bluuebunny/crossref_metadata_2025 huggingface dataset:
Behavior of a Mini Synthetic Jet in a Transverse Wall Flow: Experimental and Numerical Study
Authors: Batikh Ahmad , Baldas Lucien , Caen Robert , Colin Ste´phane , Kourta Azeddine , Boisson Henri-Claude
Published: June 2009

A synthetic jet is a time-averaged fluid motion generated by sufficient strong oscillatory flow downstream from a sudden expansion. The study of the interaction between the synthetic jet and an external flow is of great interest in particular for aeronautical applications. A network of such actuators could be used indeed on airplane wings for example to control, with a good energetic efficiency, the boundary layer separation in order to increase the lift or the laminar/turbulent transition for drag reduction. In this paper, the interaction of a sub-millimetric synthetic jet actuator with an external flow is experimentally studied. In these experiments, the actuation is ensured by acoustic excitations produced by a loud-speaker. Hot-wire anemometry is used for measurement of the velocity field in various functioning configurations (velocity of the transversal flow, actuation frequency, ...). In most of the tested configurations, the boundary layer of the transversal flow is significantly modified by the synthetic jet, which shows that these types of actuators could be efficiently used for flow control purposes.
Stochastic Dynamics of Thin Liquid Films
Authors: Willis Adam M. , Freund Jonathan B.
Published: June 2009

It has been suggested that stochastic thermal fluctuations directly affect the dynamics and stability of thin liquid films. We investigate this by comparing atomistic simulations of spreading drops with predictions from stochastically forced lubrication equations. Initially the drops spread according to Tanner’s law, l ∼ t1/7, but then accelerate as predicted for cases where fluctuating thermal stresses govern the process.
Mixing Enhancement in Electroosmotic Microchannels by Applying Periodical Secondary DC Voltage
Authors: Kim Mansuck , Jeong Siyoung
Published: June 2009

Electroosmotic flow (EOF) in microchannels induced by the electric field could purvey attractive implements to control the fluid motion in microfluidic devices such as LOCs. Most of electroosmotic flows in microchannels is laminar and the mixing occurs very slowly. In this paper, a new method is suggested to enhance the mixing in electroosmotic microchannels. In our preliminary experiments on an electroosmotic microchannel made of PDMS/glass and having the hydraulic diameter of about 150μm, interesting phenomena happened to be observed when a periodical DC voltage was applied to the secondary anode located at the middle of an electroosmotic microchannel. More specifically, a inverse flow was observed in the center area of the channels especially between the secondary anode and the existing cathode electrode when the secondary DC voltage was applied; and the flow recovered the initial direction when the secondary voltage was stopped. The reverse flow in the center area seemed to occur due to the combination of the increased flow velocity near the wall and the repulsive force which prevents the cations between the primary and secondary anode from flowing to the same direction as the primary osmotic flow. It is thought that these unsteady flow phenomena could be used to improve the mixing which is often required in microfludic devices. The effect of important parameters, such as the frequency of the secondary voltage and the location of the secondary anode, will be further investigated and discussed in detail.
Micro-Refrigerator Fabricated on Silicon Wafer
Authors: Takata Y. , Sugahara K. , Tachikawa T. , Moroe S. , Kubota H. , Kohno M. , Takahashi K. , Koshimizu T.
Published: June 2009

A prototype Joule-Thomson micro-cooler was fabricated on silicon wafer by making use of photofabrication. The micro-cooler uses ethylene as a refrigerant and it consists mainly of heat exchanger and evaporator. The cooling power of 20mW at evaporator temperature of 272K was attained at the inlet and outlet gas pressures of 2.5MPa and 0.1MPa, respectively. To understand the low cooling performance, numerical analysis of heat exchanger has been done and the effects of mass flow rate and thermal conductivity of solid on temperature profiles and effectiveness were examined. It was found that the flow rate of present experiment is too large and the decrease in flow rate gives better temperature effectiveness of heat exchanger. It was also found that the low thermal conductivity of solid improves the performance of heat exchanger.
Scaling of Flow Characteristics and Power Consumption With Profile Height for Miniature Centrifugal Fans
Authors: Walsh P. A. , Egan V. , Grimes R. , Walsh E.
Published: June 2009

This paper addresses issues that relate to downscaling the height of centrifugal fans for application in low profile technologies, such as the cooling of portable power electronics. The parameters studied throughout the paper include flow rate, pressure rise and power consumption characteristics. The former two of these are measured using a fan characterization rig and the latter by directly measuring the power supplied to the fan. These are studied for fans ranging in diameter from 15 to 30mm and with profile heights ranging from 0.3mm to 15mm. It is found that all of the phenomena encountered are best described in terms of fan aspect ratio. Overall, the results show that the conventional scaling laws cannot be accurately applied when the blade profile alone is being scaled. Indeed the only parameter that was observed to be accurately predicted by the scaling laws was the pressure rise attainable but was only accurate for fan aspect ratios greater than 0.17. Below this, the measured pressure rise characteristics fell logarithmically toward zero. The results also showed that there is no advantage to using fans with aspect ratio greater than 0.3. This was because the maximum flow rate was achieved at this aspect ratio and decreased slightly as it was further increased. Overall, the scaling phenomena described throughout this paper are invaluable to designer of efficient low profile cooling solutions that are to incorporate such fans.
Molecular Dynamics Simulation of Marangoni Flow in a Cavity
Authors: Fuchs G. , Kuhlmann H. C.
Published: June 2009

The flow in two liquids inside a rectangular side-heated cavity, driven by a thermal-gradient-induced Marangoni effect along the common interface, is investigated numerically using molecular dynamics simulations. For this configuration the surface-tension gradient drives two counter-rotating vortices inside the cavity. The model fluids are simple Lennard-Jones type fluids, selected for varying liquid-solid contact angles. The velocity field is determined and compared to analytical results of the continuum creeping-flow approximation of this configuration.
Thermal Management of Low Profile Applications
Authors: Walsh Ed , Walsh Pat , Grimes Ronan , Egan Vanessa
Published: June 2009

There is an increasing need for low profile thermal management solutions for applications in the range of five to ten watts targeted at portable electronic devices. This need is emerging due to enhanced power dissipation levels in portable electronics. This work focuses upon the optimization of such a solution within certain constraints of profile and footprint area. A number of fan geometries have been investigated where both the inlet and exit rotor angles are varied relative to the heat conducting fins on a heat sink. The ratio of fan diameter to heat sink fin length was also varied. The objective was to determine the optimal solution from a thermal management perspective within defined constraints. The results show good thermal performance for low profile thermal management solutions, and highlight the need to develop the heat sink and fan as an integrated thermal solution rather than in isolation as is the traditional methodology. It is also found that while increasing pumping power generally improves the thermal performance, only small gains are achieved for relatively large pumping power increases. This is important in optimizing portable systems which are powered by limited battery life.
Effects of Flow Field and Diffusion Layer Properties on Water Accumulation in a PEM Fuel Cell
Authors: Owejan J. P. , Trabold T. A. , Jacobson D. L. , Arif M. , Kandlikar S. G.
Published: June 2009

Water is the main product of the electrochemical reaction in a proton exchange membrane (PEM) fuel cell. Where the water is produced over the active area of the cell, and how it accumulates within the flow fields and gas diffusion layers, strongly affects the performance of the device and influences operational considerations such as freeze and durability. In this work, the neutron radiography method was used to obtain two-dimensional distributions of liquid water in operating 50 cm2 fuel cells. Variations were made of flow field channel and diffusion media properties, to assess the effects on the overall volume and spatial distribution of accumulated water. Flow field channels with hydrophobic coating retain more water, but the distribution of a greater number of smaller slugs in the channel area improves fuel cell performance at high current density. Channels with triangular geometry retain less water than rectangular channels of the same cross-sectional area, and the water is mostly trapped in the two corners adjacent to the diffusion media. Also, it was found that cells constructed using diffusion media with lower in-plane gas permeability tended to retain less water. In some cases, large differences in fuel cell performance were observed with very small changes in accumulated water volume, suggesting that flooding within the electrode layer or at the electrode-diffusion media interface is the primary cause of the significant mass transport voltage loss.
The Characteristics of CH4-Vapor Catalytic Reforming Reaction Considering CO2 and CH4 Reaction in the Micro-Chamber
Authors: Ran Jing-Yu , Niu Li-Xiang , Tang Qiang , Zhang Li
Published: June 2009

Methane and vapor catalytic-reaction is a complex reaction system, and especially CH4/CO2 reaction has an important influence to the methane/vapor reforming reaction. In this paper, the reaction character for methane and vapor catalytic reforming reaction in the micro-chamber wall with Ni catalyst is numerically investigated. The results show that the CH4/CO2 reaction has a vital influence on reactive characteristics in the different H2O/CH4 mole ratio and the mass flow-rate. With increasing the H2O/CH4 mole ratio, the concentration of H2 and CO2 increases, the concentration of CO increases and then decreases, but if the H2O/CH4 mole ratio is more than 2.5, the result is different. The reaction efficiency will descend while the flow-rate increases. The results also display that the methane conversion ratio, the vapor conversion ratio, and the hydrogen concentrations can be up to 81.73%, 69.42%, and 4.29%, while the H2O/CH4 mole ratio, flow-rate and methane/vapor mass flow-rate ratio are 2.5, 7 g/h and 0.1 respectively.
Effects of Mechanical Constraint and Heat Capacity on Vapor Explosions
Authors: Furuya Masahiro , Arai Takahiro
Published: June 2009

Small-scale experiments have been conducted to investigate the triggering mechanism of vapor explosions. In order to attain good repeatability and visibility, a smooth round water droplet was impinged onto a molten alloy surface. This configuration suppresses premixing events prior to triggering. The effect of the water droplet curvature was found to be negligibly small when the droplet diameter is larger than 4.5 mm. Vapor explosion conditions were identical for the molten tin pool depths ranging from 0.5 to 40 mm. The experimental results and the heat conduction analysis suggest that the length scale required for atomizing and fine mixing in the triggering event of the vapor explosion are sufficiently smaller than the molten tin pool depth of 0.5 mm. Six different kinds of materials were used as the pool liquid. The lower limit of the contact temperature in the vapor explosion region closely agrees with the spontaneous nucleation temperature of water. The upper limit of the initial molten alloy temperature decreases when an oxide layer forms on the surface causing an increase of the emissivity of thermal radiation that has a stabilizing effect on the vapor film. When an oxide layer formed on the surface, a water droplet was occasionally entrapped into a molten alloy dome, since the oxide layer prevents the droplet from evaporating coherently. The vapor explosion region obtained for the mirror surface is a conservative estimate, since that for the oxide surface fell into the internal region of mirror surface.
Compact Model of Slug Flow in Microchannels
Authors: Kim Dong Rip , Koo Jae-Mo , Fang Chen , Steinbrenner Julie E. , Lee Eon Soo , Wang Fu-Min , Hidrovo Carlos H. , Eaton John K. , Goodson Kenneth E.
Published: June 2009

This paper presents a theoretical investigation of the movement of liquid droplets and slugs in hydrophobic microchannels and develops a compact model for this type of two-phase flow. This model is used in the prediction of pressure drop and liquid water coverage ratio, key parameters in the operation of Proton Exchange Membrane Fuel Cells (PEMFC), the primary motivation for this work. A semi-empirical, periodic-steady two-phase separated flow compact model is formulated to characterize the slug flow behavior. The momentum equation includes the effects of acceleration, friction and surface tension on the pressure drop. The model considers spatial changes in slug velocity through the use of a force balance formulation. The model uses a departure scheme that computes slug size and shape at entrainment. The steady state slug flow compact model is capable of predicting liquid water coverage ratio and pressure drop using liquid and gas flow rates and advancing/receding triple point contact angles as its only inputs. The results indicate that the pressure drop increases as the droplet formation frequency increases.
Nanosystem Implementation Using Nanochannels of Nanoporous Membranes
Authors: Das Biswajit
Published: June 2009

We are currently developing a novel fabrication technique for the implementation of nanosystems utilizing the nanochannels in nanoporous membranes. The technique is CMOS-compatible and has the potential for volume commercial manufacturing. The technique is based on the anodization, or electrolytic oxidation, of a thin film of aluminum to form a nanoporous alumina membrane, which is used as a guide to implement the nanosystems. The underlying principle of the fabrication technique is that when aluminum is anodized in a suitable acidic electrolyte under controlled conditions, it oxidizes to form a hydrated aluminum oxide (alumina) containing a two dimensional hexagonal array of cylindrical pores. The pore diameter and the inter-pore spacing depend on the anodization conditions and the substrate parameters, and can be varied between 4 nm to 100s of nm; the pores can be several microns deep. Due to the excellent periodicity of the pores, and the ability to control the pore diameters, such anodized alumina films can be used as templates for the fabrication of periodic arrays of nanostructures. In fact, the pores in alumina templates have been used to synthesize a variety of metal and semiconductor nanostructures. In addition, the template can also be used as a mask for pattern transfer to create periodic arrays of pores on a substrate. While most of the work in this field has focused on bulk aluminum, the use of a bulk aluminum substrate precludes most photonic and electronic applications. To overcome this, we have developed a thin film alumina template technology that allows the fabrication of nanoporous membranes consisting of nanochannels with diameters ranging between 4 nm to 10s of nm. By using a novel process, we convert the nanoporous templates into an array of nanochannels supported by the membrane. These nanochannels are then used as guides to deposit nanoparticles (nanodots, nanotubes and nanopillars) to form the desired nanosystem. The nanoparticles are primarily deposited by electrophoretic techniques. We are currently using this technique to implement nanosystems based on CdSe quantum dots and carbon nanotubes with applications in broad ranging fields including multispectral detectors, photonics, gas sensors and high efficiency solar cells. In this paper, we provide a description of the fabrication technique as well as some of the nanosystems currently under development.
Flow Boiling Heat Transfer of CO2 at Low Temperatures: Challenges and Opportunities
Authors: Bansal Pradeep , Zhao Xiumin
Published: June 2009

There is a growing use of CO2 refrigeration to achieve low temperatures, particularly in the food industry, however, very limited information is available in the open literature on its boiling heat transfer characteristics below (−)30°C. This paper presents an overview of the flow boiling heat transfer characteristics of pure CO2 at low temperatures, its special thermal properties and their effect on the boiling heat transfer. The paper also presents an analysis of the experimental data collected from (−)24.3°C to (−)40°C in a novel experimental rig, specifically designed to achieve low temperatures down to (−)50°C, using 4.5m long horizontal stainless steel tube of 4.57mm inner diameter. The paper highlights the limitations of existing empirical correlations by comparing their predictions with the experimental boiling heat transfer coefficients. It is expected that the data presented in this study would be beneficial to industry and designers of compact heat exchangers for CO2 at low temperatures.
Towards an Explanation of the Mechanism of Boiling Critical Heat Flux Enhancement in Nanofluids
Authors: Buongiorno Jacopo , Hu Lin-Wen , Bang In Cheol
Published: June 2009

Nanofluids exhibit a very significant enhancement of the boiling Critical Heat Flux (CHF) at low nanoparticle concentrations. This paper reviews the nanofluid boiling database in a quest for the CHF enhancement mechanism. Briefly, buildup of a nanoparticle layer on the heated surface occurs upon boiling of nanofluids. This layer changes the surface roughness and the nucleation site density and, remarkably, can improve the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces. Significant differences are also observed in the dynamic behavior of the hot spot at CHF.
Digital Microfluidics: Connecting Biochemistry to Electronic System Design
Authors: Chakrabarty Krishnendu
Published: June 2009

Microfluidics-based biochips are revolutionizing high-throughput sequencing, parallel immunoassays, blood chemistry for clinical diagnostics, DNA sequencing, and environmental sensing. The complexity of microfluidic devices, also referred to as lab-on-a-chip, is expected to become significant in the near future due to the need for multiple and concurrent biochemical assays on multifunctional and reconfigurable platforms. This paper provides an overview of droplet-based “digital” microfluidic biochips. It presents early work on top-down system-level computer-aided design (CAD) tools for the synthesis, testing and reconfiguration of microfluidic biochips. These CAD techniques allow the biochip to concentrate on the development of the nano- and micro-scale bioassays, leaving assay optimization and implementation details to design automation tools.
Suspension Mixing and Segregation in 1D, 2D, and 3D Flows
Authors: Gilchrist James F. , Gao Changbao
Published: June 2009

Particles in concentrated flowing suspension in pressure-driven flows tend to migrate away from the walls toward the center of the channel. Demixing due to shear-induced migration inhibits mixing and near wall transport. We investigate the competition between mixing and segregation in flows with complicated geometries that generate 3D chaotic advection, resulting in nontrivial concentration patterns. Using high-speed confocal laser scanning microscopy, we directly image the microspheres to measure the interplay between chaotic advection and the 3D spatial concentration profile, local particle velocities, and suspension structure.
Experimental Study on Thermophysical Properties of Nanotubes and Nanofluids
Authors: Zhang Xing , Fujii Motoo
Published: June 2009

This paper reviews the studies of the thermophysical properties of nanotubes and nanofluids and reports the experimental studies on the thermal conductivity of individual carbon nanotubes and nanofluids containing spherical and cylindrical nanoparticles. The thermal conductivity of a single carbon nanotube has been measured by a suspended sample-attached T-type nanosensor. The size effect of the different diameters on the thermal conductivity has been observed experimentally. The effective thermal conductivity and thermal diffusivity of Au/toluene, Al2O3/water, TiO2/water, CuO/water and carbon nanofibers (CNFs)/water nanofluids have been measured by using the transient short-hot-wire technique. The measured results demonstrate that the effective thermal conductivities of CNFs/water nanofluids are much greater than those of nanofluids containing spherical nanoparticles. However, the effective thermal conductivities do not show any anomalous enhancements and can be accurately predicted by the existing formulas.
Surface Micromachined PDMS Microchannels
Authors: Ganapathy Subramani Balasubramanian , Selvaganapathy Ponnambalam
Published: June 2009

A new surface micromachining technique for Poly dimethylsiloxane (PDMS) microchannels has been developed to address the leakage problem affecting traditional PDMS microchannel process with embedded tall structures or sudden topographical transitions. Bulk micromachined PDMS cannot be conformally bonded with the surfaces having tall topological features such as thick film electrodes, porous reactor beds and other structural features. Surface micromachining technique with PDMS as structural material and photoresist as a sacrificial material allows the creation of PDMS microchannels on substrates with significant topography. Adhesion of the structural layer with the substrate was characterized for different prepolymer ratios using standard tensile test and 1:3 (Curing agent: base) combination was found to be the best with maximum adhesion strength of 7.5 MPa. The effectiveness of this technique is demonstrated by the fabrication of microchannels with embedded 6μm thick Silver electrodes. The microchannels were leak proof and conformal contact between the PDMS and electrode was confirmed through SEM. The release time for microchannels was reduced to 1 min irrespective of the length of the microchannel. The extension of this technique for fabrication of multi layered microchannel structure was demonstrated through a microfluidic valve. The valve closure occurred at 6.37 kPa.
Heat Transfer Enhancement in Evaporating Mini-Layers: Effects of an Inert Gas Flow
Authors: Iorio C. S. , Kabov O. A.
Published: June 2009

When a layer of volatile liquid is subject to a flow of inert gas, a non-uniform distribution of the evaporation rate is generated all along the interface. Being evaporation stronger at the inlet boundary of the layer, because of the maximal efficiency of the inert gas flow in removing vapor from the interface, a thermal gradient along the interface is generated. Two opposite mechanisms regulate the movement of the interface: the shear stress of the gas that entrains the interface in the direction of the flow and the thermo-capillary stress that forces the interface to move against the flow direction. Moreover, because of the overall cooling of the interface due to the evaporative process, a gradient normal to the interface is also created. It results in a potentially unstable situation that is strongly influenced by the flow rate of inert gas, the layer thickness and the liquid thermo-physical properties. The goal of the present work is to study numerically if and how the dynamic evolution of the liquid layer is driven by the above-mentioned mechanisms. The main results concern the evaluation of the influence of the thermal instability patterns, eventually generated by the concurrent action of non-uniform evaporation and thermo-capillary motion, on the heat transfer at the bottom liquid. The distribution of temperature and velocity in the gas and liquid bulk phase for different mass flow rate of inert gas has also been of interest.
Air Piston Calibrator for Microflow Delivery and Measurement Systems
Authors: Su Chun-Min , Chen Ting-Hsuan , Yang Cheng-Tsair
Published: June 2009

In view of the increasing need of microflow measurement and calibration, this paper presents a liquid flow measurement device to verify the performance of microflow sensors and pumps. The measurement principle of the air piston calibrator was based on the volumetric, time-of-flight approach. The device consisted of a micro-fabricated flow channel mounted on a base chip deployed with electrodes. A gas-liquid interface was introduced in the microchannel and moved along with the liquid flow. Upon passing by the electrodes, the interface brought about resistance variations due to the different permittivity of the gas and liquid. The time period between a pair of signal detected by the electrodes and the corresponding channel volume were then used to determine the flow rate. By the combination of different sized microchannels, a capability of measuring flow rates from 10 nL/min to 1 mL/min was demonstrated. The detection electrodes were optimized prior to their fabrication by numerical simulation. To provide a desired traceability, the volume in the channel between electrode nodes was corrected by the standard flow rate delivered by the calibrated syringe pump. Results showed that, for flow rates from 0.1 μ L/min to 1 mL/min, the mean absolute relative error and standard deviation of relative error of the measurements were lower than 1.1% and 4%, respectively.
Microfabricated Cavities for Adhesive Capture of Stem Cells Under Flow
Authors: Lee Dooyoung , Rana Kuldeepsinh , Lee Karin , DeLouise Lisa A. , King Michael R.
Published: June 2009

In previous work, we have described the adhesive capture of circulating stem cells to surfaces coated with adhesive selectin protein, both in vitro and in vivo. Here we describe PDMS surfaces microfabricated to contain an array of square 80 × 80 × 80 micron cavities. These cavities are intended to provide a local bioreactor environment to culture stem cells over extended periods of time, while sheltered from the higher shear stresses of the surrounding blood flow external of the cavities. In this paper we present in vitro flow experiments with polymeric, blood cell-sized microspheres, showing the creation of stable vortices within the microscale cavities. Computational fluid dynamics (CFD) was performed to predict the velocity field within the cavity, and for comparison with experimentally determined microsphere velocities. Future work will establish the ability to place local chemoattract molecules within the cavity interior, and the ability to accumulate viable stem cells within these cavities.
Studies on Condensation of Refrigerants in a High Aspect Ratio Microchannel and in a Novel Micro-Groove Surface Heat Exchanger: Development of Micro-Condensers in Compact Two Phase Cooling Systems
Authors: Dessiatoun Serguei , Chowdhury Sourav , Al-Hajri Ebrahim , Cetegen Edvin , Ohadi Michael
Published: June 2009

Three different refrigerants, R134a, R245fa and HFE7100 were analyzed as working fluids for two-phase cooling of high heat flux electronics in a 0.7 mm hydraulic diameter 190 mm long high aspect ratio minichannel and in a newly developed micro-groove surface condenser. The latter comprised of a micro-groove surface with rectangular grooves of 84 μm in hydraulic diameter with an aspect ratio of 10.6 and headers that directed the refrigerant flow into the grooves. It was concluded that in the minichannel R245fa provides higher heat transfer coefficients compared to R134a with a significantly higher pressure drop. The saturation temperature drop in the same channel created a significant temperature drop for HFE7100 that make the application of such minichannels for cross-flow condensers with this fluid unpractical. The microgroove surface condenser provided significantly higher heat transfer coefficients compared to the minichannel condenser. The pressure drop in the micro-groove surface condenser was extremely low and imposed just 1C temperature drop on HFE7100 at it highest heat flux. The mass flux of refrigerant in the micro-groove surface condenser is significantly lower compared to conventional mini and microchannel condensers. In its current configuration, the microgroove surface condenser benefits from the possibility of an increase in mass flux resulting in a significant increase in heat transfer coefficient and just a moderate increase in pressure drop.
The Effects of Diameter and Rotational Speed on the Aerodynamic Performance of Low Profile Miniature Radial Flow Fans
Authors: Grimes R. , Walsh P. , Walsh E. , Egan V.
Published: June 2009

Space constraints in many emerging electronic systems mean that there is a growing demand for heat sinks which are low in profile. As a result, small, low profile fans are necessary. In many instances Radial flow fans are best suited. An understanding of the design and performance of these fans is therefore necessary. For radial flow fans little work has been done to quantify the deviation of aerodynamic performance from that predicted by conventional fan laws. This paper aims to address this situation, by performing measurements of pressure rise, flow rate and power consumption for 3.5mm high radial flow fan rotors ranging in diameter from 20 to 35mm over a range of speeds. Measurements presented show variations of pressure rise and flow rate with Reynolds number to be largely in accordance with trends predicted by high Reynolds number theory, with the exception of flow rates at the lower range of Reynolds numbers which fell below the predicted values. Variations in power consumption show a similar trend to those of flow rate, with power consumption obeying the fan laws for the higher Reynolds numbers investigated, but showing a large increase at the lower Reynolds numbers.
Effect of Non Uniform Flow Distribution on Single Phase Heat Transfer in Parallel Microchannels
Authors: Bapat Akhilesh V. , Kandlikar Satish G.
Published: June 2009

The continuum assumption has been widely accepted for single phase liquid flows in microchannels. There are however a number of publications which indicate considerable deviation in thermal and hydrodynamic performance during laminar flow in microchannels. In the present work, experiments have been performed on six parallel microchannels with varying cross-sectional dimensions. A careful assessment of friction factor and heat transfer in is carried out by properly accounting for flow area variations and the accompanying non-uniform flow distribution in individual channels. These factors seem to be responsible for the discrepancy in predicting friction factor and heat transfer using conventional theory.
Manifold Microchannel Cooler for Direct Backside Liquid Cooling of SiC Power Devices
Authors: Everhart Lauren , Jankowski Nicholas , Geil Bruce , Bayba Andrew , Ibitayo Dimeji , McCluskey Patrick
Published: June 2009

A unique manifold microchannel cooler has been developed based on single phase liquid forced convection flow. It is the first of its kind to directly cool the backside of a device using a manifold microchannel design which minimizes the pressure drop across the channel while allowing the maximum cooling potential and temperature uniformity. This report illustrates the fabrication sequence, packaging procedures and test set-up required to assess the cooler’s functionality. Simulations of heat dissipation and fluid flow in the assembly were conducted using the Floworks® module of Solidworks® modeling software. The cooler has been tested using a SiC diode and initial test results show very low thermal resistivities and low thermal increases of the SiC devices. Increasing power levels and higher packaging densities of today’s microelectronics create a need for improved cooling methods to improve heat transfer from power devices. This cooler is targeted at cooling small, high power SiC devices which are seeing increased use because of their fast switching speed and higher reverse bias breakdown voltage. The manifold design uses larger channels to transport fluid into and out of the cooling region and smaller crossover microchannels that are the active cooling area. This type of design minimizes pressure drop, maximizes heat transfer and allows temperature uniformity across the device area. A 25 mm × 5 mm × 1.5 mm (thick) cooler was fabricated to cool a 4 mm × 4 mm × 0.4 mm (thick) SiC diode. The cooler has been fabricated out of a single 1 mm thick silicon wafer, using deep reactive ion etching (DRIE) to create both the microchannels and the manifold channels. The manifold channels are 200 μm wide with a 250 μm pitch with an 800 μm depth. The microchannels are 20 μm wide with a 40 μm pitch with a 200 μm depth. The dimensions were chosen based on initial modeling results to maximize the ratio of the cross sectional area of the manifold to the microchannels while staying within fabrication limitations. The SiC diode is bonded directly onto the cooler using a gold tin eutectic bond. A 1.6 mm outer diameter stainless steel capillary tube is used to flow the liquid coolant into the manifold channels and is sealed with an epoxy. Experimental results show excellent thermal results with thermal resistances less than 0.1 K/(W/cm2), heat fluxes over 600 W/cm2, and small increases in device temperature. All these results come from a cooler which has a volume less than 200 mm3. The cooler design has been shown to be very durable and has a very tight fluidic seal with no leaks throughout the experimentation. It was fabricated using standard MEMS techniques and can be easily repeated. Simulations have been correlated with experimental data for a variety of pressure drops. Simulations have also shown the cooler to be capable of keeping the device under 125 C and the water temperature under 100 C (boiling) for a heat flux of 2500 W/cm2.
Efficacy of Microscopic Interconnected Channels and Surfactants on Enhancing Pool Boiling Heat Transfer
Authors: Reyes Mazzoco Rene
Published: June 2009

Nucleate pool boiling heat transfer increases with certain liquid mixtures and some coatings over the heater’s surface. The effects of these modifications are best measured by the relative values of the convective heat transfer coefficient that quantify the ability for transferring heat. The mechanisms that increase pool boiling heat transfer are reflected in the formation of smaller bubbles that escape away from the heater’s surface at a higher velocity, than those formed under not enhanced conditions. The bubble diameter depends on a chemical effect from the liquid composition acting at the bubble’s interface, and on the physical effect of the porous coverings to break the bubbles and to allow the resulting vapor flow. The reduction in bubble diameter in liquid mixtures comes from the action of intermolecular forces at the liquid-vapor interface similar to those associated to surfactants. Several studies have concentrated on increasing the heat transfer coefficient in pool using surfactants in concentrations close to the critical micelle concentration (cmc) of the surfactant in the liquid. The surfactants achieve the highest reduction of bubble diameter by accommodating the lowest surface of their molecules at the interface. However, the mixture of 16% ethanol in water also showed an increase in the convective heat transfer coefficient by producing the lowest size of bubbles from any other ethanol-water mixture. Surface tension and sessile drop contact angle for this mixture have a behavior similar to the cmc; therefore, the mixture effect on boiling is explained through the presence of ethanol-hydrated-states accommodated at the interface. Other liquid mixtures, containing propylene glycol, ethylene glycol, ethanol and water, with cmc behavior had been found through surface tension and sessile contact angle measurements, and showed that they increased the heat transfer coefficient. The mechanical effect that increases the heat transfer coefficient with porous coverings has been explained as the breaking of emerging bubbles at the heater’s surface and the proper handling of the resulting vapor flow away from the covering. Experiments with a mesh located at a distance half the bubble diameter, at a specific power supplied, released the bubbles from the heater before finishing its formation increasing their departure frequency. An array of layers of the same mesh produced and additional increment in the heat transfer coefficient if the array is accommodated to favor the gas flow out of the heater’s region.
Towards Single-Molecule Diagnostics Using Microfluidic Manipulation and Quantum Dot Nanosensors
Authors: Yeh Hsin-Chih , Puleo Christopher M. , Ho Yi-Ping , Wang Tza-Huei
Published: June 2009

In this report, we review several single-molecule detection (SMD) methods and newly developed nanocrystal-mediated single-fluorophore strategies for ultrasensitive and specific analysis of genomic sequences. These include techniques, such as quantum dot (QD)-mediated fluorescence resonance energy transfer (FRET) technology and dual-color fluorescence coincidence and colocalization analysis, which allow separation-free detection of low-abundance DNA sequences and mutational analysis of oncogenes. Microfluidic approaches developed for use with single-molecule detection to achieve rapid, low-volume, and quantitative analysis of nucleic acids, such as electrokinetic manipulation of single molecules and confinement of sub-nanoliter samples using microfluidic networks integrated with valves, are also discussed.
Non-Intrusive Pressure Measurement in Microchannels
Authors: Fultz Derek W. , Allen Jeffrey S.
Published: June 2009

Direct pressure and flow rate measurement in microsystems has been problematic due to micro and nano length scales. Commercially available pressure and flow rate transducers are typically orders of magnitude larger than the channel cross sections making these sensors impractical for microchannels. A novel, non-intrusive measurement technique has proven to be adaptable for making such measurements. The technique, based upon backscattering interferometry, correlates changes in interferometric fringe morphology to fluid compressibility. Sub-Pascal pressure detection has been demonstrated using an unfocused laser beam impinging on a fluid-filled, glass capillary tube. Fringe movement and thickness change, resulting from fluid velocity changes and transverse tube deflection, have been correlated to pressure values via the Hagen-Poiseuille relationships for laminar flow.
Modeling and Prediction of Two-Phase Refrigerant Flow Regimes and Heat Transfer Characteristics in Microgap Channels
Authors: Bar-Cohen Avram , Rahim Emil
Published: June 2009

This keynote lecture will open with a brief review of the primary two-phase flow regimes and their impact on thermal transport phenomena in tubes and channels. The Taitel and Dukler flow regime mapping methodology will then be described and applied to the two-phase flow of refrigerants and dielectric liquids in microgap channels. The effects of channel diameter, as well as alternative transition criteria, on the prevailing flow regimes in microgaps will be explored along with available criteria for microchannel behavior. Available microgap data will then be shown to reflect the dominance of annular flow and to display a characteristic heat transfer coefficient curve in such configurations. It is found that the heat transfer coefficients in the low-quality annular flow segment of this locus can be predicted by available, microtube correlations, but that the moderate-quality transition to the axially-decreasing segment occurs at substantially.
Improvements of Roughness Control in Micromachining of Silicon Microchannels
Authors: Resnik D. , Vrtacˇnik D. , Aljancˇicˇ U. , Mozˇek M. , Penicˇ S. , Amon S.
Published: June 2009

Investigation of wet anisotropic etching of silicon microchannels on (100) silicon is presented with the emphasis to simultaneously fabricate bifurcated silicon channels in <100> and <110> directions. Silicon crystal planes that are playing major role of microchannels sidewalls are characterized for their roughness and etch rates. Beside the standardly smooth {111} crystal planes, microroughness of {110} sidewalls is of particular interest. It is shown that by implementing TMAH-Triton etchant, the roughness of {110} sidewalls was significantly improved over the standard KOH-IPA or TMAH-IPA etching system without affecting the neighboring crystal plane sidewalls. Agitation of the etching solution is found to have significant influence on both, the etch rate and surface roughness. Optimal etching conditions were determined to obtain smooth microchannel sidewalls in two major directions. Furthermore, reduced convex corner underetching was obtained compared to other etching system. Two viable solutions for the connecting through holes or vias are presented by using single or double mask layer photolithography prior to performing the etching step.
Effect of Nano-Particles on Pool Boiling Heat Transfer of Refrigerant 141b
Authors: Liu Da-Wei , Yang Chien-Yuh
Published: June 2009

Fluids with nano-sized particles have been proved that may effectively enhance the single-phase convective heat transfer performance. For pool boiling heat transfer, the published test results seems conflicted to each other. Some measured heat transfer coefficient decreased with increasing particle concentration but some showed no appreciable difference. This study provides an experimental investigation on pool boiling heat transfer performance of refrigerants R-141b with and without nano-sized Au particles on horizontal plain tubes. The test results show that the boiling heat transfer coefficients increase with increasing nano-particles concentration. At particles concentration of 1.0%, the heat transfer coefficient is more than twice higher than those without nano-particles. However, the heat transfer coefficients decreased for each test after every 5 days and finally close to those of R-141b without nano-particles. The SPM investigation shows that the test tube surface roughness decreased from 0.317 μm before boiling test to 0.162 μm after test. Further investigation by TEM and Dynamic Light Scattering particle analyzer shows that the nano-particles aggregated from 3 μm before test to 110 μm after test. This results show that the nano-sized Au particles are able to significantly increase pool boiling heat transfer of refrigerant R-141b on plain tube surface. The tube surface roughness and particle size changed after boiling test. Both of these effects degrade the boiling heat transfer coefficients.
Universal Microcarriers for Microfluidic Assays
Authors: Jensen-McMullin Cynthia , Bachman Mark , Li Guann-Pyng
Published: June 2009

Bead and cell suspension based flow-through assays are popular for high throughput biological analysis. Several technologies incorporate a tagging scheme with beads to enable multiplexing. Modern flow-through systems such as flow cytometers and cell sorters are large, bulky and expensive; consequently, much research has been performed using microfluidics to miniaturize these systems. However, several problems remain with these systems, notably it remains difficult to perform manipulations on the beads (or cells), and in the case of multiplexed systems, it remains difficult to read the tags quickly. In this paper, we present a micromachined micro-carrier, referred to as a ‘micropallet’, designed to move through a microfluidic device, which helps to solve several of these problems. Micropallets are small carrier structures, micromachined out of plastic or other materials, that are used to carry attached biological or chemical samples through a microfluidic system (e.g., DNA, RNA, proteins, antibodies, adherent cells, organisms). Similar to conventional factory pallets that carry a product through an automated manufacturing line, micropallets are engineered to carry their cargo through a micro-scale system. Thus micropallets may contain shapes, structures and materials designed to interact with and work in a microfluidic system, such as for docking, sorting, manipulation and readout. Additionally, micropallets may include bar codes or other markings, and be engineered to optimally suit the cargo they carry (for example, a micropallet might contain 3-D structures and treated sections for cells, molecules or organisms to attach). Results are presented for the use of micropallets in cell assays, DNA assays and antibody assays. Micropallets may be designed to carry a sample through a microfluidic system or for use in a static assay system, enabling versatile customisation of the micropallets and flow system for design of a programmable system that interacts with the micropallets for detection, control and manipulation.
Passive Generation of Droplets in Mini and Microchannels
Authors: Wu Liang L. , Xu Wei , Bachman Mark , Li Guann-Pynn
Published: June 2009

Microfluidic systems generating droplets by oil and aqueous solutions have shown great promise in biochemical and chemical applications, each drop acting as a micro reaction vessel. A popular approach utilizes dual flow of immiscible fluids in microchannels, producing droplets by shear forces at the liquid interfaces. These systems require precise, computer controlled flow, and the resulting drops continue to move, and so are hard to localize. We have developed a unique method to generate an array of precision microdroplets on a plate that does not require careful fluid control and places the drops at specific, predetermined locations. The method utilizes geometric design of a channel to introduce instabilities in a column of water at predetermined locations with predetermined volumes ranging from a few nanoliters to microliters. Droplets form as a result of channel design, not as a result of fluid flow. Different drops can be set up at different locations and volume, then fused together to initiate a reaction. These micro-droplet arrays can be set up by hand in under one minute and can be used for high throughput or combinatorial experiments.
Effects of Microvoids, Oxide Inclusions, and Sulfide Inclusions of the Fatigue Strength of Wheel Steels
Authors: Stone Daniel H. , Robles Francisco , Dahlman Geoffrey
Published: June 2009

The effects of microscopic discontinuities on reducing the fatigue strength of steels are well known. This paper will attempt to quantify the reduction of fatigue strength due to measured quantities of microvoids, oxide inclusions, and sulfide inclusions of the fatigue strength of wheel steels. Murikami and Endo have derived a model that relates the effects of void and inclusion area and hardness on the fatigue endurance limit. In the present work is presented a unique analysis where the effect of microcleanliness is investigated. The Murikami model was used for simplicity and because the goal in this publications is to demonstrate that inclusions play an important role on wheel performance. This work summarizes the microcleanliness analysis of 113 wheels; most of such wheels were removed from revenue service due to failure. The nature of the failure indicates that the wheels failed due to a fatigue related problem. For these reason, it is of paramount interest to determine the effects of defects (i.e. voids or inclusions) on wheel performance. For this reason a simple model (Murikami) was employed to quantify the effect of defects that were measured using the microanalysis.