bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Study of Microchannel Heat Sinks With Non-Uniform Heat Flux Conditions
Authors: Ling Ling , Fan Yanfeng , Hassan Ibrahim
Published: May 2012

Higher heat flux is produced by Micro-Electro-Mechanical Systems (MEMS) because of their reduced size and increased clock speed. At the mean time, studies of non-uniform heating conditions which are more practical than uniform heating conditions are inadequate and needed urgently. Four nonuniform heating conditions are simulated in the paper. Three heat sinks with different widths of cross-linked channels locating above the center of hotspots are studied and compared to conventional straight microchannel heat sink. Half of the module geometry is chosen to be the computational domain. Two hotspots are placed at the bottom surface. The coolant is water, whose properties are dependent on temperature. Two inlet velocities, 0.5 m/s and 1 m/s, are tested for each heat sink. Temperature profile at the hotspots, pressure drop and total thermal resistance are selected as criteria of evaluating heat sink performance. All heat sinks have better performance when there is an upstream hotspot or the upstream hotspot is subjected to a higher heat flux. Cross-linked channel width of 0.5 mm has the best benefit to obtain better temperature uniformity without increasing the maximum temperature on the bottom surface.
Experimental Investigation on Surface Particle Interactions During Pool Boiling of Nanofluids
Authors: Ganapathy Harish , Emlin V. , Parikh Anant Narendra , Sajith V.
Published: May 2012

The pool boiling characteristics of nanofluids is affected by the interaction between the nanoparticles and the heater surface which forms a sorption layer and this layer increases the surface wettability and thereby enhances the CHF. While deteriorated nucleate boiling has been attributed to the decreased activation of cavities due to the increased wettability, it fails to explain the enhanced performance observed by several researchers, which can be explained only by an increase in surface roughness and hence a direct increase in the number of cavities, thereby compensating for the increase in wettability. Attempts to characterize the roughness of heater surfaces have been restricted to magnified visualizations and intrusive probing. No non-intrusive tests have been reported on flat heaters, which are ideal to conduct surface analyses. The present work is aimed at conducting a non-intrusive experimental study to analyse the surface roughness modification due to the sorption layer on flat plate heaters. Experiments have been carried out using electro-stabilized aluminium oxide water based nanofluids of different concentrations with heaters having varying values of surface roughness. The burn-out heat flux was measured and the effect of sedimentation time was studied. The surface-particle interaction parameter (Ra/dp) was varied to capture the phenomena of plugging as well as splitting of nucleation sites. An experiment having a high value of the interaction parameter shows enhanced boiling performance and that with a value close to 1 shows deteriorated performance. Further it was seen that this behaviour is dependent on the particle concentration. Detailed surface characterization has been done using an optical measurements setup and atomic force microscopy. Boiling on nano-coated heaters has been investigated and presented as an effective solution to counter the disadvantageous transient boiling behavior of nanofluids.
Analysis of Energy and Exergy for Mixed Convection Flow in Microstructure Filled Vented Enclosures
Authors: Banna Muinul Hasan , Islam A. K. M. Sadrul , Milani Abbas S. , Hoorfar Mina
Published: May 2012

Understanding heat transfer through saturated porous media is of great importance to many engineering and geophysical systems such as cooling the electronic devices and solar power collectors, and post-accidental heat removal in nuclear reactors. Large numbers of research studies have been and are conducted on the expanding field of porous media due to the high rate of heat transfer in these systems. Despite the efforts made towards the study of the mechanics of fluid flow through porous media, little is studied the rate of exergy which is the only factor presenting the rate of reusable energy potentially produced by a heat generating body. The objective of this study is to develop a design of experiment to carry out a numerical analysis of heat transfer in a rectangular enclosure filled with a saturated porous medium. The optimum heat transfer rate will be obtained for various configuration-related parameters, namely different inlet to outlet ratios and different inlet width to cavity width ratios. These parameters will be optimized to achieve maximum rate of heat transfer and minimum rate of entropy generation. The results of this study will also help to determine relationships for predicting the heat transfer characteristics of the enclosure.
Study on Liquid-Bridging Induced Microscale Contact of Two Liquid Droplets
Authors: Shibata Yuichi , Yanai Takehiko , Okamoto Osamu , Kawaji Masahiro
Published: May 2012

The field of microfluidics is developing with advances in MEMS, biotechnology and μ-TAS technologies. In various devices, interfacial energy is a dominant factor for liquid movement in a microchannel. The surface tension and interfacial tension values are necessary to analyze the liquid behavior in the microchannel. Evaluating the values of interfacial tension is especially important for multiphase flow. A pendant drop method is usually used to measure the interfacial tension, however, this method has some inconveniences. For example, the pendant drop method demands strict accuracy for measuring the droplet size when the droplet has a non-spherical shape. Moreover, it needs an accurate value of the density difference between the two liquids. In this work, a new measurement method named “Liquid-bridging Induced Micro Contact Method” has been developed to overcome the weaknesses of the existing methods. In a previous study, we obtained the interfacial tension from bridging of two liquid droplets on the tip of opposing round metal rods. In this study, we have examined the liquid-bridging of two extruded liquid droplets out of a micro glass tube. By measuring the radii of curvature of each liquid surface and interface, we calculate the Laplace pressure on the surface and interface, and derive the interfacial tension value using the Laplace equation. To prove these two methods are reliable, we have compared the results obtained in this experiment to that of the pendant drop method. As a liquid droplet comes into contact with an opposite liquid droplet the phenomenon is recorded using a CCD camera and high speed camera. The results show that the values of interfacial tension obtained from two methods are approximately the same. Therefore, the liquid-bridging induced micro contact method has been shown to be capable of interfacial tension measurements.
Electrowetting Actuation for a Sessile Liquid Drop: Experiments and Simulations
Authors: Alavi S. , Passandideh-Fard M. , Tafteh M. H.
Published: May 2012

In this paper, experiments and numerical simulations are performed to study the effects of electric field on the contact angle of a sessile liquid drop resting on a substrate in electrowetting-on-dielectric (EWOD) application. In the experiments for studying the electrowetting, a mercury droplet of 20μL was dispensed manually on a soldermask-coated PCB (Printed Circuit Board) and different values of AC voltage were applied between the droplet and the insulator. High quality images were captured using a CCD camera in all experiments and a program was developed using the MATLAB software for image-processing purposes to obtain the contact angle and other geometrical parameters of the droplet. A numerical model was also used to simulate the drop deformation under an electric field. The continuity and momentum equations along with an equation for tracking the liquid free surface were solved. The free surface advection and reconstruction were performed based on the volume-of-fluid method using Youngs’ algorithm. To evaluate the effect of the electric field on the free surface, the electrostatic potential was first solved for the entire computational domain. Next, the electric field intensity and the surface density of the electric charge were calculated on the free surface after which the electric force could be determined. Calculated droplet shapes agreed well with those of the experiments.
Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels
Authors: Kuznetsov Vladmir V. , Shamirzaev Alisher S. , Kozulin Igor A. , Kozlov Stanislav P.
Published: May 2012

Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this paper we used the flow patterns of gas-liquid flow in rectangular microchannel to explain the regularities of refrigerants flow boiling heat transfer. The characteristics of the flow such as frequency of elongated bubbles, their length, velocity of liquid and gas phases were determined by dual laser flow scanning for the upward and horizontal nitrogen-water flow in microchannels with the size of 1500×720 μm. The flow pattern boundaries were determined also and compared with extended Mishima and Ishii correlation. Flow boiling heat transfer data were obtained for vertical and horizontal microchannel heat sink with similar channels using refrigerants R21 and R134a. The data on local heat transfer coefficients were obtained in the range of mass flow rate from 33 to 190 kg/m2s, reduced pressure from 0.03 to 0.25 and heat flux from 10 to 160 kW/m2. The flow boiling modes with nucleate and convective boiling were observed as far as heat transfer deterioration at high vapor quality and high heat flux. It was found that deterioration occurs for the annular flow when nucleate boiling was suppressed in a thin liquid film, and for elongated bubble flow pattern. The mechanism of heat transfer deterioration was discussed. The model of heat transfer deterioration was used to predict the experimental data.
Analytical Optimization of Heat Exchanger Dimensions of a Joule-Thomson Microcooler
Authors: Widyaparaga Adhika , Kuwamoto Masashi , Noda Eiji , Sakoda Naoya , Kohno Masamichi , Takata Yasuyuki
Published: May 2012

In designing a Joule-Thomson microcooler, aiming for a compact size yet maintaining good performance, it is important to find the optimum dimensions of its heat exchanger. We have developed a model capable of predicting the performance characteristics of a wiretype Joule-Thomson microcooler utilizing analytical methods and incorporating changing gas properties via gas equations of state. The model combined the heat exchanger and the JT expander, thus requiring only the inlet gas properties as input. The model results were compared to experimental measurements using C2H4 and N2O as coolant gases. Predicted mass flow rate and temperature drop were in good agreement with the measured values. The long capillary length present in the tested microcooler was revealed to maintain performance of the microcooler for longer heat exchanger lengths due to it functioning as a secondary heat exchanger. Using the calculation results it was possible to correctly estimate the optimum heat exchanger length for C2H4 and for N2O.
Two-Phase Flow in Microchannels With a Bounding Porous Medium
Authors: Nouri-Borujerdi A.
Published: May 2012

This paper investigates numerical simulation of one-dimensional homogeneous adiabatic gas-liquid two-phase flow in a rectangular microchannel with one boundary porous wall under the assumption of hydrophobic condition. Gas enters the microchannel with a uniform velocity and liquid is injected through the porous side wall. The present approach is to simulate water injection effects and developing mechanism of two-phase flow. The modeling and solution of the conservation equations provide pressure drop, vapor quality, void fraction and tow-phase mixture velocity for different water injection rates. The results show that velocity and pressure drop significantly perturbed when the water injection rate exceeds a critical value. Comparison between the results of the present work with the previous experimental work shows a good agreement.
Magnetically Mediated Formation of Ferrofluid Emulsion
Authors: Nguyen Nam-Trung , Tan Say-Hwa , Liu Jing
Published: May 2012

This paper reports both experimental and numerical investigations of the formation process of ferrofluid emulsion with and without an applied magnetic field. The droplets are formed in a flow-focusing and T-junction configuration. In the experiment, a homogenous magnetic field was generated using an electromagnet. The presence of the magnetic field in the flow direction affects the formation process and changes the size of the droplets. In both configurations, the change in the droplet size depends on the magnetic field strength and the flow rates. A numerical model was used to investigate the force balance during the droplet breakup process. The particle level set method was employed to capture the interface movement between the continuous fluid and the dispersed fluid. The process of the droplet formation and the flow field is discussed for both cases with and without the magnetic field. Finally, experimental and numerical results are compared. The results show that the polarity of the magnetic field does not affect the formation process.
Active Micromixers Based on Polarization Instability and Acoustic Streaming
Authors: Nguyen Nam-Trung , Luong Trung-Dung , Ja¨nig Oliver
Published: May 2012

This paper reports two pressure-driven high-throughput active micromixers. Mixing enhancement was achieved with applied external electrical and acoustic fields. In the first active mixer, nanoporous charge-selective Nafion membrane was used to achieve strong mixing vortices. These vortices are caused by electroconvection in the concentration polarization zone above the membrane. The required applied voltage is found to be propotional to the square root of the flow rate. In the second active mixer, surface acoustic wave lauched from an interdigitated electrode deposited on a piezoelectric substrate causes acoustic streaming and improves the mixing efficiency significantly. Surface wave with a frequency of 13 MHz was launched perpendicular to the flow. The mixing efficiency was observed to be proportional to the square of the applied voltage. Compared to conventional parallel electrodes, a focusing design of the interdigitated electrode leads to a better mixing efficiency.
An Experimental Investigation of the Miniature Loop Heat Pipe Cooling Systems for High Power Density Computer Chips
Authors: Choi Jeehoon , Yoo Junghyun , Sung Byungho , Kim Chulju , Borca-Tasciuc Diana-Andra
Published: May 2012

The implementation of high power density, multi-core central and graphic processing units (CPUs and GPUs) coupled with higher clock rates of the high-end computing hardware requires enhanced cooling technologies able to attend high heat fluxes while meeting strict design constrains associated with system volume and weight. Miniature loop heat pipe (mLHP) systems emerge as one of the technologies best suited to meet all these demands. This paper investigates experimentally a mLHP system designed for workstation CPUs. The system incorporates a two-phase flow loop with capillary driving force. Since there is a strong demand for miniaturization in commercial applications, emphasize was also placed on physical size during the design stage of the new system. Hence system weight is reduced to around 450g, significantly smaller than that of commercial coolers consisting of copper heat sinks that weight around 782g. Experimental characterization shows that the system can reach a maximum heat transfer rate of 170W with an overall thermal resistance of 0.12 K/W. The heat flux is 18.9 W/cm2, approximately 30% higher than that of larger size commercial systems. To further miniaturize the evaporator module while maintaining the same heat flux, a new structure for the porous evaporator is proposed, which consist of a porous bi-layer, with nanopores at the top surface. The role of the nanoporous layer is to provide a larger surface area for phase-change, enhancing the evaporation rate.
Test of Cross-Section Shape of Air-Liquid Interface in Microgrooves With Micro-PIV Method
Authors: Nie Xuelei , Hu Xuegong , Wang Tao , Tang Dawei
Published: May 2012

Evaporation of liquid meniscus formed in microgrooves is associated with very high heat transfer rates, but the cross-section shape of air-liquid interface has a great influence to the heat transfer in microgrooves. But the real cross-section shape of interface in microgrooves is still unknown for us. In this work, the micro-PIV (Particle Image Velocimetry) method is used to test the cross-section shape of air-liquid interface in microgrooves. In the experiment, the camera is focus on different planes from top of the microgrooves to the bottom of the microgrooves. In each plane, we can see the boundary between the air and liquid through small particles added into the liquid. The positions of boundary in each plane for a given cross section are drawn in two-dimension coordinate. Then the cross-section shape of interface in microgrooves can be seen from the fitting curve. The results show that the cross-section shapes of the interface in microgrooves are not round, but polynomial curves. The curvature of interface in microgrooves changes along a single curve. Besides, the polynomial curves also vary along axial direction of the microgrooves. The variations are more obvious in vertical microgrooves than in horizontal microgrooves.
Visualization and Evaporation Resistance Measurement for Groove-Wicked Flat Plate Heat Pipes
Authors: Wong Shwin-Chung , Chen Chung-Wei
Published: May 2012

This work experimentally studied the evaporation characteristics in groove-wicked flat-plate heat pipes. The parallel, U-shaped grooves have a width of 0.25 mm and a depth of 0.16 mm. Uniform heating was applied to the copper base plate near one end, and a cooling water jacket was connected at the other end. The evaporation resistance was calculated based on the difference of the plate temperature and the vapor temperature respectively under and above the center of the heated zone. Water was used as the working fluid. With stepwise increase of heat load, the behavior of the working fluid in the grooves was visualized, and the evaporation resistances were measured. Above a certain heat load, longitudinal liquid recession can be visualized with a steep-sloped liquid front. Behind the short liquid front is the accommodation region where the meniscus appeared to anchor on the top corners of the groove walls. Under a thermally stable situation, longitudinal oscillations of the liquid front existed in many grooves. Also, the liquid motion in different grooves seemed independent, forming a constantly varying zigzag front line. With increasing heat load, the liquid fronts gradually left the heated zone, accompanied by increasing plate temperatures. The evaporation resistance data appeared larger and more scattered than those associated with mesh or powder wicks in our published experiments, presumably due to the relatively large groove size and surface roughness from etching. No boiling was observed in all present tests. The evaporation resistances for groove wicks increase monotonically in response to the gradually enlarged dryout region with increasing heat load.
Magnetogasdynamic Flow and Heat Transfer in a Microchannel
Authors: Weng Huei Chu
Published: May 2012

The presence of current flow in an electric and magnetic field results in electromagnetic force and joule heating. It is desirable to understand the roles of electromagnetic force and joule heating on gas microflow and heat transfer. In this study, a mathematical model is developed of the pressure-driven gas flow through a long isothermally heated horizontal planar microchannel in the presence of an external electric and magnetic field. The solutions for flow and thermal field and characteristics are derived analytically and presented in terms of dimensionless parameters. It is found that an electromagnetic driving force can be produced by a combined non-zero electric field and a negative magnetic field and results in an additional velocity slip and an additional flow drag. Also, a joule heating can be enhanced by an applied positive magnetic field and therefore results in an additional temperature jump and an additional heat transfer.
Heat Conduction Law in Dielectric Nanosystems Based on the Thermomass Theory
Authors: Dong Yuan , Cao Bing-Yang , Guo Zeng-Yuan
Published: May 2012

In this paper a general heat conduction law has been proposed based on the thermomass theory, which can be derived from the Boltzmann equation for phonons in dielectrics. The Boltzmann equation for phonons gives a balance between the drift and friction parts of the distribution function. When the normal scattering term is omitted in the friction part, the Fourier’s law and Cattaneo-Vernotte thermal wave equation can be obtained by the zeroth order and first order approximations of the drift term, respectively. A second order approximation of the drift part lead to the thermomass theory based general heat conduction law, which is a nonlocal damped heat wave equation and consists of driving, inertial and resistant forces for phonon gas motion. In nanosystems, the normal scattering term of the friction part reflecting the boundary effect is required and induces a Laplacian term in governing equations by solving the phonon Boltzmann equation. A general law containing the viscosity of the thermomass fluid is obtained in analogy with the Brinkman extension in porous hydrodynamics. The general law is then applied to investigate the effective thermal conductivity of a nanosystem, which covers the effect of the ultra-high heat flux and the boundary confinement and scattering. The present research not only presents a powerful heat conduction law for heat transport in nanosystems, but also bridges the microscopic Boltzmann transport equation and the macroscopic gas dynamics of phonons in terms of the thermomass theory.
Electrical Tomography Sensing and Dielectrophoresis in Microchannel for 3D Particle Mixing
Authors: Othman Nur Tantiyani Ali , Choi Je-Eun , Takei Masahiro
Published: May 2012

The present study describes the electrical tomography sensing and dielectrophoresis (DEP) force for visualize the 3D particle mixing in the microchannel system. In the presence of non-uniform electric fields generated by point microelectrodes, the dynamic distribution behaviors of a polystyrene particle and deionized water had been investigated in this system. Microchannel was fabricated with five cross sections where 12 electrodes were installed for each measurement plane. In this experiment, the relationship between electric field frequency and DEP force of particles are calculated at different electric frequencies and diameter of particles. The applied electric field intensities are E = ±1 V/mm, ±3 V/mm and ±5 V/mm while the electric field frequencies are f = 1 kHz, 10 kHz, 100 kHz and 1 MHz and the diameter of particles are 1.3μm, 1.5μm and 2.0μm are investigated in this experiment. Simultaneously, imaged by manipulating tomography sensing at cross section A, C and D and the coupled DEP forces at cross section B and D, the particles flowing had been visualized and concentrate uniformly at near the outlets. The electrical capacitances and DEP forces between the electrode pairs of the microchannel were measured and the ECT tomograms representing the particle distribution were constructed from the measured capacitance data for each cross section in microchannel.
Experimental Study of the Effects of Ferrofluid on Thermal Performance of a Pulsating Heat Pipe
Authors: Mohammadi Maziar , Mohammadi Mohammad , Ghahremani Amir R. , Shafii M. B.
Published: May 2012

In this work, a four-turn Pulsating Heat Pipe (PHP) is fabricated and tested experimentally. The novelty of the present PHP is the capability of obtaining various thermal performances at a specific heat input by changing the magnetic field. The effects of working fluid (water and ferrofluid), charging ratio (25%, 40%, and 55%), heat input (25, 35, 45, 55, 65, 75, and 85 W), orientation (vertical and horizontal heat mode), and magnetic field on the thermal performance of PHPs are investigated. The results showed that applying the magnetic field on the water based ferrofluid reduced the thermal resistance of PHP by a factor of 40.5% and 38.3% in comparison with the pure water case for the vertical and horizontal mode, respectively. According to the experimental results, an optimum thermal resistance of 0.38 °C/W was achieved at the following conditions: water-based ferrofluid as the working fluid in the presence of magnetic field, vertical mode, charging ratio of 55%, 7% volumetric concentration, and 85 W heat input. This thermal resistance is 11.5 times better than that of the empty PHP.
A Numerical Study on Shear Stress and Heat Transfer of Segmented Flow Between Parallel Plates
Authors: Talimi V. , Muzychka Y. S. , Kocabiyik S.
Published: May 2012

The shear stress and heat transfer of moving liquid slugs between two parallel plates are studied numerically. The problem is solved initially as a steady state problem for the velocity field and shear stress, and then, the thermal problem is solved. The thermal boundary condition is constant wall temperature. The fluid properties are assumed to be constant. The finite volume method is applied using the ANSYS Fluent software package. The results show good agreement with the published literature. Effects of different interface shapes (contact angles) on wall shear stress and heat transfer is discussed. Dimensionless heat transfer plots are also presented.
Mixing Characteristics of a Two-Phase Flow (Gas-Liquid) Microchannel Mixer
Authors: Abdel-Salam Tarek , Pidugu Srikanth
Published: May 2012

Multiphase phase flows occur in many engineering and bio-medical applications. Bubble formation in microchannels can be beneficial or harmful depending upon their influence on the operation and performance of microfludic devices. Potential uses of bubble generation found in many applications such as microreactors, micropump, and micromixers. In the present work the flow and mixing process in a passive microchannel mixer were numerically investigated. Effects of velocity, and inlet width ratio (Dgas/Dliquid) on the two phase flow were studied. Numerical results are obtained for 2-dimensional and 3-dimesional cases with a finite volume CFD code and using structured grids. Different liquid-gas Reynolds number ratios (Reliquid/Regas) were used ranging from 4 to 42. In addition, three values of the inlet width ratio (Dgas/Dliquid) were used. Results for the 3-D cases capture the actual shape of the air bubble with the thin film between the bubble and the walls. Also, increasing Reliquid increases the rate of the development of the air bubble. The bubble length increases with the increase of Dgas/Dliquid. For the same values of Re, the rate of growth of the bubble increases with the increase of Dgas/Dliquid. Finally, a correlation is provided to predict the length of the bubble with liquid-gas Reynolds number ratio (Reliquid/Regas) and tube width.
Numerical Investigation of Heat Transfer Enhancement in Heat Sinks With Elongated Hexagonal Fins
Authors: Kheirandish Zahra , Shafeie Haleh , Abouali Omid
Published: May 2012

A numerical study was performed for the laminar forced convection of water over a bank of staggered micro fins with cross section of the elongated hexagon. A 3-dimensional mathematical model, for conjugate heat transfer in both solid and liquid is developed. For this aim the Navier-Stokes and energy equations for the liquid region and the energy equation for the solid region are solved simultaneously and the pressure drop as well as the heat transfer characteristics was investigated. The length and width of the studied heat sinks are one centimeter and different heights in the range of 200–500 micrometer were examined for the fluid media. The heat removal of the finned heat sink is compared with an optimum simple mirochannel heat sink. The comparison which is presented at equal pumping powers depicts the enhancement of the heat removal for some specific sizes of the finned heat sink.
CFD Analysis of Nano-Drug Transport in Vitreous Cavity due to Saccadic Eye Movements
Authors: Modareszadeh Amirreza , Abouali Omid , Ghaffariyeh Alireza
Published: May 2012

In the present research, the motion of the nano-drug in the vitreous chamber of human eye due to saccadic movements in post-vitrectomy eyes is investigated. The average radius of the vitreous cavity in human eye is equal to 12 mm. This cavity is filled with a liquid in post-vitrectomy eyes. A dynamic mesh technique was performed to model the eye motion. The unsteady 3-D forms of continuity, Navier-Stokes and concentrations of nano-drug equations were solved numerically. The numerical model was validated comparing the results of the flow field with available analytic solutions and experimental data for a sphere as an ideal model of vitreous chamber which a very close agreement was achieved. Then, the numerical simulation was performed to a real model of vitreous cavity filled with BSS (Balanced salt solution). The convection and diffusion of nano-drug in the filling fluids of post-vitrectomy eyes is computed and the results are compared with the diffusion of the nano-drug in the stagnant vitreous. The comparison depicts that the saccade movements of human eye accelerate the drug motion one to two orders of magnitude higher than that due to diffusion in stagnant vitreous chamber.
Modeling and Simulation of Unequal Droplet Splitting in Electrowetting Based Digital Microfluidics
Authors: Bhattacharjee Biddut , Najjaran Homayoun
Published: May 2012

Electrowetting-on-dielectric (EWOD) is a highly efficient technology to perform biological and medical analyses through the manipulation of pico- to nano-liter droplets on digital microfluidic systems (DMS). Droplet splitting is one of the basic fluidic operations that play a vital role in microscale mixing and concentration control. This paper presents the results of numerical investigation of unequal droplet splitting. In order to gain insight into the mechanism of droplet splitting, a three-electrode splitting system is simulated in FLOW-3D® for given geometry and material properties. When unequal voltages are applied to the adjacent electrodes on both sides of a droplet the distribution of electric field exerts spatially varying stress causing the deformation of the interface. The resulting unequal fluid flow rates towards the activated electrodes are determined by the coupled electro-hydrodynamics. The results of multiple simulation runs in terms of liquid flow rates with different ratios of the applied voltage will be very useful in developing the open-loop model of droplet splitting that can be later adopted to design a controller for unequal splitting in DMS.
Development and Analytical Treatment of 3-D Passive Mixromixer for Enhanced Microfluidics Reactions
Authors: Gosavi Suresh , Tekawade Aniket , Bodas Dhananjay , Barve Sukratu , Robert Laurent , Khan-Malek Chantal
Published: May 2012

With fast development of microfluidic systems, micro-mixing becomes a very important issue. Various attempts have been made to develop passive and active micromixers, wherein the mixing efficiency is mainly dependent on the diffusion coefficient and diffusion mechanism. This paper reports modeling and fabrication of a 3D micromixer based on the principle of sequential lamination for efficient mixing. Simulation of the square tube geometry with varying inlet velocity was performed. The improvement in the mixing efficiency is attributed to the increase in the contact surface between the different fluids decreasing the diffusion path in order to improve the molecular diffusion. The design approach in the present case is also supported by analytical treatment which indicates superiority of 2D transverse flows while designing a micromixer. The optimized parameters from CFD simulation were used for fabrication of the micromixer by standard photolithographic and soft lithographic techniques.
Coupled Electrohydrodynamic-Dielectrophoretic Pumping of Colloidal Suspensions in a Microchannel
Authors: He Guoliang , Liu Dong
Published: May 2012

Effective and versatile microfluidic pumps can be designed by utilizing various electrokinetic effects, such as electrohydrodynamics (EHD), induced-charge electroosmosis (ICEO) and dielectrophoresis (DEP). Among these, traveling-wave EHD (twEHD) has emerged as a powerful pumping mechanism due to its potential for miniaturization and the ability to pump a variety of liquids. However, when twEHD is used to deliver colloidal suspensions, the simultaneous presence of traveling-wave DEP (twDEP) effect may favorably or adversely influence the overall pumping performance, depending on the particle-fluid combination and the frequency range of the applied electric field. In this paper, the coupled EHD and DEP flows were studied numerically in a microchannel with a three-phase interdigitated microelectrode array fabricated at the bottom surface. In the numerical model, the particle-fluid interaction due to twDEP was solved using an equivalent mixture model, and the resultant velocity field was compared to that induced by either the repulsion type or the attraction type of EHD. The results show, depending on the frequency range of the traveling wave electric field and the applied thermal boundary condition, the EHD-induced flow can significantly enhance or weaken the twDEP-induced flow, vice versa.
Adiabatic Air-Water Two-Phase Flow in Circular Microchannels
Authors: Sur Aritra , Liu Dong
Published: May 2012

Gas-liquid two-phase flow in microchannels with hydraulic diameters of 100–500 μm exhibits drastically different flow behaviors from its counterpart in conventional macroscopic channels. Two particular issues are how to determine the two-phase flow patterns and how to predict the two-phase pressure drop at given flow conditions in these microchannels. This paper presents an experimental study of adiabatic two-phase flow of air-water mixture in circular microchannels with inner diameters of 100, 180 and 324 μm, respectively, to investigate the effects of channel size and phase velocity on the two-phase flow pattern and pressure drop. The air and water superficial velocities were in the range of 0.01–120 m/s and 0.005–5 m/s. Two-phase flow patterns were visualized using highspeed photographic technique. Four basic flow patterns, namely, bubbly flow, slug flow, ring flow and annular flow, were observed. The two-phase flow maps were then constructed and the transition boundaries between different flow regimes were identified. It was found that the slug flow is the dominant two-phase flow pattern in microchannels, and the transition boundaries generally shift to regions of higher gas superficial velocities as the channel dimension decreases. The experimental measurements of two-phase pressure drop were compared to the predictions from the available two-phase models in the literature. Results show that the flow pattern-based models provide the best prediction of two-phase pressure drop in microchannels.
Polymer Nanowire Arrays With High Thermal Conductivity and Superhydrophobicity Fabricated by a Nano-Moulding Technique
Authors: Cao Bing-Yang , Kong Jie , Xu Yan , Yung Kai-Leung , Cai An
Published: May 2012

High thermal conductivity and superhydrophobicity are highly desired for micro/nanofluidic devices. Good thermal conductivity is helpful for thermal control and managements. Superhydrophobicity can benefit fluid friction reduction and liquid droplet control. We report on an improved nano-moulding technique which can prepare polymer (high density polyethylene/HDPE) nanowire arrays with high thermal conductivity (more than 10W/mK) and superhydrophobicity (contact angle >150°). The thermal conductivities of the fabricated high-density polyethylene (HDPE) nanowire arrays with diameters of 100 nm and 200 nm, measured by a laser flash method, are about 2 orders of magnitude higher than their bulk counterparts. The estimated thermal conductivity of a single HDPE nanowire is as high as 26.5 W/mK at room temperature. The self-organized surfaces of polymer nanowire arrays are found to have micro-to-nanoscale hierarchical nanostructures, and have superhydrophobicity of >150° contact angles. This technique is promising for fabrication of micro/nanofluidics devices due to the advantages of simple fabrication, high-quality, low-cost, and mass-production.
Fluid Dynamics of Microfluidic Devices
Authors: Gao Z. , Ng K.
Published: May 2012

The subject of fluid dynamics of microfluidic devices such as instability, droplet formation, and control has gained considerable momentum in recently years. This is due partially to the fact that modern developments in the design and utilization of microfluidic devices for fluid transport have found many applications such as drug design and diagnostic devices in biomedicine and microdrop generators for image printing. Furthermore, the new development of nonlinear dynamics of droplets has created a new paradigm of scaling and instability theory that opened a new approach to this classic phenomenon. The utility of a microfluidic device is linked directly to its ability to control microdroplets in precision and speed for desired functionalities. An example of such a device is Kodak’s Continuous Inkjet System, which is capable of stimulating drop breakup of jets of complex fluids with unprecedented precision, speed, and selectivity. We will utilize such a microfluidic device to discuss some of the fluid dynamics topics in microfluidic devices, and to illustrate that the fluid dynamic behavior of such a device is not only influenced by the device architecture, but also by the fluidic properties and by the way the fluid is energized to induce the drop formation and movement. The topics will include a discussion of fluid properties relative to jet modulation, wavelength dependencies, thermal modulation schemes, satellite drop formation, and aerodynamic effects.
Micro- and Nano- Technologies: Roadmap Enabling More Compact, Lightweight Thermoelectric Power Generation and Cooling Systems
Authors: Hendricks Terry J.
Published: May 2012

Advanced thermoelectric (TE) energy recovery and cooling systems have critical benefits in transportation, industrial process, and military applications because of rising or uncertain energy costs and subsequent need for energy efficiency, geopolitical uncertainties impacting basic energy supplies worldwide, and the need for electrified, distributed cooling and heating systems in automotive applications. Advanced TE energy recovery and cooling technologies will require high-performance heat transfer characteristics to achieve system performance targets and requirements. However, TE energy recovery systems generally have high-temperature thermal transfer requirements (i.e., as high as 750–800 °C), while TE cooling systems require low temperature thermal transfer (i.e., 25 °C – 100 °C). Investigations have compared system power and cooling benefits and system thermal integration challenges of energy recovery and cooling systems using microchannel heat exchangers to provide high heat transfer performance in both high-temperature, high-enthalpy energy streams and low-temperature cooling streams. This work explores the roadmap and vision for using micro-technology solutions integrated with advanced thermoelectric materials in advanced TE power generation and cooling systems. Integrated system-level TE power generation and cooling system analyses demonstrate that inter-related system-level requirements on weight, volume, and performance lead to derived requirements for micro-technology solutions. Nano-technologies and micro-technologies will be presented that demonstrate where and how these technologies impact TE system designs. Of course, micro-technology manufacturing cost is critical in all energy recovery and cooling applications. Recent progress in microtechnology cost-modeling elucidates and quantifies key cost-manufacturing interdependencies, relationships, and sensitivities that will be explored in this presentation. This provides critical information on manufacturing processes, production volume dependence, material selections, and ultimately pathways forward leading to low-cost microtechnology heat and mass transfer devices that improve advanced TE energy recovery and cooling system performance (specifically including weight and volume impacts).
Lab-on-a-Chip Systems for Microchemical Synthetic Applications
Authors: Kim Dong-Pyo
Published: May 2012

Lab-on-a-chip microchemical systems were fabricated from PDMS, polyimide (PI) film and functional polymers by different lithographic techniques to look for microreactor applications in the areas of organic syntheses. In detail, efficient gas-liquid reactions in dual-channel microreactor, continuous recovery and recirculation of catalyst-immobilized magnetic particles in microfluidic system, and a flexible film microreactor will be discussed.
A Constant-Wall-Temperature Model for Prediction of Thermal Performance of Gas-to-Gas Counter-Flow and Parallel-Flow Microchannel Heat Exchangers
Authors: Koyama Kohei
Published: May 2012

Thermal performances of gas-to-gas counter-flow and parallel-flow microchannel heat exchanger have been investigated. Working fluid used is air. Heat transfer rates of both heat exchangers are compared with those calculated by a conventional log-mean temperature difference method. The results show that the log-mean temperature difference method can be employed to a parallel-flow configuration whereas that cannot be employed to a counter-flow configuration. This study focuses on the partition wall which separates hot and cold passages of the microchannel heat exchanger. The partition wall is negligibly thin for a conventional-sized heat exchanger. In contrast, the partition wall is thick compared with channel dimensions for a microchannel heat exchanger. A model which includes the effect of the thick partition wall is proposed to predict thermal performances of the microchannel heat exchangers. The heat transfer rates obtained by the model agree well with those obtained by the experiments. Thermal performances of the counter-flow and parallel-flow microchannel heat exchangers are compared with respect to one another based on temperature of the partition wall. The comparison results show that thermal performances of the counter-flow and parallel-flow microchannel heat exchangers are identical. This is due to performance degradation induced by the thick partition wall of the counter-flow microchannel heat exchanger. This study reveals that the thick partition wall dominates thermal performance of a gas-to-gas microchannel heat exchanger.
Flame Chromatography: Toward Fuel Indexing Based on Multiple Weak Flames in a Meso-Scale Channel With a Prescribed Temperature Profile
Authors: Maruta Kaoru
Published: May 2012

For understanding flame stability in microcombustor, fundamental studies on the combustion characteristics in a meso-scale channel with a prescribed wall temperature profile have been conducted. Results showed that the existence of dynamic oscillatory flames and weak flames in addition to the normal propagating flames for the first time. It is then recognized that the weak flame phenomena can be applied for examining multi-stage oxidation of hydrocarbon fuels in wide temperature range from 300K up to auto-ignition temperature. Based on the preliminary experiments with various fuels including primary referenced fuel for gasoline, research octane number (RON) of the test fuels can be clearly described by the aspects of the stabilized stationary multiple weak flames. The methodology can be termed “flame chromatography” and it is expected to be applied for fuel indexing of future alternative fuel characterizations.
Pattern Formation in Evaporating Drops With and Without Nanoparticles
Authors: Matar Omar K.
Published: May 2012

We show how asymptotic reduction techniques are used to model the motion of sessile droplets in the presence of heat transfer, evaporation and nanoparticles. When nanoparticles are present in the drop, lubrication theory is used to model the contact line dynamics and the evolution of the nanoparticle concentration. The model accounts for the effects of surface tension, Marangoni stresses, evaporation and intermolecular forces; the effect of nanoparticles on the latter endows the film with structural disjoining pressure forces near the contact line. Our numerical simulations catalogue the different types of possible contact line dynamics, which range from spreading and retraction, to pinning and ‘terracing’; the latter phenomenon is caused by the effect of nanoparticles on the intermolecular forces.
Heat Conduction in a Nano-Porous Material and Its Application
Authors: Miyazaki Koji
Published: May 2012

In this study we introduce our numerical and experimental works for the thermal conductivity reduction by using a porous material. Recently thermal conductivity reduction has been one of the key technologies to enhance the figure of merit (ZT) of a thermoelectric material. We carry out numerical calculations of heat conduction in porous materials, such as, phonon Boltzmann transport (BTE), molecular dynamics simulations (MD), in order to investigate the mechanism of the thermal conductivity reduction of a porous material. In the BTE, we applied the periodic boundary conditions with constant heat flux to calculate the effective thermal conductivity of porous materials. In the MD simulation, we calculated phonon properties of Si by using the Stillinger-Weber potential at constant temperature with periodic boundary conditions in the x,y and z directions. Phonon dispersion curves of single crystal of Si calculated from MD results by time-space 2D FFT are agreed well with reference data. Moreover, the effects of nano-porous structures on both the phonon group velocity and the phonon density of states (DOS) are discussed. At last, we made a porous p-type Bi2Te3 by using self-assembly. The average diameter of the holes was 20 nm, and the average pitch of the hexagonally arranged holes was 50 nm. The measured cross-plane thermal conductivity is 0.25W/(m·K). The thermal conductivity of the thin film is extremely lower than that of the bulk material without any major decrease in the electrical conductivity. The figure of merit of Bi0.4Te3Sb1.6 is enhanced to 1.8 at room temperature (300K) by the formation of a porous thin film.
Heat and Mass Transfer Enhancement by Active Flow Control in Micro Domains
Authors: Jung Junkyu , Elcock Daren , Kuo Chih-Jung , Amitay Michael , Peles Yoav
Published: May 2012

A flow control method is presented that employ liquid and gas jets to enhance heat and mass transfer in micro domains. By introducing pressure disturbances, mixing can be significantly enhanced through the promotion of early transition to a turbulent flow. Since heat transfer mechanisms are closely linked to flow characteristics, the heat transfer coefficient can be significantly enhanced with rigorous mixing. The flow field of water around a low aspect ratio micro circular pillar of diameter 150 μm entrenched inside a 225 μm high by 1500 μm wide microchannel with active flow control was studied and its effect on mixing is discussed. A steady control jet emanating from a 25 μm slit on the pillar was introduced to induce favorable disturbances to the flow in order to modify the flow field, promote turbulence, and increase large-scale mixing. Micro particle image velocimetry (μPIV) was employed to quantify the flow field, the spanwise vorticity, and the turbulent kinetic energy (TKE) in the microchannel. Flow regimes (i.e., steady, transition from quasi-steady to unsteady, and unsteady flow) were elucidated. The turbulent kinetic energy was shown to significantly increase with the controlled jet, and therefore, significantly enhance mixing at the micro scale.