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Keynote

2005;():1-10. doi:10.1115/ICMM2005-75074.

It has long been recognized that the fluid mechanics of gas-phase microflows can differ significantly from the macroscopic world. Non-equilibrium effects such as rarefaction and gas-surface interactions need to be taken into account and it is well known that the no-slip boundary condition of the Navier-Stokes equations is no longer valid. Following ideas proposed by Maxwell, it is generally accepted that the Navier-Stokes equations can be extended into the slip-flow regime provided the Knudsen number is less than 10−1 . Improvements in micro-fabrication techniques, however, are now enabling devices to be constructed with sub-micron feature sizes. At this scale, the flow will depart even further from equilibrium and will enter the transition regime. In recent years, there has been considerable success in the implementation of second-order slip-boundary conditions to extend the Navier-Stokes equations into the transition regime. Unfortunately, as yet, no consensus has been reached on the correct form of higher-order approach, with theoretical and experimental studies revealing large discrepancies in the magnitude of the second-order slip coefficient. It is believed that these discrepancies can be explained by the fact that continuum flow analyses neglect the Knudsen layer, which extends approximately one mean-free path from the channel wall. In addition, comparisons between kinetic and continuum slip-boundary formulations reveal another important source of error due to different definitions in the first-order slip coefficient. The paper explains how these discrepancies have arisen and describes future research directions that may help reconcile the different forms of higher-order approach.

Commentary by Dr. Valentin Fuster
2005;():11-18. doi:10.1115/ICMM2005-75075.

That surface roughness has an effect on fluid flow in networks has been understood for well over a century. The exact effect roughness has on fluid flow has not been completely understood, but a working estimate has been offered by a variety of authors over time. The work of Colebrook, Nikuradse, and Moody has provided practitioners with a method to include at least a first order estimate of roughness effects, but their work has been limited to relative roughness to diameter values of 5% or less. Modern fluidic systems at the mini and micro levels routinely violate the 5% relative roughness threshold due to the inability to control the roughness of surfaces to sufficient levels with respect to decreasing system scale. Current work by Kandlikar, et al., has extended the traditional methods of assessing surface roughness effects up to 14% relative roughness by including the effect of constricted flow diameters and modifying the Moody diagram to reflect new experimental data. The future of micro fluidics would suggest that trends for miniaturization will continue and that further understanding and experimentation will be warranted. This is especially true with regards to understanding the role of roughness on the flow in mini and micro channels.

Commentary by Dr. Valentin Fuster
2005;():19-31. doi:10.1115/ICMM2005-75081.

The present paper reviews published experimental work focusing on condensation flow regimes, heat transfer and pressure drop in minichannels. New experimental data are available with high pressure (R410A), medium (R134a) and low pressure (R236ea) refrigerants in minichannels of different cross section geometry and with hydraulic diameters ranging from 0.4 to 3 mm. Because of the influence of flow regimes on heat transfer and pressure drop, a literature review is presented to discuss flow regimes transitions. The available experimental frictional pressure gradients and heat transfer coefficients are compared with semi empirical and theoretical models developed for conventional channels and with models specifically created for minichannels. Starting from the results of the comparison between experimental data and models, the paper will discuss and evaluate the opportunity for a new heat transfer model for condensation in minichannels; the new model attempts to take into account the effect of the entrainment rate of droplets from the liquid film.

Commentary by Dr. Valentin Fuster
2005;():33-36. doi:10.1115/ICMM2005-75083.

Microfluidic devices have brought us the possibility of conducting highly efficient and automated processing and analysis of biological samples. The smaller feature sizes of the microchannels where all those operations are carried out could theoretically contribute to the physico-chemical features of miniaturized biological analyses. And also the fabrication method of the devices could lead to integration of functional elements on a single device. We’ve been working on the so-called ‘Integrated Microfluidic Devices’ for biological applications where we deal with biomolecules and living cells and tissues. In this paper, our fundamental techniques to incorporate various operations in the device such as fluidic control, detection, separation, etc. will be presented along with the actual examples of their usage in realistic applications.

Topics: Microchannels
Commentary by Dr. Valentin Fuster
2005;():37-47. doi:10.1115/ICMM2005-75084.

It is needless to say the importance of doing research on multi-phase flows in micro- and mini-channels, which is clearly seen in the growing number of researchers in this field in recent years. We started about fifteen years ago to investigate on the gas-liquid two-phase flows in circular capillary tubes in order to get fundamental information with special attention on the flow patterns, the time varying hold-up and pressure loss. The directions of flow were vertical upward, horizontal and vertical downward. After that we also did research on the flows in rectangular mini channel in the similar experimental condition. In the present paper we will present and summarize these data.

Topics: Two-phase flow
Commentary by Dr. Valentin Fuster
2005;():49-58. doi:10.1115/ICMM2005-75085.

Industrial trends are presenting major challenges and opportunities for research on two-phase flows in microchannels. Semiconductor companies are developing 3D circuits, for which multilevel microfluidic cooling is important. Gas delivery microchannels are promising for PEM fuel cells in portable electronics. However, data and modeling are needed for flow regime stability, liquid entrainment/clogging, and bubble inception/departure in complex 2D and 3D geometries. This paper provides an overview of the Stanford two-phase microfluidics program, with a focus on recent experimental and theoretical progress. Microfabrication technologies are used to distribute heaters, thermometers, pressure sensors, and liquid injection ports along the flow path. Liquid PIV quantifies forces on bubbles and fluorescence imaging detects flow shapes and liquid volume fraction. Separated flow models account for conjugate conduction, liquid injection, evaporation, and a variety of flow regimes. This work benefits strongly from interactions with semiconductor and fuel cell companies, which are seeking validated models for product design.

Commentary by Dr. Valentin Fuster
2005;():59-68. doi:10.1115/ICMM2005-75086.

Heat fluxes in IC chips and other electronics equipment have reached the current limits of air cooling technology. Some of the applications require heat fluxes well beyond the limit of 100 W/cm2 , requiring advanced cooling solutions. Liquid cooling technology has been receiving attention as the advances in single-phase liquid cooling in microchannels have shown considerable promise. The extension of compact heat exchanger technology to microscale applications offers many new possibilities. The liquid cooling technology is expected to reach heat dissipation rates as high as 10 MW/m2 (1 kW/cm2 ) with enhanced microchannels (with a junction-to-air temperature difference of 50 °C). The challenges facing flow boiling systems are also evaluated. This paper reviews the fundamental technological developments in liquid cooling as well as in flow boiling, and presents possible solutions in integrating the cooling system with a building’s HVAC unit in a large server type application. The opportunities and challenges are described in an attempt to provide the roadmap of cooling technology for cooling high flux devices in the next decade.

Topics: Heat , Microchannels
Commentary by Dr. Valentin Fuster
2005;():69-76. doi:10.1115/ICMM2005-75087.

The effects of gas and liquid inlet geometry on adiabatic gas-liquid two-phase flow in a microchannel of 100 micron diameter have been investigated using different inlet sections and methods of gas and liquid injection and mixing. Two-phase flow patterns, void fraction and friction pressure drop were found to be significantly affected by the diameter of the inlet section and how the gas and liquid phases are injected and mixed upstream of the microchannel. Using a tee junction of the same diameter as the microchannel as the inlet, the two-phase flow pattern in the microchannel is mostly intermittent with short gas and liquid slugs flowing with nearly equal velocities. The void fraction then conforms nearly to that of a homogeneous two-phase flow, and two-phase friction multiplier applicable to larger channels is obtained. However, when the diameter of the inlet section is larger than the microchannel, the two-phase flow characteristics in the microchannel become highly dependent on the flow characteristics in the inlet section. Long gas slugs become prevalent and the void fraction decreases to values far below those given by a homogeneous void fraction. The practical implications for designing microchannel devices utilizing gas-liquid two-phase flow will be described.

Commentary by Dr. Valentin Fuster
2005;():77-83. doi:10.1115/ICMM2005-75088.

Microchannels have been used in the biomedical sciences to produce a well-defined shear field in the vicinity of a reactive planar boundary. In our laboratory, we have used such systems (h∼200 microns) to probe the affinity of circulating white blood cells, human blood platelets, and stem cells for various immobilized adhesion proteins. Computer simulations are used in conjunction with in vitro flow experiments to study the behavior of cells in the context of inflammatory and cardiovascular diseases. These simulations have been instrumental in demonstrating the collective physical phenomena that occur through cell-cell hydrodynamic interactions in dense suspensions of circulating cells. Recently, we have shown that microchannels with molecular micropatterned walls are much more effective at capturing flowing cells from the free stream than equivalent uniform surfaces. These systems have great potential for biotechnology applications such as high-throughput purification of bone marrow-derived stem cells. Finally, I discuss recent results characterizing the spatial patterns of leukocyte adhesion around branched microvessels (D<40 microns) in the live mouse microcirculation, which highlight the need for engineered microchannel systems for better understanding of the transport in these geometries.

Topics: Blood , Microchannels
Commentary by Dr. Valentin Fuster
2005;():85-92. doi:10.1115/ICMM2005-75090.

In this paper, the trends and successful patterns of interdisciplinary research activities integrating thermofluid engineering and MEMS technology are discussed, through a review of recent works presented in Japan. The possible combinations of the styles of research and the roles played by these disciplines are paid attentions. Among these combinations, the patterns where “MEMS provides solutions to thermofluid problems” are considered as the main issue. Three typical forms of utilization of MEMS technology in experimental thermofluid research are focused; structural elements of test sections (microchannels), physical surface modification (artificial cavities and surface texturing) and sensors and actuators. Examples for these forms of studies are given, and the current directions of evolution and the challenges are overviewed.

Commentary by Dr. Valentin Fuster
2005;():93-102. doi:10.1115/ICMM2005-75091.

Many experimental works on the forced convection through microchannels seem to evidence that, when the hydraulic diameter is less than 1 mm, the conventional theory can no longer be considered as suitable to predict the pressure drop and convective heat transfer coefficients. This conclusion seemed valid for both gas and liquid flows. Sometimes the authors justified this claim by invoking “new” micro-effects. In the last years, this conclusion seems to be controverted by additional, more accurate experimental data. For this reason, in this lecture the explanation of the experimental results obtained for microchannels in terms of friction factors and convective heat transfer in the laminar regime is sought for within the bonds of the conventional theory. In particular, this lecture focuses on the role of viscous heating in liquids flowing through microchannels, considering them as scaling effects. The role of the cross-sectional geometry on the viscous heating is highlighted for adiabatic and diabatic channels. Design-correlations useful in defining the limit of significance of the most important scaling effects for microchannels, like viscous heating and conjugate heat transfer are also presented.

Commentary by Dr. Valentin Fuster
2005;():103-113. doi:10.1115/ICMM2005-75092.

Plate heat exchangers were first developed about 100 years ago, but have won increasing interest during the last two decades, primarily because of the development of methods of manufacturing brazed plate heat exchangers. This type of heat exchanger offers very good heat transfer performance in single-phase flow as well as in evaporation and condensation. Part of the reason is the small hydraulic diameters, typically being less than 5 mm. Other advantages of plate heat exchangers are the extremely compact design and the efficient use of the construction material. In spite of their long use, the calculation methods for predicting heat transfer and pressure drop are not widely known. It is the purpose of this article to present such calculation methods for singe phase flow and for flow boiling and to discuss some of the specifics of this type of heat exchangers.

Commentary by Dr. Valentin Fuster
2005;():115-118. doi:10.1115/ICMM2005-75093.

The high heat transfer rate and compactness of micro/minichannel mutiphase flow system associated with phase change phenomena have been increasingly attractive in thermal management and microfluidic sensor development for space applications which demand requirements such as small, light, and efficient systems. Although the numerous research efforts are given to vaporization, there has not been sufficient study to understand condensation phenomena for the applications. This paper first discusses research consideration and small scale effects on condensation, especially in microscale system. NASA’s researches related to heterogeneous condensation nucleation and wall condensation are introduced to demonstrate current practical applications and demand for condensation studies.

Commentary by Dr. Valentin Fuster
2005;():119-126. doi:10.1115/ICMM2005-75094.

Fulminant hepatic failure is a clinical syndrome associated with a high mortality rate. Orthotopic liver transplantation is the only clinically proven effective treatment for patients with end-stage liver disease who do not respond to medical management. A major limitation of this treatment modality is the scarcity of donor organs available, resulting in patients dying while waiting for a donor liver. An extracorporeal bioartificial liver (BAL) device containing viable hepatocytes has the potential to provide temporary hepatic support to liver failure patients, serving as a bridge to transplantation while awaiting a suitable donor. In some patients, providing temporary hepatic support may be sufficient to allow adequate regeneration of the host liver, thereby eliminating the need for a liver transplant. Although the BAL device is a promising technology for the treatment of liver failure, there are several technical challenges that must be overcome in order to develop systems with sufficient processing capacity and of manageable size. In this overview, the authors describe the critical issues involved in developing a BAL device. They also discuss their experiences in hepatocyte culture optimization within the context of a microchannel flat-plate BAL device.

Topics: Liver
Commentary by Dr. Valentin Fuster
2005;():127-134. doi:10.1115/ICMM2005-75095.

Fluid flow phenomena near the liquid-solid and liquid-gas interfaces are of great interest, in conjunction with heat and mass transport in microchannels. In particular, evaporation from liquid-gas interface plays an important role in the performance of micro heat pipe, capillary pumped loop, etc. Water management and flooding control in microchannels are also well known to be crucial problems in micro fuel cells. Fluid-surface interaction under no-slip or slip condition is important in design, fabrication and operation of MEMS devices. In the present manuscript, we focus on recent advances made in the visualization studies of interfacial phenomena occurring in microchannels in terms of evaporation of liquid near the meniscus, water management and two-phase flow in micro fuel cell and near-surface velocimetry using evanescent wave illumination.

Commentary by Dr. Valentin Fuster
2005;():135-151. doi:10.1115/ICMM2005-75117.

Traditional fluid mechanics edifies the indifference between liquid and gas flows as long as certain similarity parameters—most prominently the Reynolds number—are matched. This may or may not be the case for flows in nano- or microdevices. The customary continuum, Navier–Stokes modeling is ordinarily applicable for both air and water flowing in macrodevices. Even for common fluids such as air or water, such modeling is bound to fail at sufficiently small scales, but the onset for such failure is different for the two forms of matter. Moreover, when the no-slip, quasi-equilibrium Navier–Stokes system is no longer applicable, the alternative modeling schemes are different for gases and liquids. For dilute gases, statistical methods are applied and the Boltzmann equation is the cornerstone of such approaches. For liquid flows, the dense nature of the matter precludes the use of the kinetic theory of gases, and numerically intensive molecular dynamics simulations are the only alternative. The present article discusses the above issues as well as outline physical phenomena unique to liquid flows in minute devices.

Commentary by Dr. Valentin Fuster
2005;():153-161. doi:10.1115/ICMM2005-75203.

Over the past decade microfabricated chip devices have emerged as powerful tools for carrying out analytical scale separations. While these systems significantly simplify analysis procedures, their separation efficiency is largely limited due to dispersion arising from various sources. Many of these arise from the hydrodynamics of the flow through the channels themselves, including electrokinetic dispersion of solute slugs in curved geometries, dispersion due to pressure driven shear flows, the surprisingly large effect of side-walls in large aspect ratio channels, and the effect of wall absorption in open channel chromatography. All of these phenomena are essentially linked, in that they may be understood as examples of Taylor-Aris dispersion. In this paper we demonstrate how a detailed understanding of the causes of such dispersion can lead to possible remedies. In general, Taylor dispersion may be minimized by choosing microchannel designs which minimize the shear of solute slugs, or which accelerate the limiting transverse diffusion process. While optimal designs are specific to each separation process and geometry considered (and are often limited by ease of fabrication constraints), we offer several suggestions which are shown to reduce effective dispersivities by an order of magnitude or more in many instances. Such reductions directly translate into shorter required channel lengths and/or processing times.

Commentary by Dr. Valentin Fuster
2005;():163-172. doi:10.1115/ICMM2005-75240.

The present work discusses the characteristics of the thermal effects at microscale. The thermal effects are divided into two types, steady and unsteady. The unsteady thermal effect focuses on the thermal response of object during heating. It relies on the object thermal inertia. Thermal response phenomena have seldom application of practical uses in devises at conventional scale. However, the miniaturization in device size due to the development of the state-of-the-art technology makes it possible to utilize thermal responses due to its significantly reduced thermal inertia. The swift thermal response can be utilized in the designs of micro dual-layered metal membrane pumps, phase change pumps, polymerase chain reaction (PRC). For steady type the thermal effect in microchennels varies with the boundary conditions and flow parameters. The relative importance of different forces changes with the scale going down. The viscous dissipation and the work due to expansion may not be neglected for the convection inside a microchannel; the conduction in the wall has to be considered in the calculation of the Nusselt number for microchannel. The relative importance of different heat transfer mode also varies with size. The natural convection presides over the heat transport inside a microchamber and the heat through an air gap by thermal radiation may exceed that by conduction at nanoscale. All the above mentioned variations make the heat transfer at microscale different from that at conventional scale.

Commentary by Dr. Valentin Fuster
2005;():173-182. doi:10.1115/ICMM2005-75245.

The contact phase of an assembly task involving micro and nano building blocks is complicated by the presence of surface and intermolecular forces such as electrostatic, surface-tension and Van der Waals forces. Assembly strategies must account for the presence of these forces in order to guarantee successful repeatable micro and nanoassemblies with high precision. A detailed model for this electrostatic interaction is developed and analyzed. Based on the results of this analysis, dielectrophoretic assembly principles of MEMS/NEMS devices are proposed and experimentally verified with microtweezers for micro Ni parts and with nanoelectrodes fabricated with electron-beam lithography for carbon nanotube assembly. The successful manipulation and assembly of single carbon nanotubes (CNTs) using dielectrophoretic forces produced by nanoelectrodes will lead to a higher integration of CNTs into both nanoelectronics and NEMS.

Topics: Force , Manufacturing
Commentary by Dr. Valentin Fuster
2005;():183-187. doi:10.1115/ICMM2005-75256.

Cell based biosensors offer the capability for quickly detecting chemical and biological agents with high sensitivity in a wide spectrum. Membrane excitability in cells plays a key role in modulating the electrical activity due to chemical agents. However, the complexity of these signals makes the interpretation of the cellular response to a chemical agent rather difficult. It is possible to determine a frequency spectrum also known as the signature pattern vector (SPV) for a given chemical agent through analysis of the power spectrum of the cell signal. I will describe a system for the measurement of extracellular potentials from live cells isolated onto micromachined planar microelectrode arrays. Fast Fourier and wavelet transformation techniques are used to extract information related to the frequency of firing and response times from the extracellular potential. Quantitative dose response curves and response times are obtained using local time domain characterization techniques.

Commentary by Dr. Valentin Fuster
2005;():189-201. doi:10.1115/ICMM2005-75258.

Surface tension plays an important role in multi-phase systems. Its effect becomes especially dominant in micro-scales because the ratio of surface (interface) to volume is relatively large. To understand such surface tension effects, it is required to understand the surface tension itself as a thermo-physical property and at the same time, its roles in fluid dynamics and heat and mass transfer. For the better understanding of surface tension effects, it is further recommended to learn some typical surface tension phenomena So in this article, the fundamental matters relevant to surface tension are firstly described. and then the action of surface tension in fluid dynamics as well as thermal-fluid systems is discussed by addressing similarity laws and dimensionless quantities with special attention to scaling effects. Surface tension problems, besides the two sides of thermo-physical property and the effects in thermo-fluid systems, include another important related problem, that is, the interaction of liquid with contacting solid surfaces. The last part of this article includes three recent topics, each of which relates to each side of the surface tension problems categorized above: i.e. a new precise measurement method of surface tension, flow patterns and heat transfer characteristics of two-phase flow in micro conducts, and solid surface modifications and their applications to fluidic devices.

Commentary by Dr. Valentin Fuster
2005;():203-210. doi:10.1115/ICMM2005-75259.

This review describes the formation of microvortices in microfluidic systems, and discusses our experimental measurements that illustrate the velocity profiles inside such microvortices. Because of the micrometer dimensions of these vortices and the presence of high rotational velocities, we have observed a number of unique phenomena. One example is the dynamic formation of ring patterns of particles within the microvortex. The mechanism by which these patterns form relies on a balance between the centrifugal and displacement forces experienced by the re-circulating particles with a lift force exerted on the particles near the solid boundary of the microcavity. We also demonstrate the ability to orient and rotate precisely micro and nanometer -sized particles, individual DNA molecules, and single cells. Because of the high linear velocity (m/s) of fluid flow in constricted microchannels and to the small radii (< 10μm) of the microvortices, we have measured the presence of ultrahigh radial accelerations (v2 /r) in such microvortices, which can reach 107 m/s2 or 106 times the gravitational acceleration (g).

Topics: Microfluidics
Commentary by Dr. Valentin Fuster

Single-Phase Liquid Flow

2005;():211-215. doi:10.1115/ICMM2005-75009.

We previously presented results obtained for liquid flow of tap water flow in 530 to 50 μm-diameters microtubes and evidenced a clear deviation from the classical Stokes flow theory which predict a constant Poiseuille number of 64 [1]. We assumed that using the EDL theory and a constant ζ potential approach may explain our results [2]. To confirm the previous obtained results, we used our dedicated experimental set-up [3] to study other fluids: ionic (distilled water, KCl solutions) and nonionic (n-dodecane). Here, for distilled water and n-dodecane, the same range of microtubes diameters have been investigated. For KCl solutions, the experiments were realized at constant microtube diameter (152 μm) for 5 different solutions concentrations. For a given diameter, the experiments are performed using 4 microtubes’ lengths then to ensure an accurate calculation of the Poiseuille number. We evidence different behaviours for given experimental conditions (that is microtubes inner diameters and surface roughness) depending of the fluid ionic characteristics. Here, we mainly present new experimental.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2005;():217-224. doi:10.1115/ICMM2005-75019.

A technique is described on the use of un-encapsulated thermochromic liquid crystals (TLC’s) to measure the local heat transfer coefficient in microchannel geometries. Microchannel heat transfer is at the heart of the microchannel heat sink, a recent technology aimed at managing the stringent thermal requirements of today’s high-end electronics. The microencapsulated form of liquid crystals are well established for use in surface temperature mapping. Limited studies however are available on the use of the un-encapsulated form. This form is advantageous as it offers the potential for high spatial resolution which is necessary for micro geometries. The evaluation of this method and its associated difficulties is therefore the motivation for the experimental facility developed and described in the present work. Measurements are made in a closed loop facility combined with a microscopic imaging system and automated data acquisition. Results are presented for a circular tube made of stainless steel with an inner diameter of 1.0668mm. A localized TLC calibration is used to account for non-uniformities in the coating and variation of lighting conditions. Results for single-phase, thermally developing, laminar and turbulent flows using distilled water are presented. The results show that the correlations for conventional size channels are adequate for predicting the heat transfer characteristics of a nominally sized 1 mm channel.

Commentary by Dr. Valentin Fuster
2005;():225-231. doi:10.1115/ICMM2005-75024.

The flow characteristics of isopropanol in microtubes driven by a high pressure ranging from 1 MPa to 30 MPa are studied in this paper. The diameters of the microtubes are from 3 μm to 100 μm. The Reynolds number ranges from 0.1 to 1000 approximately. From the present experimental results, two reverse trends of the normalized friction coefficient C* are found for the various diameters. From the analysis of several possible factors, it may be seen that the pressure-dependence viscosity and viscous heating play the leading role. The relationship of viscosity versus pressure pointed out that the viscosity of most liquids, except the water, augmented with the increase of pressure. The analysis based on the energy equation turns out that pressure drop, specific heat, density, flow rate and heat resistance decide the average temperature rise due to viscous dissipation, Therefore, above two factors are treated as the function of pressure. An exponential function with the dependence of pressure is introduced into Hagen-Poiseuille (HP) equation to counteract the difference between experimental and theoretical values. Consequently, C* exhibits different trends which is decided by the relative importance of viscous heating and the pressure-dependent viscosity.

Commentary by Dr. Valentin Fuster
2005;():233-243. doi:10.1115/ICMM2005-75070.

An experimental facility is developed to investigate single-phase liquid heat transfer and pressure drop in a variety of microchannel geometries. The facility is capable of accurately measuring the fluid temperatures, heater surface temperatures, heat transfer rates, and the differential pressure in a test section. A microchannel test section with a silicon substrate is used to demonstrate the capability of the experimental facility. A copper resistor is fabricated on the backside of the silicon to provide heat input. Several other small copper resistors are used with a four point measurement technique to acquire the heater temperature and calculate surface temperatures. A transparent Pyrex cover is bonded to the chip to form the microchannel flow passages. The details of the experimental facility are presented. The experimental facility is intended to support the collection of fundamental data in microchannel flows. It has the capability of optical visualization using a traditional microscope to see dyes and particles. It also has the capability to perform micro-particle image velocimetry in the microchannels to detect the flow field occurring in the microchannel geometries. The experimental uncertainties have been carefully evaluated in selecting the equipment used in the experimental facility. The thermohydraulic performance of microchannels will be studied as a function of channel geometry, heat flux and liquid flow rate. Some preliminary results for a test section, with a channel width of 100 micrometers, a depth of 200 micrometers, and a fin thickness of 40 micrometers are presented.

Commentary by Dr. Valentin Fuster
2005;():245-250. doi:10.1115/ICMM2005-75082.

In a systematic approach we address the question of how important variable property effects are for flows in micro-sized channels. Due to heat transfer, the temperature dependence of fluid properties like viscosity and thermal conductivity results in deviations in a solution that accounts for that dependence compared to a solution for constant fluid properties. Compared to flows through macro-sized geometries it turns out that two distinct scaling effects lead to a strong influence of variable fluid properties in micro-sized channels. Examples are given in which Nusselt numbers differ by up to 30% depending on how the property behaviour is accounted for.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2005;():251-258. doi:10.1115/ICMM2005-75096.

Deionized water flows in fused silica microtubes, with nominal diameters of 100, 320, and 1000-μm were studied. Flow conditions produced Reynolds numbers in the range 150 < Re < 11,000. Critical Reynolds number was found to be in the range of 2180 < Recr < 2450 for the three tube sizes. The average Poiseuille number was 76.9 ± 5%. Experimental turbulent data was in excellent agreement with the Colebrook equation. Molecular tagging velocimetry was used to further validate transition; Recr was in agreement with that determined by the integral technique.

Commentary by Dr. Valentin Fuster
2005;():259-268. doi:10.1115/ICMM2005-75108.

The characteristics of fully-developed, laminar, pressure-driven, incompressible flow in rough circular microchannels are studied. A novel analytical model is developed that predicts the increase in pressure drop due to wall roughness in microtubes. The wall roughness is assumed to posses a Gaussian isotropic distribution. The present model is compared with experimental data, collected by other researchers and good agreement is observed.

Commentary by Dr. Valentin Fuster
2005;():269-280. doi:10.1115/ICMM2005-75109.

Pressure drop of fully developed, laminar, incompressible flow in smooth mini and microchannels of arbitrary cross-section is investigated. A compact approximate model is proposed that predicts the pressure drop for a wide variety of shapes. The model is only a function of geometrical parameters of the cross-section, i.e., area, perimeter, and polar moment of inertia. The proposed model is compared with analytical and numerical solutions for several shapes. Also, the comparison of the model with experimental data, collected by several researchers, shows good agreement.

Commentary by Dr. Valentin Fuster
2005;():281-289. doi:10.1115/ICMM2005-75111.

A literature review in laminar incompressible flow in microchannels has shown both early and late transitions from laminar to turbulent flow as well as no deviation from the conventional flow transition of Re = 2300. In the present work, air and water flow through rectangular channels with hydraulic diameters ranging from 325∝m to 1819∝m. Smooth surface conditions are tested to validate the test section, while a repeating roughness is used to show the effects of surface roughness and its orientation on flow characteristics in minichannels. Testing has shown that smooth channels show no deviation, while artificially roughened channels show considerable deviation from the macroscale flow transition of Re = 2300.

Commentary by Dr. Valentin Fuster
2005;():291-302. doi:10.1115/ICMM2005-75112.

The validity of friction factor theory based upon conventional sized passages for microchannel flows is still an active area of research. Several researchers have reported significant deviation from predicted values, while others have reported general agreement. The discrepancies in literature need to be addressed in order to generate a set of design equations to predict the pressure drop occurring in microchannel flow devices. The available literature on single-phase liquid friction factors in microchannels is reviewed. A database is generated to critically evaluate the experimental data available in the literature. An in-depth comparison of previous experimental data is performed to identify the discrepancies in reported literature. It is concluded that the conventional Stokes and Poiseuille flow theories apply for single-phase liquid flow in microchannel flows. New experimental data is presented and the pressure drop components are carefully analyzed. The developed procedure properly identifies the components of total pressure drop that allow for improved agreement with conventional theory.

Commentary by Dr. Valentin Fuster
2005;():303-311. doi:10.1115/ICMM2005-75126.

Large scatter in published experimental data has been observed with respect to classical theory. Recent data have confirmed that liquid, fully-developed, laminar flow in smooth microchannels of various cross-section is governed by the Navier-Stokes equations. However, when the dimensions of the channels are comparable with the wall roughness, surface effects become significant, as shown experimentally. To better assess the effect of surface phenomena such as wall roughness, sources of systematic bias must be eliminated. Some of the observed inconsistencies in data could originate from the experimental method. This paper explores and categorizes different approaches found in literature for measuring microflow characteristics and highlights the advantages and disadvantages inherent to these experimental techniques. A discussion of system components, experimental measurement and error analyses is included in the paper, with an emphasis on important issues which may have been overlooked in previous research. This study serves as a summary of experimental procedure and is a useful guideline for research in microfluidics. Moreover, several recommendations are proposed for improvement in areas requiring further study.

Commentary by Dr. Valentin Fuster
2005;():313-318. doi:10.1115/ICMM2005-75129.

In this paper, global pressure drop and velocity field were experimentally investigated for water flow in trapezoid 30mm long microchannels with a hydraulic diameter of 238μm. A micro-PIV system was used to obtain the velocity profiles at different locations of the microchannel. For Re > 500, Δp/L-Re relationship deviates from the linear relationship and the deviation depends on the Re. The experimental results of the velocity field show that the deviation is due to the entrance effect in microchannel, and the entrance length in our experiments can be predicted by Le/Dh = (0.07 ∼ 0.09)Re. Velocity profiles and root mean square values of the fluctuating centerline velocity obtained by micro-PIV indicate that the transition from laminar flow to turbulent flow has occurred between 1500 and 1800. This result is consistent with the global measurements of pressure drop.

Commentary by Dr. Valentin Fuster
2005;():319-324. doi:10.1115/ICMM2005-75130.

Poly(vinyl alcohol) (PVA) hydrogel tube was prepared (sub-millimeter inner diameter) from PVA-dimethylsulfoxide/water solution by repetitive cooling method. The inner surface of the PVA hydrogel tube was chemically modified with poly(acrylic acid) (PAA). Slip of water in intact and modified PVA hydrogel tubes was studied. The slip was inferred by observing the motion of micron-sized spherical monodispersed polystyrene particles suspended in the flowing water by using an optical microscope from very low to medium flow rate. The particle velocity was measured by both particle tracking (PT) and particle image deformation (PID) method and the slip velocity was estimated by extrapolating the experimental data of the velocity profiles on the gel surfaces. Microscopic measurements reveal that the slip velocities on the hydrogel surfaces depended on the chemical nature and the surface roughness of the hydrogel surfaces, and were proportional to flow rate. Slip velocity of modified PVA hydrogel tube was higher than that of intact PVA hydrogel tube.

Topics: Hydrogels , Water
Commentary by Dr. Valentin Fuster
2005;():325-331. doi:10.1115/ICMM2005-75164.

This paper presents a numerical method for predicting 2-D and 3-D slightly compressible flow along microchannels, in which, one dimension is much smaller than the others (such as in ink jet printerheads). Both the configuration and the slightly compressible character of the fluid are very typical in practice and are amenable to simplification of the Navier-Stokes equations for more efficient calculation. Based on assumptions of these particular configurations and the fluid property, simplified systems of Navier-Stokes equations are obtained. Bicharacteristic based numerical calculations are developed to solve the systems of simplified, slightly compressible, viscous, Navier-Stokes equations. Two dimensional results are compared with analytical solutions. Three-dimensional results are compared with the results of commercial CFD code. Satisfactory agreements have been obtained and great efficiency has been achieved.

Commentary by Dr. Valentin Fuster
2005;():333-341. doi:10.1115/ICMM2005-75183.

Incompressible flow through constriction elements (orifices, nozzles and venturis) are commonly encountered in several macro and micro scale engineering applications.. The current research endeavor experimentally investigates single-phase incompressible flows of de-ionized water through rudimentary micro-constriction configurations such as rectangular slot micro-orifices entrenched inside microchannels. Additionally, the effects of micro-orifice and microchannel size on the discharge in single-phase flows have been evaluated, and experimental data suggests that the flow rate is dictated by the constriction element opening rather than the microchannel area. The discharge coefficients associated with incompressible flows through multifarious micro-orifices have been estimated, and the effects of Reynolds number, micro-orifice size and microchannel area on the discharge coefficients have been explored. The discharge coefficients are calculated based on standardized D-D/2 (One channel diameter upstream and half channel diameter downstream) pressure tap specifications, and can be directly employed in the design of micro-valves and other micro-constriction devices. Furthermore, experimental results indicate that the discharge coefficient rises and peaks at a critical Reynolds number (200 ≤ ReCrit ≤ 500), which indicates the emergence of turbulence immediately downstream of the micro-orifice and re-laminarization further downstream. The discharge coefficient stabilizes and reaches a steady value after the critical Reynolds number has been transgressed. Finally, a correlation for the discharge coefficient, which includes the Reynolds number and the ratio between the hydraulic diameter of the micro-orifice and the microchannel, is presented to aid designers of MEMS devices involving micro-constriction components.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2005;():343-349. doi:10.1115/ICMM2005-75212.

A technique for quantitative temperature visualization of single-phase liquid flows in silicon (Si) microchannels using infrared thermography is presented. This technique offers a new way to measure, non-intrusively, local variations in wall temperature, or fluid temperature at the fluid-wall interface, in a microchannel fabricated entirely of silicon. The experimental setup and measurement procedure required to obtain a high desired signal-to-noise ratio is elaborated. A single 13-mm long and 50 μm wide by 135 μm deep Si microchannel is used in this study. Experiments were performed with a constant electrical heat input to the heat sink surface for four fluid flow rates between 0.6 g/min and 1.2 g/min, corresponding to a Reynolds number range from 200 to 300. Temperature profiles of water in contact with the visualized wall of the microchannel indicate a monotonically increasing trend from the channel inlet for all cases, which is expected of a hydrodynamically and thermally developing flow. The estimated experimental fully developed Nusselt number matches the solution provided in literature for laminar flows. Measurements of the heat sink surface temperature are performed to determine axial variation in heat flux to the visualized channel wall. Results indicate that axial non-uniformity can be significant for the larger Peclet number flows.

Commentary by Dr. Valentin Fuster
2005;():351-360. doi:10.1115/ICMM2005-75221.

Three dimensional numerical simulations of the laminar fluid flow and heat transfer of water in silicon microchannels with non-circular cross-sections were performed. Two kinds of non-circular microchannel were investigated: trapezoidal and triangular. The continuum medium assumption was adopted and the corresponding governing equations and boundary conditions were used. The finite volume method was used to discretize the differential equations. The QUICK scheme was used for the discretization of convective term and the CLEAR algorithm was adopted to deal with the coupling between velocity and pressure. The water thermal physical properties were assumed to be constants except the viscosity. For both microchannels the grid system of 82 × 42 × 142 was used. Numerical results were compared with experimental data available in the literature, and good agreements were achieved. The effects of the geometric parameters of the microchannels were investigated, and the variations of Nusselt number with Reynolds number were discussed from the field synergy principle. The simulation results reveal the effects of the geometric parameters of the microchannels, and indicate that when the Reynolds numbers are less than 100, the synergy between velocity and temperature gradient is much better than the case with Reynolds number larger than 100. There is an abrupt change in the intersection angle between velocity and temperature gradient around Re = 100. In the low Reynolds number region the Nusselt number is almost proportional to the Reynolds number, while in the high Reynolds number region, the increase trend of Nusselt number with Reynolds number is much more mildly, which showed the applicability of the field synergy principle. In addition, the fully developed Nusselt number for the microchannels simulated increases with the increasing of on Reynolds number, rather than a constant. The present study shows that for liquid such as water the heat transfer and fluid flow in microchannels with geometric dimension in the order of 10–100 micrometer without the effect of external electro-field, the continuum assumption is still valid and numerical simulation can be conducted with conventional model and approach.

Commentary by Dr. Valentin Fuster

Single-Phase Gas Flow: Experimental/Analytical

2005;():361-368. doi:10.1115/ICMM2005-75098.

In various applications such as vacuum equipment, microdevices or Chemical Vapor Infiltration, there are many situations where non isothermal flow and low pressure are required in porous media and where the pore size may be as low as some micrometers. In this context, we can have situations where the frequency of binary collisions and the frequency of collisions of gas molecules with the boundary of the pores are of the same order. For numerical simulations, we propose a macroscopic model derived by homogenization from a microscopic model based on the Boltzmann equation. This macroscopic model consists in a mass diffusion equation, with thermo-diffusion terms, coupled with heat transfer equation. For this asymptotic transport model, we must compute the effective transport coefficients from the microscopic properties of the material. These coefficients are defined through the solution of auxiliary problems posed on the unit cell of the periodic medium.

Commentary by Dr. Valentin Fuster
2005;():369-374. doi:10.1115/ICMM2005-75131.

In finite length tube or channel gas flow the pressure gradient is determined by the global mass conservation law. In the continuum and slip flow the pressure distribution determined by the global mass conservation is given analytically. In the transitional flow regime an equation containing the flow rate of the Poiseuille flow solved by the strict kinetic theory is obtained and is shown to be the degenerated Reynolds equation for the lubrication theory. The integration of the equation is illustrated in the case of full diffuse reflection of the channel wall. The pressure distribution thus obtained is shown to be in excellent agreement with experimental data of long microchannels and the simulation results of the information preservation method. The results as having the strict kinetic theoretical merit are used to confirm the unfeasibility of the Lattice Boltzmann method in the transitional flow regime.

Commentary by Dr. Valentin Fuster
2005;():375-379. doi:10.1115/ICMM2005-75135.

The performance of the micro propulsion system is determined primarily by the performance of the micro nozzles. 3D rectangular cross-section straight-convergent-divergent-straight (SCDS) micro nozzle, with throat width 16 μm and throat depth 20 μm, was fabricated and studied based on the experiment and numerical simulation. The results indicate that the first sonic point position moves away from the throat to the outlet of the microchannels, and the critical pressure ratio (defined as the local static pressure to inlet total pressure ratio when the first sonic point occurs in the internal flow of SCDS microchannel) decreases with the increasing of the S/V (surface-to-volume ratio) of micro nozzle. These behaviors might be attributed to the increased surface-to-volume ratio leads to high viscosity dissipation in micro nozzle. The relationship of critical pressure ratio and the S/V ratio is further discussed based on numerical simulation.

Commentary by Dr. Valentin Fuster
2005;():381-388. doi:10.1115/ICMM2005-75184.

This paper investigates the heat transfer of airflow in square and circular minichannels. The square channels were machined on the oxygen free copper blocks. The width of them were 0.3, 0.6, 1.0 and 2.0 mm, and the lengths were 10, 20, 50 and 100 mm. The circular channels were drilled in oxygen free copper disks. The diameters of them were 1.0, 1.27, 2.0 and 2.8 mm and the length to diameter ratios were 5 and 10. The mean heat transfer coefficient included the transfered heat upstream of the channel inlet cross-section, since the inlet was not thermally insulated. The pressure ratio of the inlet and outlet plenum was increased up to the flow choked at the channel exit. The square channel showed 7.3 times greater mean heat transfer coefficient than fully developed turbulent pipe flow for width of 2 mm and length of 10 mm channel. The so-called tube cutting method was employed to investigate the sectional heat transfer rate of the square channel. About 75 percent of the total heat transfer was completed in 10 percent inlet portion of the channel. The Stanton number was found to be principally the function of length to diameter ratio. The circular channel showed 6.79 times greater mean heat transfer coefficient than fully developed turbulent pipe flow for diameter of 1.27 mm and length of 6.35 mm channel. The heat transfer coefficient increased as the channel size (width or diameter) became smaller for constant length to diameter ratio. It implied that the result of high heat transfer coefficient had a possibility to be limited to the minichannel. Heat transfer of gas turbine film cooling hole has a possibility to be larger than ever thought.

Commentary by Dr. Valentin Fuster
2005;():389-392. doi:10.1115/ICMM2005-75254.

The measured data of mass flow rates and streamwise pressure distributions at various experimental conditions of microchannels carried out by Pong et al (1994), Harley et al (1995), Shih et al (1996), Arkilic et al (1997, 2001), and Zohar et al (2002) are normalized by the kinetic factors Ṁc and pk , respectively. The normalized data are compared each other, and they are in excellent agreement, except the few with the small differences. This demonstrates that the measured data available are generally accurate.

Commentary by Dr. Valentin Fuster
2005;():393-398. doi:10.1115/ICMM2005-75257.

Since the size of microsystems is close to the molecular mean free path, the gas rarefaction must be taken into account in modelling of microflows. However, the transition regime represents many difficulties for practical calculations because the Boltzmann equation must be applied. This approach requires great computational efforts and many engineers prefer to substitute such calculations by some estimations, which lead to quite incorrect results in many cases. At the same time, many rigorous results for the transition regime already obtained from the Boltzmann equation can be easily used in practical calculations. A brief survey of many recent books on microflows showed that all of then give just elementary information about the kinetic theory of gases, while the results on the transition flows of gases obtained during the few last decades are omitted. The aim of the present work is to review the recent results of rarefied gas dynamics and to give several examples of their applications in microchannels.

Topics: Gasdynamics
Commentary by Dr. Valentin Fuster

Single-Phase Gas Flow: Numerical

2005;():399-404. doi:10.1115/ICMM2005-75043.

Gas flows in De-Laval micronozzles are studied numerically with a 2D continuum axisymmetric model, which solves the governing equations by a control volume method. The continuum model is validated with experimental data and the comparisons on exit thrust values are in good agreement except at low Reynolds number. Parametric studies are conducted for the flows ranging from continuum to transitional flow regimes. The nozzle throat diameters are varied from 1 to 0.2 mm and the stagnation pressures from 17 to 1 Torr to investigate the effects of nozzle throat diameter, stagnation pressure and their combination. Slip and no-slip boundary conditions are employed in the studies.

Commentary by Dr. Valentin Fuster
2005;():405-409. doi:10.1115/ICMM2005-75047.

In this paper, nitrogen gas flow in long microchannels with square cross section is simulated numerically with a three-dimensional continuum model. The governing equations of the model are solved by a control volume method. Numerical results are verified with analytical solutions for the range of hydraulic diameters investigated at constant L/Dh ratio of 1000 and pressure ratio of 1.0001. As the pressure ratio is small and therefore the compressibility effect is negligible in this study. A parametric study is conducted at the same L/Dh and pressure ratio. By reducing the hydraulic diameter down from 10000 μm to 0.01 μm, the gas flow inside the microchannels switches from continuum to free molecular flows. The no-slip flow simulation results provide a constant fRe = 54.8, which is very close to the analytical result of fRe = 57. Slip flow condition reduces the friction factor and starts to drop fRe at Dh = 60 μm in the continuum regime.

Commentary by Dr. Valentin Fuster
2005;():411-418. doi:10.1115/ICMM2005-75064.

Rarefied gas flow and heat transfer in the entrance region of rectangular microchannels are investigated numerically in the slip flow regime. A control-volume based numerical scheme is used to solve the Navier-Stokes and energy equations with velocity slip and temperature jump conditions at the walls. The effects of variations in Reynolds number (0.1 ≤ Re ≤ 100 ), channel aspect ratio ( 0 ≤ α* ≤ 1 ) and Knudsen number (0 ≤ Kn ≤ 0.1) are studied on the flow development, slip flow heat transfer, and key parameters like the friction factor and the Nusselt number. For simultaneously developing flow and heat transfer, major reductions in friction coefficients and heat transfer rates are observed in the entrance region due to rarefaction effects, which also extend to the fully developed region, however at much lower levels.

Commentary by Dr. Valentin Fuster
2005;():419-425. doi:10.1115/ICMM2005-75067.

Incompressible flow through a minichannel with a basically square cross-section has been considered. It is often assumed that rounding the comers in such a channel has a significant effect on the pressure drop and heat transfer rate. In order to investigate whether this is in fact the case, attention has been given to flow in a channel in which two adjacent corners are rounded, the radius of the two corners being the same. The other two corners of the channel are not rounded. The effect of the radius of the corners of the duct on the pressure losses, on the losses associated with the developing flow near the duct entrance and on the Nusselt number has been numerically studied. It has been assumed that the flow is steady, that the flow is incompressible, that the velocity and temperature are uniform over the channel inlet plane, and that there is no slip on the boundaries. A uniform heat flux is assumed to be applied over the entire surface of the duct. The governing equations have been written in dimensionless form. Solutions to these dimensionless governing equations have obtained using a commercial finite-element software package, FIDAP. The solution has the following parameters: the Reynolds number, Re, the Prandtl number, Pr, and the dimensionless radius of the corners of the duct bend R = rc / w, where w is the width and height of the channel and rc is the radius of the rounded edges of the duct. Results have been obtained for Pr = 0.7 for Re values between 10 and 1000 and R values between 1/6 and 1/3.

Commentary by Dr. Valentin Fuster
2005;():427-432. doi:10.1115/ICMM2005-75077.

On the cathode side of a PEM fuel cell, the air usually flows through serpentine channels in a flow plate, these channels usually having a square cross-sectional shape. There is a porous diffusion layer adjacent to the flow plate. Flow cross-over of air through the porous diffusion layer from one part of the channel to another can occur. This flow cross-over is a result of the pressure differences between different parts of the channel and it causes the flow rate through the channel to vary with the distance along the channel. A numerical study of the pressure distribution and flow cross-over through the gas diffusion layer (GDL) in a PEMFC flow plate system that uses a serpentine channel system has therefore been undertaken for the case where the channel has a trapezoidal cross-sectional shape. The purpose of the present work was to study the effect of the flow plate geometry on the basic fluid flow through the plate. The flow has been assumed to be three-dimensional, steady, incompressible and single-phase. The flow through the porous diffusion layer has been described using the Darcy model. The dimensionless governing equations have been written in dimensionless form and solved by using the commercial CFD solver, FIDAP. The solution depends on (1) the Reynolds number, Re, based on the mean channel width and on the mean velocity at the channel inlet, (2) the gas Prandtl number, Pr, a value of 0.7 being assumed here, (3) the permeability of the diffusion layer, (4) the ratio, R, of the length of the wide side of the trapezoidal channel to the length of the narrow side of the channel. Values of Re between 50 and 200 and of R between 1 and 7 have been considered. The results obtained indicate that: (1) the width ratio, R, of the trapezoidal channel cross-sectional shape has a significant effect on the flow cross-over As R increases the flow cross-over through GDL increases, (2) The ratio R also has a significant effect on the pressure variation in the flow field for both cross-over and no cross-over cases. (3) Flow cross-over has a significant influence on the pressure variation through the channel, tending to decrease the pressure drop across the channel. (4) An increase in Re can lead to a slight increase in the flow cross-over.

Commentary by Dr. Valentin Fuster
2005;():433-439. doi:10.1115/ICMM2005-75123.

Two-dimensional compressible momentum and energy equations are solved to obtain the heat transfer characteristics of gaseous flows in parallel-plate micro-channels. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The computations are performed for channels with constant heat flux walls which range from 1×103 to 1×104 W·m−2 . The channel height ranges from 10 to 100 μm and the aspect ratio of the channel height and length is 200. The stagnation pressure varies from 1.2×105 to 5.0×105 Pa. The outlet pressure is fixed at the atmosphere. The predicted wall temperature by the Nusselt number for the conventional size parallel plate duct is compared with that of the results computed by numerical analysis.

Commentary by Dr. Valentin Fuster
2005;():441-446. doi:10.1115/ICMM2005-75188.

In this study, we modeled two micro-elbows with dimensions with 20×1×5810 μm3 and 90° turn at the channel centre. In case of the right angle elbow, the channel turns sharply at a right angle, and in case of the round elbow, the channel turns smoothly 60 μm radius of curvature. In order to investigate flow characteristics, numerical analyses are carried out with pressure driven condition and argon gas. Mass flow rate and pressure distribution of micro-elbows are calculated. Due to the low Reynolds number and Knudsen number Kn = 0.06, we used laminar and compressible Navier-Stokes equation and slip boundary conditions. Although the turn angle of micro-elbows is the same, 90°, mass flow rate is not the same. Mass flow rate of the right angle elbow is lower than that of the round elbow. Besides, in case of the same flow rate, the pressure drop of right angle elbow is larger than that of round elbow. As the mass flow rate decreases, the pressure drop of micro-elbows is increased. Results also show that in a right angle channel, small separation is came into existence in the sharp corner bend of inner wall.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2005;():447-452. doi:10.1115/ICMM2005-75193.

No slip wall boundary condition is known to be valid only for Knudsen number below 0.001. Most of the practical gaseous flow in microchannel, on the other hand, are characterised by higher values of Knudsen, and some slip model has to be enforced. Furthermore, in gaseous flow Kn, Ma and Re are closely related, and an increase in Kn yields an increase in Ma, for a fixed value of Re. Thus, compressibility effects may have a significant impact on microchannel performances. In the present paper, first and second order slip flow boundary conditions are enforced in an hybrid finite difference/ finite volume, compressible flow solver. A plane microchannels geometry is considered, allowing for easy validation, although the code is formulated for arbitrary geometries. Both slip velocity and temperature jump are taken into account, and some detail is given on the stability issue of second order slip at the wall, following Karniadakis and Beskok approach. The slip models are first validated against analytical solutions in the limiting case of fully developed, low Mach number flow. In order to match the literature solutions, fixed temperature is imposed at the boundaries. Then, computation at different Mach number levels are presented and discussed, highlighting the effect of compressibility along the channel length. Results are given in term of Nusselt number and Poiseuille number, and comparison between first order, second order and no-slip results are shown as a function of Knudsen value.

Commentary by Dr. Valentin Fuster

Single-Phase Gas Flow: Molecular Simulation

2005;():453-460. doi:10.1115/ICMM2005-75099.

We show that simple and efficient Monte Carlo-based solution methods for the Boltzmann equation for low-speed applications can be constructed by using appropriate variance reduction techniques. More specifically, we show that evaluation of the collision integral by sampling a representative number of collisions can be significantly accelerated by considering only the deviation from equilibrium, since this allows one to avoid considering a large number of collisions with no net effect. As the deviation from equilibrium decreases, the degree of variance reduction increases, leading to a signal to noise ratio that remains approximately constant. Thus, unlike particle techniques in which statistical sampling results in computational cost that is inversely proportional to the square of the Mach number, the approach presented here exhibits computational cost which is almost independent of the Mach number in the small Mach number limit. This is verified by numerical experiments at Mach numbers as low as O(10−5 ). We validate this approach by comparing its results with analytical and direct Monte Carlo simulations of the Boltzmann equation.

Topics: Equations
Commentary by Dr. Valentin Fuster
2005;():461-467. doi:10.1115/ICMM2005-75100.

We study the deviations for the results of the properties of a hard-sphere gas near the walls of a micro/nano channel using the hybrid MD-MC simulation method compared to the pure MD and MC results. Our model for the micro channel consists of two parallel infinite plates situated at distance L apart from each other, and of gas molecules confined between these two walls. We study the dependence of the deviations for higher densities, considering different lengths of the different simulation domains in the hybrid MD-MC method. We find that when density is increased, the deviations in the pure MC results are increasing compared to pure MD results. The deviations in the hybrid simulation results are decreasing and are very small when increasing the width of the solid-gas interface. The deviations of the pure MC simulation results from the pure MD simulation results for the number density are found to be around 0.9%, when the reduced density η = 0.1 and the width of the channel L = 50λ, where λ is the mean free path. When the hybrid method is used, the deviations are decreasing with a factor from two to three, and are between 0.32%–0.42%. For more dense gas (η = 0.2), the deviations of the MC simulation results for the number density are found to be 1.71%, and the deviations of the hybrid MD-MC simulation results between 0.246% and 0.6977%. We discuss how these deviations in case of a dense gas (η = 0.2) depend on the width of the interface, and we study it for the case when the MD domain is 10% and MC domain is 90% from the simulation domain, and also for the case when the MD domain is 50% and MC domain 50% from the whole simulation domain. For more dilute gas, the MC, MD and hybrid MC-MD simulations are in very good agreement and the deviations are negligible.

Commentary by Dr. Valentin Fuster
2005;():469-476. doi:10.1115/ICMM2005-75133.

Mixing in microchannels is an important problem because the flow is always in laminar mode even though the velocity could be high. This present paper is trying to expose the inherent effect factors on micro gases mixing by using the direct simulation Monte Carlo (DSMC) method at high Knudsen numbers. Before simulations the discretization errors of the DSMC method were discussed to ensure the numerical veracity. The simulation results show that the wall characteristics have little effects on the mixing length when the main gas flow velocities for different wall characteristics were at a same value. The gas temperature increase decreases the mixing length at a nearly inverse proportional rate. After defining a new dimensionless mixing coefficient by the mixing length to the channel height ratio, it was found that the mixing coefficient was proportional to the Mach number, and inversely proportional to the Knudsen number. This conclusion was validated in gas flows at microscale, while may be expected to expand to all laminar gas flows at all scales.

Topics: Gases , Microchannels
Commentary by Dr. Valentin Fuster
2005;():477-482. doi:10.1115/ICMM2005-75146.

The similarity between micro gas flow and rarefied gas flow was numerically investigated using a DSMC method. With compressibility and rarefaction effects, the similarity parameters are the Mach number and the Knudsen number, since the Reynolds number is dependent on the Mach number and the Knudsen number for an ideal gas. Comparisons of numerical results for various scales show that the normalized fields for the macroscopic quantities are quite similar, which shows that the microscale flow is similar to rarefied flow of an ideal gas. Therefore, existing rarefied flow results can be applied to microscale flows. However, for dense gas flows, the three dimensionless parameters are independent, so the similarity breaks down for dense gas flows. Simulation results for an ideal gas using nitrogen as an example show that the similarity holds for densities less than 7–8 times the density at standard conditions.

Commentary by Dr. Valentin Fuster
2005;():483-491. doi:10.1115/ICMM2005-75247.

The temperature driven gas flow in a two-dimensional finite length microchannel and a cylindrical tube are studied numerically with the goal of performance optimization of a nanomembrane-based Knudsen compressor. The numerical solutions are obtained using direct simulation Monte Carlo method and discrete ordinate method for BGK model kinetic equation in a wide range of Knudsen numbers from 0.05 to 50. The length-to-height ratios from 5 to 30 were examined. Three different wall temperature distributions were considered, namely, linear, stepwise, and a non-monotonic profile typical for a radiantly heated Knudsen compressor membrane. The short channel end effects are characterized, and the sensitivity of the mass flow rate to a non-monotonic temperature distribution is shown.

Commentary by Dr. Valentin Fuster

Heat Transfer Enhancement

2005;():493-501. doi:10.1115/ICMM2005-75072.

Microstructure heat exchanger are well known for their superior heat transfer capabilities and the good temperature management for applications. Most microstructure heat exchangers presented so far consisted of a number of microchannels in a defined arrangement to provide the transfer of thermal power from one fluid to another, mostly in laminar flow regime. In this publication, several crossflow microstructure heat exchangers are presented. The devices design ranges from simple linear microchannels of different dimensions and shape, complex micro column arrangements up to three dimensional flow structures like crossed sinusoidally shaped microchannels, a kind of split-and-recombine structure. By comparing experimental results of the most interesting devices, microstructure devices with good performance can be chosen.

Topics: Heat exchangers
Commentary by Dr. Valentin Fuster
2005;():503-510. doi:10.1115/ICMM2005-75160.

A numerical model has been developed for studying the flow and heat transfer characteristics of single phase liquid flow through a microchannel. In this work the heat transfer characteristics of pressure driven and electroosmotic flow through microchannels have been studied. The governing equations are the Poisson-Boltzmann and Navier-Stokes equations which have been solved numerically using the standard Galerkin and the Mixed 4-1 finite element methods, respectively. Finally the energy equation is solved numerically using the Stream-wise Upwind Petrov Galerkin (SUPG) method. Two dimensional Poisson-Boltzmann equation was first solved to find the electric potential field and net charge distribution in the microchannel. Considering the electrokinetic body forces due to interaction of an external electric field on the charged fluid elements, two dimensional Navier-Stokes equations were solved to obtain the flow field in the microchannel for a combined pressure driven-electroosmotic flow. Local and averaged heat transfer coefficients were calculated for constant wall temperature condition. The results were compared to those of pressure driven flow in the same geometry without using electroosmotic pumping. Comparisons revealed significant changes in the velocity profile and heat transfer characteristics through the channel. It was observed that the convective heat transfer rate was increased due to sharp velocity gradients in the vicinity of the microchannel walls. The influence of various effective parameters including external electric field strength and ionic concentration was also studied. It was seen that aforementioned parameters strongly affect the heat transfer rate and flow pattern through the microchannel.

Commentary by Dr. Valentin Fuster
2005;():511-517. doi:10.1115/ICMM2005-75189.

Thermocapillary convection in a liquid film inside a micro-slot bounded by a substrate wall and a cover wall is studied. At least one of the walls is structured. The walls are kept at different temperatures. A thin liquid film is wetting the substrate and is separated from the cover wall by a gas layer. The heat transfer coefficient between the liquid-gas interface and the cover wall varies together with the thickness of the gas layer. This variation causes temperature non-uniformity along the interface. The temprature non-uniformity arises also as a result of the non-uniform film thickness on a structured substrate. As a result, thermocapillary stresses are induced which act in the direction opposite to the interface temperature gradient. These stresses bring the liquid into motion and lead to the interface deformation. The film flow and film deformation in a micro-slot with structured walls is studied in the framework of the long-wave theory. The steady film deformations, as well as the velocity and temperature fields within the film are calculated. A stability analysis of the steady continuous film is performed.

Commentary by Dr. Valentin Fuster
2005;():519-528. doi:10.1115/ICMM2005-75195.

This paper reviews literature on conventional scale boiling enhancement techniques by means of reentrant cavities and discusses various avenues by which the knowledge obtained from that research can be used to enhance boiling in microchannels. Fabrication techniques developed by the Micro Thermal-Fluids Laboratory at Rensselaer Polytechnic Institute together with the Advanced Microsystems Materials Laboratory at McGill University are discussed, and preliminary data are given. These results demonstrate the potential for improving boiling heat transfer characteristics in microchannels and introduce the next generation of microchannel heat transfer technology.

Commentary by Dr. Valentin Fuster
2005;():529-536. doi:10.1115/ICMM2005-75196.

Boiling flow of de-ionized water through 227 μm hydraulic diameter microchannels with 7.5 μm wide interconnected reentrant cavities at 47 kPa exit pressure has been investigated. Average two-phase heat transfer coefficients have been obtained over effective heat fluxes ranging from 28 to 445 W/cm2 and mass fluxes from 41 to 302 kg/m2 s. A map is developed that divides the data into two regions where the heat transfer mechanisms are nucleation or convective boiling dominant. The map is compared to similar atmospheric exit pressure data developed in a previous study. A boiling mechanism transition criterion based on the Reynolds number and the Kandlikar k1 number is proposed.

Commentary by Dr. Valentin Fuster
2005;():537-543. doi:10.1115/ICMM2005-75223.

A thermal lattice Boltzmann model for investigating the flow and heat transfer process of the mixtures of the pure liquid and nanoparticles (nanofluids) in the microchannel has been developed. The external and internal forces, such as buoyancy, gravity, drag and Brownian force, and the mechanical and thermal interactions among the nanoparticles and their impact on the equilibrium velocity have been introduced. Along with a Gauss white noise model for Brownian motion, the double-distribution-function (DDF) approach is used to derive the velocities and temperatures of nanofluids in a microchannel. Some numerical computations of this model have been performed and several results have been provided in this paper. It has been found that the temperature distribution of the nanofluids in the microchannel is quite different from that of pure water flowing through a channel. Due to the random motion of the suspended nanoparticles under the action of various forces, the temperature distribution of the nanofluids seems to be irregular and the temperature distribution in the vertical direction becomes flatter compared to that for pure water in a channel. The distribution morphology and the volume fraction of the nanoparticles play a vital role in enhancing the heat transfer of the nanofluids. Numerical results also demonstrate that the distribution of the suspended nanoparticles leads to a fluctuation of the Nusselt number of the nanofluids in the direction of the main flow. Nusselt number also increases with an increase in the inlet Reynolds number.

Commentary by Dr. Valentin Fuster
2005;():545-551. doi:10.1115/ICMM2005-75232.

Much research in small-scale natural convection is targeted towards improved passive cooling of microelectronic devices (with increasing dissipative heat flux, higher operating frequencies, and increased component density). Small-scale convection also plays a central role in some more recent miniaturization efforts such as chemical analysis systems and energy conversion devices. In general, the small length-scales associated with these systems greatly inhibit natural convection heat transfer and species transport. The focus of this study is the enhancement of natural convection based heat transfer through independent modulation of heat fluxes from a planar array of distributed sources. Unsteady heat generation is common in electronic components, and more importantly, small-scale systems can be designed to induce dynamic heat fluxes. In this work, the heat transfer resulting from distributed and modulated heat sources on the order of 100μm–1000μm in 2D enclosures filled with air are investigated numerically. The heat sources are modelled as flush-mounted sources with prescribed heat flux boundary conditions. Air adjacent to the sources is heated, and eventually, the flow structure will transition from the conduction-dominated regime to the buoyancy driven convection-dominated regime. Optimum heat transfer rates and the onset of thermal instability are governed by the size and spacing of the sources, the width-to-height aspect ratio of the enclosure and the phase shift between modulated heat sources. The thermal expansion coefficient is assumed small enough that the Boussinesq approximation is appropriate, and the continuity, momentum and energy equations are solved using computational fluid dynamics (CFD). The effects of the source size and source spacing on heat transfer rates are determined. This work provides insight on the parameters required to provide enhanced heat and mass transfer in small-scale systems exhibiting distributed and modulated heat sources.

Topics: Heat
Commentary by Dr. Valentin Fuster

Adiabatic Two-Phase Flow

2005;():553-560. doi:10.1115/ICMM2005-75030.

In light water reactors each fuel rod is arranged in the shape of a square lattice with an interval of around 3 mm. Several spacers are installed on the surface of the fuel rod with arbitrary axial positions. Water flows vertically along fuel rods and is heated by those, and then many bubbles generate. In order to improve the thermal design procedure of the nuclear reactor core, it is needed to clarify velocity, pressure, temperature and void fraction distributions precisely based on the bubbly flow behavior in a vertical minichannel under the water-vapor two-phase flow condition. However, it is not easy to get those three-dimensional distributions by the experimental study. Then, large-scale two-phase flow simulations were performed to predict the three-dimensional bubby flow configurations in the simply simulated nuclear coolant channel. Regarding both mechanisms of the coalescence and fragmentation of bubbles the useful knowledge was obtained.

Topics: Bubbly flow
Commentary by Dr. Valentin Fuster
2005;():561-568. doi:10.1115/ICMM2005-75031.

In this paper, firstly a review is presented on our previous study of void fraction for adiabatic gas-liquid two-phase flows in horizontal microchannels. Water/nitrogen gas and/or ethanol-water-solution/nitrogen gas were pumped through circular microchannels of 50, 75, 100, 176, 251 and 530 μm in diameter. The concentration of ethanol in water was varied to change the surface tension and the liquid viscosity. The void fraction data for the 50 to 100 μm diameter channels showed non-linear variation against a homogenous void fraction, but the data for 251 and 530 μm diameter channels varied linearly with the homogeneous void fraction. Secondly, the data have been compared with the predictions of various correlations usually applied to mini/micro-channels as well as conventional size channels. Since no correlation could predict well all of our data, a new correlation has been proposed based on our data. It was found that the calculated void fraction by the proposed correlation agreed with all the data within 0.1, irrespective of channel diameters and the liquid properties.

Commentary by Dr. Valentin Fuster
2005;():569-576. doi:10.1115/ICMM2005-75035.

The effects of surface tension on flow characteristics of two-phase annular flows in vertical pipes of 5 to 16 mm i.d. have been studied experimentally and analytically. In the adiabatic experiments, three kinds of liquids mainly with different surface tension were used as the test liquids while air as the test gas. The data obtained were instantaneous and mean void fraction, pressure drop and flow regime, etc. at near atmospheric pressure. The effects of surface tension on each flow parameter were studied by comparing the data for the different liquids. In the analysis, the above data were compared with calculations by various correlations in literatures. Regarding the void fraction, the calculations were conducted mainly by the well-known two-phase two-fluid model. The constitutive equations of the interfacial friction force were tested against the data. The results of the above experiments and comparisons are reported in this paper.

Commentary by Dr. Valentin Fuster
2005;():577-584. doi:10.1115/ICMM2005-75042.

Experimental results of oil water two-phase flow in a horizontal microchannel with hydraulic diameter of 677 μm are presented. Two series of experiments were performed. The first one was performed with the microchannel initially saturated with a constant oil flow rate. Water injection was then started and increased stepwise. The second series of experiments was performed with the microchannel saturated with a constant water flow rate. Oil injection was then started and increased stepwise. When the steady state was reached for each experiment, the flow rate and the pressure drop were measured and the flow configuration was recorded with a digital camera. Different flow structures were observed. The flow configurations observed for the microchannel initially saturated with oil are droplet of water in oil, slug, slug-annular and annular flow. For the microchannel initially saturated with water, the observed flow structures are droplet of oil in water, semi-stratified and stratified flow. Flow maps were established and compared to the maps of the literature. Experimental results were analysed by using two models: i) the Lockhart-Martinelli model allows a good fit of pressure drop measurements, ii) the homogeneous model allows to predict correctly the results obtained with the microchannel initially saturated with oil.

Commentary by Dr. Valentin Fuster
2005;():585-589. doi:10.1115/ICMM2005-75057.

An air-water separator which uses the centrifugal force as a driving force was developed and tested for micro scale flow systems. The device is composed of a main-channel of return bend, a sub-channel running along the main-channel, and several connecting-channels which connect the main- and the sub-channel. When two-phase mixture flows through the curved main-channel, water drifts towards outside region of the curvature and partly into the sub-channel. Consequently air holdup in main-channel becomes large at its exit. The performance of the device was evaluated numerically and experimentally by means of water recovery, air recovery, and Newton efficiency.

Commentary by Dr. Valentin Fuster
2005;():591-599. doi:10.1115/ICMM2005-75060.

Concerning the two-phase gas-liquid mixtures, the present understanding of even fully developed turbulent flow of a single-phase component in close channels is not completely satisfactory. In the two-phase or multiphase flow of such heterogeneous mixture like gas-liquid many more independent parameters are involved, thereby making this process more complicated and less transparent for understanding, mathematical modeling and simulating or calculating of such parameter like the length pressure gradient. In those mathematical models for calculations and simulations, as well as for interpretation of experimental results, there is a very complicated and random phenomenon of flow patterns, which needs to be quantitatively and accurately, incorporated producing higher accuracy in calculation and description. Unfortunately, nowadays, a method of how to measure flow patterns is not available. Recognizing these challenges this paper will present an approach to incorporate flow pattern phenomenon into the two-phase flow calculation model by (1) developing a mathematical model for pressure losses in two-phase flow based on in-situ parameters, (2) developing and defining a flow pattern coefficient, which incorporates the flow pattern phenomena, and (3) present the developed mathematical model with the incorporation of flow patterns, which demonstrated significant increase of accuracy of calculations based on conducted experimental research on air-water two-phase mixture flow in a horizontal small square channel.

Commentary by Dr. Valentin Fuster
2005;():601-608. doi:10.1115/ICMM2005-75061.

The complex nature of two-phase flow itself creates obstacles in detecting, monitoring and description of flow patterns, which are directly related to spatial and temporal distributions of concentration in the mixture. In addition to this, difficulties in comparing results and noticed differences in reported results generated in different experiments require more extensive research toward finding a satisfactory explanation. Because of both high significance and difficulties in measurement of in-situ parameters including concentration measurements and existence of significant differences in the published results, one possible approach to reduce the differences is to conduct a comparative study of the measurement properties of the concomitant measurement systems for the same flow conditions (identical time and space). This reported experimental study focuses on the comparison of two different concentration measurement methods (capacitive and conductive system) including the determination of concomitancy for those two systems. In this investigation, a Computer Aided Experimentation System (CAES) is used to generate a broad range of flow conditions and flow patterns, where the in-situ concentrations are measured simultaneously in the same time and space for air-water heterogeneous mixture flow. Based on the in-situ concentration measurements (full range of concentration) from both capacitive and conductive systems, a direct comparison between the results is presented and the concomitancy between the two systems is determined.

Commentary by Dr. Valentin Fuster
2005;():609-614. doi:10.1115/ICMM2005-75069.

We study gas-liquid flows in a channel having a large aspect ratio at small capillary number. A channel of sinusoidal cross section was micro-machined in a polymeric material and covered by a transparent Plexiglas plate. The non-wetting gas (air) is injected at a controlled flow rate while the interface motion is recorded with a CCD video camera. From the convergence of asymptotic, numerical and experimental analysis, we found simple dependences for the finger width and total curvature as a function of channel aspect ratio. These results provide simple and general expressions for the pressure drop needed to over-come capillary forces and push the air finger inside the channel.

Commentary by Dr. Valentin Fuster
2005;():615-624. doi:10.1115/ICMM2005-75110.

The complex interfacial phenomena involved in two-phase gas-liquid flow have defied mathematical simplification and modeling. However, the systems are used in heat exchangers, condensers, chemical processing plants, nuclear reactor systems, and fuel cells. The present work considers a 1 mm square minichannel and adiabatic flows corresponding to practical PEM fuel cell conditions. Pressure drop data is collected over mass fluxes of 4.0 to 33.6 kg/m2 s, which correspond to superficial gas and liquid velocities of 3.19–10 m/s and 0.001–0.02 m/s respectively. The experiments are repeated with water of reduced surface tension, caused by the addition of surfactant, in order to quantify the surface tension effects, as it is recognized that surface tension is an important parameter for two-phase flow in minichannels. The accuracy of various two-phase pressure drop models is evaluated. A new model for laminar-laminar flow is developed.

Commentary by Dr. Valentin Fuster
2005;():625-633. doi:10.1115/ICMM2005-75118.

Two-phase flow and water transport in a 1.08 mm hydraulic diameter by 25-cm long gas-transport minichannel are investigated. High-speed side-view images are obtained of water droplets moving through gas diffusion media (GDM) and into a gas channel. This system simulates water transport and the flow of air and water in a polymer electrolyte membrane fuel cell (PEMFC) cathode gas channel. Advancing and receding contact angles and departure droplet diameters are measured with respect to superficial gas velocity for two GDM samples. Pressure drop is measured and compared to two-phase pressure drop correlations for three different water flow and five different airflow rates, and channel-water and GDM-water interactions are described.

Commentary by Dr. Valentin Fuster
2005;():635-642. doi:10.1115/ICMM2005-75120.

The present study investigates experimentally the evolution of two-phase flow pattern in converging and diverging, silicon-based microchannels with mean hydraulic diameter of 128 μm with CO2 bubbles produced by chemical reactions of sulfuric acid (H2 SO4 ) and sodium bicarbonate (NaHCO3 ). The microchannels are prepared by bulk micromachining and anodic bonding. Three different concentrations of 0.2, 0.5 and 0.8 mol/L of each reactant at the inlet before mixing and 10 different flow rates from 1.67×10−9 m3 /s to 16.7×10−9 m3 /s are studied. Flow visualization is made possible by using a high-speed digital camera. It is found that the acceleration effect in a converging microchannel tends to diminish chemical reactions and no bubbles are formed at low concentrations and high flow rates, while large spherical bubbles are generated in the regions near the inlet and slug flow is formed in the regions near the exit for high concentrations and low flow rates. On the other hand, the deceleration effect in a diverging microchannel tends to promote chemical reactions, especially in the regions near the exit, and many bubbles are produced in those regions. Slug flow with large bubble slugs tends to appear in most parts of the diverging microchannel for low concentrations. Merger of bubbles appears frequently in the diverging microchannel. The results demonstrate the strong effects of concentration and acceleration or deceleration on the evolution of two-phase flow pattern in a converging or diverging microchannel.

Commentary by Dr. Valentin Fuster
2005;():643-649. doi:10.1115/ICMM2005-75138.

This paper describes the experimental results of Marangoni flow in a microchannel. The microchannel that we examined was made of polydimethylsiloxane (PDMS), shaped by photolithography and molding techniques. The typical channel size was about 100 μm width and 55–70 μm hight. A single-component liquid with ethanol and two-component liquids with mixtures of ethanol and distilled water were used as test liquids. Both liquids contained 1–5 μm diameter tracer particles to visualize the flow-field. Velocity distribution for the single-component liquid shows a fountain-like flow under the reference frame of the gas-liquid interface position. In stark contrast, the distribution for the two-component liquids shows a strong rolling motion near the interface caused by Marangoni flow resulting from a concentration difference. In most cases, this rolling motion is symmetrical to the central axis of the channel. On the interface, the liquid flows from the center to the edge (contact line) position, and the velocity is independent of interfacial velocity and direction. In some cases, we found asymmetric rolling motion. To observe this phenomenon more precisely, we are developing a 3-dimensional (3-D) flow observation system. The flow structures are discussed with the 3-D observation. This phenomenon seems to offer large capability for microfluidic applications such as in a micromixer.

Commentary by Dr. Valentin Fuster
2005;():651-658. doi:10.1115/ICMM2005-75139.

In this paper, the behavior of a micron-scale fluid droplet on a heterogeneous surface is investigated using a two-phase lattice Boltzmann method. The solid surface is uniform hydrophilic substrate separated by a hydrophobic strip on the central line. The dependence of spreading behavior on wettability, the width of hydrophobic strip and gravity is investigated. A decrease in contact angle of the liquid on a hydrophilic surface leads to break up of the droplet for certain substrate patterns. The two-phase Lattice Boltzmann Method (LBM) permits the simulation of the time dependent threedimensional motion of a liquid droplet on solid surface patterned with hydrophobic and hydrophilic strips. A nearest-neighbor molecular interaction force is used to model the adhesive forces between the fluid and solid surface. The model is validated by demonstrating consistency of the measured dynamic contact line with experimentally measured surface properties and observed surface shapes. The simulations suggest that the present lattice Boltzmann model can be used as a reliable way to study fluidic control on heterogeneous surfaces and other wetting related subjects.

Commentary by Dr. Valentin Fuster
2005;():659-664. doi:10.1115/ICMM2005-75147.

The frictional pressure drops of single-phase and two-phase flows in mini-pipes and mini-rectangular channels were investigated experimentally. The friction factors and the critical Reynolds number were measured by using water single-phase and gas-water two-phase flows through circular and rectangular channels respectively. Diameter of the circular pipes was 0.5, 0.25 and 0.17 mm, respectively; dimension of the rectangular channels was 0.2 × 20 mm, 0.2 × 10 mm, 0.2 × 5 mm and 0.2 × 2 mm, respectively. The experimental results for water single-phase flow in the circular tubes show that the measured friction factor agreed well with the conventional Poiseuille’s equation (λ = 64/Re) in laminar flow regime; the laminar-turbulent transition Reynolds number was approximately 2300 in a range of the present experimental conditions for each diameter. On the other hand, the experimental results for water flow in the rectangular channels slightly differed from the conventional equation (λ = 96/Re). For the two-phase flow experiments, pressure drops and flow patterns were collected over 0.01 < jG < 15 m/s for the superficial gas velocity and 0.01 < jL < 2 m/s for the superficial liquid velocity. Test gas was pressurized argon; test liquid was water. The argon gas was mixed with water through a coaxial annular nozzle to make two-phase flow. The observed flow patterns were slug, churn and annular flows; bubbly flow pattern was not observed in a range of the present experimental conditions. Time-averaged void fraction and two-phase friction pressure drops were also obtained. The two-phase friction multipliers were shown to be in good agreement with a correlation presented by Mishima-Hibiki in the experimental range considered in the present report.

Commentary by Dr. Valentin Fuster
2005;():665-669. doi:10.1115/ICMM2005-75158.

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.

Commentary by Dr. Valentin Fuster
2005;():671-678. doi:10.1115/ICMM2005-75159.

The rapid development of microfabrication techniques creates new opportunities for applications of microchannel reactor technology in chemical reaction engineering. The extremely large volume-to-surface ratio and the short transport path in microchannels enhance heat and mass transfer dramatically and hence provide many potential opportunities in chemical process development and intensification. Multiphase reactions involving gas/liquid reactants with a solid as a catalyst are ubiquitous in the chemical and pharmaceutical industries, and the hydrodynamics play a prominent role in reactor design and performance. For gas/liquid two-phase flow in a microchannel, the Taylor slug flow regime is the most commonly encountered flow pattern, therefore the present study deals with the numerical simulation of gas and liquid slugs in a microchannel. A T-junction microchannel (empty or packed) with varying cross-sectional width (0.25, 0.5, 0.75, 1, 2 and 3 mm) served as the model micro-reactor, and a finite volume based commercial CFD package, FLUENT, was adopted for the numerical simulation. The gas and liquid slug lengths at various operating conditions were obtained and found to be in good agreement with the literature data.

Commentary by Dr. Valentin Fuster
2005;():679-684. doi:10.1115/ICMM2005-75190.

In this article, the effect of surface tension on a stratified flow of two immiscible fluids between two parallel plates is examined. The level-set method is used to model the interface evolution between the fluids. To ensure mass conservation of both fluids, a localized mass correction scheme is employed. For demonstration purposes, the Finite-Volume method is used to solve the governing equations. Two different situations namely, the flows of two fluids of identical properties and two fluids of different properties are examined. These results are compared with that of the VOF method.

Commentary by Dr. Valentin Fuster
2005;():685-689. doi:10.1115/ICMM2005-75198.

Although water is produced on the cathode side of a polymer electrolyte membrane fuel cell (PEMFC), it is known to migrate to the anode, where the two-phase hydrogen-water interactions become critical to keep the channels clear for effective reactant delivery. Hydrogen-water flow regime transitions were experimentally investigated and compared to air-water transitions in a 1.0 mm square channel. Gas superficial velocities were evaluated for an anode stoichiometric ratio of 2.0 over a current density range from 0.1 A/cm2 to 2.0 A/cm2 . Liquid superficial velocities were controlled at the corresponding cathodic water production rate. It is shown that the annular transition in a hydrogen system occurs at as much as twice the gas velocity required for the same transition in an air system. At the low liquid flux expected in the anode channels of a PEMFC, a transition from slug to annular two-phase flow will occur at an unobtainable velocity for efficient fuel cell operation.

Commentary by Dr. Valentin Fuster
2005;():691-695. doi:10.1115/ICMM2005-75244.

For the internal fluid distribution in a fuel cell, channels of about 1 mm are usually used. The type of two-phase flow (reactive gases, liquid and vapor water) in these mini-channels has an important influence on the fuel cell performances. Thus it is important to optimize this two-phase flow to avoid the flooding of channels and electrodes and the instabilities in the fuel cell. To understand the type of two-phase flow, depending of the mini-channel size and section, and for different fluid velocities, several flow visualizations have been done, in channels of circular (2 mm diameter) and square (1 and 4 mm2 ) cross-section for different channel slope. Two main classes of flow patterns have been identified: separated flow (annular flow with or without waves, stratified flow), and intermittent flow (plug and slug flow). It has been also observed that the pressure gradient presents a specific behavior near the transition between separated flow and intermittent flow. A model for predicting the flow pattern transition is proposed.

Commentary by Dr. Valentin Fuster
2005;():697-702. doi:10.1115/ICMM2005-75251.

In this paper, the flow through a serpentine microchannel with obstructions on wall is studied. Various obstruction geometries ranging from rectangular to triangular are considered. For each geometry pressure drop across single obstruction is studied at various Reynolds numbers. Also the effect of obstruction height on the pressure drop is investigated. A parametric study is conducted for different obstruction heights, geometries and Reynolds numbers.

Commentary by Dr. Valentin Fuster
2005;():703-708. doi:10.1115/ICMM2005-75255.

The field of microfluidics is developing with advances in MEMS (micro electro-mechanical system) and μ-TAS (micro total analysis system) technologies. In various devices, controlling the flow rate of liquid or gas accurately at micro or nanoliter volume levels is required. By using a ferrofluid, the flow of a liquid or gas in a microchannel can be controlled by the driving power exerted on the ferrofluid. In this study, an unsteady flow of a liquid slug caused by the driving force exerted by a ferrofluid has been investigated in a 200μm circular microchannel under the influence of an external magnetic field. The motion and behavior of the liquid slugs were observed. Thus, by visualizing the behavior and interface of the ferrofluid and the liquid slug, we could study the characteristics of fluid transport phenomena in the microchannel. The relationship between the movements of the liquid slug and the ferrofluid was investigated experimentally and analytically. The surface tension and kinematic viscosity of the liquid significantly affected the movement of the liquid slug. Therefore, the velocity of the ferrofluid is affected by the physical properties of the liquids pulled. Combinations of various liquids with a ferrofluid were examined in this study. The velocity of a short liquid slug was obtained and compared with the velocity of the ferrofluid. Also an apparent surface tension of the sample liquid was examined analytically.

Commentary by Dr. Valentin Fuster
2005;():709-715. doi:10.1115/ICMM2005-75261.

In this paper we present a first order study of liquid water detachment and entrainment into air flows in hydrophobic microchannels. A silicon based microfabricated test structure was used for this purpose. It consists of a 500 μm wide by 45 μm deep U-shaped channel 23 mm in length through which air is flown. The structures are treated with a Molecular Vapor Deposition (MVD) process that renders them hydrophobic with a nominal contact angle of 108° (in situ contact angles inside the channels are measured directly during testing). Liquid water is injected through a single side slot located two-thirds of the way downstream from the air channel inlet. The side slot extends the whole depth of the air channel while its width is varied from sample to sample. Visualization of the water slugs that form as water is injected into the air channel was performed. Slug dimensions at detachment are correlated against superficial gas velocity. Proper dimensionless parameters are postulated and examined to compare hydrodynamics forces against surface tension. It is found that for Re below 200 slug detachment is dominated by pressure gradient drag arising from confinement of a viscous flow in the channel. On the other hand, for Re above 200 the predominant drag is inertial in nature with stagnation of the air due to flow obstruction by the slugs.

Commentary by Dr. Valentin Fuster

Condensation

2005;():717-728. doi:10.1115/ICMM2005-75248.

A model for predicting heat transfer during condensation of refrigerant R134a in horizontal microchannels is presented. The thermal amplification technique is used to measure condensation heat transfer coefficients accurately over small increments of refrigerant quality across the vapor-liquid dome. A combination of a high flow rate closed loop primary coolant and a low flow rate open loop secondary coolant ensures the accurate measurement of the small heat duties in these microchannels and the deduction of condensation heat transfer coefficients from measured UA values. Measurement were conducted for three circular microchannels (0.5 < Dh < 1.5 mm) over the mass flux range 150 < G < 750 kg/m2 -s. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to interpret the results based on the applicable flow regimes. The heat transfer model is based on the approach originally developed by Traviss et al. (1973) and Moser et al. (1998). The multiple-flow-regime model of Garimella et al. (2005) for predicting condensation pressure drops in microchannels is used to predict the pertinent interfacial shear stresses required in this heat transfer model. The resulting heat transfer model predicts 86% of the data within ±20%.

Commentary by Dr. Valentin Fuster
2005;():729-735. doi:10.1115/ICMM2005-75260.

The paper gives a progress report on a theoretical study of film condensation in microchannels. The model takes account of surface tension, vapor shear stress and gravity. The effect of channel shape is investigated for condensation of R134a in channels with cross sections: square, triangle, inverted triangle, rectangle with longer side vertical and rectangle with longer side horizontal. The case considered here is where the channel wall temperature is uniform and the vapor is saturated at inlet. For a given mass flux, the local condensate film profile around the cross section is calculated together with the mean heat-transfer coefficient at different distances along the channel. Results are presented here for one vapor mass flux, one vapor temperature and one wall temperature.

Commentary by Dr. Valentin Fuster

Biological Systems

2005;():737-741. doi:10.1115/ICMM2005-75155.

In this paper, we present a microscale impedance-based technique for detecting different levels of blood plasma coagulation triggered by tissue factor thromboplastin. Impedance-based detection relies on measuring changes in the ac impedance between two electrodes due to the formation of favorably insoluble fibrin after treated with thromboplastin. Both gold and carbon parallel electrodes were tested and the optimum operating frequency for most sensitive coagulation detection was investigated. Micro fabricated gold electrodes could be used to detect the impedance difference between different levels of plasmas and the result was compared with optical measurement. A peculiar conductance maximum occurs at the optimal frequency for carbon electrodes and is a sensitive indicator of blood coagulation. Our optimized small electrode sensors are ideal for point-of-care applications.

Commentary by Dr. Valentin Fuster
2005;():743-749. doi:10.1115/ICMM2005-75173.

Pulsed electromagnetic field (PEMF) treatment is a potentially non-invasive method for tissue engineering. In this paper, a theoretical model is established to simulate the regeneration of articular cartilage for Osteoarthritis by means of pulsed electromagnetic fields (PEMF). The electrical field, flow field, single particle motion and concentration field during the growth of chondrocyte are obtained by solving the theoretical model numerically, which accounts for cell distribution in the culture dish. The induced electric field strength can be numerically obtained by Maxwell’s equation and then the potential distribution by the Poisson equation and Laplace equation. The chondrocytes can be driven to move once the electric field is built up. In the calculation of the flow field, the continuity and momentum equation are applied to obtain the bulk electroosmotic velocity field which will affect the motion of the charged cell due to viscous drag forces. The motion of a single particle can be obtained by the classic Newton’s second law. In addition to a single particle, the concentration distribution of particles which indicates the migration of chondrocytes can be described by the conservation law of mass. Boundary conditions are required to solve these sets of equations numerically. A comparison between model results and actual experimental data for the growth and migration of chondrocytes is performed. The results presented here allow a better understanding of the role PEMF in the treatment of Osteoarthritis.

Commentary by Dr. Valentin Fuster
2005;():751-759. doi:10.1115/ICMM2005-75217.

Microfluidic parallel plate flow chambers provide a method for evaluating the role of molecular mechanics in the dynamic adhesion of cells or bacteria. We use optical microscopy for real-time monitoring of adherent cells in laminar flow conditions to study a counter-intuitive observation. Escherichia coli, our most common intestinal bacteria, require shear stress to bind to surfaces coated with mannose, a carbohydrate expressed on many host cells. In fact, even cells that are already bound will detach if the flow is turned down. Because these flow chambers involve bacteria binding to surfaces similar to biomaterial or tissue surfaces in our body, these experiments shed light on bacterial adhesion in vivo. However, flow chamber experiments can be carefully controlled so that they can also be used to determine the mechanism behind this unusual behavior. We show that the effect of shear stress on transport of bacteria to the surface cannot explain these results. Rather, increased shear stress causes bacteria to roll instead of detach and to switch to a stationary mode of adhesion.

Commentary by Dr. Valentin Fuster
2005;():761-767. doi:10.1115/ICMM2005-75220.

Tissue engineering is a promising, innovative approach to repair or replace injured cartilage in the joints. Bioreactors used in tissue engineering serve to maintain controlled environments that provide many of the conditions necessary for cells to produce functional cartilaginous tissues (Darling et al, 2003, Freed et al, 2000). The use of computational fluid dynamics (CFD) can accelerate the development of optimal bioreactor configurations and provide insight into underlying fluid characteristics within the bioreactor (Williams et al, 2002). This study utilizes Fluent CFD analysis software to optimize the design of a microfluidic bioreactor device. Interstitial pressure and flow-induced shear stress are two factors that have been shown to enhance chondrocyte matrix molecule biosynthesis. Characterization of these and other hydrodynamic properties within the bioreactor provides insight into optimal design configurations as well a predictive mechanism for understanding biophysical phenomena and fluid forces affecting chondrogenesis.

Commentary by Dr. Valentin Fuster

Biomolecule Separation

2005;():769-778. doi:10.1115/ICMM2005-75246.

Processing large volume liquid samples for PCR-based infectious agent testing is ubiquitous among a wide variety of environmental samples. A number of different extraction protocols and devices are available to purify the nucleic acids from these complex samples. However, most of these approaches are optimized for specific kinds of samples and typically involve benchtop equipment and highly skilled personnel. Among the most common purification techniques are those that utilize a combination of chaotropic agents and random surfaces of glass (packed beds of micro-beads, fibers, particles, etc.) in a simple disposable plastic device. As an alternative surface, we have exploited the glass-surface properties of oxidized single crystal silicon in high-surface-area microstructures for silicon oxide-mediated nucleic acid extraction, purification, and concentration from large volume samples. One particular microstructure, a silicon pillar chip, provides highly-ordered and controlled surface interactions. The high aspect ratio (∼40) of the pillars provides an immense surface area within a relatively small volume, thus allowing for the capability to concentrate nucleic acids by several orders of magnitude. Here, we report on the effectiveness of this microfluidic chip in processing Francisella DNA in wastewater. The flow-through properties of the microfluidic chip enabled the entire procedure to be automated by embedding the chip in a reusable microfluidic cassette.

Topics: DNA
Commentary by Dr. Valentin Fuster

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