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IN THIS VOLUME


2004;():1-6. doi:10.1115/ICMM2004-2314.

Integrated Nano-Technologies, LLC (INT) has spent the last three and a half years developing a novel electronic based DNA identification system. In addition to combining DNA with microelectronics to form sensors that respond to, and identify DNA sequences in target organisms; this system relies heavily on the manipulation and delivery of small volumes of fluids. These fluids facilitate both the biological and chemical reactions required by the devices processes. The Company has encountered fluidic engineering challenges throughout the development of the system including: • pump accuracy and delivery; • material incompatibilities; • adhesive intrusion; • reaction waste collection; • micro mixing/channel design. In this presentation, the challenges and the approaches taken to overcome them will be presented, as well as a device overview.

Topics: DNA
Commentary by Dr. Valentin Fuster
2004;():7-11. doi:10.1115/ICMM2004-2315.

There is considerable market interest for miniature blood diagnostic kits for cholesterol, ovulation, glucose, bacteria, leukemia cells, coagulation tests, drugs etc. However, the presence of blood cells in the blood samples introduces an array of micro-fluidic issues that have become main obstacles to commercialization of these products. We discuss some of these anomalous micro-fluidic features of blood here. Some devices and designs developed at the Notre Dame Center for Micro-fluidics and Medical Diagnostics will be presented as solutions to these micro-fluidic challenges.

Topics: Microfluidics , Blood
Commentary by Dr. Valentin Fuster
2004;():13-24. doi:10.1115/ICMM2004-2316.

An analysis of the different significant length scales allows us to show the major part played by rarefaction in gas microflows, and the different flow regimes encountered in microchannels. The main theoretical and experimental results from the literature about steady pressure-driven gas microflows are summarized. Because it is very frequent in microchannels, the slip flow regime is more detailed and the question of appropriate choice of boundary conditions is discussed. It is shown that using second-order boundary conditions allows us to extend the applicability of the slip flow regime to higher Knudsen numbers that are usually relevant to transition regime. The case of pulsed flows is also presented, for this kind of flow is frequently encountered in micropumps. The influence of slip on the frequency behavior (pressure gain and phase) of microchannels is illustrated. When subjected to sinusoidal pressure fluctuations, microdiffusers reveal a diode effect which depends on the frequency. This diode effect may be reversed, when the depth is shrunk from a few hundred to a few micrometers. Thermally driven flows in microchannels are also described. They are particularly interesting for vacuum generation, using microsystems without moving parts.

Commentary by Dr. Valentin Fuster
2004;():25-31. doi:10.1115/ICMM2004-2317.

Micro and macrosystems, as represented by microchannels and millichannels, have characteristic length scales much longer than those required to observe “size effects” manifested in nanostructures. How can one explore nanoscale effects to benefit micro and macrosystems? This paper first discusses nanoscale heat transfer effects and then uses a few examples to illustrate some potential ways to integrate nanoscale effects into micro and macrosystems.

Commentary by Dr. Valentin Fuster
2004;():33-44. doi:10.1115/ICMM2004-2318.

In recent years, thermal management of microelectronics is becoming a major feasibility bottleneck and has shown the limitations and shortcomings of conventional solutions. Market expetations are posing a simultaneous challenge of increased power levels coupled with high heat fluxes. Active and passive systems incorporating mini-/micro channel flows are gaining ground to meet the challenges. Both single phase and two-phase flows are under consideration with the latter proving to be better alternatives. Inline with these developments are the Pulsating Heat Pipes (PHPs), which are very attractive entrants in the family of closed passive two-phase heat transfer systems. Research activity in this area has steadily increased after their introduction. These apparently simple looking cooling devices have offered considerable challenges in phenomenological and theoretical understanding. These devices have already shown very high promise for terrestrial applications. They also have a potential for thermal control applications for space. Yet, complete design rules and optimization procedures are still not available. This paper highlights major progress and milestones achieved in the development of this promising technology of pulsating heat pipes in the last decade. A comprehensive review of design rules and modeling strategies available so far is presented. All the influence parameters affecting the thermal performance are explained in detail. Some recommendations for future research are also made.

Topics: Heat pipes
Commentary by Dr. Valentin Fuster
2004;():45-55. doi:10.1115/ICMM2004-2319.

An overview is given of current developments in micro mixing technology, where the emphasis is on liquid mixing in passive micro mixers. The mixers presented are differentiated by the hydrodynamic principle employed, and four important principles are discussed in some detail: hydrodynamic focusing, flow separation, chaotic advection and split-and-recombine flows. It is shown that these principles offer an excellent mixing performance in various dynamical regimes. Hydrodynamic focusing is a concept working very much independently of the Reynolds number of the flow. Flow separation offers a rich dynamical behavior over a Reynolds number scale of several hundred, with superior performance compared to purely diffusive mixing already found at low Reynolds numbers. For chaotic advection different implementations tailor-made for low and comparatively high Reynolds numbers exist, both leading to an exponential increase of the interface between two fluids. Split-and-recombine flows can only be realized in a close-to-ideal form in the low Reynolds number regime. Corresponding mixers can be equipped with comparatively wide channels, enabling a favorable ratio of throughput to pressure drop. The overview given in this article should enable a potential user of micro mixing technology to select the most favorable concept for the application envisaged, especially in the field of chemical process technology.

Commentary by Dr. Valentin Fuster
2004;():57-66. doi:10.1115/ICMM2004-2320.

Quantitative liquid crystal thermography was used to investigate boiling incipience and nucleate flow boiling in rectangular mini-channels with channel heights of 2 mm to 500 μm. Distributions of surface temperature along the heated surface were measured from the liquid crystal images, and streamwise profiles of heat transfer coefficient on the heated surface were calculated. The working fluid was the refrigerant R-11. Observations of the boiling incipience superheat excursion, the hysteresis phenomenon, and saturated flow boiling are presented. Comparisons to established two-phase heat transfer correlations are performed to investigate the existence of “thin channel” effects.

Commentary by Dr. Valentin Fuster
2004;():67-76. doi:10.1115/ICMM2004-2321.

The increased circuit density on today’s computer chips is reaching the heat dissipation limits for air-cooled technology. Direct liquid cooling of chips is being considered as a viable alternative. This paper reviews liquid cooling in terms of technological options and challenges. The possibilities presented herein indicate a four-to ten-fold increase in heat flux over the air-cooled systems. The roadmap for single-phase cooling technology is presented to identify research opportunities in meeting the cooling demands of future IC chips. The use of three-dimensional microchannels that incorporate either microstructures in the channel of grooves in the channel surfaces may lead to enhancement in single-phase cooling. A simplified fabrication process is described that can build both classes of three-dimensional microchannels. Proof-of-concept microchannels are presented to demonstrate the efficacy of the fabrication process.

Commentary by Dr. Valentin Fuster
2004;():77-85. doi:10.1115/ICMM2004-2322.

Most microfluidic processes in lab-on-a-chip devices are electrokinetic processes. Fundamental understanding of the electrokinetic based microfluidic processes is key to the design and process control of lab-on-a-chip devices. This paper will review basics of the electrical double layer field, and three key on-chip microfluidic processes: electroosmotic flow, sample mixing and sample dispensing.

Topics: Microfluidics
Commentary by Dr. Valentin Fuster
2004;():87-95. doi:10.1115/ICMM2004-2323.

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

Topics: Heat
Commentary by Dr. Valentin Fuster
2004;():97-108. doi:10.1115/ICMM2004-2324.

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

Commentary by Dr. Valentin Fuster
2004;():109-118. doi:10.1115/ICMM2004-2325.

Remote cooling is the established cooling scheme in notebook computers, and increasingly, other computing sectors like desktops and servers are evaluating this approach as an option for cooling future platforms. While remote cooling facilitates a larger heat exchanger than the space directly over the processor would allow, it introduces an additional thermal resistance, in particular, θp-f (plate to fluid resistance) — the resistance in getting the heat from the cold plate to the fluid. For any remote cooling system, this resistance needs to be carefully evaluated and minimized. Pumped fluid loops incorporating microchannel heat exchangers are a viable option to achieve low plate-to-fluid resistances. In this paper we will identify a reasonable target for θp-f and subsequently describe two similar but fundamentally different thermal systems to accomplish this target performance: single-phase and two-phase pumped loops. Although two phase flows are traditionally thought of as the way to accomplish the highest heat transfer coefficients and thus the lowest resistances, with microchannel heat sinks the contrast is not so acute. We will present results from our experimental work on single- and two-phase heat transfer from microchannel heat sinks and demonstrate a transition where single-phase performance matches that of two-phase operation. This will be followed by the analysis methods used to predict the heat transfer and the pressure drop data. Moreover, we will discuss system level issues and other hurdles that need to be overcome in commercialization of microchannel technology for cooling computer systems.

Commentary by Dr. Valentin Fuster
2004;():119-127. doi:10.1115/ICMM2004-2326.

This paper provides an overview of the application of minichannels, typically on the order of 1 mm hydraulic diameter, in the design of polymer electrolyte membrane (PEM) fuel cells. In these electrochemical devices, minichannels deliver reactant hydrogen and oxygen to the anode and cathode electrodes, respectively, while transporting product water out of the cell. The channels must be designed for low pressure drop, to avoid excessive parasitic power losses from gas handling equipment. However, the channels also need to operate in a flow regime in which the overall water balance in the fuel cell can be maintained. The various aspects of minichannel design, including size and cross-sectional shape, are discussed, with particular emphasis on fuel cell water management. In addition to reviewing these fundamental aspects of minichannel design, examples are given of new experimental tools currently under development which are applied to relate channel water transport and accumulation to fuel cell performance.

Commentary by Dr. Valentin Fuster
2004;():129-140. doi:10.1115/ICMM2004-2327.

The possible applications of micro-thermofluid control are overviewed, mainly on the basis of the total of seventeen proceedings of IEEE International Workshops or Conferences on Micro Electro Mechanical Systems held since 1987. The contents consist of three aspects. The first is thermofluid control in microsystems; particularly, the attention is focused on the various types of micropumps. The second is thermofluid control in miniaturized thermofluid machines such as a microturbine and a microcooler. The third is micro-thermofluid control for conventional-sized flow phenomena such as turbulent shear flows and a flapping-wing micro-aerial vehicle. The author stresses that, in this field, considerable advances have been achieved by relatively young researchers with various backgrounds other than the classical theromofluid dynamics, and many challenging works are in progress which will lead to new possibilities in the field of thermofluid dynamics. Also, the author has gained the impression that the researchers’ byword has recently changed from “what can MEMS research do?” to “what should MEMS research do?”

Topics: Thermofluids
Commentary by Dr. Valentin Fuster
2004;():141-148. doi:10.1115/ICMM2004-2328.

The single-phase heat transfer enhancement techniques are well established for conventional channels and compact heat exchangers. The major techniques include flow transition, breakup of boundary layer, entrance region, vibration, electric fields, swirl flow, secondary flow and mixers. In the present paper, the applicability of these techniques for single-phase flows in microchannels and minichannels is evaluated. The microchannel and minichannel single-phase heat transfer enhancement devices will extend the applicability of single-phase cooling for critical applications, such as chip cooling, before more aggressive cooling techniques, such as flow boiling, are considered.

Commentary by Dr. Valentin Fuster
2004;():149-156. doi:10.1115/ICMM2004-2329.

This paper is the first part of a two part study into the pressure-flow characteristics of a range of microchannels measured over a range of typical Reynolds numbers. Here the manufacture of the channels and their resulting quality is addressed. The target application is silicon cooling. Wet Etching, Deep Reactive Ion Etching (DRIE) and Precision Sawing have been used to create microchannels in silicon and thermoset plastic. Anodic bonding has been used to bond covers onto the DRIE and Wet Etched channels. Wet etching a (100) silicon wafer using a KOH solution produced trapezoidal channels of width 577 μm and height 413μm. DRIE using the Bosch process produced rectangular channels in (100) silicon of width 304μm and height 332μm. Mechanical sawing using a Disco Dicing Saw produced near rectangular channels in both silicon and plastic. The silicon channels were 52μm wide and 423μm deep, and the plastic channels were 203μm wide by 344 or 382μm deep. Channel dimensions were measured using a scanning electron microscope. Silicon was the main material chosen, since it is possible to cut cooling channels directly into one side of a silicon device, while the electronic parts are deposited on the other, giving effective cooling with minimal thermal resistance. The plastics chosen are commonly used to encapsulate electronic packages and will also be in close proximity to the heat producing regions of the device it protects. Embossed channels on a plastic encapsulant also potentially offer a low cost mass producible means of cooling electronic devices with a low overall thermal resistance. A glass cover was anodically bonded over the silicon channels to prevent channel to channel leakage and provide optical access. The plastic channels were also covered by a glass slide, bonded in position using SU8 Photoresist spun on the glass. This paper demonstrates the feasibility of producing relatively large microchannels in two materials by three methods. Part two of this paper will describe the modular flow test system and analyze the flow friction through the channels.

Commentary by Dr. Valentin Fuster
2004;():157-164. doi:10.1115/ICMM2004-2330.

Part 1 of this paper (Eason et al 2004) investigates the manufacturing of a variety of microchannels, produced by wet and dry etching in silicon, as well as precision mechanical sawing in silicon and thermoset plastic. This paper describes the experimental equipment and methods used to measure the pressure flow characteristics of the manufactured channels. A custom designed test system has been built in order to test each sample using the same inlet and outlet manifolds, pressure tappings, pumping system and instrumentation. The pressure drop across each set of channels was measured using an inductive pressure transducer. The mass flow rate through the system is measured by weighing the flow from the system in a given time. The measured pressure flow behaviour was compared with theoretical values as calculated from macro scale theory. Channel dimensions used for this calculation are as measured in part 1 of this paper. Error analysis was then carried out in order to determine the overall accuracy of the experimental work and determine whether any deviation from theoretical values is of experimental significance. This step is essential in any attempt to determine whether microchannel flows are indeed different to macro scale flows in a fundamental way. The deep reactive ion etched (DRIE) channels show the most significant lack of correlation with theoretical predictions. Compensation must be introduced to deal with the difference in cross section between the perfectly rectangular channels used for the theoretical prediction and the actual cross section of the channels.

Commentary by Dr. Valentin Fuster
2004;():165-171. doi:10.1115/ICMM2004-2331.

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

Commentary by Dr. Valentin Fuster
2004;():173-182. doi:10.1115/ICMM2004-2332.

Steady, incompressible flow through a basic minichannel system has been considered. Slip at the wall has been assumed to be negligible. The channel has a rectangular cross-section. The flow enters the channel with a uniform velocity and temperature and passes through a straight channel section that has a length that is 30 times the width of the channel. The flow then passes around a 180° bend. Following the bend, the flow passes down another straight channel which also has a length of 30 times the size of the channel. A uniform heat flux is applied over the entire surface of the duct. The flow geometry does not represent any that is likely to occur in minichannel system applications but is adequate for the evaluation of the assumptions often adopted regarding pressure losses, flow development lengths and heat transfer rates in the preliminary design real such systems. The governing equations have been written in dimensionless form and solutions to these 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, the dimensionless height-to-width ratio of the channel H = h / w, and the dimensionless radius of the bend Ri = ri / w. Here w is the width of the channel, h is the height of the channel and ri is the radius of the inside surface of the bend. Results are only presented for P r = 0.7 and Ri = 1. Re values between 10 and 500 and H values between 0.5 and 2 have been considered.

Commentary by Dr. Valentin Fuster
2004;():183-190. doi:10.1115/ICMM2004-2333.

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

Commentary by Dr. Valentin Fuster
2004;():191-198. doi:10.1115/ICMM2004-2334.

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

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2004;():199-204. doi:10.1115/ICMM2004-2335.

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

Commentary by Dr. Valentin Fuster
2004;():205-212. doi:10.1115/ICMM2004-2336.

The paper presents both three and two-dimensional numerical analysis of convective heat transfer in microchannels. The three-dimensional geometry of the microchannel heat sink followed the details of the experimental facility used during a previous research step. The heat sink consisted of a very high aspect ratio rectangular microchannel. Two channel heights, namely 1mm and 0.3mm (0.1mm), were used for 3D (2D) numerical model respectively. Water was employed as the cooling liquid. The Reynolds number ranged from 200 to 3000. In the paper, the thermal entrance effect is analyzed in terms of heat transfer efficiency. Finally, the comparison between measured and computed heat flux and temperature fields is presented. Contrary to the experimental work, the numerical analysis did not reveal any significant scale effect in heat transfer in microchannel heat sink up to the smallest size considered (0.1 mm).

Commentary by Dr. Valentin Fuster
2004;():213-220. doi:10.1115/ICMM2004-2337.

Fluid flow and heat transfer in microchannels have been important research area during the past decade. The understanding and explanation of the fundamental mechanisms of flow and heat transfer are critical to the application of microchannel systems to many important industrial and research projects. We present a review of the literatures on fluid flow and heat transfer of single-phase liquid in microchannels. Recent experimental and theoretical studies are both covered. The emphasis has been on studies on single-phase liquid flows. As a conclusion, although further work needs to be done, carefully designed experiments have obtained data that agree well with the conventional theory developed for larger channels. The theoretical explanation of some experimental results, which deviate the conventional theory for larger channels, is still under development.

Commentary by Dr. Valentin Fuster
2004;():221-228. doi:10.1115/ICMM2004-2338.

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

Commentary by Dr. Valentin Fuster
2004;():229-235. doi:10.1115/ICMM2004-2339.

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

Commentary by Dr. Valentin Fuster
2004;():237-243. doi:10.1115/ICMM2004-2340.

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

Commentary by Dr. Valentin Fuster
2004;():245-249. doi:10.1115/ICMM2004-2341.

The axial-heat-transport characteristics of oscillatory flow of water in mini-tubes were investigated to provide possible next-generation electronics cooling in thermal management devices. The mini-tubes were made of SUS304 with inner diameters of 0.3, 0.5, and 0.8 mm, which were smaller than those of reported in the literature. The oscillation frequency of the liquid column ranged from 0.38 to 12.5 Hz and its tidal displacement ranged from 25 to 100 mm. The effective thermal diffusivity was defined to evaluate heat transport rate for each experimental conditions and to compare with existing analytical-model results. The measured effective thermal diffusivities divided by the square of tidal displacement and angular frequency drew “tuning-curves” in the graph, which are similar to the curve drawn by the analytical-model. Regarding the magnitudes of the diffusivities, although the agreement between the measured diffusivities and calculated diffusivities are fairly good, the measured values are roughly twice higher than the prediction results at the frequencies of about 3 Hz and the lower.

Topics: Flow (Dynamics) , Heat
Commentary by Dr. Valentin Fuster
2004;():251-258. doi:10.1115/ICMM2004-2342.

For small Reynolds numbers, conductive heat transfer in the wall of mini-micro channels can become quite multidimensional: the wall heat flux density does not stay uniform and heat transfer mainly occurs at the entrance of the channels. The use of a ID model to invert measurements designed for estimating the convective heat transfer coefficient can lead to misinterpretations such as a variation of the Nusselt number with the Reynolds number. Three analytical models of conjugated heat transfer in channels are proposed, and the potential inversion of measurements is considered. A non-dimensional number M quantifying the relative part of conductive axial heat transfer in walls is introduced.

Commentary by Dr. Valentin Fuster
2004;():259-266. doi:10.1115/ICMM2004-2343.

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

Commentary by Dr. Valentin Fuster
2004;():267-271. doi:10.1115/ICMM2004-2344.

We discuss and validate a recently proposed second-order slip model for dilute gas flows. Our discussion focuses on the importance of quantitatively accounting for the effect of Knudsen layers close to the walls. This is important, not only for obtaining an accurate slip model but also for interpreting the results of the latter since in transition-regime flows the Knudsen layers penetrate large parts of the flow. Our extensive validation illustrates the above points by comparing direct Monte Carlo solutions to the slip model predictions for an unsteady flow. Excellent agreement is found between simulation and the slip model predictions up to Kn = 0.4, for both the velocity profile and stress at the wall. This demonstrates that the proposed second-order slip model reliably describes arbitrary flowfields (and related stress fields) in a predictive manner at least up to Kn = 0.4 for both steady and transient problems.

Topics: Gas flow
Commentary by Dr. Valentin Fuster
2004;():273-280. doi:10.1115/ICMM2004-2345.

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

Topics: Compressibility , Jets
Commentary by Dr. Valentin Fuster
2004;():281-288. doi:10.1115/ICMM2004-2346.

Low Knudsen number isothermal slip flow past a confined spherical particle has been investigated using a specially adapted Navier-Stokes solver. Knudsen numbers covering the continuum and slip-flow regimes (Kn ≤ 10−1 ) are considered while the Reynolds number is varied between 10−3 ≤ Re ≤ 0.5. In addition, blockage effects are studied by varying the ratio between the diameter of the pipe (H) and the diameter of the particle (D). A particularly important aspect of the present study is the proper formulation of the slip-velocity boundary condition over the curved surface of the particle. This is achieved by recasting Maxwell’s conventional velocity-slip equation as a function of the wall shear stress in order to account correctly for the curvature. The results show that blockage effects are extremely important in the continuum regime and cause amplification in the hydrodynamic drag on the particle. However, blockage phenomena are shown to be less important as the Knudsen number is increased. At the upper limit of the slip-flow regime, Kn ≈ 10−1 , blockage amplification effects are reduced by almost 50% for a pipesphere geometry of H/D = 2.

Commentary by Dr. Valentin Fuster
2004;():289-296. doi:10.1115/ICMM2004-2347.

A mathematical model has been developed to characterize the effect of packing of molecules of a hard-sphere dense gas near the hard walls of a microchannel. Analytical techniques, Monte Carlo (MC) methods and Molecular Dynamics (MD) simulation methods have been used to characterize the influence of the characteristic parameters such as number density, reduced density, length of the system and molecular diameter on the equilibrium properties of the gas near the hard walls of the microchannel. The height and the position of the density oscillation peaks near the wall are characterized. Comparisons between MD and MC results for particles having different diameter are presented. For the same size of the particles and moderately dense gas, MC and MD results are similar, differences in the density profiles being limited only to the oscillatory region. For different particle sizes, MD and MC results are limited to a short distance near the wall for long size systems and moderately dense fluids. The effect of the boundary (particle size) on the simulation results is increasing with η (reduced density) and it is very small in case of a dilute gas. For small η and small particle size (R) relative to length of the system L, the height of the oscillations peaks is slowly increasing with R/L, and for high densities is always decreasing with R/L. The position of these peaks depends only on the size of the particles and when R is much smaller than L, it shows a small dependence on L.

Topics: Microchannels
Commentary by Dr. Valentin Fuster
2004;():297-304. doi:10.1115/ICMM2004-2348.

Computational simulations of experiments on turbulent convection heat transfer of carbon dioxide at supercritical pressures in a vertical tube of diameter 0.948 mm have been carried out using low-Reynolds number eddy viscosity turbulence models. The simulations were able to reproduce the general features exhibited in the experiments. The modelling study has provided valuable information on the detailed flow and turbulence fields. It has been shown that for mini tubes such as the one used in the current study, the buoyancy effect is generally insignificant. Heat transfer can be significantly impaired when the heating is strong. This is due to the reduced turbulence production, induced by the flow acceleration which is in turn caused by strong heating.

Commentary by Dr. Valentin Fuster
2004;():305-311. doi:10.1115/ICMM2004-2349.

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

Commentary by Dr. Valentin Fuster
2004;():313-318. doi:10.1115/ICMM2004-2350.

The current research investigates the effect of Reynolds number and Knudsen number on the coefficient of skin friction and reattachment length for a micro-scale fluid flow over a step mounted on a lower wall of a micro-channel. Five Reynolds numbers are studied Re = 1, 10, 25, 50, and 75 and the Knudsen number is varied from 1×10−3 to 0.1. Finite difference method with non-uniform grid is used to solve the incompressible Navier Stokes equations accompanied with velocity slip boundary condition. As Knudsen number (Kn) decreases the magnitude of modified local shear stress (1/2 Cf Re), on the upper wall of the channel, increases. In the circulation zone behind the step and for the case of high Reynolds number (Re = 50 and Re = 75) the modified local shear stress increases as Knudsen number increases. Results show that the modified total skin friction (1/2 CD Re) decreases as the Knudsen number increases. The modified total skin friction drops significantly with Knudsen number for Kn >= 1 × 10−2 . However, (1/2 CD Re) is relatively independent of Knudsen number for Kn < 1 × 10−2 . Finally, for 1 × 10−2 < Kn < 0.1, as Knudsen number increase the reattachment length increases.

Commentary by Dr. Valentin Fuster
2004;():319-324. doi:10.1115/ICMM2004-2351.

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

Commentary by Dr. Valentin Fuster
2004;():325-332. doi:10.1115/ICMM2004-2352.

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

Commentary by Dr. Valentin Fuster
2004;():333-340. doi:10.1115/ICMM2004-2353.

Convection heat transfer of CO2 at supercritical pressures in a vertical mini tube with a diameter of 0.948 mm was investigated experimentally and numerically. The local heat transfer coefficients, bulk fluid temperatures and wall temperatures were measured and presented. The effects of inlet fluid temperature, fluid pressure, mass flow rate, heat flux and wall thickness on the convection heat transfer in the mini tube were investigated. The experimental results were compared with calculated results using well-known correlations and numerical simulations. The results showed that the variable thermophysical properties of supercritical CO2 significantly influenced the convection heat transfer in the vertical mini tube and that for the studied conditions the influence of the wall thickness on the convection heat transfer in the mini tube was not great. For bulk fluid temperatures higher than the pseudo-critical temperature, the simulation results and the correlation results for the convection heat transfer coefficients in the mini tube corresponded well to the experimentally measured results.

Commentary by Dr. Valentin Fuster
2004;():341-350. doi:10.1115/ICMM2004-2354.

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

Commentary by Dr. Valentin Fuster
2004;():351-358. doi:10.1115/ICMM2004-2355.

This paper is devoted to analyzing the friction factor of incompressible rarefied gas flow through microchannels. A theoretical investigation is conducted in order to underline the conditions for experimentally evidencing rarefaction effects on the pressure drop. It is demonstrated that for a fixed geometry of the microchannel cross section it is possible to calculate the minimum value of the Knudsen number for which the rarefaction effects can be observed experimentally, taking into account the experimental uncertainties on the evaluation of the friction factor.

Commentary by Dr. Valentin Fuster
2004;():359-366. doi:10.1115/ICMM2004-2356.

Gaseous flow characteristics in fused silica microtubes and square microchannels are studied experimentally. The existing works in the literature on experimental gaseous flow are analyzed. The data in fused silica micro circular tubes with diameters ranging from 50 μm to 201 μm and the data in fused silica micro square channels with hydraulic diameter ranging from 52 μm to 100 μm show that the flow friction factors are in good agreement with the theoretical prediction for conventional tubes and no distinguishable deviation is observed. The transition Reynolds number is around 2000 and a slight early transition from laminar to turbulent is observed due to the compressibility effect. For the helium flow in fused silica microtubes with inner diameters ranging from 10 μm to 20 μm, the decrease in friction factor is observed. In addition, factors including roughness, compressibility and rarefaction that may have significant effects on flow characteristics in microchannels are discussed.

Commentary by Dr. Valentin Fuster
2004;():367-372. doi:10.1115/ICMM2004-2357.

The two phase flow of air-water mixture in circular and rectangular channels with hydraulic diameters of 100 μm to 1 mm has been modeled via computational tools. Two phase flow patterns obtained from the computed results have been compared to those of the ones reported in literature and a reasonable agreement has been obtained. After the validation of the computational procedure, keeping the dimensionless numbers relevant to the small hydraulic diameters constant, the size of the channel has been changed to understand the transition to the flow regime in which the “conventional” governing equations fail. Gas and liquid superficial velocities range from 0.083 m/s to 70 m/s and from 0.043 m/s to 5.67 m/s, respectively.

Commentary by Dr. Valentin Fuster
2004;():373-380. doi:10.1115/ICMM2004-2358.

Both vertical upward and horizontal gas-liquid two-phase flows in a flat capillary rectangular channel were studied to clarify the flow phenomena, the holdup and the frictional pressure drop. The dimension of the channel used was 9.9 mm × 1.1 mm. The orientations of the channel were with the wide side vertical and the wide side horizontal. The differences between the flow characteristics in such orientations were investigated. New correlations of holdup and frictional pressure drop for flat capillary channels are proposed, in which the effect of aspect ratio has been taken into consideration.

Commentary by Dr. Valentin Fuster
2004;():381-388. doi:10.1115/ICMM2004-2359.

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

Commentary by Dr. Valentin Fuster
2004;():389-396. doi:10.1115/ICMM2004-2360.

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

Commentary by Dr. Valentin Fuster
2004;():397-403. doi:10.1115/ICMM2004-2361.

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

Commentary by Dr. Valentin Fuster
2004;():405-412. doi:10.1115/ICMM2004-2362.

In microchannel flow, gas-liquid interface behavior is important for many applications, e.g. micro-reactors, micro heat pipes to name only two of them. Microfluidic channels shape are generally rectangular or triangular with associated solid corner. Those corners are interesting for they drastically increase capillary effects of wetting liquids compared to smooth solid boundaries. We study capillarity driven gas–liquid flows in a flat triangular channel. A channel of triangular cross section was micro-machined in a polymeric material and covered by a transparent plexiglas plate. The wetting fluid is injected at a controlled flow rate and the interface motion along the corners is recorded with a CCD video camera. A simple lubrication theory predicts the temporal evolution of the liquid-gas interface. A good agreement is found between those predictions and experimental results. the theory also predicts when the invasion of the channel bulk occurs. The dynamics of the bulk meniscus is discussed. The results suggest that the bulk meniscus dynamics is affected by the growth of the liquid fingers that develop along the edges of the channel.

Topics: Capillarity , Motion
Commentary by Dr. Valentin Fuster
2004;():413-420. doi:10.1115/ICMM2004-2363.

In this paper an experimental study was performed for relation between two-phase pressure drop and flow distribution in compact heat exchanger using small diameter tubes. We performed the experimental study in non-heating mode. A test section was consisted of the horizontal header (circular tube: φ 5 mm × 80 mm) and 10 upward circular channels (φ 1.5 mm × 850 mm) using acrylic tube. Three different types of tube insertion depth were tested for the mass flux and inlet quality ranging from of 50–200 kg/m2 s and 0.1–0.3, respectively. Air and water were used as the test fluids. Two-phase pressure drop of each channel and three type of distribution header was measured. As whole, single-phase and two-phase, pressure drop in rear channel is found to be lower than that in front channel. In conclusion, we can claim that principle of distribution is almost same pressure drop in each channel. Comparing pressure drop in branch tube with correlation equation, it was found that in single-phase flow, experimental value was 10% lower than Hagen-Poiseuille, Blasius equation (Eq. 40) in two-phase flow.

Commentary by Dr. Valentin Fuster
2004;():421-427. doi:10.1115/ICMM2004-2364.

In the present study, we have carried out the experimental and numerical studies for the single- and two- phase flow characteristics and the corresponding pressure drop in the single- and multi-channels. We used the finite volume method to solve the mass and momentum conservation equations. The volume of fluid model is used to predict the two-phase flow in the channel. The calculated results for the single- and two-phase flow are partly compared with the present experimental data, showing relatively good agreement between them. The numerical scheme used in this study predicts well characteristics of single- and two-phase flow in a multi-channel. Thus we expect that system performances could be improved by obtaining the optimal conditions from the present calculation.

Commentary by Dr. Valentin Fuster
2004;():429-437. doi:10.1115/ICMM2004-2365.

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

Commentary by Dr. Valentin Fuster
2004;():439-444. doi:10.1115/ICMM2004-2366.

Present study demonstrates the head loss and the flow characteristics as the open microchannel makes turns. The microchannels are of various aspect ratios of depth/width ranging from 0.5 to 2, and makes turns with different angles ranging from 60 to 120 degrees. The investigations are performed both by experiments and numerical simulations based on first principle equations. For the open channel system, the flow is mainly driven by surface tension under same pressure atmosphere. For liquid flow in open microchannel without turn, the liquid front velocity decreases but the interface area of liquid-gas increase as flow moves downstream. For turning flows, liquid front velocity is decreased firstly and then increased sharply at the turning point. Furthermore, the liquid front velocity can be increased for higher aspect ratio of channel height and width, and the effect of aspect ratio is significant up to aspect ratio between 1.0–1.5. Detailed flow characteristics as well as the head loss coefficient due to microchannel turning are discussed.

Commentary by Dr. Valentin Fuster
2004;():445-452. doi:10.1115/ICMM2004-2367.

Flow boiling heat transfer characteristics of water and hydrocarbons in mini and microchannels are experimentally studied. Two different test section geometries are employed; a circular channel with a hydraulic diameter of 1500 μm, and rectangular channels with height values of 300–700 μm and a width of 10mm. In both facilities the fluid flows upwards and the test sections, made of the nickel alloy Inconel 600, are directly electrically heated. Thus the evaporation takes place under the defined boundary condition of constant heat flux. Mass fluxes between 25 and 350 kg/(m2 s) and heat fluxes from 20 to 350 kW/m2 at an inlet pressure of 0.3 MPa are examined. Infrared thermography is applied to scan the outer wall temperatures. These allow the identification of different boiling regions, boiling mechanisms and the determination of the local heat transfer coefficients. Measurements are carried out in initial, saturated and post-dryout boiling regions. The experimental results in the region of saturated boiling are compared with available correlations and with a physically founded model developed for convective boiling.

Commentary by Dr. Valentin Fuster
2004;():453-460. doi:10.1115/ICMM2004-2368.

In the present study, heat transfer in a falling film micro groove evaporator has been simulated by an analytical model. The flow and thermal fields were divided in two regions, i.e. macroscopic flow inside the groove and the microscopic flow where intensive evaporation takes place at the thin film interline region. For the micro region model, pressure in the liquid film was expressed as a sum of surface tension and disjoining pressure effects. The film thickness profile was obtained by solving the 4th order differential equation by Runge-Kutta method. Then, this micro region model was combined with the macro region model. Macro region model solves one dimensional bulk flow inside the groove with gravitational effect taken into account. Constant curvature of the liquid vapor surface was assumed for the macro flow. It is shown that the gravitational force is essential for providing the liquid to wide range of heat transfer area. Thus, diverging branch evaporator is investigated. It is demonstrated that this concept has large potentiality for improving the performance of the micro groove falling film evaporator.

Commentary by Dr. Valentin Fuster
2004;():461-468. doi:10.1115/ICMM2004-2369.

This paper discusses the effect of diameter on both flow boiling heat transfer and transition from macro to microchannel evaporation. A recently proposed three-zone flow boiling model based on evaporation of elongated bubbles in microchannels is briefly described and used for the present analysis. In the microscale range, the model predicts an increase in the two-phase heat transfer coefficient with a decrease of d for low values of the vapor quality and a decrease of h for larger values of x. This behavior is explained by the influence of the liquid film thickness, deposited periodically behind passing liquid slugs.

Commentary by Dr. Valentin Fuster
2004;():469-474. doi:10.1115/ICMM2004-2370.

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

Commentary by Dr. Valentin Fuster
2004;():475-481. doi:10.1115/ICMM2004-2371.

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

Commentary by Dr. Valentin Fuster
2004;():483-490. doi:10.1115/ICMM2004-2372.

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

Commentary by Dr. Valentin Fuster
2004;():491-497. doi:10.1115/ICMM2004-2373.

Experiments on flow boiling of water are carried out for a rectangular vertical channel with dimension (width × depth) 0.86 × 2.0 mm2 (hydraulic diameter 1.2 mm). The Confinement number, Co = [σ/(g(ρL − ρG )dh 2 )]0.5 , for the investigated range of operating parameters is about 2.2. So, applying the criterion Co > 0.5 for micro channels, the investigated channel has to be classified as “micro”. Water as working fluid is used with different mass and heat fluxes. Investigations of heat transfer and pressure drop were carried out. Results of flow visualisation are presented. On the basis of observations, a flow pattern map is obtained and the results are compared with existing flow pattern transition boundaries proposed by Taitel et al. (1980) and Mishima and Ishii (1984). The paper contains recent experimental results of an ongoing EU-project. Previous results have been presented in Shuai et al. (2002, 2003).

Commentary by Dr. Valentin Fuster
2004;():499-505. doi:10.1115/ICMM2004-2374.

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

Commentary by Dr. Valentin Fuster
2004;():507-513. doi:10.1115/ICMM2004-2375.

Convective boiling heat transfer coefficients of R-22 were obtained in a flat extruded aluminum tube with Dh = 1.41 mm. The test range covered mass flux from 200 to 600 kg/m2 s, heat flux from 5 to 15 kW/m2 and saturation temperature from 5°C to 15°C. The heat transfer coefficient curve shows a decreasing trend after a certain quality (critical quality). The critical quality decreases as the heat flux increases, and as the mass flux decreases. The early dryout at a high heat flux results in a unique ‘cross-over’ of the heat transfer coefficient curves. The heat transfer coefficient increases as the mass flux increases. At a low quality region, however, the effect of mass flux is not prominent. The heat transfer coefficient increases as the saturation temperature increases. The effect of saturation temperature, however, diminishes as the heat flux decreases. Both the Shah and the Kandlikar correlations underpredict the low mass flux and overpredict the high mass flux data.

Topics: Aluminum , Boiling
Commentary by Dr. Valentin Fuster
2004;():515-522. doi:10.1115/ICMM2004-2376.

The experimental investigations cover heat transfer of refrigerants R 123 and R 11 flowing through vertical minichannels of 40 mm wide rectangular section and depths of 1 mm, 1.5 mm and 2 mm. The heating foil, supplied with controlled direct current, constitutes one of the surfaces of the minichannel. The liquid crystal thermography technique is applied in order to measure the two-dimensional temperature field of the heating surface. The investigations focus on the transition from single-phase forced convection to nucleate boiling, i.e. in the zone of boiling incipience. The present work aims to examine and analyze how the selected parameters (inlet pressure, inlet liquid subcooling, liquid flow velocity) affect nucleate boiling incipience for various geometry (changeable depth) of the minichannel. Furthermore, the investigations are intended to develop a correlation for the calculations of the Nusselt number under the conditions of boiling incipience in the minichannel. The equations are derived as modifications of the already developed ones [Piasecka, 2002; Piasecka and Poniewski, 2003b,c; Piasecka et al., 2004] and as a function of changeable parameters in the experimental investigations.

Commentary by Dr. Valentin Fuster
2004;():523-530. doi:10.1115/ICMM2004-2377.

Bubble behavior and pressure fluctuation during boiling are modeled for enhanced surfaces with surface pores and sub-surface channels. The hydraulic diameter of the channels is about 0.42 mm. It is assumed that the latent heat flux is solely generated by thin-film evaporation on the channel surface. The evaporation rate is given by a semi-empirical correlation including effects of surface tension and liquid viscosity. The activation pressure and the bubble departure size are different for different pores. The vapor pressure inside the bubble is related to the channel pressure by the orifice equation. The bubble growth rate is determined by a modified Rayleigh equation. The vapor mass in a given channel volume is given by the mass conservation equation. Thus the instantaneous channel pressure and vapor volume can be determined which, in turn, govern the dynamic processes of bubble initiation and growth on various pores. This model demonstrates for the first time the dynamic picture of boiling phenomena on enhanced surfaces, e.g. the instantaneous activation, growth and detachment of bubbles on various pores, the fluctuation of channel pressure and vapor mass etc.. The model has been tested for boiling of propane and isobutane on an enhanced tube. The predictions agree reasonably well with the experiments.

Commentary by Dr. Valentin Fuster
2004;():531-537. doi:10.1115/ICMM2004-2378.

In the present study, the local heat transfer and pressure drop characteristics are investigated experimentally for the flow boiling of refrigerant HFC134a in a multi-port extruded tube of 1.06mm in hydraulic diameter. The test tube is 865mm in total length made of aluminum. The pressure drop is measured at an interval of 191 mm, and the local heat transfer coefficient is measured in every subsection of 75mm in effective heating length. Experimental ranges are as follows: the mass velocity of G = 100–700 kg/m2 s, the inlet temperature of Tin = 5.9–11.4 °C and inlet pressure of about 0.5 MPa. The data of pressure drop are compared with a few previous correlations for small diameter tubes, and the correlations can predict the data relatively good agreement. The data of heat transfer coefficient is compared with the correlations of Yu et al. proposed for relatively large diameter tubes. It is found that there are some differences about two phase multiplier factor of convective heat transfer between the circular channel and rectangular channel.

Commentary by Dr. Valentin Fuster
2004;():539-550. doi:10.1115/ICMM2004-2379.

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

Commentary by Dr. Valentin Fuster
2004;():551-558. doi:10.1115/ICMM2004-2380.

Using microstructured wall surfaces may improve the heat transfer performance of falling film or shear-driven film cooling devices enormously. The advantages of the structured surface include the prevention of the formation of dry patches on hot surfaces, the promotion of ultra-thin film evaporation, and a wavy motion of the film that enhances mixing of the liquid. We develop a model describing the hydrodynamics and heat transfer by evaporation of gravity- and gas flow-driven liquid films on grooved surfaces. For low Reynolds numbers or low liquid mass fluxes the heat transfer is governed by the evaporation of the ultra-thin film at a micro region, in the vicinity of the three-phase contact line. We investigate the hydrodynamic stability of the film flow using the long-wave theory. In addition to the films completely covering the wall structure, we study the stability characteristics of a thin liquid film partly covering the grooved wall, so that the flow region is bounded by contact lines. Two cases are analyzed: fully wetting liquids and liquids which form a small but finite contact angle with the wall material.

Commentary by Dr. Valentin Fuster
2004;():559-564. doi:10.1115/ICMM2004-2381.

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

Commentary by Dr. Valentin Fuster
2004;():565-572. doi:10.1115/ICMM2004-2382.

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

Commentary by Dr. Valentin Fuster
2004;():573-580. doi:10.1115/ICMM2004-2383.

Flow boiling in micro- and mini-channels has attracted much attention in recent years. But the phenomena is such confined channels have not been fully understood and explained. Some conclusions reached by different authors are even contradictory. The present research is trying to study some aspects of flow boiling in mini- and micro-channels. In the present paper boiling heat transfer and two-phase flow patterns in rectangular narrow channels were studied. The gap size of the channel was varied as 2, 1, 0.5 and 0.2 mm with the channel width and length being kept at 20 mm and 100 mm, respectively. In the present mini- and micro-channels, four flow patterns were identified; bubbly, intermittent, wavy and annular flow. They can be also divided into several sub-flow patterns. Flow patterns showed strong channel gap size dependence. Smaller gap size deleted bubbly flow, thus induced simpler flow patterns to shift the annular flow at lower vapor quality. The channels can be divided into two groups depending on the gap size; the larger gap group of 2 and 1 mm, and the smaller gap group of 0.5 and 0.2 mm. The larger gap group showed similar heat transfer behavior as conventional size of tubes. The smaller gap group indicated some peculiar phenomena. Heat transfer coefficient in the smaller gap group was relatively high in the low quality region. Then heat transfer coefficient decreased monotonously with increasing vapor quality. This behavior was considered attributable to the micro-bubble generation in the channel corners and an early partial dryout of thin liquid film. Thus the relationship between heat transfer coefficient and flow pattern should be carefully pursued in micro- and mini-channels to develop heat transfer correlations based on flow patterns.

Commentary by Dr. Valentin Fuster
2004;():581-587. doi:10.1115/ICMM2004-2384.

Boiling heat transfer and corresponding two-phase flow phenomena are of significant interest for the design of a compact evaporator. The present work investigates experimentally, using a high-speed digital CCD camera, the two-phase flow phenomena for boiling in a silicon-based, two parallel trapezoid microchannels, which were prepared by the combination of silicon bulk micro machining and Pyrex-silicon wafer bonding. Onset of nucleate boiling, bubbly flow, slug flow, and partial dry out slug flow are typically observed along the flow direction. The appearance of the partial dryout slug flow may degrade the nucleate boiling heat transfer in the microchannel. At a low flow rate, reversed vapor flow is observed. In such a flow pattern, liquid droplets are formed intermittently on the inner wall of top Pyrex glass due to vapor condensation. Moreover, the reversed vapor flow usually accompanies with large magnitude two-phase flow oscillations.

Commentary by Dr. Valentin Fuster
2004;():589-594. doi:10.1115/ICMM2004-2385.

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

Commentary by Dr. Valentin Fuster
2004;():595-599. doi:10.1115/ICMM2004-2386.

In this study, the nucleus formation and bubble growth at beginning of boiling process in microchannels was investigated. Canonical molecular distribution was introduced to analyze characteristics of nucleate boiling when liquid molecular number is very small. The minimum bulk phase volume in which phase change was enable to occur was determined from thermodynamic theory of bubble formation in a superheated liquid and from the energy distribution of the molecules in the bulk phase at a given temperature and pressure. The comparison of the free energy decrease during nucleus formation on a flat wall and at a corner of walls indicates that nuclei more easily form at a corner than on a flat wall for wetting liquids. The experimental results demonstrated that the superheat temperature increased as diameter of microchannels decreased. The experimental measurements are in a quite agreement with theoretical predictions.

Commentary by Dr. Valentin Fuster
2004;():601-608. doi:10.1115/ICMM2004-2387.

Heat transfer and pressure drop, are experimentally recorded for flow boiling water in a single 706 μm circular copper channel 158.75 mm long. Heat is supplied by heat transfer oil at specified temperatures to a helical channel in the test section. In contrast to other current experimental techniques for flow boiling in small diameter tubes, a uniform temperature boundary condition is employed rather than a constant heat flux condition. The principal results of these experiments are two-phase flow boiling heat transfer rates and an analysis of the time-dependent pressure drop signature during two-phase flow in a minichannel. The range of experiments includes mass fluxes of 43.8–3070 kg/m2 s and wall temperatures of 100°C–171.2°C. In all cases the test section water inlet is subcooled to between 72.9°C and 99.6°C. The inlet pressures used are 1.1–230.5 kPa (gage).

Commentary by Dr. Valentin Fuster
2004;():609-615. doi:10.1115/ICMM2004-2388.

A global model simulating the behavior in steady-state of a refrigerating system using a small channel condenser, is presented in this paper. Its objective is to optimize the design of this system in term of improving its efficiency and minimizing the refrigerant charge, in order to reduce the greenhouse effect. We were interested mainly on the small channel condenser model in which heat exchange and pressure drop were calculated with laws adapted, when it is necessary, to the small channel configuration. The refrigerant charge contained in the condenser was determined from correlations giving the local void fraction. The numerical results were compared with the experimental ones obtained with a device conceived for this objective. This comparison has shown a good agreement. Then, the model was used as a design tool for an optimal refrigerating system with a minimum TEWI. The optimization mainly concerned the small channel condenser geometry. The results have shown an absence of limit to the channel diameter reduction, in term of TEWI: an increase in the total tube number makes it possible to limit the system pressure drop. Moreover, the global model permits to determine the optimum tube distribution for a configuration of a condenser with two passes: this optimum is about 80% in the first pass and 20% in the second one.

Commentary by Dr. Valentin Fuster
2004;():617-624. doi:10.1115/ICMM2004-2389.

Preliminary experimental results of measuring velocity fields of a transparent liquid flow in a closed circuit, through a 100 μm deep flat cell with heat exchanger microchannel elements are presented. The resolution and possible errors of the microscopic particle image velocimetry system are discussed in relation with the evaluation results. Particle fouling phenomenon, which proved to be the main difficulty in performing velocity field measurements in microchannels in the past, are widely overcome by techniques which avoid or limit it. The test object, which is aimed at being exposed to real technical conditions (pressures up to 0.6 MPa leading to flow velocities up to 15 m/s, as well as temperatures up to 100°C), was up to now operated at a Reynolds number of about 5. The obtained information allows for starting the test loop upgrade.

Commentary by Dr. Valentin Fuster
2004;():625-632. doi:10.1115/ICMM2004-2390.

In this paper the experimental heat transfer coefficients measured during condensation of R134a and R410A inside multiport minichannels are presented. The need for experimental research on condensation inside multiport minichannels comes from the wide use of those channels in automotive air-conditioners. The perspective for the adoption of similar channels in the residential air conditioning applications also calls for experimental research on new high pressure refrigerants, such as R410A. Heat transfer data are compared against models to show the accuracy of the models in the prediction of heat transfer coefficients inside minichannels.

Commentary by Dr. Valentin Fuster
2004;():633-640. doi:10.1115/ICMM2004-2391.

By using unique experimental techniques and careful construction of the experimental apparatus, the characteristics of the local heat transfer were investigated using the condensing R134a two-phase flow, in horizontal single mini-channels. The circular channels (Dh = 0.493, 0.691, and 1.067 mm) and rectangular channels (Dh = 0.494, 0.658, and 0.972 mm) were tested and compared. Tests were performed for a mass flux of 100, 200, 400, and 600 kg/m2 s, a heat flux of 5 to 20 kW/m2 , and a saturation temperature of 40°C. In this study, effect of heat flux, mass flux, vapor qualities, hydraulic diameter, and channel geometry on flow condensation were investigated and the experimental local condensation heat transfer coefficients are shown. The experimental data of condensation Nusselt number are compared with existing correlations.

Commentary by Dr. Valentin Fuster
2004;():641-648. doi:10.1115/ICMM2004-2392.

A mathematical model for vapor condensation in cylindrical longitudinally finned minichannel has been proposed taking into account all temperature non-uniformity. The model describes annular and rivulet flow regimes until complete grooves flooding. It is shown that when the model does not account for the wall heat conductivity the maximal calculated enhancement of heat transfer is predicted for the “sharp” trapezoidal fins. The most enhanced heat transfer in the case of non-isothermal fins has been obtained for the curvilinear fins of expanded Adamek’s parametric family. When reducing the heat conductivity the curvilinear fins become still more effective in comparison with the trapezoidal fins. The wall non-isothermity is a factor, which cannot be neglected when modeling condensation in a minichannels with finned surfaces.

Commentary by Dr. Valentin Fuster
2004;():649-656. doi:10.1115/ICMM2004-2393.

This paper presents a multiple flow-regime model for pressure drop during condensation of refrigerant R134a in horizontal microchannels. Two-phase pressure drops were measured in five circular channels ranging in hydraulic diameter from 0.5 mm to 4.91 mm. For each tube under consideration, pressure drop measurements were first taken over the entire range of qualities from 100% vapor to 100% liquid for five different refrigerant mass fluxes between 150 kg/m2 -s and 750 kg/m2 -s. Results from previous work by the author on condensation flow mechanisms in microchannel geometries were used to assign the applicable flow regime to the data points. Pressure drop models for intermittent (Garimella et al. 2002) and annular (Garimella et al. 2003a) flow reported earlier by the authors were modified and combined to develop a comprehensive model that addresses the entire progression of the condensation process from the vapor phase to the liquid phase. This combined model accurately predicts condensation pressure drops in the annular, disperse wave, mist, discrete wave, and intermittent flow regimes. Overlap and transition regions between the respective regimes are also addressed using an appropriate interpolation technique that results in relatively smooth transitions between the predicted pressure drops. The resulting model predicts 82% of the data within ±20%.

Commentary by Dr. Valentin Fuster
2004;():657-660. doi:10.1115/ICMM2004-2394.

A simultaneous visualization and measurement experiment was carried out to investigate condensation flow patterns of steam flowing through an array of parallel microchannels with a hydraulic diameter of 82.8 μm and a length of 30mm in a <100> silicon wafer. These microchannels were covered with a thin transparent pyrex glass from the top that enabled the visualization of flow patterns in the test section. The degassed and deionized water steam flowing in the microchannels was cooled by cooling water of 8°C at the bottom of the wafer. Experiments were performed for different inlet pressures while the outlet pressure was maintained at a value of 105 Pa (the atmospheric pressure). When the inlet pressure was decreased to the value of 1.45×105 Pa and the corresponding mass flux was decreased to 23.6g/cm2 s, a succession of droplet/injection/slug-bubble flow was observed in the microchannels. Under this condition, the upstream, midstream, and downstream of the microchannels were occupied by the droplet flow, injection flow, and slug-bubble flow, respectively. This concurring droplet/injection/slug-bubble flow appeared periodically in the microchannels that caused large fluctuations of wall temperatures and fluid temperatures with respect to time. The droplet/injection/slug-bubble flow pattern in microchannels has never been reported in the literature.

Commentary by Dr. Valentin Fuster
2004;():661-666. doi:10.1115/ICMM2004-2395.

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

Commentary by Dr. Valentin Fuster
2004;():667-672. doi:10.1115/ICMM2004-2396.

This work presents an active Micro Heat Spreader (MHS) for use in electronic cooling. The device includes four reservoir chambers and is actuated using membrane deflection. An investigation is performed using Finite Element Analysis to determine the performance characteristics of the MHS. Two operating conditions for the actuation order of the membranes are considered. Dimensional similitude is used to obtain a single maximum temperature versus Reynolds number curve for the device at the given operating condition and Prandtl number. Fabrication of the MHS is proposed using the Silicon-on-Insulator (SOI) technique and actuation of the membranes can be achieved using thin film PZT.

Commentary by Dr. Valentin Fuster
2004;():673-677. doi:10.1115/ICMM2004-2397.

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

Commentary by Dr. Valentin Fuster
2004;():679-685. doi:10.1115/ICMM2004-2398.

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

Commentary by Dr. Valentin Fuster
2004;():687-694. doi:10.1115/ICMM2004-2399.

The conjugated two-dimensional model, based on long-wave theory, of a steady laminar flow of liquid film and co-current gas flow in plane channel with the height varied from 150 to 500 μm is performed. A chip with the several millimeters length is located on the bottom wall of channel. The linearised approximation of the problem is obtained analytically. Numerical calculations are executed for liquid FC-72 and Nitrogen gas flow. In contrast to a case of a large channel, there is essential an influence of liquid film deformations on pressure and velocity in a gas phase.

Commentary by Dr. Valentin Fuster
2004;():695-700. doi:10.1115/ICMM2004-2400.

Channel networks designed with constructal theory are compared. The efficiency of the networks when used for cooling a uniformly heated surface is compared. Three networks are compared and it is found that the two constructal designs with two and three constructal levels have similar performance. It is shown that for a given pumping power, the constructal designs give a heat transfer coefficient of the surface which is almost a factor of magnitude higher than the one obtained for a parallel channel system.

Topics: Cooling , Networks , Heating
Commentary by Dr. Valentin Fuster
2004;():701-708. doi:10.1115/ICMM2004-2401.

Recently, drop on demand inkjet printers have been used to deposit ceramic containing inks to develop ceramic components for several strategic applications (for sensors, fuel cells and for intelligent inks to be used as self assembling particles to interact with incident wave forms). It seems that the availability of literature with respect to the studies on fluid-structure interaction in a drop on demand inkjet printer is limited, though enough information is available on the preparation of ceramic inks. The design of nozzle for drop on demand inkjet printing involves transient interaction between fluids and structures to eject ink droplets. Study of phenomena that contribute to the droplet formation, ejection and deposition on a substrate for several combination of physical properties of constituents of the ink and the characteristics of actuation mechanism is relevant for understanding and effective utilization of direct ceramic inkjet printing (DCIJP). This paper focuses on the simulation of formation and ejection of a ceramic ink droplet (paraffin wax loaded with different volume fraction of alumina particles) from a reservoir using piezoelectric actuation. The properties of ceramic ink are found in literature and they are used for simulation. Simulations were performed with computational fluid dynamics software (CFD-ACE+) which can solve multi-physiscs problems as encountered in DCIJP. This study gives details of the tight interaction among different physical phenomena that contribute to he droplet formation and ejection process. The results from this study will be useful for the preparation of ceramic inks to achieve desired droplet characteristics.

Commentary by Dr. Valentin Fuster
2004;():709-715. doi:10.1115/ICMM2004-2402.

A numerical study is conducted to examine the flow characteristics of the inkjet print-head with special attentions on the refilling process. By solving the full set of three-dimensional transient Navier-Stokes equations and considering the process of bubble growth and collapse as a movable membrane, it is found that the double refilling channels can reduce the flow surge phenomenon considerably due to the imposed friction. However, for the additional cylinder obstacle placed at the filling channel, the flow surge phenomenon is still present. This is because of the jet-like flow along the cylinder leading to a collision and eruption of fluid angled towards the plane boundary with the presence of cylinder. The calculated results also indicated the flow surge can be moderately suppressed for fluid having larger dynamic viscosity.

Commentary by Dr. Valentin Fuster
2004;():717-721. doi:10.1115/ICMM2004-2403.

A new mechanism for generating a micron-sized bubble jet in an electrolyte within a micro-fluidic device is reported. A three-dimensional electrode tip is placed at the center of a micro-channel. All but the electrode tip is insulated with a dielectric film. A counter insulated electrode sits across the tip electrode and a large high-frequency (> 100 kHz) and high intensity (>1000 V) AC field is applied across the electrodes. The large voltage generates micron-sized bubbles at the tip which snaps off intermittently or continuously to produce a micro-bubble jet by a unique tip streaming mechanism. The ejected bubbles continue to be driven by an electrostatic force in the tip direction and move at a velocity in excess of 10 cm/sec. They drag a large surrounding liquid column around them to move at the same velocity and hence generate a pumping action stronger than all reported electro-kinetic micro-pumps.

Commentary by Dr. Valentin Fuster
2004;():723-730. doi:10.1115/ICMM2004-2404.

A new high-frequency (> 10 kHz) AC electrospray phenomenon which behaves distinctly to DC electrosprays is investigated. Unlike DC electrosprays, the drops do not emanate from the usual well-defined Taylor cone-jet and carry no net charge. Instead, the meniscus vibrates at a resonant frequency associated with its capillary-inertia vibration time, periodically ejecting drops with dimensions of order 10 μm under the action of the Maxwell-Wagner electric stress at the meniscus tip. Above a crossover frequency, the polarization and direction of the Maxwell force reverses and an apparent electrowetting effect is observed. A simple lubrication model is used to generate spatio-temporal evolution profiles for the stretched liquid meniscus tip as a function of the Maxwell-Wagner stress and a capillary number. From a self-similar scaling technique, the meniscus is shown to advance as t1/2 .

Commentary by Dr. Valentin Fuster
2004;():731-738. doi:10.1115/ICMM2004-2405.

Performance of MEMS-based nozzles at moderate and low temperatures is numerically analyzed using the direct simulation Monte Carlo method. Considering the intermolecular attractive potential due to low temperature, the generalized soft sphere collision model is introduced. The Larsen-Borgnakke model for the generalized sphere model is used to model the energy exchange between the translational and internal modes. The results for nozzle flows at an initial temperature of 300 K show that the temperature behind the throat is quite low and the intermolecular attractive potential cannot be ignored. Different working conditions in two-dimensional nozzles are simulated using the present method, including exit pressure, inlet pressure, initial temperature, nozzle geometry, and gas species. The effect factors on the nozzle performance are analyzed. A 3D nozzle flow simulation shows the increased surface-to-volume ratio which leads to high viscosity dissipation cause a much lower flow characteristic and performance comparing with the 2D case.

Commentary by Dr. Valentin Fuster
2004;():739-743. doi:10.1115/ICMM2004-2406.

The knowledge of the fundamental aspects of hydrodynamics at microscales is an exciting challenge. Some authors have published conflicting results concerning the friction and the thermal exchange coefficients, the transition to a turbulent flow regime (Qu et al. 2000, Mala and Di 1999, Papautsky et al. 1999). Some explanations, based on surface effects, have been proposed, but microeffects, if they are, are probably hidden by experimental artefacts. We aim at performing local measurements of pressure drops in monophasic microstreams. Precedent works (Baviere et al. 2003) have shown that a great care has to be taken with the intrepretation of anomalous or unexpected results, and that the metrological set up of these experiments is crucial. We have performed and tested cupro-nickel strain gauges micromachined on different sorts of silicon nitride membranes. The design of the gauges obeys an electrical Wheatstone bridge configuration. The experimental signals are in good agreement with the expected electromechanical response of the bridge. The sensitivity ranges from 50 to 100 μV/V/bar with a thermal drift below 0.011%.°C−1 . Such sensors have been integrated along smooth and rough silicon microchannels with hydraulic diameter of 15 μm, and no deviation from the laminar regime has been observed with such local pressure sensors. The micromachining of these devices is described and the first local pressure drops measurements performed with deionized water of low electrical resistivity are presented and discussed.

Commentary by Dr. Valentin Fuster
2004;():745-752. doi:10.1115/ICMM2004-2407.

The viscous micropump consists of a cylinder placed eccentrically inside a microchannel, where the rotor axis is perpendicular to the channel axis. When the cylinder rotates, a net force is transferred to the fluid due to the unequal shear stresses on the upper and lower surfaces of the rotor. Consequently, this causes the surrounding fluid in the channel to displace towards the microchannel outlet. The simplicity of the viscous micropump renders it ideal for micro pumping, however, previous studies have shown that its performance is still less than what is required for various applications. The performance of the viscous micropump, in terms of flow rate, pressure head and efficiency, may be enhanced by implementing more than one rotor into the configuration. The present study will numerically investigate the performance of various configurations of the viscous micropumps with multiple rotors, namely the dual-horizontal rotor, the triple-horizontal rotor, the symmetrical-dual-vertical rotor, and the 8-shaped dual-vertical rotor. The development of drag force with time, as well as the viscous resisting torque on the cylinders were studied. In addition, the corresponding drag and moment coefficients were calculated. Results show that the symmetrical-dual-vertical rotor configuration yields the best efficiency, and generates the highest flow rate. The steady state performance of the single-stage micropump was compared with the available experimental and numerical data, and was found to be in very good agreement. This work provides a foundation for future research on the subject of fluid phenomena in viscous micropumps.

Topics: Micropumps
Commentary by Dr. Valentin Fuster
2004;():753-760. doi:10.1115/ICMM2004-2408.

Numerical simulations based on first principle equations is used to study the crosstalk phenomena and elimination mechanism for the parallel microchannel systems as microarray. The cases of this study focus on the surface tension driven flow include inkjet printing system, stamping by stamper protein microarray with embedded microchannels and the filling of H-shaped parallel microchannels. The parallel arrangements of microfluid components in terms of configuration as well as the parallel operation mode are the key issues to ensure the least crosstalk.

Commentary by Dr. Valentin Fuster
2004;():761-768. doi:10.1115/ICMM2004-2409.

Miniaturisation of modern electronics means that future compact electronic systems are likely to be too hot to be held in the users hand. Simultaneous increases in heat dissipation will also require the development of novel compact cooling technologies. In systems such as mobile phones and palmtop computers, macro scale fans cannot be used to overcome this problem, as they are too large. As a solution, the implementation of micro fan technology is proposed. Previous investigators have shown that reduction of the Reynolds number of turbomachinery results in reduced efficiency. To experimentally investigate this predicted phenomenon, a series of geometrically similar axial flow fans have been fabricated. These range in size from the macro to the micro scale with the Reynolds numbers varying linearly with fan dimensions. Through detailed Particle Image Velocimetry (PIV) measurements and pressure flow characterization of these fans, this investigation aims to quantify the reduction in efficiency, which occurs as the Reynolds number is reduced. This paper concludes that the extent to which fan efficiency is reduced by Reynolds number is in surprisingly good agreement with relatively simple predictions developed by the authors in previous investigations. Reduced Reynolds number was also seen to alter the velocity distribution at the fan outlet. This is an important point as it indicates a change in the physics of the flow with reducing scale.

Topics: Reynolds number
Commentary by Dr. Valentin Fuster
2004;():769-772. doi:10.1115/ICMM2004-2410.

An experimental model of Joule-Thomson micro-cooler has been fabricated on silicone wafer of 39mm×15.5mm in size. The micro-cooler uses ethylene as the refrigerant and it works in the pressure range between 2.3MPa and 0.1MPa. The micro-cooler consist of condenser, evaporator and capillary tube. Flow passage of the refrigerant was fabricated by etching process. Peltier device was used to pre-cool and condense the refrigerant. Temperature decrease by 3K was obtained at maximum flow rate of 3mg/s.

Topics: Joules
Commentary by Dr. Valentin Fuster
2004;():773-779. doi:10.1115/ICMM2004-2411.

Flow in microchannels is usually slow and the mixing of several fluids is poor if only relying on diffusion. A microchannel mixer with a simple design that is capable of rapid mixing is important for practical applications such as biochemistry analysis. In this study, a general numerical approach to analyze microchannel flow is proposed. The method is based on numerical particle tracking and computer graphics techniques. As a case study, an electro-osmotic driven microchannel mixer is considered with four geometric configurations. The microchannel has a repeated oblique-angled stripe pattern of zeta potential coatings on its floor. The comparison of the particle residence time distributions gives some indication of the mixers’ performances. The efficiency of mixing two flow streams at Péclet numbers 2 × 105 and 2 × 104 is then evaluated using the developed procedure. The results indicate that the stripe length ratio is one of the parameters that can be optimized for better mixing. Of the three stripe length ratios evaluated, the configuration with the stripe length ratio 2.0 is found to be the best. In addition, this case study demonstrates that the developed numerical technique is suitable for expeditious parametric optimization of mixer design.

Topics: Fluids , Microchannels
Commentary by Dr. Valentin Fuster
2004;():781-788. doi:10.1115/ICMM2004-2412.

This work shows the application of convective fluid flow caused by flow-induced secondary vortices to fluidic single-phase micro mixers. As an example we used simple static T-shaped micro mixers. The convective flow was observed both by simulations and by experiments and is suitable for enhancing the mixing quality. Concerning micro reactors, it is necessary that the mixing is faster than the chemical reaction to be induced so that the creation of unwanted side products is minimized. The mixing model by Bourne is slightly modified for continuous flow reactors and applied to our mixers. Using this model, timescales for the mixing in our micro mixers are calculated. A first test reaction — the iodide-iodate reaction by Villermaux and Dushman — to check the validity of the timescales is outlined. These overall results will help to achieve a deeper understanding of micro reactors.

Commentary by Dr. Valentin Fuster
2004;():789-793. doi:10.1115/ICMM2004-2413.

Microreactors and narrow channel reactors have found an increasing number of applications in the last few years for their enhanced heat and mass transfer properties if compared to traditional process equipment. In this investigation, mixing efficiency in a narrow channel reactor system has been studied by using the iodide-iodate scheme of parallel competing reactions that leads to the formation of iodine. The tested system is constituted by two reactors machined in Perspex. The two channels have identical configuration and a square cross section with diagonal lines of 1·10−3 m and 2·10−3 m respectively. Influence of flow rate on the selectivity towards iodine has been studied for both reactors. This allows the characterization of mixing intensity at varying operating conditions. The results obtained reflect the expected influence of flow rate and channel characteristic dimension on mixing efficiency. This investigation has been carried out on the same reactor system that had been previously used for studying the precipitation of calcium carbonate from solutions of sodium carbonate and calcium nitrate. In fact, a study on mixing efficiency is particularly useful in the case of precipitation reactions as poor mixing can lead to a final product that does not respect marketing requirements in terms of particle size and particle size distribution. The information acquired in the two investigations can constitute the basis for the design of modules based on narrow channel technology for the production of powders and slurries with controlled properties.

Commentary by Dr. Valentin Fuster
2004;():795-801. doi:10.1115/ICMM2004-2414.

A new electrolyte AC micro-pump and mixer design based on Faradaic reaction polarization is investigated. The non-uniform AC electric field generated by electrode reaction polarizes the electrode surface, exerts a net electric force on the polarized layer and drives a net AC electro-osmotic flow of the electrolyte on the planar microelectrodes. At large electric potentials (>10V) and frequencies of 100 kHz to 1MHz, the flow direction is shown to be opposite to (previously reported) AC pumps due to capacitive charging. Moreover, the velocity has an exponential dependence on the applied electric potential due to the reaction and is hence an order of magnitude higher than the velocity driven by capacitive charging, which has a quadratic dependence. Theoretical analyses demonstrating the possibility of this Faradaic mechanism and some preliminary experimental results are presented.

Commentary by Dr. Valentin Fuster
2004;():803-807. doi:10.1115/ICMM2004-2415.

In this study a newly fabricated micromixer is proposed. This design comprises periodically arranged simple blocks. In this configuration, the stirring is greatly enhanced at a certain parameter set. This device is fabricated by rapid prototyping technology, stereolithography method, so that we can reduce the R&D time and cost. To characterize the flow field and the stirring effect both the numerical and experimental methods were employed. To obtain the material deformation, three-dimensional numerical computation to the Navier Stokes equations are performed by using a commercial code, FLUENT 6.0. Numerical results show that materials are deformed by the counter clockwise spiral motion of the secondary flows. In the experiment, flow visualization for the stirring effect is performed by using pure water in one reservoir and water mixed with a fluorescent dye in the other, so that we can see the flow motion inside the microchannel. The numerical and experiment results show that the stirring is significantly enhanced at larger block-height. We assert that we can apply the rapid-prototyping technology in the micro fabrication.

Commentary by Dr. Valentin Fuster
2004;():809-815. doi:10.1115/ICMM2004-2416.

A number of papers have been published on the computational approaches to electrokinetic flows. Nearly all of these decoupled approaches rely on the assumption of the Poisson-Boltzmann equation and do not consider the effect of velocity field on the electric double layers. By means of a charge continuity equation, we present here a numerical model for the simulation of pressure driven flow with electrokinetic effects in parallel-plate microchannels. Our approach is similar to that given by van Theemsche et al. [Anal. Chem., 74, 4919 (2002)] except that we assumed liquid conductivity to be constant and allows simulation to be performed in experimental dimension. The numerical simulation requires the solution of the Poisson equation, charge continuity equation and the incompressible Navier-Stokes equations. The simulation is implemented in a finite-volume based Matlab code. To validate the model, we measured the electrical potential downstream along the channel surface. The simulated results were also compared with known analytical solutions and experimental data. Results indicate that the linear potential distribution assumption in the streaming direction is in general not valid, especially when the flow rate is large for the specific channel geometry. The good agreement between numerical simulation and experimental data suggests that the present model can be employed to predict pressure-driven flow in microchannels.

Commentary by Dr. Valentin Fuster
2004;():817-823. doi:10.1115/ICMM2004-2417.

Electroosmotic flow with Joule heating effects was examined numerically and experimentally in a micro capillary. Fluorescence-based thermometry and velocimetry techniques were employed to visualize the liquid temperature and the electroosmotic velocity profile, respectively. Sharp temperature drops close to the two ends and a high-temperature plateau in the middle of the capillary were observed. Correspondingly, concave-convex-concave velocity profiles were observed in the inlet-middle-outlet regions of the homogeneous capillary. The measured liquid temperature distributions and electroosmotic velocity profiles along the capillary agree well with the predictions of a theoretical model developed in this paper.

Commentary by Dr. Valentin Fuster
2004;():825-832. doi:10.1115/ICMM2004-2418.

Surface roughness has been considered as a passive means of enhancing the species mixing in electroosmotic flow through microfluidic systems. It is highly desirable to understand the synergetic effect of the 3D roughness and the surface heterogeneity on the electrokinetic flow through microchannels. In this study, we developed a three-dimensional, finite-volume-based numerical model to simulate electroosmotic transport in a slit microchannel (formed between two parallel plates) with numerous heterogeneous prismatic roughness elements arranged symmetrically and asymmetrically on the microchannel walls. The results showed that, the rough channel’s geometry and the electroosmotic mobility ratio of the roughness elements’ surface to that of the substrate, εμ , have dramatic influence on the induced pressure field, the electroosmotic flow patterns and the electroosmotic flow rate in the heterogeneous rough microchannels. The associated sample species transport in the heterogeneous rough microchannels presents tidal-wave-like concentration field at the intersection between four neighboring rough elements when under low εμ values, and presents the concentration field similar to that of the smooth channels when under high εμ values.

Commentary by Dr. Valentin Fuster
2004;():833-840. doi:10.1115/ICMM2004-2419.

This paper considers the electrophoretic motion of multiple spheres in an aqueous electrolyte solution in a straight rectangular microchannel, where the size of the channel is close to that of the particles. This is a complicated 3-D transient process where the electric field, the flow field and the particle motion are coupled together. The objective is to numerically investigate how one particle influences the electric field and the flow field surrounding the other particle and the particle moving velocity. It is also aimed to investigate and demonstrate that the effects of particle size and electrokinetic properties on particle moving velocity. Under the assumption of thin electrical double layers, the electroosmotic flow velocity is used to describe the flow in the inner region. The model governing the electric field and the flow field in the outer region and the particle motion is developed. A direct numerical simulation method using the finite element method is adopted to solve the model. The numerical results show that the presence of one particle influences the electric field and the flow field adjacent to the other particle and the particle motion, and that this influences weaken when the separation distance becomes bigger. The particle motion is dependent on its size, with the smaller particle moving a little faster. In addition, the zeta potential of particle has an effective influence on the particle motion. For a faster particle moving from behind a slower one, numerical results show that the faster moving particle will climb and then pass the slower moving particle then two particles’ centers are not located on a line parallel to the electric field.

Commentary by Dr. Valentin Fuster
2004;():841-848. doi:10.1115/ICMM2004-2420.

The electroosmotic flow in a microchannel packed with microspheres under both direct and alternating electric fields is analyzed. In the case of the steady DC electroosmosis in a packed microchannel, the so-called “capillary model” is used, in which it is assumed that a porous medium is equivalent to a series of intertwined tubules. The interstitial tubular velocity is obtained by analytically solving the Navier-Stokes equation and the complete Poisson-Boltzmann equation. Then using the volume averaging method, the solution for the electroosmotic flow in a single charged cylindrical tubule is applied to estimate the electroosmosis in the overall porous media by introducing the porosity and tortuosity. Assuming uniform porosity, an exact solution accounting for the electrokinetic wall effect is obtained by solving the modified Brinkman momentum equation. For the electroosmotic flow under alternating electric fields in a cylindrical microchannel packed with microspheres of uniform size, two different conditions regarding the openness of channel ends are considered. Based on the capillary model, the time-periodic oscillating electroosmotic flow in an open-end microchannel in response to the application of an alternating electric field is obtained using the Green’s function approach to the Navier-Stokes equation. When the two ends of the channel are closed, a backpressure is induced to generate a counter flow, resulting in a new zero flow rate. Such induced backpressure associated with the counter-flow in a closed-end microchannel is obtained analytically by solving the transient modified Brinkman momentum equation.

Commentary by Dr. Valentin Fuster
2004;():849-856. doi:10.1115/ICMM2004-2421.

The effect of the electric double layer (EDL) on the bypass transition mechanism in the linear evolution stage is explored through direct numerical simulations. An initial perturbation velocity field consisting of a pair of counterrotating vortices is introduced in Poiseuille and EDL flows and the time-space evolution of the perturbed field is analysed for short times at half the critical Reynolds numbers (3000 for Poiseuille and 150 for EDL). The wall normal and spanwise perturbation velocities development are both quantitatively and qualitatively similar in macro and micro flows. The streamwise velocity, which is initially zero and set up by the generation of the wall normal vorticity is twice larger under the EDL effect. Both flows develop inclined strong streamwise shear layers. Overall is the close similarity of the disturbance evolution showing that the three dimensional linear mechanism in EDL flow lead to the structures that are at least as strong as in Poiseuille flow.

Commentary by Dr. Valentin Fuster
2004;():857-864. doi:10.1115/ICMM2004-2422.

A common application in microfluidic devices is on-chip capillary electrophoresis (CE). In this process, sample species are transported by electroosmotic flow and separated based on their electrophoretic mobilities. Separated analytes are typically detected using laser-induced fluorescence. It has been found that the sample shape and size, which is critical to the later detection processes or the quality of other analytical techniques, depends on many parameters, such as the sample diffusion coefficient, the applied voltages, and the electrical conductivity difference between sample and buffer. The conductivity difference can alter the electric field strength, which is the driving force behind both the electroosmotic bulk flow and the electrophoretic velocity of individual species. Therefore, the manipulation technique is required to consider the transport processes with conductivity differences. A numerical model presented in this paper is used to simulate the sample transport process with the consideration of conductivity gradient in order to develop the sample manipulation techniques. There are two situations studied here, which are sample pumping (where bulk transport is increased and analyte separation is delayed using a relatively high conductivity sample), and sample stacking (where bulk transport is decreased and analyte separation is expedited using a relatively low conductivity sample). The effects of applied electrical potential, sample diffusion coefficient and the extent of conductivity difference on the sample control are investigated through the developed model. The simulation results show that the sample transport with the consideration of conductivity gradient differs significantly from that of uniform conductivity case.

Commentary by Dr. Valentin Fuster
2004;():865-868. doi:10.1115/ICMM2004-2423.

Microfludic devices with integrated electrical detection will enable fast, low-cost or portable sensing and processing of biological and chemical samples. As an inherent property of microfabrication, micro-electrical impedance spectroscopy detectors can take advantage of AC electrokinetics for particle manipulation, leading to enhanced sensitivity. Preliminary experiments on particle detection were carried out using microelectrode pairs, and impedance spectra are compared with respect to opposite effects of dielectrophoresis and electrode polarization. The values of cell equivalent circuit are extracted for electrode optimization.

Commentary by Dr. Valentin Fuster
2004;():869-874. doi:10.1115/ICMM2004-2424.

Enhancing species mixing in microfluidic applications is key to reducing analysis time and increasing device portability. Mixing in electroosmotic flows is usually diffusion-dominated requiring extended channel lengths and retention times. Recent numerical studies have indicated that the introduction of electrically charged surface heterogeneities may augment mixing efficiencies by creating localized regions of flow circulation. In this study, we experimentally visualize the effects of surface charge patterning and develop an optimized electrokinetic micromixer applicable to the low Reynolds number regime. Using the optimized micro-mixer, mixing efficiencies were improved between 22% and 68% for applied potentials ranging from 70–555 V/cm when compared with the homogeneous case. For 95% mixture, this equates to a potential decrease in required mixing channel length of up to 88% for flows with Péclet numbers between 190 and 1500.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2004;():875-881. doi:10.1115/ICMM2004-2425.

The mechanisms of coupled heat transfer and flow are modeled to describe the looped pulsating heat pipe of high heat flux. The latent heat transfer produces the pressure difference between the heating section and cooling section. This can provide the operational driving force to overcome the total flow resistances. While the sensible heat transfer contributes more to the transferred power. The results demonstrate that the circulation flow velocity can balance the heat and mass transfers automatically. And the ratio of latent heat transfer to sensible heat transfer is within 30 percent.

Commentary by Dr. Valentin Fuster
2004;():883-890. doi:10.1115/ICMM2004-2426.

The fluid flow and heat transfer characteristics of micro heat pipes are analyzed theoretically, in order to understand the physical phenomena and quantify the influence of various parameters on overall thermal performance of these devices. A one-dimensional model is utilized to solve the governing equations for the liquid/vapor flow and the heat transfer in the heat pipe channel. Variations in the liquid and vapor cross-sectional areas along the axial length of the heat pipe are included and the equations are solved using an implicit finite difference scheme. Appropriate models for fluid friction in small passages with varying cross-sectional areas have been incorporated to yield the axial distribution of the meniscus radius of curvature and the velocity, temperature and pressure in both the liquid and the vapor phases. Using this information, the effective thermal conductivity of the micro heat pipe is modeled, and parametric studies are performed by changing the heat load and cooling rate. The results of the analysis are discussed and compared with other theoretical models and experimental results found in the literature. By so doing, this analysis provides greater insight into the physical phenomena of flow and heat transfer in micro heat pipes and identifies a methodology for optimizing the design of these devices.

Commentary by Dr. Valentin Fuster
2004;():891-897. doi:10.1115/ICMM2004-2427.

Micro heat pipes (MHPs) are, essentially, miniature heat transfer devices which use phase change to transfer thermal energy. In recent years, there have been numerous proposals for their applications in cooling electronic devices. In this paper, the axial liquid distribution of a triangular MHP is investigated for the case of inclined orientation. The study is limited to the case of positive inclination, whereby the condenser section is elevated from horizontal position. In this case, the inclusion of gravity renders the governing equation unsolvable analytically, and the 4th order Runge-Kutta method has been selected to solve it numerically. The results show that for a horizontally oriented MHP, so that the effect of gravity can be neglected, the liquid distribution along the axial direction increases monotonically from the minimum value at the evaporator end to the maximum at the condenser end. However, if the MHP is positively inclined, the axial distribution of the liquid phase is changed qualitatively. While the liquid distribution still increases monotonically starting from the evaporator end, it reaches its maximum value not at the condenser end but at a certain point in the condenser section, beyond which the liquid distribution decreases monotonically. Moreover, as the angle of inclination is increased, the maximum-distribution point moves further away from the condenser end. This maximum point, where potentially flooding will first take place, results from the balance between the effects of gravity and the heat load on the MHP, the former having the propensity to move all the liquid from the condenser towards the evaporator while the latter the tendency to place more liquid in the condenser section. As the liquid distribution assumes its greatest value at the maximum point, a throat like formation appears there. This formation is detrimental to the performance of an MHP, because it hinders, and at worst may block, the axial flow of the vapor phase.

Topics: Heat pipes
Commentary by Dr. Valentin Fuster
2004;():899-903. doi:10.1115/ICMM2004-2428.

We develop a mathematical model for heat transfer and fluid flow near a contact line on a heated surface in the presence of thermocapillary flow and evaporation. The coupled heat transfer and flow problem is reduced to an equation for local thickness, which is then solved numerically. The steady-state results indicate that thermocapillary stresses act to reduce the rate of liquid flow towards the contact line and increase interfacial curvature there. We also discuss solutions than involve moving contact lines, applicable to studies of start-up and shut-down operations of heat pipes. The velocity of the contact line and the apparent contact angle are found as functions of the Marangoni number. Thermocapillary effect is shown to reduce contact line speed and increase the apparent contact angle. Finally, the local solution is incorporated into global solutions for curvature variations of an evaporating three-dimensional meniscus in a corner. This configuration is typically encountered in proposed designs of micro heat pipes. Interface curvature is found as a function of the axial coordinate for the case of linear axial temperature variation in the corner.

Commentary by Dr. Valentin Fuster
2004;():905-910. doi:10.1115/ICMM2004-2429.

Based on the principle of electric dipole radiation and the Planck’s spectral distribution of emissive power, the enhancement of thermal radiation between two planar semi-infinite media or two nano-spheres was studied in this paper by the Monte Carlo method. By this simple method, some parameter’s influence on the radiative heat transfer was investigated, such as the distance between two semi-infinite media, the particle’s radius, the distance between two particles and the difference in temperature between two particles, and so on. This solution is not rigorous but simple. The results show that heat transfer can be enhanced by several orders of magnitude for the near field effect. And the radiative heat transfer is decreasing sharply with the increasing of the distance.

Commentary by Dr. Valentin Fuster
2004;():911-916. doi:10.1115/ICMM2004-2430.

When blood suspension penetrates a capillary radius by wetting, the advancing meniscus decelerates rapidly when the blood cell volume fraction is above a certain critical concentration. Below the critical concentration, blood suspension behaves like a homogeneous liquid and the wetted length increases as the 0.5 power of time. We attribute the former deceleration dynamics to a unique packing mechanism behind the meniscus that is driven by radial migration of the deformable blood cells. Unlike rigid particle suspensions, a concentrated slug develops behind the meniscus of blood suspension and its concentration increases linearly with respect to the meniscus position downstream due to this packing mechanism. As the suspension viscosity blow up with a −2 power with respect to blood concentration φ at maximum packing, viscous dissipation at the slug quickly controls the meniscus speed if the slug length is comparable to the total wetted length, thus significantly delaying the meniscus penetration dynamics. The critical concentration is measured empirically and shown to be a linear function of the capillary radius R with a simple scaling theory. For 40% whole blood, penetration rate is too slow, in the order of μm /s at 2cm from the entrance, to be widely used in sample loading for miniature diagnostic kits with diameter less than 26 micron.

Commentary by Dr. Valentin Fuster
2004;():917-923. doi:10.1115/ICMM2004-2431.

Biosensors and more specifically biochips exploit the interactions between a target analyte and an immobilized biological recognition element to produce a measurable signal. Systems based on surface phase nucleic acid hybridization, such as modern microarrays, are particularly attractive due to the high degree of selectivity in the binding interactions. One drawback of this reaction is the relatively long time required for complete hybridization to occur, as a result of the diffusion limited reaction kinetics. In this work an electrokinetically controlled DNA hybridization microfluidic chip will be introduced. The electrokinetic delivery technique provides the ability to dispense controlled sample sizes to the hybridization array while serving to increase the mass transfer rate and therefore the reaction speed. The focus of this paper will be on the design and microfabrication of the chip, the unique H-type channel structure and electrokinetic sample delivery and washing technique, and development of the on-line hybridization scanning. Initial hybridization results presented here demonstrate that less than 5 minutes and 4.9nL of 0.5μM ssDNA sample was required (35s dispensing period followed by a 4 minute wash) for complete hybridization.

Topics: DNA
Commentary by Dr. Valentin Fuster
2004;():925-930. doi:10.1115/ICMM2004-2432.

A novel velocity measurement method for microscale flow field characterization is reported, particle linear image velocimetry (PLIV). The method records a series of one-dimensional images that represent the trace of particles in the flow across a one-dimensional imager. Linear imaging results in a faster frame rate than planar imaging, allowing observation of larger microscope magnification or measurement of faster flow rates in real-time than comparable techniques. In contrast to particle image velocimetry (PIV), PLIV does not require high-speed cameras or shutters. Furthermore, PLIV is adaptable to multiple linear imager formats and, as one example, can use laser scanning confocal microscopes (LSCM) that acquire images slowly but with high spatial resolutions and optical sectioning ability. Higher resolution can be obtained for flows where in-plane velocity gradient in the direction of the optical path (z-direction) is important. This paper presents the PLIV algorithm, and demonstrates its utility by measuring Poiseuille flow with 1-μm resolution in a microfluidic environment.

Commentary by Dr. Valentin Fuster
2004;():931-938. doi:10.1115/ICMM2004-2433.

The micro pumping technology is one of the major and growing research fields in microfluidics. The use an electric voltage to induce electro-kinetic fluid flow is an efficient and reliable mechanism for micro pumping systems. The prediction of quantities such as the mass flow rate or the mean species concentration for this type of flow, termed the Electro-Osmotic flow, plays a crucial role in the design and control process of the entire microfluidic system. To this end, numerical techniques are efficient to evaluate these quantities but accuracy depends on the mesh utilizes. The a posteriori finite element output bound method is used to calculate these quantities while offering information regarding accuracy. The bound method applied here-in is based on the flux-free approach and provides relevant, inexpensive, and asymptotic lower and upper bounds to the mass flow rate of an Electro-Osmotic flow in a cross-intersection of a two-dimensional microchannel. To obtain shaper bounds, the flux-free approach is further enhanced by an adaptive mesh refinement strategy. This work focuses on the development of the numerical procedure for Electro-Osmotic flows and reports performance of the method in terms of numerical accuracy and computational cost.

Commentary by Dr. Valentin Fuster
2004;():939-946. doi:10.1115/ICMM2004-2434.

Thermal management is one of the greatest challenges in maintaining the functionality and reliability of high-speed micro-electronic systems such as MEMS and NEMS. This requires development of high performance heat transfer media, which can not only flow through micro- and nano-channels under local operating conditions, but also carry as more heat as possible out of the system. Recent work has shown that suspensions of nanoparticles with a size considerably smaller that 100nm but with thermal conductivity orders of magnitudes higher that the base liquids have a greater potential as a high energy carrier for the micro- and nano-systems. However, it is also known that particles in a suspension undergoing a shearing action may migrate, hence lead to non-uniformity. This indicates that the efficiency of heat transfer in the micro- and nano-channels may not be as superior as expected, which bears significance to the system design and operation. This work aims at addressing this issue by examine the effect of particle migration on heat transfer in small channels. This involves development of both flow and heat transfer models, and numerical solution to the models. The flow model takes into account the effects of the shear-induced and viscosity gradient-induced particle migrations, as well as self-diffusion due to the Brownian motion, which is coupled with an energy equation. The results show that particle migration leads to concentration of particles in the wall region can be much lower than that in the core region. Particle migration is also shown to increase the Nusselt number under both constant temperature and constant heat flux conditions.

Commentary by Dr. Valentin Fuster
2004;():947-953. doi:10.1115/ICMM2004-2435.

A Lattice Boltzman Model (LBM) with the Poisson-Boltzmann equation for charge distribution is presented for the simulation of electroosmotic transport in straight rectangular micro and nanochannels. Our results from the LBM are in excellent agreement with the corresponding analytical solution. We have shown that the Lattice Boltzmann Model in the presence of an external force may be used an effective computational tool to simulate the electroosmotic transport phenomena in micro- and nanochannels.

Commentary by Dr. Valentin Fuster
2004;():955-959. doi:10.1115/ICMM2004-2436.

We presented a lattice Boltzmann method (LBM) using a mean-field representation of the free energy for fluid systems. This free-energy approach provides more realistic contact angles and fluid density profiles near the vicinity of an impenetrable wall, which cannot be easily obtained by other LBM schemes. Our method was tested against various criteria and the results are in good agreement with those from thermodynamics and molecular dynamics considerations. This mean-field approach to LBM can have important implication on studies where the solid-fluid interactions are crucial to fluidic behaviors.

Commentary by Dr. Valentin Fuster
2004;():961-966. doi:10.1115/ICMM2004-2437.

Proteinaceous bubbles of 185 nm in average diameter were synthesized by a sonochemical treatment of bovine serum albumin in aqueous solution and the nanoparticles (TiO2 ) solution was made by ultrasonic irradiation. To study the macroscopic flow behavior associated with the changes in the state of microparticles, a flow test of these solutions in microchannels was done. Also the size distributions of the proteinaceous bubbles in solution before and after the flow test were measured by a light scattering method. Test results show that the air-filled proteinaceous bubbles in solution adjust their size to reduce the shear stress encountered in the flow through the microchannel. On the other hand, the flow rate of the solution with nanoparticles suspensions becomes smaller than that of deionized water above the flow rate of 6 cm3 /min in the microchannel with a dimension of 100×150 μm2 .

Commentary by Dr. Valentin Fuster
2004;():967-975. doi:10.1115/ICMM2004-2438.

At the Institute for Micro Process Engineering of the Forschungszentrum Karlsruhe, micro heat exchangers are manufactured out of single foils of base metal alloys. The characterisation of the thermohydraulic properties of microchannel heat exchangers is done using water as heat transfer medium on both passages at temperatures of 10°C and 95°C. The present publication will give an overview of the numerical simulation as well as experimental results for crossflow and counterflow microchannel heat exchangers. A comparison of three crossflow heat exchangers with different microchannel structures, and two different types of counterflow microchannel heat exchangers is shown. For comparison, the heat transfer rate, the overall heat transfer coefficients and efficiencies as well as pressure drop obtained from experiment and theory is shown. For numerical simulation, two models have been used. An easily accessibly method is to use classical engineering codes based on the Nusselt theory (VDI Wärmeatlas, 1994). A more detailed model is to use computational fluid dynamics (CFD) with the commercially available tool FLUENT ®, where best estimation codes have been applied for numerical simulation. Both numerical calculations are a helpful complement to predict thermal and hydrodynamic behaviour of the microchannel heat exchangers.

Commentary by Dr. Valentin Fuster
2004;():977-981. doi:10.1115/ICMM2004-2439.

Non-metallic, flexible, thin, microchannel heat exchangers made from heat-sealable polyimide films have recently been developed for refrigeration and air-conditioning applications. In order for these heat exchangers to function properly and independently, robust and reliable connectors are needed. The connectors must be easy to manufacture and assemble, hold high pressures without leaking, and have the ability to connect to standard tubing and piping and different heat exchangers together in series and parallel. Three uniquely different connector designs have been developed, manufactured, and tested. The first includes a machined metal connector internally embedded within the heat exchanger that is capable of holding pressures up to 1.03 MPa. The second connector design includes a two-piece polymer assembly containing an O-ring used to seal around the inlet and outlet holes of the heat exchanger. The design incorporates a 10-degree wedge that creates the sealing force when fully assembled with the heat exchanger and has been pressure tested to 2.07 MPa without leaking. A variation of the design allows multiple heat exchangers to connect in series and in parallel. Finally, the third design contains no additional parts and connects two heat exchangers together by thermally bonding the inlet of one heat exchanger to the outlet of another. The thermally bonded connection was able to hold pressures greater than 2.07 MPa.

Commentary by Dr. Valentin Fuster
2004;():983-987. doi:10.1115/ICMM2004-2440.

Each stack of microstructured stainless steel and aluminum plates was coated and brazed in vacuum. As a supporter of catalyst, the coating layer was formed by solgel method and anodizing respectively. Though the number of brazed plates extended to a hundred, the thickness of coating layer on the respectively coated plate was relatively uniform and the leakage of assembly was effectively minimized. The critical variables for brazing were both thickness and shape of filler metal. Consequently, brazing method had good resolution for 200μm three dimensional multi layer structures. Through these results, coating to support catalysts in highly stacked layers by brazing can be applied to micro catalytic heat exchanger where the reaction and heat transfer occur simultaneously.

Commentary by Dr. Valentin Fuster
2004;():989-994. doi:10.1115/ICMM2004-2441.

In the search for more compact air/liquid heat exchangers, one possible way is to increase the heat transfer coefficient and surface area by a decrease of the size of the fluid channels. A practical example could be air/water cross-flow heat exchangers used in cars. These exchangers are designed so that air pressure drop is minimised at a given thermal power exchanged from water to air. In this case, minimisation of the total volume leads to a very thin structure with a large frontal area, with a lot of small and short air channels. This configuration is very inconvenient for most practical applications and also difficult to manufacture at low cost. Using this rationale, we have designed and patented a cross-flow heat transfer surface with microchannels that has such a structure, but can be manufactured industrially at reasonable cost by extrusion either in aluminium or in polymers. Moreover, the arrangement of the heat transfer surfaces is very flexible and allows for different configurations (accordion, serpentine, cylindrical, star...) so that various geometric configurations, adapted to specific applications, can be obtained. The thermo-hydraulic performance of the structure has been simulated using standard correlations and CFD codes. Prototypic structures made by stereolithography have been manufactured in glass reinforced polymer and are currently being tested on a test bench. In order to validate our simulation code, a single structure and an accordion arrangement heat exchanger are under investigation. Compared to classical heat exchangers, our design is superior in flexibility and compactness for air/liquid applications. An additional interest of our design would be to increase performance in humid air cooling applications, since our structure may drain condensates more easily. We are currently looking for a partnership to develop this design for industrial applications.

Commentary by Dr. Valentin Fuster
2004;():995-1002. doi:10.1115/ICMM2004-2442.

Joule heating is present in electrokinetically driven flow and mass transport in microfluidic systems. Specifically, in the cases of high applied voltages and concentrated buffer solutions, the thermal management may become a problem. In this study, a mathematical model is developed to describe the Joule heating and its effects on electroosmotic flow and mass species transport in microchannels. The proposed model includes the Poisson equation, the modified Navier-Stokes equation, and the conjugate energy equation (for the liquid solution and the capillary wall). Specifically, the ionic concentration distributions are modeled using (i) the general Nernst-Planck equation, and (ii) the simple Boltzmann distribution. These governing equations are coupled through temperature-dependent phenomenological thermal-physical coefficients, and hence they are numerically solved using a finite-volume based CFD technique. A comparison has been made for the results of the ionic concentration distributions and the electroosmotic flow velocity and temperature fields obtained from the Nernst-Planck equation and the Boltzmann equation. The time and spatial developments for both the electroosmotic flow fields and the Joule heating induced temperature fields are presented. In addition, sample species concentration is obtained by numerically solving the mass transport equation, taking into account of the temperature-dependent mass diffusivity and electrophoresis mobility. The results show that the presence of the Joule heating can result in significantly different electroosomotic flow and mass species transport characteristics.

Commentary by Dr. Valentin Fuster

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