective Seeding
Authors: Mielnik Michal M. , Sætran Lars R.
Published: September 2008

A novel seeding method, permitting high out-of-plane resolution and instantaneous (time-resolved) velocity field measurements using a standard Microscale Particle Image Velocimetry (micro-PIV) setup, is presented. The method relies on selective seeding of a thin fluid layer within an otherwise particle-free flow. The generated particle sheet defines the depth and position of the measurement plane, independently of the details of the optical setup. Therefore, for low magnification objectives in particular, considerable improvement in the measurement depth is possible. Selectively seeded micro-PIV (SeS-PIV) is applied to a microchannel flow, and the measured instantaneous velocity fields are in excellent agreement with the theoretical solution for the flowfield. The currently presented measurements have a depth-wise resolution 20% below the estimated optical measurement depth of the micro-PIV system. In principle, a measurement depth corresponding to the diameter of the tracer particles may be achieved.
The Effect of Creep Flow on Two-Dimensional Isoflux Microchannels
Authors: van Rij Jennifer , Harman Todd , Ameel Tim
Published: September 2008

Micro channel convective heat transfer and friction loss characteristics are numerically evaluated for gaseous, two-dimensional, steady state, laminar, constant wall heat flux flows. The effects of Knudsen number, accommodation coefficients, second order slip boundary conditions, creep flow, and thermal/hydrodynamic developing flow are considered. These effects are compared through the Poisuelle number and Nusselt number. Numerical values for the Poisuelle and Nusselt numbers are obtained using a continuum based three-dimensional, unsteady, compressible computational fluid dynamics algorithm that has been modified with slip boundary conditions. To verify the numerical results, analytic solutions for the hydrodynamically and thermally fully developed Poisuelle and Nusselt numbers have been derived. The fully developed analytic Poisuelle and Nusselt numbers are given as a function of Knudsen number, the first and second order velocity slip and temperature jump coefficients, the Brinkman number, and the ratio of the thermal creep velocity to the mean velocity. Excellent agreement between the numerical and analytical data is demonstrated. Second order slip terms and creep velocity are shown to have significant effects on the Poisuelle and Nusselt numbers.
Multi-Objective Design Optimization of Topology and Performance of Branching Networks of Cooling Passages
Authors: Gonzalez Maickel , Moral Ramon J. , Martin Thomas J. , Sahoo Debasis , Dulikravich George S. , Jelisavcic Nenad
Published: September 2008

The objective of this study was to develop an automatic, self-sufficient, preliminary design algorithm for optimization of topologies of branching networks of internal cooling passages. The software package includes a random branches generator, a quasi 1-D thermo-fluid analysis code COOLNET, and multi-objective hybrid optimizer. COOLNET analysis software has the same trends as shown in an earlier publication depicting the results of a similar analysis code used by Pratt & Whitney. The hybrid multi-objective optimization code was verified against classical test cases involving multiple objectives. The number of branches per level was optimized in order to minimize coolant mass flow rate, total pressure drop, and maximize total heat removed. Optimization with four levels of fractal branching channel networks was tested. This optimization varied the number of branching channels extending from each single channel. COOLNET needed approximately forty iterations on average to analyze each configuration. The number of iterations necessary for each geometry depended on the number of branches per configuration. The hybrid multi-objective optimizer needed 500 iterations to create a converged Pareto front of optimized branching network configurations for the case of four branching levels. A population of 60 designs was used. The total number of function evluations analyzed was 30,000. The entire design optimization process takes approximately 3 hours on a single 3.0 GHz Pentium IV processor. In this work the total number of Pareto-optimal designs was 100. After finding the Pareto front points, the user has to decide which optimized cooling network configuration is the best for the desired application. It was demonstrated that this can be accomplished by utilizing Pareto-optimal solutions to create a curve representing pumping power vs. total heat removed and by observing which designs provide favorable break-even energy transfer. The magnitude of the ratio of heat transferred to total pressure drop and ratio of heat transfer to pumping power could be further increased by incorporating the channel’s hydraulic diameter, cross sectional area, lengths, and wall roughness as optimization variables.
Electroosmotic Flow in “Click” Surface Modified Microfluidic Channels
Authors: Prakash Shaurya , Long Timothy M. , Wan Jonathan , Moore Jeffrey S. , Shannon Mark A.
Published: September 2008

A rapid, facile, and modular surface modification scheme for the covalent attachment of pre-formed polymer moieties to self-assembled monolayers via ‘click’ chemistry within glass microfluidic channels (3 cm long, 110 μm wide and 15 μm deep) is described. The effect that different moieties have on the electroosmotic flow (EOF) within the microchannels is evaluated. The application of linear polymers such as poly(ethylene glycol) (PEG) generates hydrophilic surfaces that reduce the analyte-wall interactions, thereby increasing separation efficiency and improving resolution, especially in bio-separations. Dendritic polymers such as poly(amido amine) (PAMAM) on channel walls can provide high-surface area structures with tunable surface charge depending on the generation of the dendrimer coating. Modified surfaces are characterized by X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared-Attenuated Total Reflection spectroscopy (FTIR-ATR), and contact angle measurements. EOF measurements in modified and unmodified channels provide information about wall-analyte interactions. A PAMAM dendrimer coated channel presents an amine terminated surface with a positive charge in contrast to a negatively charged bare-glass surface. Use of surface coatings can lead to an increase of the EOF by 15% as is the case for an azide terminated surface or reverse the direction of EOF as is the case for the PAMAM coatings by changing the surface charge polarity.
The Entrance Effect on the Electric Potential and Field in the Microchannel With Two Reservoirs
Authors: Suh Yong Kweon , Heo Hyeong Seok , Park Jae Hyun
Published: September 2008

In this paper we consider the effect of the entrance into the microchannel having two reservoirs on the electric potential and the electric field. For the case when the channel width is much smaller than the reservoir size, as usual, we designed a method to patch the numerical solutions obtained for the reservoir domain and the asymptotic solutions valid for the region near the entrance to the channel which is attached to the reservoir. Based on the assumption of small channel size, we also derived two asymptotic solutions for the potential and the electric field applicable to the reservoir region and to the channel region. Then the working formula has been established which can predict the effect of the electrode and channel size relative to the reservoir on the electric field built inside the channel. It was shown that the working formula is robust and applicable to the wide range of the parameters.
Scale Effect on the Gaseous Flow Around a Micro-Scaled Gas Turbine Blade
Authors: Yamada Toru , Asako Yutaka
Published: September 2008

Two-dimensional compressible momentum and energy equations are solved on gaseous flows around a micro-scaled gas turbine blade (GE-E3) whose axial chord ranges from 86.1μm to 86.1mm to obtain the scale effect. The numerical methodology is based on Arbitrary-Lagrangian-Eulerian (ALE) method. The flow is assumed to be ‘no heat conduction’ flow. The computations were performed for gaseous flow around a single blade with periodical conditions imposed along the boundaries in the pitch directions. The study is focused on the effect of the scale of the turbine blade on the performance. The predicted pressure distribution on both the pressure and suction sides of the conventional sized blade and both the inlet and outlet Mach numbers were compared with available experimental data to verify the code and the scale effect was discussed.