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8th International Conference on Nanochannels, Microchannels, and Minichannels

2010;():1-7. doi:10.1115/FEDSM-ICNMM2010-30168.

The development of novel methods for the isolation of primary stem and progenitor cells is important for the treatment of blood cancers, tissue engineering, and basic research in the biomedical sciences. Our lab has previously shown that microtubes coated with P-selectin protein can be used to capture and enrich hematopoietic stem and progenitor cells from a mixture of cells perfused through the tube at physiologically-relevant shear stresses[1][2], and that using a surface coating of colloidal silica nanoparticles (12 nm diameter, 30% by weight SiO2 ) increased cell capture and decreased rolling velocity[3]. Here we show that 50 nm colloidal silica nanoparticle coatings may similarly increase cell capture, and that these protocols are effective for enrichment of human adult CD34-positive HSCs from primary apheresis and bone marrow aspirate samples. Future research may include long-term colony-forming assays to confirm stem cell activity of enriched cells, and transplantation in immune-deficient mice.

Topics: Nanoparticles , Cancer
Commentary by Dr. Valentin Fuster
2010;():9-19. doi:10.1115/FEDSM-ICNMM2010-30177.

White blood cell (WBC) sequestration in lung capillaries is a key step in the inflammatory response to lung infection. P-selectin and ICAM-1 have well-defined roles in WBC adhesion in venules but their role in pulmonary capillaries is still unclear. Here, a novel in vitro Micropipette Cell Adhesion Assay used P-selectin, ICAM-1 or BSA-coated capillary-sized glass micropipettes as an in vitro model of a lung capillary. WBC were aspirated into adhesion molecule-coated vessels of varying diameters. Cell velocities and activation times were determined under pressures representative of lung capillaries. WBC velocities in this assay were significantly lower on P-selectin than BSA and decreased with increasing P-selectin concentration. These results demonstrate that P-selectin at low density mediates WBC adhesion in the pulmonary capillary geometry. WBC can also become activated upon aspiration into micropipettes and under some circumstances can be seen to exhibit a cyclic migratory behavior. This work was supported by grant BES-0547165 from the National Science Foundation and by an award from the American Heart Association.

Topics: Motion , Lung
Commentary by Dr. Valentin Fuster
2010;():21-30. doi:10.1115/FEDSM-ICNMM2010-30559.

The pressure drop in microchannels filled with porous media formed by square arrays of cylinders (micro-porous channels) is investigated. Combining the Brinkman equation and the existing models for permeability of regular arrays of cylinders, the pressure drop in the considered micro-porous channels is calculated theoretically. Soft lithography method is used to fabricate several Polydimethylsiloxane (PDMS) microporous-channels with porosities in the range of 0.35 to 0.95, fiber diameters varying from 50 to 400 μm, and channel depth of approximately 100 μm. Distilled water is pushed through the samples using a syringe pump with steady flow rate and the resulting pressure drops are measured for several flow rates. The developed model captures the trends of experimental data for all of the samples. Our analysis indicates that a competing behaviour exists between the permeability and the channels dimensions for controlling the pressure drop. Therefore, the Darcy number should be used to determine the dominating parameter.

Commentary by Dr. Valentin Fuster
2010;():31-39. doi:10.1115/FEDSM-ICNMM2010-30605.

Wall shear stress acting on arterial walls is an important hemodynamic force determining vessel health. A parallel-plate flow chamber with a 127 μm-thick flow channel is used as an in vitro system to study the fluid mechanics environment. It is essential to know how well this flow chamber performs in emulating physiologic flow regimes especially when cultured cells are present. Hence, the objectives of this work are to computationally and experimentally study the characteristic of the flow chamber in providing a defined flow regime and shear stress to cultured cells and to map wall shear stress distributions in the presence of an endothelial cell layer. Experiments and modeling were performed for the nominal wall shear stresses of 2 and 10 dyn/cm2 . Without endothelial cells, the flow field is uniform over 95% of the chamber cross-section and the surfaces are exposed to the target stress level. Using PIV velocity data, the endothelial cell surfaces were re-constructed and flow over these surfaces was then simulated via FLUENT. Once endothelial cells are introduced, local shear variations are large and the velocity profiles are no longer uniform. Due to the velocity distribution between peaks and valleys, the local wall shear stresses range between 47–164% of the nominal values. This study demonstrates the non-uniform shear stress distribution over the cells is non-negligible especially in small vessels or where blockage is important.

Commentary by Dr. Valentin Fuster
2010;():41-49. doi:10.1115/FEDSM-ICNMM2010-30661.

3-D models of both sides of human nasal passages were developed to investigate the effect of septal deviation on the flow patterns and nano particles deposition in the realistic human nasal airways. 3-D computational domain was constructed by a series of coronal CT scan image before and after septoplasty from a live 25-year old nonsmoking male with septal deviation in his right side nasal passage. For several breathing rates corresponding to low or moderate activities, the steady state flow in the nasal passages was simulated numerically. Eulerian approach was employed to find the nano particles concentrations in the nasal channels. The flow field and particles depositions depend on the passage geometry. The abnormal passage has more particles deposition comparing with the normal side and post-operative passages for nano particles because of rapid change in geometry. However, regional depositions have the same behavior for the nano particles in the three different studied passages. Despite the anatomical differences of the human subjects used in the experiments and computer model, the simulation results are in qualitative agreement with the experimental data.

Commentary by Dr. Valentin Fuster
2010;():51-55. doi:10.1115/FEDSM-ICNMM2010-30717.

Creating monodispersed hydrogel microparticles is advantageous for drug delivery applications. We explore microfluidic flow focusing as a method for generating such particles. Our hydrogel has a unique composition that makes it biodegradable and mechanically strong. We designed and manufactured a polymer microfluidic chip that mixes three viscous precursor solutions and generates a steady stream of microparticles from that solution. We found that microfluidic flow focusing produce particle with a coefficient of variance around 9%; this was a four times improvement over traditional methods.

Commentary by Dr. Valentin Fuster
2010;():57-63. doi:10.1115/FEDSM-ICNMM2010-30857.

Autonomous micro-swimming robots can be utilized to perform specialized procedures such as in vitro or in vivo medical tasks as well as chemical surveillance or micro manipulation. Maneuverability of the robot is one of the requirements that ensure successful completion of its task. In micro fluidic environments, dynamic trajectories of active micro-swimming robots must be predicted reliably and the response of control inputs must be well-understood. In this work, a reduced-order model, which is based on the resistive force theory, is used to predict the transient, coupled rigid body dynamics and hydrodynamic behavior of bio-inspired artificial micro-swimmers. Conceptual design of the micro-swimmer is biologically inspired: it is composed of a body that carries a payload, control and actuation mechanisms, and a long flagellum either such as an inextensible whip like tail-actuator that deforms and propagates sinusoidal planar waves similar to spermatozoa, or of a rotating rigid helix similar to many bacteria, such as E. Coli. In the reduced-order model of the micro-swimmer, fluid’s resistance to the motion of the body and the tail are computed from resistive force theory, which breaks up the resistance coefficients to local normal and tangential components. Using rotational transformations between a fixed world frame, body frame and the local Frenet-Serret coordinates on the helical tail we obtain the full 6 degrees-of-freedom relationship between the resistive forces and torques and the linear and rotational motions of the swimmer. In the model, only the tail’s frequency (angular velocity for helical tail) is used as a control input in the dynamic equations of the micro-swimming robot. The reduced-order model is validated by means of direct observations of natural micro swimmers presented earlier in the literature and against; results show very good agreement. Three-dimensional, transient CFD simulations of a single degree of freedom swimmer is used to predict resistive force coefficients of a micro-swimmer with a spherical body and flexible tail actuator that uses traveling plane wave deformations for propulsion. Modified coefficients show a very good agreement between the predicted and actual time-dependent swimming speeds, as well as forces and torques along all axes.

Topics: Biomimetics
Commentary by Dr. Valentin Fuster
2010;():65-72. doi:10.1115/FEDSM-ICNMM2010-31080.

The complex rheology of red blood cell (RBC) in microcirculation has been a topic of interest for many decades. As RBC is highly deformable, shape change affects the microcirculation and such effect should be accounted accurately to understand the rheology of blood flow. A particle based model is developed to construct the red blood cell (RBC) based on the minimum energy principle. A bead-spring network is utilized to represent the cross-sectional plane of RBC membrane. The total energy of the RBC is associated with spring stretch/compression, bending and constraint of fixed area. Shape optimization of swollen RBC due to continuous deflation is performed. A bi-concave RBC shape is accurately achieved when the circular shape is deflated to 65%. Dissipative particle dynamics (DPD), a coarse-grained Mesoscopic particle simulation is used to simulate the flow. RBC in its equilibrium shape is placed inside a microchannel of height 10 μm to study the deformation of the cell under shear. Force exerted on RBC particles by plasma particles were determined and solved as the external force in the DPD equation to calculate the position and velocity of each particle. As the simulation started, the RBC experienced the shear and drag force by surrounding plasma and evolved to the characteristic parachute type shape as observed in experiments. Once the RBC reached the steady deformation, it continued with the same shape and stayed in the center of the channel. It is observed that the parachute shape and its motion along the centerline of the flow help reducing the drag and subsequently achieving the state of minimum energy. Formulation and results were validated against the experimental and computational results reported in the literature.

Commentary by Dr. Valentin Fuster
2010;():73-78. doi:10.1115/FEDSM-ICNMM2010-31090.

In this study micro and nano scale measurement techniques are applied to platelet studies and determination of factors in platelet aggregation and thrombus formation. Conventionally it has been assumed that platelets are stimulated by blood clotting factors and platelet activators to aggregate and form a thrombus at sites of vascular injury. We have recently shown that a primary factor in initiating platelet aggregation is hemodynamic shear. This paper presents the effect of shear rate on the time evolution of thrombus formation and the final geometry of a mature thrombus. A relationship between maximum mature thrombus height and local shear rate is formulated. We have shown that the shear rate is not only an important factor in initiating platelet aggregation but is also one of the main inhibitors of platelet aggregation and thrombus formation. We propose that when the platelets reach a critical height, they encounter a specific local hemodynamic range, which prevents further thrombus growth.

Commentary by Dr. Valentin Fuster
2010;():79-84. doi:10.1115/FEDSM-ICNMM2010-30139.

This paper presents results of numerical simulations of various processes that demonstrate phase change heat transfer at high heat fluxes using the level-set method. The model used for the purpose has been first validated for the growth of an evaporating bubble in infinite medium, and fim boiling in 2D and 3D. It has then been applied to simulate the nucleation and departure of a single bubble from a solid body subject to conductive heat transfer. Unlike our previous investigations where phase change induced evaporation rate was incorporated like a sub-grid scale heat transfer model applied to the triple contact line, the present work reports simulations with direct phase change modelling by integrating energy fluxes at the interface. The effect of the conductive heat transfer in the solid from which the bubble departs is also taken into account. Comparison with visual images suggests that accounting for conjugate heat transfer is important to capturing micro-hydrodynamics in nucleate boiling, at least qualitatively.

Commentary by Dr. Valentin Fuster
2010;():85-94. doi:10.1115/FEDSM-ICNMM2010-30218.

In this paper, the experimental flow boiling visualization results of a microchannel are presented and discussed. A series of visualization experiments have been conducted in a horizontal, circular, uniformly heated microchannel, to record the two-phase flow patterns evolved during the boiling process and to study the ebullition process. A high speed camera (REDLAKE HG50LE) with a maximum of 100000 fps together with tungsten lights was used to capture the images along the test section. Microchannel was made of circular fused silica tube having an internal diameter of 0.781 mm and a uniformly heated length of 191 mm. Outside of the test tube was coated with a thin, electrically conductive layer of Indium Tin Oxide (ITO) for direct heating of the test section. Refrigerant R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 25 °C and 30 °C. Mass flux was varied from 100 kg/m2 s to 400 kg/m2 s and heat flux ranged from 5 kW/m2 to 45 kW/m2 . Visualization results show that the bubble growth is restricted by the tube diameter which results in very short existence of isolated bubbly flow regime except essentially restricted to a very short length of test tube. Flow patterns observed along the length were: Isolated bubble, elongated bubble, slug flow, semi annular and annular flow. Rigorous boiling and increased coalescence rates were observed with increase in heat flux. Bubble frequency was observed to increase with both heat and mass flux. A comparison with our previous flow boiling visualization studies, carried out for a test tube of 1.33 mm internal diameter, shows that the number of active nucleation sites is less while the bubble frequency is higher for the current study. Mean bubble length and bubble velocity during elongated bubble flow pattern have also been calculated from the images obtained during the tests.

Commentary by Dr. Valentin Fuster
2010;():95-99. doi:10.1115/FEDSM-ICNMM2010-30230.

The paper reports preliminary results from a new research programme for making accurate heat transfer and pressure drop measurements during condensation in microchannels. While commissioning the apparatus a dummy test section was used with identical channel and header geometry to that to be used in the main test program (The final test section will comprise a relatively thick copper test section containing 98 accurately located thermocouples for measuring the temperature distribution from which local heat flux and temperature at the microchannel surface will be obtained). While using the dummy test section (without embedded thermocouples) the opportunity was taken to make accurate pressure drop measurements while measuring the vapor flow rate and total heat transfer rate based on coolant measurements. Data have been obtained for FC72 and steam. Approximate comparisons with available pressure drop calculation methods are presented.

Commentary by Dr. Valentin Fuster
2010;():101-108. doi:10.1115/FEDSM-ICNMM2010-30273.

Cooling systems incorporating convective boiling in mini- and microchannels achieve very high thermal performance. Although many investigations related to the subject have already been conducted, the basic phenomena of the heat transfer mechanisms are not yet fully understood. The development of empirical correlations based only on flow pattern maps does not lead to a deeper knowledge of the mechanisms. In this study a comprehensive measurement technique that was successfully adapted in pool boiling experiments [8,9] was used for the investigation of forced convective boiling of FC-72 in a single rectangular minichannel. This technique allows the measurement of the local temperature with very high spatial and temporal resolution. High speed video recording was used to observe the flow inside the minichannel. The inlet Reynolds number was kept constant for the first measurements to Re = 200 corresponding to a hydraulic diameter of the minichannel of 800 μm. The Bond number for the proposed setup is about Bo ≈ 1.2. Several flow pattern regimes such as bubbly flow, slug flow and partially dryout were observed for heat fluxes up to 25 kW / m2 . From an energy balance at each pixel element of the thermographic recordings the local transient heat flux could be calculated and compared to the flow pattern video recordings. The results of the first experiments already give an indication about the heat transfer mechanisms at different flow regimes.

Commentary by Dr. Valentin Fuster
2010;():109-114. doi:10.1115/FEDSM-ICNMM2010-30281.

Experiments were performed using the laser extinction method to directly measure the thickness of the liquid film between the bubbles and heating plates in a mini-gap formed by two parallel vertical quartz glass plates for the dielectric fluid HFE-7200 at gap sizes of 0.15, 0.3 and 0.5mm. High-speed movies were also taken to measure the velocity of bubble forefront simultaneously. It was confirmed that the microlayer thickness was determined by the gap size and velocity of bubble forefront. At the region of small Weber number, microlayer thickness of HFE-7200 was obviously thicker than that of water, toluene and ethanol at the same velocity and gap size for its small surface tension. Furthermore, by nondimensional analyzing of experimental data, the empirical correlation proposed in previous study which was based on water, toluene and ethanol is still reliable for HFE-7200.

Commentary by Dr. Valentin Fuster
2010;():115-120. doi:10.1115/FEDSM-ICNMM2010-30428.

To increase the nucleate boiling efficiency, many nucleate boiling experiments have been conducted and could get brilliant and challengeable results. A consensus was that CHF and heat transfer were affected by a modified heating surface which change the micro roughness, thermophysical properties of heating surface, or the wettability. Of the many parameters, the wettability study is regarded as the most powerful factor. For finding the optimized condition at the nucleate boiling (high heat transfer and high CHF), we design the special heaters to examine how two materials, which have different wettabilities, affect the boiling phenomena. The special heaters have several types of hydrophobic patterns which have the precise size because they were made by MEMS techniques on the silicon oxide surface. In the experiments with patterned surface, hydrophobic dots lead to an early bubble inception and induce the better heat transfer. These experiments are compared with classic and recent models for bubble inception. The all experiments are conducted under the saturated pool boiling condition with distilled water at 1 atm pressure. The peculiar Teflon (AF1600) is used as the hydrophobic material. The hydrophilic part is performed by silicon oxide through the furnace procedure. The experiments using the micro-sized patterns and milli-sized patterns are performed, and the results are compared with the reference surface. These mixed-wettability studies are expected to induce the development of the nucleate boiling condition.

Commentary by Dr. Valentin Fuster
2010;():121-127. doi:10.1115/FEDSM-ICNMM2010-30478.

Experiments were performed to study the effects of surface wettability on flow boiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 kg/m2 s and the base heat flux varied from 30 to 80 kW/m2 . Water enters the test channel under subcooled conditions. The samples are silicone oxide (SiOx), titanium (Ti), diamond-like carbon (DLC) and carbon-doped silicon oxide (SiOC) surfaces with static contact angles of 26°, 49°, 63° and 103°, respectively. The results show significant impacts of surface wettability on heat transfer coefficient.

Commentary by Dr. Valentin Fuster
2010;():129-136. doi:10.1115/FEDSM-ICNMM2010-30524.

A network analysis was established to model the array of evaporative micro-channels with possible non-uniformity heating as well as branching of the channels. Iterative solution of the evaporative micro-channel network can be obtained using the Hardy-Cross method together with accurate two-phase head-loss correlations. Based on the experimental evidence, cross-cutting micro-channels reduce the uneven flow distribution in parallel micro-channels at non-uniform heating. Through this network analysis, it is also evident that cross-cutting grooves on parallel micro-channels are effective in reducing non-uniform heating effects and enhancing the uniform wall temperature distribution.

Commentary by Dr. Valentin Fuster
2010;():137-146. doi:10.1115/FEDSM-ICNMM2010-30603.

This paper verified the macro-to-micro-scale transitional criterion BoRel0.5 = 200 proposed by Li and Wu [W. Li, Z. Wu, A general criterion for evaporative heat transfer in micro/minichannels, International Journal of Heat and Mass Transfer 53 (2010) 1967–1976] because data points where BoRel0.5 ≤ 200 and BoRel0.5 > 200 show very different trends for the entire database (1,672 data points). For the 859 data points with BoRel0.5 ≤ 200, the boiling number at CHF decreases greatly with length-to-diameter ratio Lh /dhe when Lh /dhe is small, while Lh /dhe presents negligible effect on the boiling number when Lh /dhe > 150. For the region where Lh /dhe ≤ 150 and BoRel0.5 ≤ 200, a simple saturated CHF correlation was proposed by using the boiling number, length-to-diameter ratio, and exit quality. Heated length and heated equivalent diameter were adopted in the length-to-diameter ratio, considering the actual heat transfer conditions. A combined dimensionless number WemCal0.8 was introduced to correlate the micro/mini-channel database for the region: Lh /dhe > 150 and BoRel0.5 ≤ 200. The new method can predict the overall micro/mini-channel database accurately on the whole. It can predict almost 95.5% of the non-aqueous data and 93.5% of the water data within the ± 30% error band.

Commentary by Dr. Valentin Fuster
2010;():147-154. doi:10.1115/FEDSM-ICNMM2010-30700.

Evaporation of liquids is of major interest for many topics in process engineering. One of these is chemical process engineering, where evaporation of liquids and generation of superheated steam is mandatory for numerous processes. Generally, this is performed by use of classical pool boiling and evaporation process equipment. Another possibility is creating mixtures of gases and liquids, combined with a heating of this haze. Both methods provide relatively limited performance. Due to the advantages of microstructure devices especially in chemical process engineering [1] the interest in microstructure evaporators and steam generators have been increased through the last decade. In this publication several microstructure devices used for evaporation and generation of steam as well as superheating will be described. Here, normally electrically powered devices containing micro channels as well as non-channel microstructures are used due to better controllability of the temperature level. Micro channel heat exchangers have been designed, manufactured and tested at the Institute for Micro Process Engineering of the Karlsruhe Institute of Technology for more than 15 years. Starting with the famous Karlsruhe Cube, a cross-flow micro channel heat exchanger of various dimensions, not only conventional heat transfer between liquids or gases have been theoretically and experimentally examined but also phase transition from liquids to gases (evaporation) and condensation of liquids. However, the results obtained with sealed microstructure devices have often been unsatisfying. Thus, to learn more onto the evaporation process itself, an electrically powered device for optical inspection of the microstructures and the processes inside has been designed and manufactured [2]. This was further optimized and improved for better controllability and reliable experiments [3]. Exchangeable metallic micro channel array foils as well as an optical inspection of the evaporation process by high-speed videography have been integrated into the experimental setup. Fundamental research onto the influences of the geometry and dimensions of the integrated micro channels, the inlet flow distribution system geometry as well as the surface quality and surface coatings of the micro channels have been performed. While evaporation of liquids in crossflow and counterflow or co-current flow micro channel devices is possible, it is, in many cases, not possible to obtain superheated steam due to certain boundary conditions [4]. In most cases, the residence time is not sufficiently long, or the evaporation process itself can not be stabilized and controlled precisely enough. Thus, a new design was proposed to obtain complete evaporation and steam superheating. This microstructure evaporator consists of a concentric arrangement of semi-circular walls or semi-elliptic walls providing at least two nozzles to release the generated steam. The complete arrangement forms a row of circular blanks. An example of such geometry is shown in Figure 8. A maximum power density of 1400 kW · m−2 has been transferred using similar systems, while liquid could be completely evaporated and the generated steam superheated. This is, compared to liquid heat exchanges, a small value, but it has to be taken in account that the specific heat capacity of vapor is considerably smaller than that of liquids. It could also be shown that the arrangement in circular blanks with semi-elliptic side walls acts as a kind of micro mixer for the remaining liquid and generated steam and, therefore, enhances the evaporation.

Topics: Evaporation , Water
Commentary by Dr. Valentin Fuster
2010;():155-161. doi:10.1115/FEDSM-ICNMM2010-30878.

Two-phase microfluidic heat exchangers have the potential to provide high-heat flux cooling with lower thermal resistance and lower pumping power than single-phase heat exchangers. However, the process of phase change in two-phase heat exchangers can cause flow instabilities that lead to microchannel dryout and device failure [1–3]. Modeling these flow instabilities remains challenging because the key physics are highly coupled and occur over disparate time and length scales. This work introduces a new approach to capture transient thermal and fluidic transport with a reduced-order model consisting of fluidic, thermal, and phase-change submodels. The present study presents a reduced-order, transient, multichannel fluidic circuit submodel for integration into this proposed modeling approach. The fluidic submodel is applicable in flow regimes in which a thin liquid film exists around the bubble. Flow response to boiling is modeled considering bubble overpressure. An adaptive time step approach is used to treat the rapid flow response at short time scales after initial bubble vaporization. Using a seeded bubble technique for testing two-phase flow response, the model predicts a stability threshold at 0.015 W of localized superheating for two 100-micron square channels in parallel with a pump flow rate of 0.15 ml/min. Once integrated with the proposed reduced-order thermal and phase change models, this fluidic circuit model will yield criteria for stable two-phase heat exchanger operation considering factors such as pumping pressure, channel geometry, and applied heat flux that can be compared to experimental observations.

Commentary by Dr. Valentin Fuster
2010;():163-172. doi:10.1115/FEDSM-ICNMM2010-31147.

Pool boiling is of interest in high heat flux applications because of its potential for removing large amount of heat resulting from the latent heat of evaporation and little pressure drop penalty for circulating coolant through the system. However, the heat transfer performance of pool boiling systems is not adequate to match the cooling ability provided by enhanced microchannels operating under single-phase conditions. The objective of this work is to evaluate the pool boiling performance of structured surface features etched on a silicon chip. The performance is normalized with respect to a plain chip. This investigation also focuses on the bubble dynamics on plain and structured microchannel surfaces under various heat fluxes in an effort to understand the underlying heat transfer mechanism. This work is expected to lead to improved enhancement features for extending the pool boiling option to meet the high heat flux removal demands in electronic cooling applications.

Commentary by Dr. Valentin Fuster
2010;():173-180. doi:10.1115/FEDSM-ICNMM2010-30010.

Hydrogen production through autothermal reforming (ATR) of hydrocarbons, such as methane, is one option of interest for mobile applications of hydrogen fuel cells. In the present study, a numerical investigation of catalytic autothermal reforming of methane in a surface microreactor is presented. A three-dimensional ATR reactor model is developed to simulate the flow and surface reactions in a microchannel of rectangular cross section with 340-μm sides, and total length of 8.5 mm. A four-reaction mechanism is implemented to simulate the surface reactions on a Ni/Al2 O3 catalyst. The governing equations in the model include conservations of mass, momentum, energy and chemical species. A CFD code based on the finite-volume method has been developed in-house to solve the governing equations. Validation of the results against available data confirms the accuracy of the numerical approach. The simulation results reveal the dependency of hydrogen yield on space velocity (SV), air/fuel molar ratio (A/F), water/fuel molar ratio (W/F), and the gas feed temperature.

Commentary by Dr. Valentin Fuster
2010;():181-191. doi:10.1115/FEDSM-ICNMM2010-30239.

A three-dimensional model is developed to simulate the behavior of a single-channel three-way catalytic converter. The flow regime is assumed to be steady and laminar, and the channel walls are considered as isothermal. A multi-step, global heterogeneous reaction mechanism with 16 reactions and 11 species is used in this investigation to enhance the accuracy of the results. The chemical reactions are assumed to occur only on the reactor walls. The developed model is validated against available experimental data for stoichiometric operating conditions. The effect of the feed temperature on the conversion efficiency of the main pollutant components is studied. The light-off temperature for the stoichiometric A/F is found to be about 530 K for CO, NO and UHC, and 425 K for H2 conversion. The model is also applied to predict the effect of reactor length and inlet mixture space velocity on the conversion efficiency at two different temperatures. By using the same kinetics a well-stirred, unsteady model is also developed to identify the sensitivity of the multi-step kinetic mechanism to the mixture composition. The effect of mole fraction variation of each species on the conversion of other mixture components is investigated.

Commentary by Dr. Valentin Fuster
2010;():193-199. doi:10.1115/FEDSM-ICNMM2010-30389.

Chemical reactions in gas-liquid systems are often occurring and embrace many issues, especially the contacting of the gas and liquid stream and generation of gas bubbles. Continuous mixing and generation of a large interface between the phases is very important for maintaining and intensifying a chemical reaction between the two phases. The generated heat from the chemical reaction has to be removed very quickly from the flowing stream, which is also a challenging task. Recent applications of microreactors at Lonza Ltd. are described with gas-liquid mass transfer and highly exothermic chemical reactions. The proper description and understanding of convective flow, heat transfer and reaction kinetics are essential for the successful application of microstructured devices.

Commentary by Dr. Valentin Fuster
2010;():201-207. doi:10.1115/FEDSM-ICNMM2010-30532.

The chemical reaction yield was predicted by using Monte Carlo simulation. The targeted chemical reaction of a performance evaluation using the microreactor is the consecutive reaction. The main product P1 is formed in the first stage with the reaction rate constant k1. Moreover, the byproduct P2 is formed in the second stage with the reaction rate constant k2. It was found that the yield of main product P1 was improved by using a microreactor when the ratio of the reaction rate constants became k1/k2 >1. To evaluate the Monte Carlo simulation result, the yields of the main products obtained in three consecutive reactions. It was found that the yield of the main product in cased of k1/k2 >1 increased when the microreactor was uesd. Next, a pilot plant involving the numbering-up of 20 microreactors was developed. The 20 microreactor units were stacked in four sets, each containing five microreactor units arranged. The maximum flow rate when 20 microreactors were used was 1 × 104 mm3 /s, which corresponds to 72 t/year. Evaluation of the chemical performance of the pilot plant was conducted using a nitration reaction. The pilot plant was found to capable of increasing the production scale without decreasing the yield of the products.

Commentary by Dr. Valentin Fuster
2010;():209-216. doi:10.1115/FEDSM-ICNMM2010-31192.

Numerical modeling of methane-steam reforming is performed in a mini/microchannel with heat input through Nickel-deposited channel walls. The low-Mach number, variable density Navier-Stokes equations together with multicomponent reactions are solved using a parallel numerical framework. Methane-steam reforming is modeled by three reduced-order reactions occurring on the reactor walls. The surface reactions in the presence of Nickel catalyst are modeled as Neumann boundary conditions to the governing equations. Effects of the total heat input, heat flux profile, and inlet methane-steam molar concentration on production of hydrogen are investigated in detail.

Commentary by Dr. Valentin Fuster
2010;():217-222. doi:10.1115/FEDSM-ICNMM2010-30070.

Photolithography is one of the main mass nano-production processes. Smaller devices are always aimed to save material and energy. Manufacturing small devices by photolithography is a challenge, due to the risk of collapse of patterns during the drying of rinse liquid. One of the main pattern shapes is the two-line parallel. In our previous study, an analytical model was developed for predicting the collapse of large (L/d, LAR>20; see Fig. 1) two-line parallel patterns [1]. This model assumes the rinse interface shape is cylindrical. Knowledge of the rinse interface shape is needed to define the forces contribute to collapse, i.e. Laplace pressure and surface tension force at the three-phase line. In the current study, a Finite Element (FE) model is developed to predict the collapse of short (LAR<20) and large (LAR>20) two-line parallel patterns. Rinse liquid shape and its curvature are found using Surface Evolver (an interactive program for the study of surfaces shaped by surface tension, gravitational and other energies). Another finite element method (i.e. ANSYS 11.0) is used to find the pattern deformation. It was found that the pattern deformation decreases by decreasing the LAR value. It is important as for the cases that due to the design specifications, selection of the pattern material and rinse liquid is restricted, by changing the LAR value one may resolve the collapse problem.

Commentary by Dr. Valentin Fuster
2010;():223-232. doi:10.1115/FEDSM-ICNMM2010-30171.

Due to the high intrinsic thermal conductivity of carbon allotropes, there have been many attempts to incorporate such structures into existing thermal abatement technologies. In particular, carbon nanotubes (CNTs) and graphitic materials (i.e., graphite and graphene flakes or stacks) have garnered much interest due to the combination of both their thermal and mechanical properties. However, the introduction of these carbon-based nanostructures into thermal abatement technologies greatly increases the number of interfaces per unit length within the resulting composite systems. Consequently, thermal transport in these systems is governed as much by the interfaces between the constituent materials as it is by the materials themselves. This paper reports the behavior of phononic thermal transport across interfaces between isotropic thin films and graphite substrates. Elastic and inelastic diffusive transport models are formulated to aid in the prediction of conductance at a metal-graphite interface. The temperature dependence of the thermal conductance at Au-graphite interfaces is measured via transient thermoreflectance from 78 to 400 K. It is found that different substrate surface preparations prior to thin film deposition have a significant effect on the conductance of the interface between film and substrate.

Commentary by Dr. Valentin Fuster
2010;():233-237. doi:10.1115/FEDSM-ICNMM2010-30222.

The method described in this paper introduces a new multiplexing format for cross-referencing of DMF systems through the simultaneous use of threshold-based voltage actuation (which sets a minimum voltage to initiate droplet motion) and bi-polar voltage activation on the overlying and underlying electrodes. The design makes use of bi-polar voltage activation and threshold effects to eliminate inter-droplet interference and overcome addressability limitations. In the proposed DMF multiplexer structure, these two requirements must both be satisfied for 2-D multiplexed addressability. Experimental characterization of the threshold voltage associated with the first requirement is presented. With regard to requirement two, the bi-polar voltage activation scheme is applied to a fabricated DMF multiplexer, and independent microdroplet motion is shown. The technique can be applied in actuating isolated microdroplets or microdroplet groups (simultaneously) in large-scale/highly-parallel DMF devices.

Commentary by Dr. Valentin Fuster
2010;():239-248. doi:10.1115/FEDSM-ICNMM2010-30710.

This paper deals with ElectroWetting-On-Dielectric using AC voltage (AC EWOD) in both open and closed configuration. The open configuration is more convenient for fundamental studies and observations of electrowetting associated phenomena whereas the closed configuration is particularly adapted for practical use in a Digital lab-on-chips (DLC). Software developed for the study of electrowetting and droplet oscillations in coplanar electrode configuration are presented. Particular features of electrowetting are outlighted. Droplet oscillations and hydrodynamic induced flows in open coplanar configuration are considered and applications for DLC are illustrated in closed configuration.

Topics: Electrodes
Commentary by Dr. Valentin Fuster
2010;():249-253. doi:10.1115/FEDSM-ICNMM2010-30754.

In this paper a novel numerical algorithm is proposed for modeling the transient motion of microdroplets in digital microfluidic systems. The new methodology combines the effects of the electrostatic and hydrodynamic pressures to calculate the actuating and opposing forces and the moving boundary of the microdroplet. The proposed model successfully predicts transient motion of the microdroplet in digital microfluidic systems, which is crucial in the design, control and fabrication of such devices. The results of such an analysis are in agreement with the expected trend.

Commentary by Dr. Valentin Fuster
2010;():255-260. doi:10.1115/FEDSM-ICNMM2010-31212.

Electrowetting-on-dielectric (EWOD) is a new method for handling droplets on the microfluidic chips. By applying electrical potential, the interfacial energy of liquid-solid interface changes, results altering of droplet contact lines. To increase the flow rate of such a digital microfluidic system one way is to raise the droplet velocity. One important factor for enhancing droplet velocity in EWOD systems is the proper switching the electrodes or “switching sequence”. To examine the effect of switching in EWOD, the EDEW 1.0 simulation tool is used in this paper. By simulating the motion of a 1μL water droplet in a 1D electrode array, the resultant surface energy curves during the motion of droplet in different electrode switching sequences are obtained. The results show proper electrode switching has a remarkable effect on increasing of droplet velocity. To enhance the droplet velocity, the electrode, which is placed next to the droplet at forward direction, should be powered after droplet passed over it. In addition, it would be more efficient to first turn on the next electrode, and then turn off the previous one.

Commentary by Dr. Valentin Fuster
2010;():261-262. doi:10.1115/FEDSM-ICNMM2010-30258.

The presence of considerable amounts of free charge dispersed in a liquid is the basis for electrokinetic phenomena which are related to the existence of an electrical double layer (EDL). In polar liquids, the dissociation of electrolytes into ionic species is well understood and numerous electrokinetic phenomena are known; a good overview is given by e.g. Delgado et al. [1]. In nonpolar liquids it is known that electrical charges can exist as well. The presence of these electrical charges is utilized, for example, in colloid science to stabilize particle suspensions [2]. For this purpose, surfactants are added which enhance the zeta potential of the particles in order to prevent their agglomeration. Additionally to the manipulation of surface charges, it is reported that the electrical conductivity of nonpolar liquids essentially increases when surfactant is added and traces of water are present [3]. Such ternary solutions of nonpolar liquid-water-surfactant are known to contain surfactant agglomerations, so-called inverted micelles with a size of several nanometers, detectable for instance by quasielastic lightscattering measurements. Figure 1 sketches the generation and structure of an inverted micelle. In general, surfactants are macromolecules consisting of different functional groups, e.g. a polar “head” and a nonpolar “tail”. Above the critical micelle concentration (cmc), surfactant molecules attach with their polar head at a water droplet forming the inverted micelle. It is assumed that electrical charges are dissolved in the polar core of the inversed micelles enabling opposite charges to be held sufficiently far apart and preventing an agglomeration of different micelles [4].

Commentary by Dr. Valentin Fuster
2010;():263-269. doi:10.1115/FEDSM-ICNMM2010-30465.

Recently, we have demonstrated electrothermal hydrodynamics with an external heating source of a highly focused 1,064 nm laser beam [1]. This phenomenon, when coupled with particle-electrode electrokinetic interactions, has led to the rapid and selective concentration of suspended colloids [2–6]. This technique, termed Rapid Electrokinetic Patterning (REP) was demonstrated without any additional surface modification or patterning of the electrodes. This dynamic, optically induced fluid and particle manipulation technique could be used for a variety of lab-on-a-chip applications. However, there are additional effects that have yet to be investigated that are important for a complete understanding of REP. This paper showcases experimental particle-particle behavior observations by varying particle diameter, electrode material, and preliminary results of varying fluid electrical conductivity.

Commentary by Dr. Valentin Fuster
2010;():271-272. doi:10.1115/FEDSM-ICNMM2010-30499.

Since the capillary electrophoresis was proposed to be run in the chip format, tremendous studies have been performed by covering many different aspects of this technology. One key element is the sample plug generated between the injection and separation process, because it will play a governing role on the final separation performance, i.e., the separation efficiency depends on the initial sample plug and its further dispersion development. In literature, some work has been done previously to generate various sample plugs, or optimize them by means of either channel design or operational control. However, little work has been reported to characterize the sample plug with evaluating parameters. Usually, the well-defined and reproducible sample plug is anticipated for high quality separation. By experience, thin-rectangular sample plugs are normally assumed, but not technically proved yet, to have superior performance in electrophoretic separation. Quantitative study is necessary to be performed to demonstrate the relevant qualitative estimation or analysis. All above stated are the motivation of current work.

Commentary by Dr. Valentin Fuster
2010;():273-274. doi:10.1115/FEDSM-ICNMM2010-30533.

The electrokinetic transport phenomena are to be numerically studied based on cross-linked microchannel networks, which have been commonly employed for on-chip capillary electrophoresis applications. Applied potential field, flow field and concentration field should be solved to predict the species transport process under electrokinetic flows. Together with the well-designed channel geometry, a detailed physical model was firstly formulated through a series of governing equations and corresponding boundary/initial conditions, which was briefly re-presented from our previous publications. The emphasis of current work was to justify the simplest non-dimensional scheme and identify the most beneficial parameters so that an effective and simplified non-dimensional model was developed for numerical studies.

Commentary by Dr. Valentin Fuster
2010;():275-279. doi:10.1115/FEDSM-ICNMM2010-30696.

In capacitive deionization (CDI), salt water is passed through two polarized electrodes, whereby the salt is adsorbed onto the electrode surface and removed from the water stream. This approach has received renewed interest for water desalination due to the development of new high-surface area carbon-based nanomaterials. However, there is currently limited understanding as to how electrode geometry, surface properties, and capacitance affect ion capture. In this work, we experimentally investigate various standard carbon-based electrode materials, including activated carbon and carbon cloths, as well as microfabricated silicon structures for CDI. Electrochemical characterization through cyclic voltammetry was used to determine the electrochemical properties of each material. In addition, a mini-channel test cell was fabricated to perform parametric studies on ion capture. By controlling electrode geometry and chemistry in these studies, the work helps elucidate transport mechanisms and provide insight into the design of optimal materials for capacitive deionization.

Topics: Carbon , Water
Commentary by Dr. Valentin Fuster
2010;():281-288. doi:10.1115/FEDSM-ICNMM2010-30734.

The behavior of a liquid jet in an electrostatic field is numerically simulated. The simulations performed correspond to a transient liquid jet leaving a capillary tube maintained at a high electric potential. The surface profile of the deforming jet is defined using the VOF scheme and the advection of the liquid free surface is performed using Youngs’ algorithm. Surface tension force is treated as a body force acting on the free surface using continuum surface force (CSF) method. To calculate the effect of the electric field on the shape of the free surface, the electrostatic potential is solved first. Next, the surface density of the electric charge and the electric field intensity are computed, and then the electric force is calculated. Liquid is assumed to be a perfect conductor, thus the electric force only acts on the liquid free surface and is treated similar to surface tension using the CSF method. To verify the simulation results, a simplified case of electrowetting phenomenon is simulated and free surface shape in stable state is compared with experimental results. Then the electrostatic atomization in spindle mode is simulated and the ability of the developed code to simulate this process is demonstrated.

Commentary by Dr. Valentin Fuster
2010;():289-295. doi:10.1115/FEDSM-ICNMM2010-30750.

In general, the modeling of electroosmotic flows can be approached in two fundamentally–different ways. (i) The thickness of electrical double layer (EDL) is ignored and the effect of the electrical forces within the EDL is imaged into a modified kinematic boundary condition, the so-called Helmholtz–Smoluchowski slip condition. This approach is numerically simple and inexpensive, but implies several restrictions. (ii) The EDL is fully resolved, using a first–principle approach based on differential conservation equations for mass, momentum, and charge. This approach is enormously elaborate and numerically expensive, but appears to be applicable for a much wider range of problems. As an example, the treatment of internal electrodes, adjacent to insulating walls at defined zeta potential, appears difficult with the simple approach (i), since any non–continuous potential distribution at the wall leads to a singularity of the electrical field strength. To avoid these difficulties, we develop a hybrid model which, on the one hand, electrically resolves the EDL to reveal a perfectly-continuous potential distribution in the complete microchannel. On the other hand, the flow equations are solved in the fluid bulk only, not comprising the EDL. Hence, the effect of the EDL is still incorporated by means of modified kinematic boundary conditions. The advantage of this hybrid model is, firstly, to avoid artificial singularities of the electrical field strength, where regions of different surface charge meet. These singularities are clearly artificial, since they result from neglecting the extend of the EDL. Secondly, the hybrid model, at each time step, needs to solve only once for the potential distribution, which makes it numerically inexpensive and simple. Hence, systematic parameter studies are within reach. We apply the hybrid model to investigate the influence of internal electrodes onto the flow field, driven by electroosmosis in a modular rectangular microchannel. As internal electrodes can be positioned at lower distances (if compared to external electrodes), they can be operated at lower voltages and still ensure strong electrical field strength. Systematic studies on the effect of different electrode positions and voltages are presented, leading to optimized settings for specific tasks as pumping or mixing. Further, a comparison to first-principle simulations using the approach (ii) is presented for selected cases. This demonstrates that the hybrid model perfectly captures the dominant physics.

Commentary by Dr. Valentin Fuster
2010;():297-303. doi:10.1115/FEDSM-ICNMM2010-30778.

This paper investigates the effects of velocity slip in the presence of an electric double-layer on fluid flow and heat transfer in a parallel plate hydrophobic microchannel. The electric potential filed is determined through the Poisson-Boltzmann equation together with the Debye-Hückel (D-H) approximation, while the velocity field is obtained by solving the Navier-Stokes equations under fully developed conditions. In most previous studies, zeta-potential has been considered as an independent variable for the analysis of induced voltage. However, experimental findings show that in electrokinetic slip flows with constant wall potential, the zeta potential is related to the slip coefficient and the D-H parameter. Therefore, in the present study, the wall potential is considered as an independent variable and the zeta potential is determined from an available experimental correlation. The effects of velocity slip, the D-H parameter, the wall potential and the Brinkman number on the induced voltage and the velocity and temperature fields are examined in detail. Results indicate that the slip effects on the zeta potential dramatically affect the flow and temperature fields.

Commentary by Dr. Valentin Fuster
2010;():305-312. doi:10.1115/FEDSM-ICNMM2010-31153.

Concrete is a highly porous material which is susceptible to the migration of highly deleterious species such as chlorides and sulfates. Various external sources including sea salt spray, direct sea water wetting, deicing salts and brine tanks harbor chlorides that can enter reinforced concrete. Chlorides diffuse into the capillary pores of concrete and come into contact with the rebar. When chloride concentration at the rebar exceeds a threshold level it breaks down the passive layer of oxide, leading to chloride induced corrosion. Application of electrokinetics using positively charged nanoparticles for corrosion protection in reinforced concrete structures is an emerging technology. This technique involves the principle of electrophoretic migration of nanoparticles to hinder chloride diffusion in the concrete. The re-entry of the chlorides is inhibited by the electrodeposited assembly of the nanoparticles at the rebar interface. In this work electrochemical impedance spectroscopy (EIS) combined with equivalent circuit analysis was used to predict chloride diffusion coefficients as influenced by nanoparticle treatments. Untreated controls exhibited a diffusion coefficient of 3.59 × 10−12 m2 /s which is slightly higher than the corrosion initiation benchmark value of 1.63 × 10−12 m2 /s that is noted in the literature for mature concrete with a 0.5 water/cement mass ratio. The electrokinetic nanoparticle (EN) treated specimens exhibited a diffusion coefficient of 1.41 × 10−13 m2 /s which was 25 times lower than the untreated controls. Following an exposure period of three years the mature EN treated specimens exhibited lower chloride content by a factor of 27. These findings indicate that the EN treatment can significantly lower diffusion coefficients thereby delaying the initiation of corrosion.

Commentary by Dr. Valentin Fuster
2010;():313-321. doi:10.1115/FEDSM-ICNMM2010-31165.

Time-dependent laminar liquid flow and thermal characteristics in a square cross-section microchannel were numerically investigated using computational fluid dynamics code. In the numerical model developed the upper and bottom microchannel substrate properties, Joule heating caused by applying electric potential, pressure driven flow, electroosmosis, heat transfer coefficients on the microchannel bottom wall and variations in the liquid thermophysical properties were all taken into account. Liquid flow velocity distribution and temperature fields were calculated by solving both Navier-Stokes and energy equations, and electric field distribution was determined based on their electric potential. The results obtained demonstrate the impact that applied potential, pressure difference, heat transfer coefficient and microchannel dimensions have on liquid flow and thermal behaviors in a square microchannel. Finally, the results with the model developed were then compared with those of a liquid having constant thermophysical properties.

Commentary by Dr. Valentin Fuster
2010;():323-329. doi:10.1115/FEDSM-ICNMM2010-31213.

Scaling down the biochemical analytical system has been an important topic of research recently. Minimizing the energy requirement for the microfluidic transportation is essential for the realization of a Lab-on-a-chip (LOC) that can perform the Point-of-Care Testing (POCT). In this work, modeling and analysis of a low voltage Electro-osmotic (EO) micropump applicable for the Bio-Microfluidic systems using COMSOL Multiphysics software package is presented. In the previously reported low voltage EO micropump (3), position of electrodes makes the fabrication process a tedious task. Here, we investigate the effects of placing the electrodes tangential to the microchannel since such designs can be easily fabricated using PDMS/glass fabrication process, and also comparing the effect of different electrode configurations on the pump performance. In addition, the effects of geometrical parameters of micropump on volumetric flow rate and velocity profiles are investigated.

Commentary by Dr. Valentin Fuster
2010;():331-333. doi:10.1115/FEDSM-ICNMM2010-31241.

PDMS (Polydimethylsiloxane) is widely used as a microfluidic chip material for various applications due to its desirable properties [1, 2]. However PDMS has several drawbacks that limit its utilization in a number of microfluidic applications [1–4]. Properties such as the hydrophobic nature, sample absorption, and low electrokinetic properties (low zeta potential) are some issues that must be considered before using PDMS for numerous applications [3]. In many PDMS based chips electroosmotic pumping is used for fluid flow and sample transport along the microchannel networks. Simplicity of implementation in microfluidic chips, fast response time, and the plug-like velocity profile are the major advantages of electroosmotic flow compared to other fluid pumping techniques [2]. This type of flow utilizes the formation of electric double layer (EDL) in microchannels and the movement of ions under an applied external electric field. Thus, the surface properties of the channel material and liquid properties (ionic concentration, pH, and viscosity) play major roles in electroosmotic pumping for different solutions in microchannels.

Commentary by Dr. Valentin Fuster
2010;():335-341. doi:10.1115/FEDSM-ICNMM2010-30179.

This study conducts an experimental study concerning the airside performance of highly compact heat sinks under cross flow condition. The test fin patterns can be classified into four categories, namely the base plain fin heat sink (Type I), interrupted fin geometry (Type II), dense vortex generator (Type III), loose vortex generator (Type IV) and their combinations. It is found that the heat transfer performance is strongly related to the arrangement of enhancements. The interrupted and dense vortex generator configurations normally contribute more pressure drop penalty than improvements of heat transfer. This deterioration becomes especially evident at a lower frontal velocity. The oblique VG with cannelure structure shows an appreciable lower pressure drop than that of plain fin geometry. In the meantime, the presence of interrupted surface may also jeopardize heat conduction path due to constriction. The results indicate that the vortex generators operated at a higher frontal velocity is more beneficial than that of plain fin geometry. In summary of the test results, it is therefore concluded that augmentation via various fin patterns like interrupted or vortex generator is quite effective only at developing region. However, the conventional enhanced fin patterns lose its superiority at the fully developed region. To tackle this problem, some techniques employing swing flow or unstable flow field accompanied with the asymmetric design, shows potential to resolve this problem.

Commentary by Dr. Valentin Fuster
2010;():343-351. doi:10.1115/FEDSM-ICNMM2010-30749.

An experimental study of mini-jet impingement boiling is presented for saturated conditions. Unique to this study is documentation of boiling characteristics of a submerged water jet under sub-atmospheric conditions. Data are reported at a fixed nozzle-to-surface distance that corresponds to a monotonic decrease in heat transfer coefficient for single-phase jet impingement. A mini nozzle is used in the present study with an internal diameter of 1.16 mm. Experiments are performed at three sub-atmospheric pool pressures of 0.2 bar, 0.3 bar and 0.5 bar. At each pressure, jet impingement boiling at four Reynolds numbers are characterized and compared with the pool boiling heat transfer. Enhancements in critical heat flux with increasing Re are observed for all pressures.

Topics: Boiling , Water
Commentary by Dr. Valentin Fuster
2010;():353-358. doi:10.1115/FEDSM-ICNMM2010-30921.

In the present study, a numerical model was developed for laminar flow in a microchannel with a suspension of microsized phase change material (PCM) particles. In the model, the carrier fluid and the particles are simultaneously present, and the mass, momentum, and energy equations are solved for both the fluid and particles. The particles are injected into the fluid at the inlet at a temperature equal to the temperature of the carrier fluid. A constant heat flux is applied at the bottom wall. The temperature distribution and pressure drop in the microchannel flow were predicted for lauric acid microparticles in water with volume fractions ranging from 0 to 8%. The particles show heat transfer enhancements by decreasing the temperature distribution in the working fluid by 39% in a 1 mm long channel. Meanwhile, particle blockage in the flow passage was found to have a negligible effect on pressure drop in the range of volume fractions studied. This work is a first step towards providing insight into increasing heat transfer rates with phase change-based microparticles for applications in microchannel cooling and solar thermal systems.

Commentary by Dr. Valentin Fuster
2010;():359-366. doi:10.1115/FEDSM-ICNMM2010-31098.

In this study, we simulate rarefied gas flow through a confined nano-impinging jet using direct simulation Monte Carlo (DSMC) method. The effects of geometrical parameters, pressure ratio, and wall conditions on the heat transfer from a hot surface are examined. Hot surface modeled via diffusive constant wall temperature. Various inlet/confining surface conditions such as specular, adiabatic, and constant temperature are implemented and the effects of them on the wall heat flux rates are studied. The results show that Knudsen number, velocity slip, and temperature jump are main reasons which specify magnitudes of wall heat flux rates. Among all geometrical parameters, H/W ratio has the greatest effect on heat transfer, where H is jet distance from the hot surface and W is the jet width. For different values of pressure ratio, the biggest quantity of wall heat flux rate relates to the lowest velocity slip case. Also for inlet/confining walls with constant temperature condition equal to coolant flow temperature, heat transfer from the hot surface was the maximum.

Commentary by Dr. Valentin Fuster
2010;():367-374. doi:10.1115/FEDSM-ICNMM2010-31167.

This study numerically investigates the feasibility and advantages of using a multilayer pin-fin heat sink to increase the overall performance of the heat sink. For the purpose of determining overall performance of the pin-fin heat sink a figure of merit (FOM) term is introduced in this paper, which constituted of both the thermal resistance and the pumping power of the heat sink. Higher the FOM of a heat sink better is its overall performance. A computational fluid dynamics software CoventorWARE™ is used for the analysis of micro heat sink performance. A small portion of the entire heat sink is modeled in this study assuming repeatability towards both sides for the ease of analysis. The developed models consist of two sections, the substrate (silicon) and the fluid (water at 278K). A uniform heat flux is applied to the base of the heat sink. A single layer micro pin-fin heat sinks with same dimensions as of the multi layer heat sink was also modeled for the comparison purpose. Temperature distribution at five different locations from the inlet to the outlet section is also analyzed to study the temperature distribution over the heat sink. Circular pin-fins were used in both the multilayer and single layer micro heat sinks. Feasibility of using micro channels as the second layer was also investigated in this paper and it proved to have advantages over using pin-fin structures on both layers. A geometric optimization based on the substrate thickness of the second layer of the double layer heat sink showed that the substrate thickness of the second layer doesn’t have any effect on the overall thermal resistance of the heat sink.

Topics: Fins , Heat sinks
Commentary by Dr. Valentin Fuster
2010;():375-382. doi:10.1115/FEDSM-ICNMM2010-31209.

Thin and very thin (less than 10 μm) liquid films driven by a forced gas/vapor flow (stratified or annular flows), i.e. shear-driven liquid films in a narrow channel is a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Development of such technology requires significant advances in fundamental research, since the stability of joint flow of locally heated liquid film and gas is a rather complex problem. The paper focuses on the recent progress that has been achieved by the authors through conducting experiments. Experiments with water in flat channels with height of H = 1.2–2.0 mm (mini-scale) show that a liquid film driven by the action of a gas flow is stable in a wide range of liquid/gas flow rates. Map of isothermal flow regime was plotted and the length of smooth region was measured. Even for sufficiently high gas flow rates an important thermocapillary effect on film dynamics occurs. Scenario of film rupture differs widely for different flow regimes. It is found that the critical heat flux for a shear driven film can be 10 times higher than that for a falling liquid film, and exceeds 400 W/cm2 in experiments with water for moderate liquid flow rates. This fact makes use of shear-driven liquid films promising in high heat flux chip cooling applications.

Commentary by Dr. Valentin Fuster
2010;():383-396. doi:10.1115/FEDSM-ICNMM2010-30167.

Accurate modeling of gas microvection is crucial for a lot of MEMS applications (micro-heat exchangers, pressure gauges, fluidic microactuators for active control of aerodynamic flows, mass flow and temperature micro-sensors, micropumps and microsystems for mixing or separation for local gas analysis, mass spectrometers, vacuum and dosing valves[[ellipsis]]). Gas flows in microsystems are often in the slip flow regime, characterized by a moderate rarefaction with a Knudsen number of the order of 10−2 –10−1 . In this regime, velocity slip and temperature jump at the walls play a major role in heat transfer. This paper presents a state of the art review on convective heat transfer in microchannels, focusing on rarefaction effects in the slip flow regime. Analytical and numerical models are compared for various microchannel geometries and heat transfer conditions (constant heat flux or constant wall temperature). The validity of simplifying assumptions is detailed and the role played by the kind of velocity slip and temperature jump boundary conditions is shown. The influence of specific effects, such as viscous dissipation, axial conduction and variable fluid properties is also discussed.

Commentary by Dr. Valentin Fuster
2010;():397-405. doi:10.1115/FEDSM-ICNMM2010-30178.

Kinetic theory of gases, as described by the Boltzmann or model kinetic equations, provides a solid theoretical approach for solving microscale transport phenomena in gases. Due to significant advancement in computational kinetic theory and due to the availability of high speed parallel computers, kinetic equations may be solved numerically with modest computational effort. In this framework, recently developed upgraded discrete velocity algorithms for solving linear and nonlinear kinetic equations are presented. In addition, their applicability in simulating efficiently and accurately multidimensional micro flow and heat transfer problems is demonstrated. Analysis and results are valid in the whole range of the Knudsen number.

Commentary by Dr. Valentin Fuster
2010;():407-412. doi:10.1115/FEDSM-ICNMM2010-30212.

High pressure gradient driven micro-channel flow modelling with very the high ratios of absolute pressure and temperature (see Agrawal et al. 2005 [1]) define the difference between physical and computational results using continuum approaches (see Maurer et al. 2003, Durst et al. 2006, Dongari et al. 2008 [3, 4, 8]). In the present paper this deviation of the computational results is explained by the statistical correlation of the molecular number density and the single molecule velocity inside a compressible gas flow. Classical solutions of Navier-Stokes equations do not satisfy the physical conditions of compressible, dilute molecular flows (see Brenner 2005, Greenshields and Reese 2007, Mizzi et al. 2008 [2, 6, 9]). Furthermore the consistent entropy production and the comparison between macroscopic physical values and the molecular diffusion closure are shown. Finally the computational results using this statistical model are compared with algebraic solutions verifying the thermodynamic consistence of the present statistical moment closure model.

Commentary by Dr. Valentin Fuster
2010;():413-419. doi:10.1115/FEDSM-ICNMM2010-30316.

Microscale flow simulation is considered in this paper for a microchannel flow geometry. Lattice Boltzmann Model (LBM) was used as the numerical method for flow simulation, in which an effective mean free path was used in relaxation time appeared in LBM. The effective mean-free-path makes it possible to investigate flow characteristics in transition flow regime, for which Knudsen number varies from 0.1 to 10. Such implementation does not change the computational efficiency of LBM significantly. Results are obtained for flow configuration in a long microchannel. The slip velocity was predicted in this flow configuration with good accuracy. Good correspondence with Direct Simulation Monte Carlo (DSMC) method was observed.

Commentary by Dr. Valentin Fuster
2010;():421-431. doi:10.1115/FEDSM-ICNMM2010-30320.

Slip flow in noncircular microchannels has been examined and a simple model for normalized Poiseuille number is proposed to predict the friction factor and Reynolds number product fRe for slip flow. The developed model for normalized Poiseuille number has an accuracy of 4.2 percent for all common duct shapes. As for slip flow, no solutions or graphical and tabulated data exist for most geometries, the developed simple model can be used to predict friction factor, mass flow rate, and pressure distribution of slip flow in noncircular microchannels for the practical engineering design of microchannels such as rectangular, trapezoidal, double-trapezoidal, triangular, rhombic, hexagonal, octagonal, elliptical, semielliptical, parabolic, circular sector, circular segment, annular sector, rectangular duct with unilateral elliptical or circular end, annular, and even comparatively complex doubly-connected microducts.

Commentary by Dr. Valentin Fuster
2010;():433-441. doi:10.1115/FEDSM-ICNMM2010-30399.

We present experiments on the isothermal gas flow at relatively high Mach numbers in microfabricated channels of small aspect ratios. The microchannels were fabricated by deep etching on silicon wafers, bonded to a Pyrex wafer to cover and seal them; the microchannels were 10 microns deep with a variety of widths. The accurate determination of the small flow rates was performed by measuring the displacement of a bead of mercury in a precision bore glass tube in a controlled environment. The experiment setup has been specially designed to account for inlet and outlet loss. The inferred friction coefficient at different values of Knudsen, Reynolds and Mach numbers shows that the flow inside the microchannel follows the classical laminar behavior over the range of experiments.

Commentary by Dr. Valentin Fuster
2010;():443-450. doi:10.1115/FEDSM-ICNMM2010-30488.

A newly designed device for the experimental characterization of rarefied pressure driven gas flows is presented. The device is intended for both the thermal and the hydrodynamic analysis of gas flowing inside a single micro channel. The innovative feature of the present design is the integration of temperature and pressure sensors inside the micro channel itself. The sensors are fabricated as an array on longitudinal thin film membranes, installed on a polymeric supporting frame. A peculiar multi layer configuration of the device allows, on the one hand, the mounting of the sensor layer as a separate channel upper wall and, on the other hand, the interchangeability of the test sections. The sensors directly face the gas stream, registering the temperature and the pressure profiles along the channel. This gives additional information about the gas behavior compared to the simplified assumption of a linear profile developing between the measured inlet and outlet values. Examples of integrated sensors in micro channels have been already realized; however the majority of them are in silicon channels, as only with silicon fabrication technologies the sensor sizes can be reduced down to the micron range. This of course limits the field of application of the obtained results. In the present case, although silicon technology is still employed for the sensor manufacturing, this refers to the sensor wall only, while the remaining three-wall-channel is machined into an exchangeable foil. The foil, in principle, can be made of any material and can be easily replaced. Several channel dimensions and materials, as well as different surface roughness levels can be tested with the same device, making it very flexible and suitable for a broad characterization of gas wall interactions. A future experimental campaign will investigate the influence of roughness and material on the flow and the heat transfer characteristics. This is a very critical point in rarefied gas applications in MEMS, as the similarities with conventional flows as well as new features need to be identified and analyzed. With the described configuration it is possible to have insight into the flow parameters and to understand the actual behavior of gases under specific flow conditions. This is important in order to obtain a proper modeling and to validate the results of simulations and calculations on rarefied gas flows in micro channels.

Commentary by Dr. Valentin Fuster
2010;():451-458. doi:10.1115/FEDSM-ICNMM2010-30489.

Numerical investigation of a gas flow through microchannels with a sharp, 90 degrees bend is carried out using Navier-Stokes (N-S) equations with the classical Maxwell first-order slip boundary condition, including the tangential gradient effect due to the wall curvature, and Smoluchowski first order temperature jump definition. The details of the flow structure near the corner are analyzed, investigating the competing effects of rarefaction and compressibility on the channel performances. The flow characteristics in terms of velocity profiles, slip velocity distribution along inner and outer wall, pressure, average Mach number along central line of the channel have been presented. The results showed that impact of the bend on the channel performances is smaller at high rarefaction levels. The behaviour of pressure and velocity away from the bend is similar to that of a straight microchannel; however, the asymmetry in the flow at the bend, with high velocities and high velocity gradients on its inner side, has a strong impact on wall slip velocities. The presence of a recirculation is detected on both the inner and outer walls of the corner for larger Reynolds. However, rarefaction may delay the onset of recirculation. It is also observed that the mass flux through a bend microchannel can even be slightly larger than that through a straight microchannel of the same length and subjected to the same pressure difference.

Commentary by Dr. Valentin Fuster
2010;():459-466. doi:10.1115/FEDSM-ICNMM2010-30587.

The effects of variable physical properties on the flow and heat transfer characteristics of simultaneously developing slip-flow in rectangular microchannels with constant wall heat flux are numerically investigated. A co-located finite-volume method is used in order to solve the mass, momentum and energy equations in their most general form. Thermophysical properties of the flowing gas are functions of temperature, while density and Knudsen number are allowed to change with both pressure and temperature. Different Knudsen numbers are considered in order to study the effects of slip-flow. Simulations indicate that the constant physical property assumption can result in under/over-prediction of the local friction and heat transfer coefficients depending on the problem configuration. Density and thermophysical property variations have significant effects on predicting flow and heat transfer characteristics since the gas temperature constantly changes as a result of the applied wall heat flux. Heat transfer coefficient is affected both due to the change in the velocity field and change in thermophysical properties. Also temperature dependence of the local Knudsen number can significantly alter the friction coefficients due to its strong dependence on slip conditions. The degree of discrepancy varies for different cases depending on the Knudsen number, and the applied heat flux strength and direction (cooling versus heating).

Commentary by Dr. Valentin Fuster
2010;():467-473. doi:10.1115/FEDSM-ICNMM2010-30622.

In the case of micro-channels, the boundary layer is formed on the walls and it plays a role of a wall of a converging and diverging nozzle. Then, the outlet Mach number is beyond unity even if the tube is straight. Therefore, in the present study, the successive incident and reflected shock waves on underexpanded state jet from a straight micro-tube whose diameter ranges from 150μm to 500μm were visualized by Schlieren and Shadowgragh methods. The stagnation pressure ranges from 597 to 963 kPa. The flow characteristics on supersonic jet at the micro-tube outlet were also obtained. Also, it is confirmed that Mach number at a straight micro-tube outlet is beyond unity since the shock wave generates from the needle and the Mach number fluctuates in the jet. The experimental correlation for the distance from the micro-tube outlet to the Mach disk as a function of the ratio of stagnation pressure to ambient back pressure was proposed and compared with available correlations in literature.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2010;():475-480. doi:10.1115/FEDSM-ICNMM2010-30652.

This paper presents a numerical investigation based on the Lattice Boltzmann method for gaseous flow in a microchannel with rectangular grooves on the walls. Firstly, the prepared computer code is validated with comparison the obtained results with analytic solution for fully developed rarefied gas flow in a simple micro channel. The effects of the rectangular grooves on the flow characteristics and heat transfer behavior are discussed and the results are compared with a simple mircochannel. For this purpose, a two-dimensional constant wall temperature microchannel with air as the coolant is investigated. The results show that the heat removal increases for the grooved channel more than 50% compared with a simple microchannel. But the pressure drop is also increases due to the effects of grooves. The increase in friction factor is more than twice for some Knudsen numbers.

Commentary by Dr. Valentin Fuster
2010;():481-490. doi:10.1115/FEDSM-ICNMM2010-30743.

We investigate whether a power-law form of probability distribution function better describes the free paths of dilute gas molecules in a confined system. An effective molecular mean free path model is derived, which allows the mean free path to vary close to bounding surfaces. Our model is compared with molecular dynamics simulation data, and also other classical mean free path models. As gas transport properties can be related to the mean free path through kinetic theory, the Navier-Stokes constitutive relations are then modified and applied to various benchmark test cases. Results for isothermal pressure-driven Poiseuille flows in micro-channels are reported, and we compare our results with conventional hydrodynamic models, solutions of the Boltzmann equation, and experimental data. Our new approach provides good results for mean free path and cross-sectional flow velocity profiles up to Knudsen numbers around 1, and for integral flow parameters such as flow rate and friction factor up to Knudsen number of 10. We discuss some limitations of our power-law model, and point to the way forward for further development.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2010;():491-501. doi:10.1115/FEDSM-ICNMM2010-30968.

A numerical investigation of transient performance of 3 D linear micronozzles has been performed. The baseline model for the study is derived from the NASA/Goddard Space Flight Center MEMS-based hydrogen peroxide micro-thruster prototype. The 3 D micronozzles investigated here have depths of 25 μm, 50 μm, 100 μm, and 150 μm and employ expanders with a 30° half-angle. A hyperbolic-tangent actuation profile is used to model the opening of a microvalve in order to simulate start-up of the thruster. The inlet stagnation pressure when the valve is fully opened is 250 kPa and generates a maximum throat Reynolds number of Remax ∼ 800 . The complete actuation occurs over 0.55 ms and is followed by 0.25 ms of steady-state operation. The propulsion scheme employs 85 % pure hydrogen peroxide as a monopropellant. Simulation results have been analyzed and thrust production as a function of time has been quantified along with the total impulse delivered. Micronozzle impulse efficiency has also been determined based on a theoretical maximum impulse achieved by a quasi-1 D inviscid flow responding instantaneously to the actuation profile. It is found that both the flow and thrust exhibit a response ‘lag’ to the time-varying inlet pressure profile. Simulations are compared to previous 2 D results and indicate that thrust per unit nozzle depth, impulse, and efficiency increase with nozzle depth and approach the 2 D results for nozzle depths greater than 150 μm.

Commentary by Dr. Valentin Fuster
2010;():503-514. doi:10.1115/FEDSM-ICNMM2010-31069.

The entropy generation rate has become a useful tool for evaluating the intrinsic irreversibilities associated with a given process or device. This work presents an analytical solution for entropy generation in hydrodynamically fully developed thermally developing laminar flow in a microtube. The rarefaction effects as well as viscous heating effects are taken into consideration, but axial conduction is neglected. Using fully developed velocity profile, the energy equation is solved by means of integral transform. The solution is validated by comparing the local Nusselt numbers against existing literature data. From the results it is realized that the entropy generation decreases as Knudsen number increases, while the effect of increasing values of Brinkman number and the ratio of Brinkman number to dimensionless temperature difference is to increase entropy generation. The average entropy generation number over the cross section of channel increases with increasing values of axial coordinate, until it reaches a constant value at fully developed conditions.

Commentary by Dr. Valentin Fuster
2010;():515-526. doi:10.1115/FEDSM-ICNMM2010-31070.

The issue of entropy generation in laminar forced convection of a Newtonian fluid through a slit microchannel is analytically investigated by taking the viscous dissipation effect, the slip velocity and the temperature jump at the wall into account. Flow is considered to be hydrodynamically fully developed but thermally developing. The energy equation is solved by means of integral transform. The results demonstrate that to increase Knudsen number is to decrease entropy generation, while the effect of increasing values of Brinkman number and the group parameter is to increase entropy generation. Also it is disclosed that in the thermal entrance region the average entropy generation number over the cross section of channel is an increasing function of axial coordinate.

Commentary by Dr. Valentin Fuster
2010;():527-529. doi:10.1115/FEDSM-ICNMM2010-31232.

Along with the progress in micro- and nano-technologies, such as Micro Electro Mechanical Systems (MEMS) and μ-TAS (Micro-Total Analysis Systems), the Knudsen number, which is a non dimensional parameter for rarefaction, of the flow around and inside the systems becomes large. In such high Knudsen number flows, gas-surface interaction has become important for flow field analyses. To illustrate overall gas-surface interaction without any detailed processes, an accommodation coefficient, α, is the most widely used as an empirical parameter for a practical purpose. One of accommodation coefficients, the tangential momentum accommodation coefficient (TMAC) αt , is in closely related to the loss of the pressure through a micro channel. Therefore, TMAC is an important coefficient for flow inside micro/nano fluidic devices. To obtain TMAC from experiments, the mass flow rate measurements in a microtube were carried out using the constant volume method. The results obtained from the experiments were analyzed in frame of the Navier-Stokes equation associated with the second order velocity slip boundary condition. The mean Knudsen number was less than 0.3, where the velocity slip boundary condition is applicable. From the mass flow rates, the slip coefficient of the boundary condition was obtained, and then, TMAC was determined. The experimental apparatus showed very low leakage rate, and TMAC was determined with a high degree of accuracy. The TMACs of the same surface material with different dimensional parameters were compared for validation of the system.

Topics: Momentum
Commentary by Dr. Valentin Fuster
2010;():531-539. doi:10.1115/FEDSM-ICNMM2010-30473.

Micro heat sinks have a broad applicability in many fields such as aerospace applications, micro turbine cooling, micro reactors electronics cooling and micro biological applications. Among different types of micro heat sinks, those with micro pin-fins are becoming popular due to their enhanced heat removal performance. However, relevant experimental data is still scarce and few optimization studies are present in the literature. In order to effectively optimize their performance an extensive parametric study is necessary and should be based on a realistic model. Moreover, micro pin fin heat sinks should be optimized according to appropriate performance criteria depending on the application. The objective of this paper is to fill the research gap in micro pin fin heat sink optimization based on realistic configurations. In this paper, the parameters for micro pin optimization are the pin-fin height over diameter ratio (0.5<H/D<5) and the longitudinal and transverse pitch ratios (1.5<(SL, ST)/D<5), while Reynolds number and heat flux provided from the base of the micro heat sink are in the range of (1<Re<100) and (20<q(W/cm2 )<500) respectively. In this research micro pin fin heat sinks are three dimensionally modeled on a one-to-one scale with the use of commercially available software COMSOL Multiphasics 3.5a. Full Navier-Stokes equations subjected to continuity and energy equations are solved for stationary conditions. To have increased computational efficiency, half of the heat sink is modeled with the use of a symmetry plane. In order to validate the use of numerical models parametric values from previous experimental data available in the literature are exactly taken and simulated. The numerical and experimental results show a good agreement. After this validation optimization study is performed using the three dimensional numerical models.

Commentary by Dr. Valentin Fuster
2010;():541-549. doi:10.1115/FEDSM-ICNMM2010-30513.

As an alternative to massive CFD, a hybrid technique, which has the advantage of accounting for all of the three-dimensional features of the flow field, but with a limited computational effort, is used for the solution of conjugate convection-conduction heat transfer problems in cross-flow micro heat exchangers. The key feature of the proposed method is represented by the separate computation of the velocity fields in single microchannels and on the subsequent mapping of such velocity fields onto the three-dimensional grid used to solve the thermal problem. The cross-flow micro heat exchangers considered in the paper consist of a number of layers of rectangular microchannels. A parametric study is carried out on the combined effect on cross-flow micro heat exchanger thermal performances due to the variation of the microchannel cross-section and of the ratio of solid to fluid thermal conductivity.

Commentary by Dr. Valentin Fuster
2010;():551-558. doi:10.1115/FEDSM-ICNMM2010-30711.

Heat transfer and flow behavior in a mini-tube bank was examined. The tube bank was simulated with wires of 1 mm diameter. The wires were arranged in the 5×5 in-line array and the 5×5 staggered array with the arranging pitch = 3. Experiments were performed in the range of the tube Reynolds number Re = 4 ∼ 3,500. Numerical analyses were also performed with the commercial CFD code of STAR-CD. The heat transfer coefficient of the tube of the first row was well expressed with the existing heat transfer correlations. In the case of the in-line array, unlike usual sized tube banks, the measured heat transfer coefficients of the tubes after the second row were lower than those of the first row and the difference between those increased as the Reynolds number was increased. At approximately Reynolds number ≃ 50, the difference turned to decrease; the heat transfer coefficients initiate to recover to the first row value. Then, the heat transfer coefficient in the rear row became larger at approximately Re ≃ 1,000 than that of the first row. In the case of the staggered array, the decrease in the heat transfer coefficient in the rear row was smaller than that in the case of the in-line array. The recovery of the heat transfer coefficient to the first row value started at a little bit lower Reynolds number and it exceeded the first row value at approximately Re ≃ 700. The flow visualization results and also the STAR-CD analytical results indicated that when the Reynolds number was low, the wake region of the preceding tube was stagnant. This flow stagnation caused the heat transfer deterioration in the front part of the rear tube, which resulted in the lower heat transfer coefficient of the rear tube than that of the first row. As the Reynolds number was increased, the flow state in the wake region changed from the stagnant condition to the more disturbed condition by periodical shedding of the Karman vortex. This change caused the recovery of the heat transfer in the front region of the rear tube, which resulted in the recovery of the heat transfer coefficient of the rear tube.

Commentary by Dr. Valentin Fuster
2010;():559-567. doi:10.1115/FEDSM-ICNMM2010-30797.

Chaotic fluid mixing is generally considered to enhance fluid-wall heat transfer and thermal homogenisation in laminar flows. However, this essentially concerns the transient stage towards a fully-developed (thermally-homogeneous) asymptotic state and then specifically for high Péclet numbers numbers Pe (convective heat transfer dominates). The role of chaos at lower Pe under both transient and asymptotic conditions, relevant to continuous thermal processes as e.g. micro-electronics cooling, remains largely unexplored to date. The present study seeks to gain first insight into this matter by the analysis of a representative model problem: heat transfer in the 2D time-periodic lid-driven cavity flow induced via non-adiabatic walls. Transient and asymptotic states are investigated in terms of both the temperature field and the thermal transport routes. This combined Eulerian-Lagrangian approach enables fundamental investigation of the connection between heat transfer and chaotic mixing and its ramifications for temperature distributions and heat-transfer rates. The analysis exposes a very different role of chaos in that its effectiveness for thermal homogenisation and heat-transfer enhancement is in low-Pe transient and asymptotic states marginal at best. Here chaos may in fact locally amplify temperature fluctuations and thus hamper instead of promote thermal homogeneity. These findings reveal that optimal thermal conditions are at lower Pe not automatic with chaotic mixing and may depend on a delicate interplay between flow and heat-transfer mechanisms.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster
2010;():569-574. doi:10.1115/FEDSM-ICNMM2010-30990.

Attributed to its high heat transfer coefficient, evaporating cooling involving the use of micro heat exchangers is considered a possible thermal management solution for cooling of high heat flux electronic devices. The desire to develop high-performance micro heat exchangers operating in the evaporation regime provides a major motivation for the present work. Methanol evaporated in two micro heat exchangers with chevron flow passages and straight flow passages respectively were tested in the present study. The test results show that the heat transfer coefficient increased with increasing flow rate in both chevron and straight flow passages micro heat exchangers. However, the effect of vapor quality on the heat transfer coefficient in the straight passages heat exchanger is in adverse to that in the chevron passages heat exchanger. The heat transfer coefficient increased with increasing vapor quality in the chevron passages heat exchanger but decreased in the straight passages heat exchanger. The flow visualization through transparent cover heat exchangers showed that the liquid film inside channel is partially dry out in the straight passages heat exchanger. The dryout portion area increased with increasing heating rate and exit vapor quality. This degraded the average heat transfer performance for evaporation in the straight passages heat exchanger. Because of the surface tension effect, the liquid film was dragged at the intersection corner of the upper and lower plate chevron passages. There is no significant dryout portion in the chevron passages heat exchanger. The relation of vapor quality with heat transfer performance in chevron passages heat exchanger is therefore similar to the boiling in a single channel prior to critical heat flux condition.

Commentary by Dr. Valentin Fuster
2010;():575-583. doi:10.1115/FEDSM-ICNMM2010-31268.

A parametric investigation has been performed to study the different operating limits of heat pipes employing a novel type of metal foam as wick for chip cooling applications. These foams have a unique spherical pore cluster microstructure with very high surface to volume ratio compared to traditional metal foams and exhibit higher operating limits in preliminary tests of heat pipes, suggesting high cooling rates for microelectronics. In the first part of this parametric study, widely used correlations are applied to calculate the five types of heat transfer limits (capillary, boiling, viscous, entrainment and sonic) as a function of temperature, type of foam, and porosity. Results show that the dominant limit is mostly the capillary limit, but for 50 pore-per-inch (PPI) foam, the boiling limit will be dominant. Also, 50 and 60 PPI foams have higher heat transfer limits than sintered copper powder. In the second part of this study, thermodynamic steady state modeling of a flat heat pipe has been done to study the effect of the different parameters on the dominant limit (capillary). A dimensionless number has been proposed to evaluate the balance between the pressure loss in the vapor and liquid phases as an additional design guideline to improve the capillary limit in flat heat pipes.

Commentary by Dr. Valentin Fuster
2010;():585-590. doi:10.1115/FEDSM-ICNMM2010-30114.

We consider the two-dimensional problem of steady natural convection in a narrow horizontal cylindrical annulus filled with porous medium due to a constant temperature variation on the outer and adiabatic conditions at the inner boundaries with constant volumetric heat flux. The solution is expanded in powers of a single combined similarity parameter, which is the product of the gap ratio to the power of two, and Rayleigh number. The series is extended by means of symbolic calculation up to 28 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is small, but Pade approximation leads our results to be good even for much higher value of the similarity parameter.

Commentary by Dr. Valentin Fuster
2010;():591-598. doi:10.1115/FEDSM-ICNMM2010-30127.

We have developed a model capable of predicting the performance characteristics of a wiretype Joule-Thomson microcooler intended for use within a cryosurgical probe. Our objective was to be able to predict evaporator temperature, temperature distribution and cooling power using only inlet gas properties as input variables. To achieve this, the model incorporated changing gas properties due to heat transfer within the heat exchanger and isenthalpic expansion within the capillary. In consideration of inefficiencies, heat in-leak from free convection and radiation was also considered and the use of a 2D axisymmetric finite difference code allowed simulation of axial conduction. Two types of microcoolers differing in inner tube material, poly-ether-ether-ketone (PEEK) and stainless steel, were tested and simulated. CO2 was used as the coolant gas in the calculations and experimental trials for inlet pressures from 0.5 MPa to 2.0 MPa. Heat load trials of up to 550 mW along with unloaded trials were conducted. Comparisons to experiments show that the model was successfully able to obtain a good degree of accuracy. For the all PEEK microcooler in a vacuum using 2.0 MPa inlet pressure, the calculations predicted a temperature drop of 57 K and mass flow rate of 19.5 mg/s compared to measured values of 63 K and 19.4 mg/s therefore showing that conventional macroscale correlations can hold well for turbulent microscale flow and heat transfer as long as the validity of the assumptions is verified.

Topics: Joules
Commentary by Dr. Valentin Fuster
2010;():599-604. doi:10.1115/FEDSM-ICNMM2010-30176.

Nanostructures exhibit both nanofluidic and nanophotonic phenomena that can be exploited in sensing applications. In the case of nanohole arrays, the role of surface plasmons on resonant transmission motivates their application as surface-based biosensors. Research to date, however, has focused on dead-ended (or ‘blind’) holes, and therefore failed to harness the benefits of nanoconfined transport combined with plasmonic sensing. A flow-through nanohole array format presented here enables biomarker sieving and rapid transport of reactants to the sensing surface. Proof of concept operation is demonstrated and compared with previous methods. The various transport mechanisms are characterized with the aim to utilize the metallic plasmonic nanostructure as an active element in concentrating as well as detecting analytes. The invited presentation will provide an overview of all our experimental, computational and analytical work in this area. This paper is focused on the analysis and evaluation of flow-through nanohole arrays for analyte sensing.

Commentary by Dr. Valentin Fuster
2010;():605-612. doi:10.1115/FEDSM-ICNMM2010-30181.

Though uncommon in most microfluidic systems due to the dominance of viscous and capillary stresses, it is possible to drive microscale fluid flows with considerable inertia using surface acoustic waves (SAWs), which are nanometer order amplitude electro-elastic waves that can be generated on a piezoelectric substrate. Due to the confinement of the acoustic energy to a thin localized region along the substrate surface and its subsequent leakage into the body of liquid with which the substrate comes into contact, SAWs are an extremely efficient mechanism for driving fast microfluidics. We demonstrate that it is possible to generate a variety of efficient microfluidic flows using the SAW. For example, the SAWs can be exploited to pump liquids in microchannels or to translate free droplets typically one or two orders of magnitude faster than conventional electroosmotic or electrowetting technology. In addition, it is possible to drive strong microcentrifugation for micromixing and bioparticle concentration or separation. In the latter, rich and complex colloidal pattern formation dynamics have also been observed. At large input powers, the SAW is a powerful means for the generation of jets and atomized aerosol droplets through rapid destabilization of the parent drop interface. In the former, slender liquid jets that persist up to centimeters in length can be generated without requiring nozzles or orifices. In the latter, a monodispersed distribution of 1–10 micron diameter aerosol droplets is obtained, which can be exploited for drug delivery and encapsulation, nanoparticle synthesis, and template-free polymer array patterning.

Commentary by Dr. Valentin Fuster
2010;():613-623. doi:10.1115/FEDSM-ICNMM2010-30349.

In this paper, the experimental investigation on friction factor and heat transfer of single phase liquid flow and two-phase boiling flow in single zigzag micro-channel with micro-orifice at inlet has been conducted. The dimension of the micro-orifice is 1mm×0.227mm×0.25mm. The experiment was conducted in copper rectangle zigzag micro-channel with the hydraulic diameter of 0.321mm and the length of 29 mm. The experimental results of friction factor and heat transfer coefficient were provided and the effect of the micro-orifice was discussed. It was found that the friction factor of flow in zigzag micro-channel deviated largely from the predictive value in the laminar flow, while it coincided well with the correlation for turbulent flow. In addition, the variation of local heat transfer coefficient showed that the inlet restrictor has significant effect on heat transfer of boiling flow in micro-channel.

Commentary by Dr. Valentin Fuster
2010;():625-632. doi:10.1115/FEDSM-ICNMM2010-30364.

How the energy transfers during electron-phonon nonequilibrium in thin metal films is still an open question, and how to measure the intrinsic thermal transport properties of the material under the covering layer is another challenge. In this paper, the heat transfer process from electron-phonon nonequilibrium in thin gold film to borosilicate glass substrate has been studied by resorting to different segments of the transient thermoreflectance signal, which is obtained from the rear-pump front-probe transient thermoreflectance technique. The gold film, which has a thickness of 23.1 nm, is deposited on the borosilicate glass substrate using using a physical vapor deposition (PVD) approach. Within the framework of the two-temperature model (TTM), the electron-phonon (e-ph) coupling factors of the gold film, which reflect the strength of heat flow from hot electrons to cold phonons, are derived from the signal taken after the first several picoseconds with different pump fluences, and the measured value is (1.95–2.05)×1016 W m−3 K−1 . The electron-phonon coupling factor does not significantly change in response to the pump pulse fluence variation and exhibits little change compared to the bulk gold value 2.4×1016 W m−3 K−1 . Furthermore, the thermal conductivity of the glass substrate is obtained through the thermoreflectance signal between 20 to 140 picoseconds and the value is W m−1 K−1 .

Topics: Heat transfer ,