bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamical Behavior and Flow Mechanism of Fluid in Nanochannel by Molecular Dynamics Simulation
Authors: Liu Juanfang , Liu Chao , Li Qin
Published: September 2010

The flow properties and dynamical behavior of fluid in a nanochannel were investigated by nonequilibrium molecular dynamics simulation. First of all, the locale distribution of molecules in the channel is found to be strongly inhomogeneous compared to the bulk fluid. In the vicinity of the wall, portion of the fluid molecules are absorbed on the surface of wall due to the strong interaction of the atoms between the wall and liquid, so that the fluid density in the contact region would be much larger than one of the bulk fluid. But in the other region, the local density value approaches one of the bulk fluids with the increasing distance from the wall. This oscillatory behavior of density resulted in different motion behavior of molecules in the different region of nanochannel. The molecular behavior in the interfacial region is remarkably different from those of fluid atoms in the center of channel and wall atoms, which posses both the motion properties of bulk liquids and a solid atom. At the molecular level, macroscopic continuum hypothesis failed, that is, the results predicted by the Navier-Stoke equations deviate from the simulation data adopted by molecular dynamics simulation. In the paper, the velocity profiles for the channels with different width were plotted, which demonstrated that the time-averaged velocity profiles was not quadratic when the channel width was less than 10 molecular diameters. But on the other cases, the velocity profiles will agree well with the analytical solution based on the NS theory. The molecular dynamics simulation method can withdraw the important microscopical information from the simulation process, which benefit to analyze the flow mechanism at such length scale channel.
Study of Gas Flow in Micronozzles Using an Unstructured DSMC Method
Authors: Roohi Ehsan , Darbandi Masoud , Mirjalili Vahid
Published: September 2010

The current research uses an unstructured direct simulation Monte Carlo (DSMC) method to numerically investigate supersonic and subsonic flow behavior in micro convergent–divergent nozzle over a wide range of rarefied regimes. The current unstructured DSMC solver has been suitably modified via using uniform distribution of particles, employing proper subcell geometry, and benefiting from an advanced molecular tracking algorithm. Using this solver, we study the effects of back pressure, gas/surface interactions (diffuse/specular reflections), and Knudsen number, on the flow field in micronozzles. We show that high viscous force manifesting in boundary layers prevents supersonic flow formation in the divergent section of nozzles as soon as the Knudsen number increases above a moderate magnitude. In order to accurately simulate subsonic flow at the nozzle outlet, it is necessary to add a buffer zone to the end of nozzle. If we apply the back pressure at the outlet, boundary layer separation is observed and a region of backward flow appears inside the boundary layer while the core region of inviscid flow experiences multiple shock-expansion waves. We also show that the wall boundary layer prevents forming shocks in the divergent part. Alternatively, Mach cores appear at the nozzle center followed by bow shocks and an expansion region.
Electroosmotic Flow of Power-Law Fluids in a Slit Microchannel
Authors: Zhao Cunlu , Yang Chun
Published: September 2010

Electroosmotic flow of power-law fluids in a slit channel is analyzed. The governing equations including the linearized Poisson–Boltzmann equation, the Cauchy momentum equation and the continuity equation are solved to seek analytical expressions for the shear stress, dynamic viscosity and velocity distributions. Specifically, exact solutions of the velocity distributions are explicitly found for several special values of the flow behavior index. Furthermore, with the implementation of an approximate scheme for the hyperbolic cosine function, approximate solutions of the velocity distributions are obtained. In addition, a mathematical expression for the average electroosmotic velocity is derived for large values of the dimensionless electrokinetic parameter, κH, in a fashion similar to the Smoluchowski equation. Hence, a generalized Smoluchowski velocity is introduced by taking into account contributions due to the finite thickness of the electric double layer and the flow behavior index of power-law fluids. Finally, calculations are performed to examine the effects of κH, flow behavior index, double layer thickness, and applied electric field on the shear stress, dynamic viscosity, velocity distribution, and average velocity/flow rate of the electroosmotic flow of power-law fluids.
In-Situ Diagnostics of PEFCs
Authors: Friedrich Kaspar Andreas , Wagner Norbert , Schulze Mathias
Published: September 2010

Polymer electrolyte fuel cells (PEFCs) are one of the most interesting alternatives for a pollution-free electrical energy production in many applications where a highly reliable source of electricity is needed. One of the major challenges in the development of PEFCs is to exploit the whole capacity that is inherent to a given membrane electrode assembly (MEA). In practice, certain obstacles remain to be overcome like local mass transport effects, non-uniformly manufactured MEAs, locally varying contact resistances, water management and temperature gradients. All these parameters lead to an inhomogeneous electrochemical activity over the electrode area. Consequently, a variation and a gradient of the current density over the cell area occurs which tends to result in inferior performance and low durability of a PEFC. For the determination of current density distribution different in-situ methods and measurement techniques are applied. Results can be used to improve cell components, to validate models and to detect inappropriate detrimental operating conditions of the fuel cell.
Detection of Non-Diffusion-Limited Enzymatic Surface Reaction in Nanofluidic Channels
Authors: Duan Chuanhua , Chen Yu-Feng , Kim Dong-Kwon , Majumdar Arun
Published: September 2010

Using nanofluidic channels to detect enzymatic surface reactions can overcome diffusion-limited patterning as enzyme accelerates surface reactions without being consumed. In this paper, Trypsin proteolysis reaction is used to demonstrate this idea. Trypsin (enzyme) cleaves Poly-L-Lysine (PLL) coated on the surface of silica nanochannels, resulting in a change of surface charge density and channel height. This change is detected by monitoring the electrical conductance along the nanochannels. 50 μg/ml Trypsin has been detected in 90 nm-height nanochannels within one hour, which is 30 times faster compared to that of a diffusion-limited surface binding reaction. The effect of nanochannel height and the detection limitation are also discussed. Our results indicate that nanofluidic channels can be used for detecting enzymatic-based surface reactions.
Two-Phase Flow Experimental Study: Influence of Gravity Level on Local Boiling Heat Transfer Inside a Minichannel
Authors: Luciani Se´bastien , Brutin David , Le Niliot Christophe , Tadrist Loune`s
Published: September 2010

Flow boiling in minichannels is the most complex convective phase change process. Indeed, there are a lot of physical parameters that influence the two-phase flow during boiling. Here, we will focus on the influence of one of this factor: the gravity level. Actually, there are not many mechanisms that have been proposed for the role of this bound on boiling phenomena. In fact, there is not complete agreement on the importance of gravity on heat and mass transfers with phase change because there is a lack of experimental data at this small scale and because reproducing different gravity levels during parabolic flights has a cost. In this line, the goal of this work is to obtain benchmark data on the local heat transfer coefficient in a minichannel during hyper and microgravity. We want to acquire a better knowledge of the elementary phenomena which control the heat and mass transfers during convective boiling. Indeed, boiling in microscale geometry is a very efficient mode of heat transfer since high heat and mass transfer coefficients are achieved. Actually, minichannels and microchannels are widely used in industry and they are already attractive in many domains such as design of compact evaporators and heat exchangers. They provide an effective method of fluid movement and they have large heat dissipation capabilities. In these situations, their compact size and heat transfer abilities are unrivalled. In this communication, the objective is to acquire better knowledge of the conditions that influence the two-phase flow under microgravity. The expected results will contribute to the development of microgravity models. To perform these investigations, we used an experimental data coupling with an inverse method based on BEM (Boundary Element Method). This non intrusive approach allows us to solve a 3D multi domain IHCP (Inverse Heat Conduction Problem). With this analysis, we are able to quantify the local heat flux, the local temperature and the local heat transfer coefficient in a minichannel (254 μm) by inversing thermocouples data without disturbing the established flow.
Experimental and Theoretical Investigation of Inorganic Scale Deposition in Carbonated Micromodels
Authors: Mosavati Negar , Kharrat Riyaz , Farzaneh Seyed Amir , Ghazanfari Mohammad Hossein
Published: September 2010

In this study, a series of experiments have been conducted on glass micromodel to investigate the effect of different parameters on the gradual process of Barium Sulphate precipitation. Due to the visual nature of the glass micromodel, the shape, size and distribution of scaling formation in a carbonate-rock-look-alike pattern with water-wet characteristics is precisely observable. In addition, an exponential functionality (correlation) was proposed which incorporates all physical parameters affecting the behavior of the system in a dimensionless form. Reynolds number and deviation from equilibrium conditions are the backbones of this correlation. The adjustable exponents of the equation were determined by using a suitable optimization algorithm. This meaningful correlation can also predict the core extracted data with reasonable accuracy. The microscopy observation results confirmed that the deposition is initiated and extended in the middle space of pores and throats. In addition, by increasing the temperature and brine concentration as well as by decreasing the flow rate, the size of crystals was grown. Moreover, the experimental results indicated that by increasing the temperature and brine concentration as well as by decreasing the flow rate, the scaling tendency was increased.
Analytical Approach for Finding Velocity and Temperature Distribution of Electroosmotic Flow in Micro- and Transitional Nano-Channels
Authors: Movahed Saeid , Kamali Reza , Eghtesad Mohammad
Published: September 2010

The past decade has seen tremendous growth in areas of micro- and nano-fluidics, and MEMs flow control. Nowadays, there is considerable interest in micro- and nano/technologies consisting of small structures in contact with liquid media. By increasing the motivations of using miniaturized devices such as MEMS and NEMS and inventing new methods of their manufacturing, the inspirations of their study and analysis have been increased more and more. One of the most important characteristics of these devices which have undeniable impacts on their performances is miniaturized-channel flow field. By decreasing the dimensions of channels, the influence of surface effects becomes prominent and cannot be ignored. One of the most charismatic categories of these phenomena is elecrokinetic effect which can results in electroosmotic flow field (EOF) that has many advantages such as being vibration free, being much more compact, having flat-form velocity and etc. These beneficiaries lead to the increasing stimulus of using this type of flow field. One of the most important disadvantages of EOF is the Joule heating effect, the generation of heat due to the electroosmosis effect. Besides, miniaturized-channels are usually used as heat sink in miniaturized devices. By considering these facts, it can be concluded that heat characteristics of EOF must be studied carefully in order to manage the Joule heating effect and to utilize the cooling characteristics of miniaturized-channels. By reviewing the studies that have been performed in this field of study, it can be concluded that there is not any analytical approaches in dealing with heat transfer of EOF in miniaturized-channels though analytical formulas are completely essential for investigating, monitoring and controlling of any systems. In this regards, having some analytical studies on heat transfer analysis of miniaturized-channel flow field is completely essential. In the present study, by using the Schwartz-Christoffel mapping, an analytical tactic will be proposed in order to find electroosmotic velocity and consequently temperature distribution of EOF in micro- and transitional nano-channels.
Thermal Rectification in Bi-Layered Nanofilm by Molecular Dynamics
Authors: Wang Shuai-Chuang , Liang Xin-Gang
Published: September 2010

A thermal rectifier has such nature that its thermal conductance or thermal conductivity has different values with reversed heat flux direction. This work investigates the rectification of the cross-plane thermal conductivity and interfacial thermal resistance of nanoscale bi-layered films using the nonequilibrium molecular dynamics (NEMD) method. The effects of the thickness of the single layer with the total thickness constant, the ratio of the atomic mass and temperature difference in the two ends on the thermal rectification are all considered. The results of the simulations show that the thermal conductivity and the interfacial thermal resistance are different for the heat flux with opposite directions. For the composite film with two layers of the same thicknesses, the thermal conductivity is larger when the heat flux direction is from the light layer to the heavy one. The difference becomes larger when the ratio of the atomic mass in the two layers increases. Increasing the heat flux makes the rectification of thermal conductivity larger, which means that the rectification is dependent on the temperature. For the composite film with fixed total thickness, the rectification becomes smaller when the thickness of the light layer increases. When the light layer is thick enough, the rectification is found reversed, which means that the thermal conductivity is larger with the heat flux direction from the heavy layer to the light one. The phonon density of states is also calculated to explain the phenomenon, and it is found that the overlap of the phonon density of states for the two layers is almost same even if the rectification of the thermal conductivity is reversed.
Characteristics of Two-Phase Flows in a Rectangular Microchannel With a T-Junction Type Gas-Liquid Mixer
Authors: Kawahara Akimaro , Sadatomi Michio , Nei Keitaro , Matsuo Hideki , Masuda Takatoshi
Published: September 2010

In this study, gas-liquid two-phase flows in a horizontal rectangular microchannel have been investigated. The rectangular microchannel has the hydraulic diameter of 0.235 mm, and the width and the depth of 0.24 mm and 0.23 mm, respectively. A T-junction type gas-liquid mixer was used to introduce gas and liquid in the channel. In order to know the effects of liquid properties, distilled water, ethanol and HFE7200 were used as the test liquids, while nitrogen gas as the test gas. The flow pattern, the bubble length, the liquid slug length and the bubble velocity in two-phase flow were measured with a high speed video camera, and the void fraction was determined from the bubble velocity data and the superficial gas velocity data. In addition, the pressure drop was also measured with a calibrated differential pressure transducer. The bubble length data were compared with the calculation by the scaling law proposed by Garstecki et al (2006). The bubble velocity data and/or the void fraction data were well correlated with the well-known drift flux model (Zuber and Findlay, 1965) with a new distribution parameter correlation developed in this study. The frictional pressure drop data were also well correlated with Lockhart-Martinelli method with a correlation of two-phase friction multiplier.
Infrared Thermal Image on Vapor-Liquid Interface in Capillary Microgrooves Heat Sink
Authors: Wang Tao , Hu Xuegong , Tang Dawei , Guo Chaohong
Published: September 2010

An infrared thermoviewer is utilized to measure the temperature distribution on solid walls and vapor-liquid interfaces of the rectangular capillary microgrooves heat sink, which is made of borosilicate glass. The infrared thermal image clearly shows that the solid wall temperature of microgroove top is lower than the average temperature of vapor-liquid interface. The results indicate that heat source position has a significant influence on the microgrooves surface temperature distribution, besides working liquid, tilt angle (the angle between microgroove surface and gravity direction) and heat flux.
Experimental Study of Boiling Phenomena by Micro/Milli Hydrophobic Dot on the Silicon Surface in Pool Boiling
Authors: Jo Hang Jin , Kim Hyungmo , Ahn Ho Seon , Kim Seontae , Kang Soon Ho , Kim Joonwon , Kim Moo Hwan
Published: September 2010

Many pool boiling experiments to enhance the nucleate boiling condition have been conducted and could get brilliant and challengeable results. A consensus was that CHF and heat transfer were affected by a modified heating surface. One of the efforts was the nanofluids experiments, and they have exhibited an incredible enhancement of CHF when nanofluids have been used as a working fluid in pool boiling. The results also have showed clearly that such large CHF enhancement came from the deposition of nanoparticles on the heating surface changing the surface condition. The surface covered by oxidized metal nanoparticles has a high wettability, and so it affects CHF. The fact that the wettability effect is significant to the enhancement of CHF is also supported by other kinds of boiling experiments. In addition, many researchers reported that wettability enhances not only CHF but also nucleate boiling heat transfer coefficient. In this regard, the excellent boiling performance (a high CHF and a high heat transfer coefficient) in pool boiling could be achieved through some favorable surface modification which satisfies the optimized wettability condition. For finding the optimized condition, we design the special heaters to examine how two materials, which have different wettabilities, affect the boiling phenomena. The special heaters have hydrophobic dots on the silicon surface. The hydrophobic dots lead to an early bubble inception. The bubble interface is bounded on the material boundary. The peculiar teflon(AF1600) is used as the hydrophobic material. The contact angle of the heating surface which is made by teflon is 120° to water at the room temperature. The contact angle of the silicon surface is 60° at the room temperature. The experiments using the micro hydrophobic dots and milli hydrophobic dot are performed, and the results are compared with the reference surface.
Effect of Channel Length on the Gas-Liquid Two-Phase Flow Phenomena in a Microchannel
Authors: Kimura Ryuji , Ide Hideo , Hashiguchi Hiroshi , Kawaji Masahiro
Published: September 2010

An optical measurement system was used to investigate the effect of microchannel length on adiabatic gas-liquid two-phase flow characteristics. Experiments were conducted with 146 mm and 1,571 mm long, circular microchannels of 100 micron diameter. Two-phase flow patterns, void fraction, gas and liquid plug lengths and their velocities were measured for two inlet configurations and gas-liquid mixing, i.e., (a) reducer and (b) T-junction. The test section length was found to have a significant effect on the two-phase flow characteristics measured at the same axial location in the microchannel test section typified by the void fraction data. The mean void fraction data obtained in the shorter (146 mm) microchannel with the reducer inlet agreed well with the equation by Kawahara and Kawaji which was previously proposed. On the other hand, the mean void fraction obtained at 36 mm from the inlet in the longer (1,571 mm) microchannel corresponded well with the homogeneous flow model and Armand’s equation for both reducer and T-junction inlet configurations. In the present experimental ranges of superficial gas velocity, jG = 0.03 ∼ 14 m/s, and superficial liquid velocity, jL = 0.04∼0.7 m/s, the gas and liquid plugs obtained in the longer microchannel had relatively shorter lengths and higher velocities than those in the shorter channel. Thus, both the microchannel length and inlet geometry were found to affect the two-phase flow characteristics in a microchannel.
Impedance Measurement in Multilayer Connecter System of Micro Channel With Different Frequency
Authors: Choi Je-Eun , Takei Masahiro
Published: September 2010

In investigating a microchannel, understanding regarding dispersion reaction are need to be acquire first down to the view point of multiphase flow dynamics. Usually, various measurements methods and in-depth analyzation are necessary to understand thoroughly the inside reaction occurs in this kind of system. In this experiment, fabricated multilayer microchannel with commercial connecter system was made for a tomography system with electrodes attached. This fabricated multilayer microchannel measures impedance using 12 electrodes from 5 different positions continuously. In measuring the impedance, it is necessary to consider the electric field caused by the electrodes and the force distribution inside the particles. Because of a non-uniform electric field distribution inside the system, forces inside the particles are not equal thus net force is present. This study, discuss how the frequency of the electric fields affects the particle position as a driving force to the microchannel system.
Visualization of Water Distribution in Operating PEMFC Using X-Ray Microscopy
Authors: Kim Jongrok , Je Junho , Kaviany Massoud , Son Sang Young , Kim MooHwan
Published: September 2010

In this investigation, X-ray microscopy (7B2) in Pohang Accelerator Laboratory was employed to visualize the water distribution in operating PEMFC which had 2cm × 2cm active area. This X-ray microscopy has 1μm spatial resolution with 1.5mm × 1.2mm view area. Each image spent about 1.3 second, 0.65 second for exposure and about 0.6 second for data read out. The resistance of the electric loader was changed and electric current and voltage was measured during images were recorded. The water distribution in PEMFC was analyzed with this I-V curve. Water distribution was depend on current density and aggregated on interfaces of layers (GDL, MPL, MEA).
Influence of Interfacial Effect Between a Porous Wall and an Air Region on Natural Convection
Authors: Chen Baoming , Liu Fang , Liu Aimin , Geng Wenguang
Published: September 2010

VOCs natural convective flow driven by thermo, solute and humidity buoyancy forces from a porous wall to indoor room was studied numerically in this paper, including coupled diffusion effects by three gradients interactions. The physical model for the fluid flow made use of Brinkman-Forchheimer extended Darcy equation in the porous wall and Navier-Stokes equation in the clear region. Finite element method was used to solve governing equations. Effect of interface at the interface on flow, heat and mass transfer was studied, which varied with stress jump coefficient β and permeability. The results showed that flow velocity varied greatly with increase of coefficient β in the boundary. Decrease in the value of dimensionless permeability Da also had influence on velocity variation at the interface. However, there are little influence of interface on the distribution of temperature and concentration in the whole region.
Effect of Coupled Diffusion on Heat and Mass Transfer in Partially Porous Cavities
Authors: Liu Fang , Chen Baoming , Geng Wenguang , Liu Aimin
Published: September 2010

VOCs natural convective flow from the porous layer to an air region, including thermo-diffusion and diffusion-thermo effects was studied in this paper. Two domain model was used for problem formulation. The partial differential equations, governing the problem under consideration, were solved by applying a finite element approach. Weak form equation was used to link interface between the porous wall and the air region. The effects of thermo-diffusion and diffusion-thermo had been examined on flow, temperature and concentration fields. The results showed that the above mentioned effects had to be taken into flow, heat and mass transfer processes.
Molecular Dynamics Simulation of Nano Cluster-Seed Effects on Heterogeneous Nucleation
Authors: Suh Donguk , Jung Seung-chai , Yoon Woong-sup
Published: September 2010

A three-dimensional heterogeneous nucleation is simulated by classical molecular dynamics, where the Lennard-Jones gas and solid nano cluster-seed molecules have argon and aluminum properties, respectively. All dimensions of the wall are periodic and a soft core carrier gas within the system controls the temperature rise induced by latent heat of condensation. There are three shapes of cluster-seeds being cube, rod, and sphere, three classes of masses, and the simulation took place under nine supersaturation ratios, making a total of 81 calculations. An analysis of variance was performed under a three-way layout to analyze the cluster-seed and supersaturation ratio effects on the system. For supersaturation ratios above the critical value nucleation rates were evaluated, below growth rates, and overall liquefaction rates were each defined and calculated. Results show that the supersaturation ratio dominantly controls all rates, but seed characteristics are important for the growth of the largest cluster under the critical supersaturation ratio. Overall liquefaction increases subject to an escalation of supersaturation ratio and seed mass. However, the significance of the supersaturation ratio for overall liquefaction suggests that thermal diffusion is more dominant than mass interactions for this system. Homogeneous characteristics are also compared with the heterogeneous system to find that though nucleation may occur for an insufficient supersaturation ratio when a seed is within the system, the addition of a seed does not in fact facilitate the increase in rates of the phenomena at high supersaturation ratios. Finally a comparison with the classical nucleation theory asserts a 3 to 4 order of magnitude difference, which is within the lines of deviation when it comes to theory and molecular simulations.
Numerical Investigation of a Fiber Web Effect on the Pressure Drop and Particles Collection in a Microchannel
Authors: Dastan Alireza , Abouali Omid
Published: September 2010

In this paper pressure drop and particle deposition in a microchannel with a hydraulic diameter of 225 micrometer is investigated numerically. Several hundred micron length fibers caught at the entrance of the channels making a “fiber web” also is modeled in this research. Governing equations for the flow field are solved with an Eulerian approach while the equations of particle motion in the flow are solved by a Lagrangian approach. Assuming the symmetry in the domain, one channel and the corresponding plenum are studied in the computational domain. For studying the effects of fibers in the flow, two fiber webs with four and six solid fibers are studied. The increase of pressure drop in the microchannel because of the entrance fiber web is computed and discussed. Also deposition and collection of the particles with various diameters at the fiber webs are also presented.
Dynamic Spreading of Platelets on Collagen in Microchannels
Authors: Lee Dooyoung , Fong Karen P. , Brass Lawrence F. , Hammer Daniel A.
Published: September 2010

Platelets come contact collagen exposed on the subendothelial matrix at sites of vascular injury that triggers their activation and the formation of a hemostatic plug. Glycoprotein VI and integrin α2β1 are major collagen receptors on the platelet surface. Although the spreading of platelets on the collagen is important to function, it has been difficult to study the dynamics of spreading over a relevant time scale. Here we focus on the early stages of murine platelet spreading on collagen and/or fibrinogen under both static conditions and flow and then probe their dynamics by quantitative visualization using real-time reflection interference contrast microscopy. In this study, we found under static conditions the spreading area of platelets on collagen initially increased quickly by following the power law A ∼ t0.7 before slowing. Interestingly, we observed in microchannels under flow that single platelets as well as aggregates that were spread on the collagen contracted over time under shear. This contraction was not observed under static conditions in our system. This effect might help to maintain the hemostatic plug under the shear force. Future studies will be aimed at investigating the spatial-temporal cytoskeletal dynamics of platelets and the functions of collagen receptors on the platelets surface using platelets derived from genetically engineered mice.
Growth of Ultrafine Particles Through a Minichannel With Capillary Structure
Authors: Choi Jin Young , Son Sang Young
Published: September 2010

The development of personal monitoring devices with the size of a human palm so that people can carry or wear with them has been of importance over a decade. Many small apparatuses to be able to detect the chemical and biological agent have been developed and commercialized, but the portable and wearable device to detect small particles, especially with the size of nanometer has not been developed yet up to our knowledge. In the present work, a new miniature device which is small enough for the application to a personal ultrafine particle monitoring sensor is described. The enlargement of particle size through heterogeneous condensational growth is used for the whole system. And, for miniaturization, microscale multiphase fluid control and microstructured surface modification technologies are employed. In order to evaluate the performance of the developed device, the condensational growth of ultrafine aerosol particles in a minichannel made of capillary structure was investigated through a laser sheet visualization technique. The main body of the device was constructed by cutting a commercially produced capillary structure. The physical dimension of the developed device is less than 1/10 of the smallest design commercially available. The polydisperse aerosol particles with the mean diameter of 20nm and concentration of more than 60,000 particles/cc and purified water as condensing fluid were used for test. The image of condensed droplets grown by the device was compared with those of standard size PSL particles. The experimental results show that the entered ultrafine particles successfully grew to micron size droplets through the developed condensation chamber and then could be counted with the optical detection system. Furthermore, the counting efficiency and the change of average size of condensed droplets were also investigated with the operating condition.
Numerical Study of Fluid Flow and Heat Transfer for Al2O3-Water Nanofluid Impinging Jet
Authors: Vaziei Parisa , Abouali Omid
Published: September 2010

In this study a circular confined and submerged jet impinging on a horizontal hot plate is numerically simulated. Water and 36nm Al2O3-water nanofluid with various particle volume fractions are used as a working fluid for cooling the hot plate. Both laminar and turbulent impinging jets in various nozzle to plate distances and Reynolds numbers are considered. For laminar cases Navier-Stokes and energy equations and for turbulent cases RANS and time averaged energy equations were solved numerically to obtain the flowfield and temperature distribution. The turbulence effect was considered with a two equations model. The properties of nanofluid such as thermal conductivity, viscosity and density are modified using the appropriate models. The present study reports the Nusselt number on the hot plate for investigated cases. Temperature difference between the inlet fluid and the hot plate are obtained for different mass flow rates and particle volume fractions and are compared with experimental data for turbulent jets. The results show that using Al2O3 nano-particles in laminar jets enhances the heat transfer but for the turbulent jets Al2O3-water nanofluid has a lower performance for heat removal compared with clear fluid.
Numerical Simulation of Single Phase Liquid Flow in Narrow Rectangular Channels With Structured Roughness Walls
Authors: Srivastava Rishabh R. , Schneider Nicholas M. , Kandlikar Satish G.
Published: September 2010

Laminar flow in rough channels is receiving considerable attention recently due to its application in microfluidic devices. Developing a proper understanding of the fundamental effects of roughness in narrow channels is essential. In the present study, single-phase laminar flow of an incompressible fluid through narrow rectangular channels with structured roughness walls is investigated numerically using CFD software, FLUENT. The results for smooth channel geometries are first compared with the previously validated experimental data of Brackbill [1]. Triangular ribbed roughness structures, defined by pitch and height, are incorporated in the CFD model and the fully developed region is analyzed. Pressure drop results from CFD simulations are presented and the friction factor, f, is calculated for comparison with published experimental results of Brackbill and Kandlikar [2–4] and Brackbill [1]. The validated numerical scheme will be used in future work for the evaluation of the effect of different roughness features on laminar flow.
Cooling of Microelectronic Devices Packaged in a Single Chip Module Using Single Phase Refrigerant R-123
Authors: Pasupuleti Tushara , Kandlikar Satish G.
Published: September 2010

An approach towards practical application of microchannel cooling system is necessary as the demand of high power density devices is increasing. Colgan et. al. [1] have designed a unit known as Single Chip Module (SCM) by considering the practical issues for packaging a microchannel cooling system with a microelectronic device. The performance of the SCM has already been investigated by using water as working fluid by Colgan et. al. [1]. Considering the actual working conditions, water cannot be used in electronic devices as the working fluid because any leakage may lead to system damage. Alternative fluids like refrigerants were considered. In this research, the performance of SCM has been studied by using refrigerant R-123 as working fluid and compared with water cooled system. Cooling of 83.33 W/cm2 has been achieved for a powered area of 3 cm2 by maintaining chip temperature of 60°C. The heat transfer co-efficient obtained at a flowrate of 0.7 lpm was 34.87 kW/m2-K. The results obtained indicate that from a thermal viewpoint, R-123 can be considered as working fluid for microelectronic cooling devices.
The Effect of Surface Wettability on Viscous Film Deposition
Authors: Herescu Alexandru , Allen Jeffrey S.
Published: September 2010

The viscous deposition of a liquid film on the inside of a capillary has been experimentally investigated with a focus on the relationship between the film thickness and surface wettability. With distilled water as a working fluid tests were run in a 622 microns diameter glass tube with contact angles of 30° and 105°, respectively. In the first set of experiments the tube was uncoated while in the second set a fluoropolymer coating was applied to increase the contact angle. A film thickness dependence with the contact angle θ (surface wettability) as well as the Capillary number in the form hR ∼ Ca2/3/cosθ is inferred from scaling arguments. For partial wetting it may explain the existence of a thicker film for nonzero contact angle. It was further found that the non-wetting case of 105° contact angle deviates significantly from the existing theories, the film thickness presenting a weak dependence with the Capillary number. This deviation as well as the apparent non-uniqueness of the solution is thought to be caused by the film instability (rupture) observed during the tests. The thickness of the deposited film as a function of the Capillary number was estimated from the liquid mass exiting the capillary and the gas-liquid interface (meniscus) velocity, and compared to Bretherton’s data and a correlation proposed by Quere. The film thickness measurements as well as the meniscus velocity were determined with the aid of a Photron high speed camera with 10000 frames per second sampling capability coupled with a Nikon TE-2000 inverted microscope and a Precisa electronic balance.
A New Switching Method of Two-Dimensional EWOD-Based Droplet Translation on Single-Plate Configuration
Authors: Park Jun Kwon , Lee Seung Jun , Kang Kwan Hyoung
Published: September 2010

In electrowetting-on-dielectric (EWOD), droplet can be transported using either parallel-plate configuration or single-plate configuration of electrodes. Single-plate configuration, i.e. without cover plate, is less popular due to its difficulty in successive translation of droplet and slow translation speed, in spite of its enhanced flexibility in interfacing with other components. This paper investigates the effect of switching method on droplet translation for the single-plate configuration. Droplet was transported on a two-dimensional four by four electrode array by EWOD actuation. In previous works, high voltage and ground condition were applied on two activating electrodes, maintaining other surrounding electrodes in floating state. Here, we propose a new switching method in which all the electrodes are grounded except one activating electrode. We compared the performance of the present switching method with conventional ones using the same electrode array. Performance of each method was experimentally examined with respect to maximum translation speed in which successive translation was possible. Our method showed much better performance under various experimental conditions. In addition, the present method does not necessitate the floating condition, which leads to the reduction of the number of switches and controllers to a half of conventional one.
Switching Time of Electrowetting-Based Devices
Authors: Hong Jiwoo , Oh Jung Min , Kang Kwan Hyoung
Published: September 2010

The switching time is important factor for enhancing performance of electrowetting-based devices, such as electronic display, liquid lens, and RF switches. These devices require fast spreading, receding time of droplet. In this work, we suggested an analytical method to predict the switching time. We considered the case of a liquid droplet on an insulator-coated electrode surrounded by air. The properties of droplet such as volume, density, viscosity, and interfacial tension affects the switching time. First we explored the influence of drop volume on switching time. To predict the switching time, the dynamics of a droplet are modeled by using the domain perturbation method. It is shown that the overall dynamics is governed by P2 shape mode. Based on theoretical analysis, we introduced a parameter which represents the switching time. We compared the theoretical prediction with experimental results, and then discussed the effect of contact-line hysteresis.
Experimental Study on Behavior of Bubbles and Temperature Fluctuation of Heat Transfer Surface by Using Heat Transfer Surface With Artificial Cavities Created by MEMS Technology
Authors: Sato Takato , Koizumi Yasuo , Ohtake Hiroyasu
Published: September 2010

Pool nucleate boiling heat transfer experiments were performed for water using heat transfer surfaces having a unified cavity. A single cylindrical hole of 10 μm in diameter and 40 μm in depth was formed on a mirror-finished silicon wafer of 0.2 mm in thickness using the Micro-Electro Mechanical Systems (MEMS) technology. This silicon plate was used as the heat transfer surface. The back side of the heat transfer surface was heated by a semi-conductor laser beam. The back-side surface temperature was measured by a radiation thermograph with a temperature resolution of 0.08 K and a time resolution of 3 ms/line. Experiments were conducted in the range up to 1.35 × 105 W/m2. The standard deviations of the local fluctuating heat transfer surface temperature were calculated. So the cross-correlation coefficients between the cavity center and a certain point were calculated by using the standard deviations and the time-series surface temperature data. Then, the intensity of the thermal influence exerted by the boiling bubbles on the local position was derived. The thermal influence extents determined from the intensity were 2.1 – 3.3 times larger than the mean diameter of all departure bubbles in the present experimental range.
Liquid-Liquid Segmented Flows in Polymer Microfluidic Channels
Authors: Kim Namwon , Murphy Michael C. , Soper Steven A. , Nikitopoulos Dimitris E.
Published: September 2010

Liquid-liquid segmented flows in microchannels fabricated on polymer test chips were investigated experimentally. Polymer test chips were prepared using hot embossing of polycarbonate (PC) sheets with micro-milled brass mold inserts. Three different configurations of microchannels were prepared with injection to test channels expansion ratios of 16, 4 and 2 and a fixed test channel geometry. Deionized water with blue food-coloring dye (1% v/v) was used as a dispersed fluid at flow rates (QD) between 0.5 and 60 μl/min. The carrier fluid was perfluorocarbon (FC 3283) with nonionic fluorous-soluble surfactant (Perfluorooctanol, 10% v/v) at flow rates (QC) between 3 and 25 μl/min. The two fluids were injected separately into the chips. Droplet and Plug flows with transient Irregular Segmented flows between two flow regimes were mainly observed in the test channels of the three different chips. Flow pattern maps and transitions between flow regimes were determined in terms of a fixed homogeneous carrier fluid volumetric flow ratio (βC) to compare the effect of the expansion ratios from the injection to the test channels. The droplet and plug regimes were shifted to higher carrier and lower dispersed fluid superficial velocities and the plug flow regime was broader with the lower expansion ratio channels. The transient irregular segmented flow was favored in the higher expansion channel ratio and the interval of transient irregular segmented flow between droplet and plug flow regimes were shorter for the low expansion channel ratios. This is evidence that flow regime maps in micro-channels are not universal and depend on the configuration part of the micro-injection system. The length of the dispersed segmented flows and the distance between consecutive droplets or plugs as a function of βC were determined by image processing of frames acquired via CCD camera with bright field illumination. The average length of the dispersed fluid was shown to scale approximately with βC to the −1.2 power. Velocities of the dispersed droplet and plug flows were measured using double-pulsed laser illumination and were found to be 1.25 ± 0.049 and 1.46 ± 0.077 times faster than the superficial velocity of the segmented flow respectively. Two-phase pressure drop measurements were also carried out for all flow regimes and associated trends were correlated with changes in flow topology. Comparisons of experimental pressure drop with the predictions for a modified Lockhart-Martinelli correlation were also made.
Novel Boundary Condition Implementation to Model Electroosmotic Phenomenon in Microchannels
Authors: Darbandi Masoud , Farzinpour Pouyan
Published: September 2010

This Paper presents simulation of Electroosmotic phenomenon in a capillary using an extended hybrid finite-volume-element method. Most of the last electroosmotic phenomenon studies in microchannels do not take into consideration the effect of reservoirs and pressure drops occurred at the inlet and outlet of the microchannel. This neglect may lead to a few drawbacks. For example, the incorrect pressure drops can significantly alter the flow patterns and mass flow rates in microchannels. As a remedy the use of reservoirs at the two ends of a microchannel has been recently practiced as a novel strategy to overcome this drawback. However, this remedy needs careful attention because the resulting solutions need to be independent of the reservoirs’ sizes. In this paper, we introduce a novel geometry and new boundary conditions, which resolves the difficulties encountered with inserting the reservoirs at the two ends of a microchannel. The results of applying the new geometry and implementing the new boundary conditions are satisfactory. Therefore, using the outcomes of this paper, the real simulation of electroosmotic field is possible.
Development of Cooling System for a Large Area at High Heat Flux by Using Flow Boiling in Narrow Channels
Authors: Miura Shinichi , Inada Yukihiro , Shinmoto Yasuhisa , Ohta Haruhiko
Published: September 2010

Advance of an electronic technology has caused the increase of heat generation density for semiconductors densely integrated. Thermal management becomes more important, and a cooling system for high heat flux is required. It is extremely effective to such a demand using flow boiling heat transfer because of its high heat removal ability. To develop the cooling system for a large area at high heat flux, the cold plate structure of narrow channels with auxiliary unheated channel for additional liquid supply was devised and confirmed its validity by experiments. A large surface of 150mm in heated length and 30mm in width with grooves of an apex angle of 90 deg, 0.5mm depth and 1mm in pitch was employed. A structure of narrow rectangular heated channel between parallel plates with an unheated auxiliary channel was employed and the heat transfer characteristics were examined by using water for different combinations of gap sizes and volumetric flow rates. Five different liquid distribution modes were tested and their data were compared. The values of CHF larger than 1.9×106W/m2 for gap size of 2mm under mass velocity based on total volumetric flow rate and on the cross section area of main heated channel 720kg/m2s or 1.7×106W/m2 for gap size of 5mm under 290kg/m2s were obtained under total volumetric flow rate 4.5×10−5m3/s regardless of the liquid distribution modes. Under several conditions, the extensions of dry-patches were observed at the upstream location of the main heated channel resulting burnout not at the downstream but at the upstream. High values of CHF larger than 2×106W/m2 were obtained only for gap size of 2mm. The result indicates that higher mass velocity in the main heated channel is more effective for the increase in CHF. It was clarified that there is optimum flow rate distribution to obtain the highest values of CHF. For gap size of 2mm, high heat transfer coefficient as much as 7.4×104W/m2K were obtained at heat flux 1.5×106W/m2 under mass velocity 720kg/m2s based on total volumetric flow rate and on the cross section area of main heated channel. Also to obtain high heat transfer coefficient, it is more useful to supply the cooling liquid from the auxiliary unheated channel for additional liquid supply in the transverse direction perpendicular to the flow in the main heated channel.