bluuebunny/crossref_metadata_2025 huggingface dataset:
A Novel Methodology for Combined Parameter and Function Estimation Problems
Authors: Molavi Hosein , Hakkaki-Fard Ali , Rahmani Ramin K. , Ayasoufi Anahita , Molavi Mehdi
Published: September 2010

This article presents a novel methodology, which is highly efficient and simple to implement, for simultaneous retrieval of a complete set of thermal coefficients in combined parameter and function estimation problems. Moreover, the effect of correlated unknown variables on convergence performance is examined. The present methodology is a combination of two different classical methods: The conjugate gradient method with adjoint problem (CGMAP) and Box–Kanemasu method (BKM). The methodology uses the benefit of CGMAP in handling function estimation problems and BKM for parameter estimation problems. One of the unique features about the present method is that the correlation among the separate unknowns does not disrupt the convergence of the problem. Numerical experiments using measurement errors are performed to verify the efficiency of the proposed method in solving the combined parameter and function estimation problems. The results obtained by the present approach show that the combined procedure can efficiently and reliably estimate the values of the unknown thermal coefficients.
On the Condition for Thermal Rectification Using Bulk Materials
Authors: Go David B. , Sen Mihir
Published: September 2010

In conduction, thermal rectification occurs when a material or structure transfers heat asymmetrically. In this work, it is shown that, in the bulk, a necessary condition for thermal rectification is that the thermal conductivity of the material or structure be a function of both space and temperature, and that this function be nonseparable. Practically, this can be achieved using a composite structure consisting of many materials although it could also be achieved with a properly developed single material.
3D Integrated Water Cooling of a Composite Multilayer Stack of Chips
Authors: Alfieri Fabio , Tiwari Manish K. , Zinovik Igor , Poulikakos Dimos , Brunschwiler Thomas , Michel Bruno
Published: September 2010

New generation supercomputers with three dimensional stacked chip architectures pose a major challenge with respect to the removal of dissipated heat, which can reach currently as high as 250 W/cm2 in multilayer chip stacks of less than 0.3 cm3 volume. Interlayer integrated water cooling is a very promising approach for such high heat flux removal due to much larger thermal capacity and conductivity of water compared with air, the traditional cooling fluid. In the current work, a multiscale conjugate heat transfer model is developed for integrated water cooling of chip layers and validated with experimental measurements on an especially designed thermal test vehicle that simulates a four tier chip stack with a footprint of 1 cm2. The cooling heat transfer structure, which consists of microchannels with cylindrical pin-fins, is conceived in such a way that it can be directly integrated with the device layout in multilayer chips. Every composite layer is cooled by water flow in microchannels (height of 100 μm), which are arranged in two port water inlet-outlet configuration. The total power removed in the stack is 390 W at a temperature gradient budget of 60 K from liquid inlet to maximal junction temperature, corresponding to about 1.3 kW/cm3 volumetric heat flow. The computational cost and complexity of detailed computational fluid dynamics (CFD) modeling of heat transfer in stacked chips with integrated cooling can be prohibitive. Therefore, the heat transfer structure is modeled using a porous medium approach, where the model parameters of heat transfer and hydrodynamic resistance are derived from averaging the results of the detailed 3D-CFD simulations of a single streamwise row of fins. The modeling results indicate that an isotropic porous medium model does not accurately predict the measured temperature fields. The variation of material properties due to temperature gradients is found to be large; therefore, variable properties are used in the model. It is also shown that the modeling of the heat transfer in the cooling sublayers requires the implementation of a porous medium approach with a local thermal nonequilibrium, as well as orthotropic heat conduction and hydrodynamic resistance. The improved model reproduces the temperatures measured in the stack within 10%. The model is used to predict the behavior of multilayer stacks mimicking the change of heat fluxes resulting from variations in the computational load of the chips during their operation.
Laminar Boundary Layer Development Around a Circular Cylinder: Fluid Flow and Heat-Mass Transfer Characteristics
Authors: Alper Ozalp A. , Dincer Ibrahim
Published: September 2010

This paper presents a comprehensive computational work on the hydrodynamic, thermal, and mass transfer characteristics of a circular cylinder, subjected to confined flow at the cylinder Reynolds number of Red=40. As the two-dimensional, steady and incompressible momentum and energy equations are solved using ANSYS-CFX (version 11.0), the moisture distributions are computed by a new alternating direction implicit method based software. The significant results, highlighting the influence of blockage (β=0.200–0.800) on the flow and heat transfer mechanism and clarifying the combined roles of β and moisture diffusivity (D=1×10−8–1×10−5 m2/s) on the mass transfer behavior, are obtained for practical applications. It is shown that the blockage augments the friction coefficients (Cf) and Nusselt numbers (Nu) on the complete cylinder surface, where the average Nu are evaluated as Nuave=3.66, 4.05, 4.97, and 6.51 for β=0.200, 0.333, 0.571, and 0.800. Moreover, the blockage shifts separation (θs) and maximum Cf locations (θCf−max) downstream to the positions of θs=54.10, 50.20, 41.98, and 37.30 deg and θCf−max=51.5, 53.4, 74.9, and 85.4 deg. The highest blockage of β=0.800 encourages the downstream backward velocity values, which as a consequence disturbs the boundary layer and weakens the fluid-solid contact. The center and average moisture contents differ significantly at the beginning of drying process, but in the last 5% of the drying period they vary only by 1.6%. Additionally, higher blockage augments mass transfer coefficients (hm) on the overall cylinder surface; however, the growing rate of back face mass transfer coefficients (hm−bf) is dominant to that of the front face values (hm−ff), with the interpreting ratios of h¯m−bf/h¯m=0.50 and 0.57 and h¯m−ff/h¯m=1.50 and 1.43 for β=0.200 and 0.800.
Heat Transfer Characteristics of Oscillating Heat Pipe With Water and Ethanol as Working Fluids
Authors: Xian Haizhen , Yang Yongping , Liu Dengying , Du Xiaoze
Published: September 2010

In this paper, experiments were conducted to achieve a better understanding of the oscillating heat pipe (OHP) operating behavior with water and ethanol as working fluid. The experimental results showed that there existed a necessary temperature difference between the evaporator and the condenser section to keep the heat pipe working. The maximum effective conductivity of the water OHP reached up to 259 kW/m K, while that of the ethanol OHP is of 111 kW/m K. Not all the OHPs are operated in the horizontal operation mode. The heat transfer performance of the ethanol OHP was obviously affected by the filling ratio and the inclination angle but the influence law is irregular. The effect of the filling ratio and the inclination angle of the water OHP were smaller than that of the ethanol one. The heat transfer performance of the OHP was improved with increase of operating temperature. The startup characteristics of the OHP depended on the establishment of the integral oscillating process, which was determined by the operating factors. The startup temperature of the ethanol OHP varied from 40°C to 50°C and that of the water, OHP varied from 40°C to 60°C without considering the horizontal operating mode. The water OHP showed a better performance and more stable heat transfer characteristics than the ethanol OHP, which had no obvious advantages of the startup capability as well.
Entropy Generation Analysis for Nanofluid Flow in Microchannels
Authors: Li Jie , Kleinstreuer Clement
Published: September 2010

Employing a validated computer simulation model, entropy generation is analyzed in trapezoidal microchannels for steady laminar flow of pure water and CuO-water nanofluids. Focusing on microchannel heat sink applications, local and volumetric entropy rates caused by frictional and thermal effects are computed for different coolants, inlet temperatures, Reynolds numbers, and channel aspect ratios. It was found that there exists an optimal Reynolds number range to operate the system due to the characteristics of the two different entropy sources, both related to the inlet Reynolds number. Microchannels with high aspect ratios have a lower suitable operational Reynolds number range. The employment of nanofluids can further minimize entropy generation because of their superior thermal properties. Heat transfer induced entropy generation is dominant for typical microheating systems while frictional entropy generation becomes more and more important with the increase in fluid inlet velocity/Reynolds number.
Novel In-Situ Laser Based Diagnostics for Measurement of Water Vapor and Carbon Dioxide Concentrations in the Gas Distribution Channels of a PEM Fuel Cell
Authors: Lambe Derek E. , Seleski Kyle , Kumar Ranganathan , Basu Saptarshi
Published: September 2010

A novel method has been implemented for measuring the concentration of various gas species (water vapor, carbon dioxide) within fuel cell gas channels and other minichannel applications in a non-invasive manner through the use of tunable diode laser absorption spectroscopy (TDLAS). An optically accessible test cell has been designed to allow for the passage of 1–0.5 millimeter diameter laser beams along 12 mm-12 cm long flow paths, while also allowing for visual observation of the channels in order to detect the formation of liquid water. Concentrations of water vapor and carbon dioxide have been measured in situ within the test cell with a temporal resolution of 0.5 secs and 2.5 secs respectively. The technique is portable to high aspect ratio channels yielding concentration measurements of species over 1 mm long passages with an experimental uncertainty of 5%.
The Numerical Investigation of a New Passive Micromixer With Improved Tesla Structure
Authors: Fan YanFeng , Hassan Ibrahim
Published: September 2010

In this paper, 3D numerical simulations are performed to investigate the mixing process within an improved Tesla micromixer. This improved Tesla micromixer applies the flow separation/recombination and converging/diverging principles to enhance mixing. A portion of the working fluid, which separates from the main flow, enters the Tesla side branch and mixes with the main flow again at the exit of the Tesla unit. The tested volume flow rate ranges from 1 μL/min to 100 μL/min. Grid independence is carried out to minimize the effect of numerical diffusion. Optimization is done to determine three parameters, which are the gap ratio (H/W), the mixing cell number (N), and the angle at the gap inlet (β). The effects of these three parameters on mixing are investigated at a volume flow rate of 100 μL/min. The simulation results show that the gap ratio is the most important factor. Three parameters are selected as H/W = 50/200, N = 10 and β = 90° for further investigation. The traditional Tesla micromixer is also simulated for comparison with the present design. The mixing efficiency is approximately 60% in the range of the tested volume flow rate. The improved micromixer has better mixing efficiency than the traditional Tesla micromixer when the volume flow rate is less than 50 μL/min.
Modeling and Simulation of a Rollerball Microfluidic Device
Authors: Rojas-Solo´rzano Luis R. , Anna Shelley L. , Bradeddine Bourtal , Amon Cristina H.
Published: September 2010

The fluid delivery process through a rollerball device is investigated by means of physical modeling and numerical simulations. The microfluidic device is intended to deliver liquid above a substrate interacting with the surrounding air. While the fluid is delivered, air entrainment occurs through the capillary gap, creating a two-phase liquid-gas mixture whose composition and properties affect significantly the quality of the continuous fluid deposition. For the numerical solution of the 2D two-phase flow governing equations, the finite volume-based finite element method is used with 2nd order time-space schemes for the fully coupled system of equations. The quality of the liquid micro-volume delivery proves to be largely affected by both the speed of the roller and fluid properties. It is found that only under very low speed and some fluid properties, it is possible to guarantee a gas free liquid deposition. Envisioning the potential use of this convenient and popular device in the deployment of microfluid layers or substances at very small quantities with controlled quality, it is apparent the need for handling and channeling out the air entrainment without perturbing the liquid quality.
Laminar Forced Convection in Annular Microchannels With Slip Flow Regime
Authors: Sadeghi Arman , Asgarshamsi Abolhassan , Saidi Mohammad Hassan
Published: September 2010

Fluid flow and heat transfer at microscale have attracted an important research interest in recent years due to the rapid development of microelectromechanical systems (MEMS). Fluid flow in microdevices has some characteristics which one of them is rarefaction effect related with gas flow. In this research, hydrodynamically and thermally fully developed laminar rarefied gas flow in annular microducts is studied using slip flow boundary conditions. Two different cases of the thermal boundary conditions are considered, namely: uniform temperature at the outer wall and adiabatic inner wall (Case A) and uniform temperature at the inner wall and adiabatic outer wall (Case B). Using the previously obtained velocity distribution, energy conservation equation subjected to relevant boundary conditions is numerically solved using fourth order Runge-Kutta method. The Nusselt number values are presented in graphical form as well as tabular form. It is realized that for the case A increasing aspect ratio results in increasing the Nusselt number, while the opposite is true for the case B. The effect of aspect ratio on Nusselt number is more notable at smaller values of Knudsen number, while its effect becomes slighter at large Knudsen numbers. Also increasing Knudsen number leads to smaller values of Nusselt number for the both cases.
Numerical Simulation of Fluid Flows and Mixing in Microchannels Induced by Internal Electrodes
Authors: Wa¨lter Bettina , Ehrhard Peter
Published: September 2010

We investigate the influence of internal electrodes onto the flow field, governed by electroosmosis and electrophoresis in a modular rectangular microchannel. As internal electrodes can be positioned at lower distances, they can be operated at lower voltages and still ensure strong electrical field strength. Even at lower voltages, electrode reactions influence the species concentration fields, and the crucial question arises, whether at the electrodes all species can be kept in dissolution or whether some species are released in gaseous form. The position and charge of multiple internal electrodes is a further focus of our investigations: wall-tangential electrical field components are responsible for pumping, wall-normal electrical field components are responsible for mixing. Hence, an optimized position and charge of all electrodes will lead to an optimized electrical field, designed to fulfill the desired tasks of the modular microchannel. The mathematical model for the numerical treatment relies on a first-principle description of the EDL and the electrical forces caused by the electrical field between the internal electrodes. Hence, the so-called Debye-Hu¨ckel approximation is avoided. The governing system of equations consists of a Poisson equation for the electrical potential, the continuity and Navier-Stokes equations for the flow field, species transport equations, based on the Nernst-Planck equation, and a charge transport equation. Further, a model for the electrode reactions, based on the Butler-Volmer equation, is in place. The simulations are time-dependent and two-dimensional in nature and employ a FVM.
Numerical Investigation for Determining Temperature Peaks in a Finite Slab With Volumetric Heat Generation Based on the Hyperbolic Model of Heat Conduction
Authors: Goharkhah Mohammad , Amiri Shahin , Baghapour Behzad
Published: September 2010

In this paper, the non-Fourier heat conduction problem in a finite slab with volumetric heat generation is investigated numerically. A linear dependence on space for heat generation has been considered. MacCormack’s explicit predictor-corrector scheme is used for solving the wave propagation problem. Results show that the space-dependent heat generation strongly affects the temperature distribution. A temperature peak in the slab has been observed due to superposition of thermal waves. It has been shown that the magnitude of the temperature peak increases with increasing the slope of dimensionless heat generation function. In order to validate the approach, the results have been compared with the analytical solution which shows an excellent agreement.
Impact of Drops on Various Non-Wetting Surfaces
Authors: Brunet Philippe , Merlen Alain
Published: September 2010

We have carried out an experimental study of liquid drop impact on various superhydrophobic substrates. Our surfaces are of two kinds (1) a carpet of chemically coated nanowires and (2) a smooth warm substrate. In the latter case, the Leidenfrost effect (also called ‘boiling crisis’) ensures the existence of a thin layer of air coming from the evaporation of the drop, thus preventing the drop to touch the warm surface. Technically, in this latter situation the contact angle can then be considered as equal to 180 degrees, with no hysteresis. Due to its initial inertia, the drop experiences a flattening phase after it hits the surface, taking the shape of a pancake. Once it reaches its maximal lateral extension, the drop begins to retract and bounces back. We have extracted the lateral extension of the drop, and we propose a model that explains the trend. We find a limit initial velocity beyond which the drop (1) protrudes into the nanowire carpet (2) touches the hot plate, provoking a local violent boiling. We discuss the relevance of practical issues in terms of self-cleaning surfaces or spray-cooling.
Gas Circulation Due an Azimuthal Temperature Distribution Over a Micro-Tube Wall
Authors: Sharipov Felix
Published: September 2010

In many applications, a temperature of fixed cross section of micro-tube is not uniform, but it can deviate from its equilibrium value. In this case the gas flow is not axi-symmetrical any more, but the flow is two dimensional. Under such conditions, the density cannot be considered as uniform across the section and gas circulations can occur. The aim of the present work is numerical calculation of distributions of velocity, density and temperature caused by an azimuthal temperature distribution over the tube wall. The problem is solved on the basis of the kinetic equation over a wide range of the gas rarefaction.
Impact of the Temperature of Twin Inclined Tandem Jets on Their Dynamic Interaction With a Cooler Oncoming Crossflow
Authors: Radhouane Amina , Mahjoub Nejla , Mhiri Hatem , Lepalec George , Bournot Philippe
Published: September 2010

We deal in the present paper with the commonly known “jets in crossflow” configuration. The interest raised by this configuration comes from its wide application in several fields and from its dependence on numerous parameters. Its well understanding is crucial in the way it helps solve problems in relation with the mixing processes, the heating and cooling performances, the control of fume dispersion, etc... These problems are also faced in small-scale applications like electronic devices cooling or the pulverization of fuel in combustion chambers through infinitely small nozzles, etc... The jets handled in our study are elliptic, aligned in the same direction of the oncoming crossflow, undergoing the same initial conditions and their dimensions are within the micro-scale. In addition to these primer conditions, an injection ratio of R=2 is imposed between the velocities of the jets and the main flow. The jet nozzles adopt a streamwise inclination of 60° and are separated with a distance of three diameters (the little ellipse diameter). The configuration is examined both experimentally by means of the Particle Imagery Velocity (PIV) technique and numerically with the finite volume method. The latter is based on the resolution of the Navier Stokes equations by means of the RSM second order turbulent closure model and a non uniform mesh system that is particularly refined near the injection nozzles. Once validated by confrontation to the experimental data, our numerical model will be generalized by the introduction of further conditions that make it more realistic. A non reactive fume is injected within the jets and a variable temperature gradient is imposed between the jets and the crossflow. Our aim is precisely to evaluate the impact of the introduced temperature gradient on the reigning flow field, and particularly on its dynamic features (the different velocity components).
Influence of Surface Wettability on Pool Boiling Heat Transfer
Authors: Phan Hai Trieu , Caney Nadia , Marty Philippe , Colasson Stephane , Gavillet Je´roˆme , Marechal Alain
Published: September 2010

Although boiling process has been a major subject of research for several decades, its physics still remain unclear and require further investigation. This study aims at highlighting the effects of the surface wettability on pool boiling heat transfer. Nanocoating techniques were used to vary the water contact angle from 20 to 110° by modifying nanoscale surface topography and chemistry. The experimental results obtained disagree with the predictions of the classical models. A new approach of nucleation mechanism is established to clarify the nexus between the surface wettability and the nucleate boiling heat transfer. In this approach, we introduce the concept of macro- and micro-contact angles to explain the observed phenomenon.
The Thermal Transport Properties of Ethylene Glycol Based MGO Nanofluids
Authors: Yu Wei , Xie Hua-Qing , Li Yang , Chen Li-Fei
Published: September 2010

Ethylene glycol based nanofluids containing MgO nanoparticles (MgO-EG) were prepared, and the transport properties including thermal conductivity and viscosity were measured. The results show that the thermal conductivity of MgO-EG nanofluid depends strongly on particle concentration, and it increases nonlinearly with the volume fraction of nanoparticles. The thermal conductivity of MgO-EG nanofluids is larger than that of nanofluids containing the same volume fraction of TiO2, ZnO, Al2O3 and SiO2, maybe due to its lowest viscosity among the tested metallic oxide nanofluids. Thermal conductivity enhancement of MgO-EG nanofluids appears weak dependence on temperature from 10 to 60°C, and the enhanced ratios are almost constant. Viscosity measurements show that MgO-EG nanofluids demonstrate Newtonian behavior, and the viscosity significantly decreases with temperature. The thermal conductivity and viscosity increments of nanofluids are higher than those of the existing classical models for the solid-liquid mixture.
Single-Nozzle Micropumps
Authors: Huang X. Y. , Wang S. S. , Yang C.
Published: September 2010

We demonstrate a new design of valveless micropump with a single nozzle and driven by piezoelectric disk. The nozzle is formed by a planar channel with an acoustic resonator profile, and the pump is fabricated by lamination of layers made of polymethyl-methacrylate (PMMA) and dry adhesives. Both experimental characterization and numerical simulation show that pump works well at low frequency in terms of relatively high pressure head and flowrate. Moreover, the pumping flow direction can be altered by simply adding buffer areas at the inlet and outlet. The pump is suitable for microfluidic integrations.
Liquid Metal Based Mini/Micro Channel Cooling Device
Authors: Deng Yue-Guang , Liu Jing , Zhou Yi-Xin
Published: September 2010

Effective heat dissipation is of great importance in many engineering fields. In this paper, we investigated a newly emerging method to significantly improve the cooling capability of micro channel devices, through implementing liquid metal with low melting point as the powerful coolant. A series of experiments with different working fluids and volume flow were performed, and the different cooling effects between liquid metal and water were compared. In order to better evaluate the cooling capability of liquid metal based micro channel cooling device, the hydrodynamic and heat transfer theory involved was discussed. The results indicated that, when the system operated in a relatively high velocity, micro channel cooling devices with liquid metal as coolant could produce higher convective heat transfer coefficient compared to those with traditional cooling fluids. And under the same pump power, not only the thermal resistance of liquid metal based micro channel could be much smaller, but also the coolant volume flow could be decreased. What is more, the liquid metal can be driven by a highly efficient electromagnetic pump without any noise. Therefore, more compact and energy-saving micro channel cooling devices with better cooling capability may come into reality. This new method is rather practical, and is expected to be important for realizing an extremely high heat dissipation rate.
Capillary Filling in Nanochannels
Authors: Phan Vinh Nguyen , Yang Chun , Nguyen Nam-Trung
Published: September 2010

Capillary filling is the key phenomenon in planar chromatography techniques such as paper chromatography and thin layer chromatography. While capillary filling in channels of micrometers scale are experimentally verified that obeys well to Washburn’s law, there is evident show that the speed of capillary filling in nanochannels is noticeable lower than described by Washburn’s formula. This paper describes a model for capillary filling phenomenon in nanochannel. Experiments on the filling of electrolytic and nonelectrolytic solutions in polymeric nanochannels were carried out. The filling processes were observed and recorded. Filling distances were measured, from which filling speeds were derived. Formation of air bubbles was also observed with some channel’s geometry. A mathematical model to calculate the electroviscous effect was established. This model shows that contribution of electroviscous effect in the reduction of filling speed is small. This result also agrees well with other previous theoretical works on the electroviscous effect. That means beside electroviscous effect, there are other phenomena that contribute in the reduction of capillary filling speed in nanochannel, such as air bubbles formation. Those phenomena need to be described qualitatively and quantitatively in order to understand more on the capillary filling in nanochannel.
The Optimum Design of Traveling-Wave Electroosmotic Micropumps
Authors: Hsu Shou-Ping , Ye Hong-Jun , Luo Win-Jet , Chen Jauh-Shyong
Published: September 2010

This study investigates the performace of electroosmotic micropumps with arrays of microelectrodes for pumping electrolyte in a microchannel. Traveling-wave potentials are applied to the microelectrode arrays. Ions accumulate in the double layer in response to the applied signal. The electric field acts on the charge pulling the fluid in the direction of the traveling wave. In this study, the effects of the widths and heights of the electrodes, the gap size between the electrodes and the frequency of the applied traveling-wave potential on the pumping velocity of the micropump are investigated in order to obtain the optimum design of the micropump. The optimum operating frequency of the traveling-wave potential is about 1 kHz for the micropumps with different electrode widths and gap sizes. The pumping velocities increase with the decrease of the electrode widths and gap sizes for the micropumps. For the micropumps with different electrode widths and gap sizes, it is found the optimum electrode heights are about 5.5 μm when the gap sizes are less than the electrode widths, and the optimum electrode heights are about 10.4 μm when the gap sizes are greater than the electrode widths.
A Numerical Investigation of Thermally Mediated Droplet Formation in a T-Junction
Authors: Ho Peng-Ching , Yap Yit Fatt , Nguyen Nam-Trung , Chai Chee Kiong John , Wong Teck Neng , Yobas Levent
Published: September 2010

The present article presents a numerical investigation on the effect of thermal forcing for droplet formation in a T-junction. Thermal forcing, generated by a heater embedded into the channel wall, induces a non-uniform temperature field which results in the variation the fluids’ properties and affects the droplet formation process in a desirable manner. In the present article, droplet formation process is posed as an incompressible immiscible two-phase flow problem with the motion of the two-phases strongly coupled via the related interfacial conditions. It is governed by the three-dimensional Navier-Stokes and the energy equations. The interface is captured with a narrow-band particle level-set method. Solutions are obtained using a finite volume method on a staggered mesh. The numerical model is validated against droplet formation in a cross junction. With the formation of water droplet in oil within the squeezing formation regime as a case study, the physics underlying droplet formation process in a T-junction affected by a thermal forcing is investigated. The combined effect of variations in both viscosities and surface tension result in a larger droplet. It is believed that the behavior of fluids system under an imposed thermal forcing depends strongly on the characteristics of temperature dependent viscosities and surface tension.
Fabrication of Nanochannels on Polymer Thin Film
Authors: Phan Vinh-Nguyen , Abgrall Patrick , Nguyen Nam-Trung , Shao Peige , Van Kan Jeroen Anton
Published: September 2010

Recent advances in nanotechnology allow the fabrication of structures down to the nanometer range. Various theoretical and experimental studies on the characteristics of fluid in nanochannels have been carried out in recent years. The results show that transport phenomena in nanoscale promise a wide range of applications in biological and chemical analysis. Practical applications require fabrication of nanochannels with a short production time and at a low cost. Polymer is considered as a suitable material for mass production of nanochannels due to the wide range of properties available, as well as the low cost of material and fabrication process. This paper reports the fabrication of planar nanochannels using hot embossing and thermal bonding technique on a polymer thin film. The mold for hot embossing was fabricated on a silicon wafer using photolithography and Reactive Ion Etching (RIE). Polymethylmethacrylate (PMMA) thin film with a thickness of 250 μm was used as the base material to emboss the nanochannels from the silicon mold. Temperature and pressure were controlled and recorded continuously during the embossing process. The channels then were examined by Atomic Force Microscope (AFM) in tapping mode to verify the width and the depth of the channel. Next, another piece of PMMA thin film was bonded to the first piece by thermal bonding process to make closed nanochannels. Temperature and pressure during the bonding process were controlled and recorded. Access to the channels was made on the thin film by a laser cutter before embossing. The results showed that open planar channels with the depth down to 30nm can be fabricated on PMMA thin film with a process time less than 30 minutes. Width and depth of the channels agree well with appropriate dimensions on the mold. Bonding can be achieved within 40 minutes. Closed planar channels with the depth of 300nm were fabricated successfully by a combination of embossing and thermal bonding processes. This project demonstrates the possibility of fabricating nanochannels with low cost and short processing time using polymer material. The processes are suitable not only for nanochannels but also for more complicated nanostructures. The presented technique allows the fabrication of nanodevices with various designs.
Experimental Study on a Honeycomb Micro Channel Cooling System
Authors: Liu Yonglu , Luo Xiaobing , Liu Wei
Published: September 2010

A honeycomb porous micro channel cooling system for cooling of electronic chips was proposed. Experimental investigation was conducted to determine the heat transfer characteristics and cooling performance of this micro channel cooling system. The heat transfer capabilities of the cooling system with different pipe diameters, different working media and various pumping power were evaluated. The influences of working flow rate and test system on the cooling performance were also analyzed experimentally. The results showed that the better cooling capability of the system not only relied on the increasing pump power, but also the agreement between pumps diameters and system pipes diameters. The thermo-physical characters and mass flow rate of the working fluid were also important to the system performance.
Numerical Study on Mixing in a Chaotic Serpentine Mixer Using a Mapping Method
Authors: Kang T. G. , Singh M. K. , Anderson P. D. , Meijer H. E. H.
Published: September 2010

We introduce a chaotic serpentine mixer (CSM), which is motivated by the three-dimensional serpentine channel [Liu et al., 2000, J. Microelectromech. Syst. 9, pp. 190–197], and demonstrate a systematic way of utilizing the mapping method [Singh et al., 2008, Microfluid Nanofluid 5, pp. 313–325] to find out an optimal set of design variables for the new mixer. The new mixer shows globally chaotic mixing even in the Stokes flow regime, while maintaining the benefits of the original design. One geometrical period of the mixer consists of two functional units, inducing two flow portraits with crossing streamlines. Each half period of the mixer consists of an “L-shaped” bend and a bypass channel. The two flow portraits may be either co-rotational or counter-rotational. As a preliminary study, first of all, mixing in the original serpentine channel has been analyzed to demonstrate the flow characteristics and to find out a critical Reynolds number showing chaotic mixing above the limit. The working principle of the newly proposed mixer is explained by the manifold of the deforming interface between two fluids. To optimize the mixer, we choose three key design variables: the sense of rotation of the two flows, the aspect ratio of the rectangular channel, and the lateral location of the bypass channel. Then, simulations for all possible combinations of the variables are carried out. At proper combinations of the variables, almost global chaotic mixing is observed in the creeping flow regime. The design windows, provided as a result of the parameter study, can be used to determine a proper set of the design variables to fit with a specific application. The deforming interface of the two fluids shows that, even in a poor mixer in Stokes flow regime, as the Reynolds number increases, more efficient mixing is resulted in due to the enhanced cross-sectional vertical motion and back flows near the bends.
Experiment Investigation of R134A Flow Boiling Process in Micro-Channel With Cavitation Structure
Authors: Cao Hongzhang , Xu H. B. , Liang N. , Tian C. Q.
Published: September 2010

One cavitation structure that the channel cross section expanded suddenly was introduced in single straight microchannel. The experiment was carried out with R-134a as the fluid medium, which was drove by gear pump. The flow pattern was observed by CCD and microscope. The average boiling heat transfer coefficient was estimated with the calculation method proposed in this paper. The experimental results present the boiling began at the cavitation structure, and stable flow boiling was maintained. When heat power grew up the boiling became strong then the pressure drop in the micro-channel increased and the heat transfer was enhanced. The liquid percentage in two-phase flow increased and the length of boiling area became small when mass flux grew up with the fixed heat power.
Flow Simulations in a Sub-Micro Porous Medium by the Lattice Boltzmann and the Molecular Dynamics Methods
Authors: Takenaka S. , Suga K. , Kinjo T. , Hyodo S.
Published: September 2010

In order to devise and establish a cost-effective strategy to simulate flows in continuum to slip and transitional regimes, present study focuses on evaluating a lattice Boltzmann equation [Niu et al., Phys. Rev. E 76, 036711, 2007] formerly proposed by the present authors’ group. The main test flow case is a flow around a square cylinder situated in a sub-micro channel. Since a rather shorter streamwise domain size and a periodic streamwise boundary condition are imposed, the flow regime is regarded as a part of an infinite cylinder array set in a narrow channel which can be considered as a kind of micro-porous media. For the assessment of the lattice Boltzmann simulations, the molecular dynamics (MD) simulations using Leonard-Jones potential are also performed. In the MD simulations, novel boundary treatments are applied. The results of the square cylinder flow by both the simulations at Knudsen number Kn=0.08 show reasonable agreement and confirm the reliability of the present lattice Boltzmann method (LBM). The fact that the computational time of the present MD simulation to obtain the reliable statistics is 1800 times as much as that of the LBM clearly indicates the usability of the LBM for engineering applications even in the slip and transitional flow regimes.
Numerical Study of an Ion Injection EHD Micropump
Authors: Ghazi R. , Saidi M. S. , Saidi M. H.
Published: September 2010

The purpose of this study is to carry out a numerical investigation of injection type EHD pumps. To this end, the flow is considered laminar, steady and incompressible flow. The flow model is based on the assumptions that the fluid is Newtonian and the fluid properties are constant. The results show that all of the ions emitted from emitter are not collected by next collector electrode, but some of this injected ions move to the previous one, which causes back flow. Although this back flow is smaller than net pumping flow but it would reduce the net flow. Because of existence of this back flow, finding the optimum distance of electrodes from each other is necessary. This study shows that when the distance from emitter electrode to next collector is half of its distance from previous one, the maximum flow rate will be achieved. The results also show that flow rate reduces linearly as the pressure increases.
Characterisation of Flow and Mass Transfer in Cross Shape and T-Shape Micromixers
Authors: Ait Mouheb Nassim , Montillet Agnes , Solliec Camille , Comiti Jacques , Legentilhomme Patrick , Snita Dalimil
Published: September 2010

The understanding of physical phenomena such as flow behaviour and mass transfer performance is needed in order to develop appropriate micromixers for industrial or biomedical applications. In this work, CFD is used to characterize the flow and the liquid mixing quality in a micromixer as a function of the Reynolds number. Two micromixers are studied in steady flow conditions; they are based on two geometries, respectively T-shaped (⊤) and cross-type (+). Simulations allow, in the case of ⊤ micromixers, to chart the topology of the flow and to describe the evolution of species concentration downstream the crossing. The streamlines layout and the mixing quality curves reveal three characteristic types of flow previously reported in the literature, depending on Reynolds number: stratified, vortex and engulfment flows. In the case of cross-type micromixers, the structure of the flow is strongly three-dimensional and is characterized by symmetrical vortices in both output channels. The results show that the + shaped system can improve the mixing process in comparison with the micromixers having ⊤ geometry. The second part of the study is experimental. Two cells are constructed, for both geometries (T-shaped and cross) using square channels with 400 μm hydraulic diameter. In order to use particle image velocimetry (PIV), a system has been adapted to measure velocity fields for various channel plans at different channel depths. This allows observing the evolution of the flow and the vortices development along the microchannels. A second experimental technique, the electrochemical one involving microelectrodes implemented at several positions on the channel wall located near the crossing, has been used. The electrochemical method can locally characterize the formation of swirling flows. These two complementary experimental results will be analysed and a comparison with the CFD results will be performed.
Flow and Heat Transfer Characteristics of Supercritical Nitrogen in Mini-Tube
Authors: Zhang P. , Huang Y. , Wang R. Z.
Published: September 2010

Fluids with the state near the critical points-supercritical fluids show many tremendously strange characteristics, such as singularities in compressibility and viscosity, diminishing difference in vapor and liquid phases and so on which have attracted a lot of investigations both experimentally and theoretically for fundamental research. In the present study, experimental investigations of the heat transfer and fluid flow characteristics of supercritical nitrogen in a mini-tube of 2 mm in diameter and 250 mm in length are carried out, the effects of many influential factors on fluid flow and heat transfer are studied. Meanwhile, the numerical analysis of the fluid flow and heat transfer characteristics of supercritical nitrogen is also conducted by using FLUENT. It is found that the agreement between the experimental and numerical results is quite good and the further interpretation and discussion are carried out.
Electrode Arrangement Effect on Natural Convection in a Horizontal Enclosure
Authors: Ghazi R. , Saidi M. S. , Saidi M. H.
Published: September 2010

The heat transfer enhancement of natural convection, using electrohydrodynamic technique inside a horizontal enclosure heated from below, is studied numerically. The interactions between electric field, flow field, and temperature field are investigated by CFD methods. The flow and temperature fields are affected by voltage applied at the wire electrodes. For different voltages and number of electrodes, it is noticed that the Nusselt number increased in all cases and the best enhancement is obtained at lower Rayleigh numbers. It is also shown that increasing the number of electrodes doesn’t always cause an increase in the heat transfer enhancement. Actually, when the number of electrodes is equal to the number of Be´nard cells, the best heat transfer enhancement is obtained.
Numerical Study of Gaseous Microchannel Flows on the Dimensional and Physical Effects
Authors: Lin Kuan-Hung , Wu Jiunn-Chi
Published: September 2010

In this article, we perform a series of simulations to analyze the gaseous flow in two-dimensional (2D) and three-dimensional (3D) microchannels. The geometry effects of entrance and exit, applied pressure ratios and rarefaction effects on the flow characteristics are thoroughly investigated. In addition, a modified Poiseuille number correlation for air flows is obtained. This calculation solves the compressible Navier-Stokes and energy equations under velocity slip and temperature jump conditions with varying inlet to outlet pressure ratios (from 1.76 to 20), the outlet Knudsen numbers (from 0 to 0.22) and the aspect ratios (from 0 to 0.47). The calculated mass flow rate, pressure distribution and friction factor are compared with analytic solutions and experimental data in both the slip flow and earlier transition flow regimes. In the case of higher applied pressure ratio, both experiments and numerical modeling show pressure drop at upstream and downstream. Finally, we discuss the adequacy of the friction factor correlation for the 2D flow and the 3D flow in microchannels with both inlet and outlet chambers.
Heat Transfer Characteristics and Cooling Performance of Microchannel Heat Sinks With Nanofluids
Authors: Chen Chien-Hsin , Ding Chang-Yi
Published: September 2010

This paper presents a numerical study on the heat transfer characteristics and cooling performance of a microchannel heat sink with water-γAl2O3 nanofluids having different nanoparticle volume fraction. In view of the small dimensions of the microstructures, the microchannel heat sink is modeled as a fluid-saturated porous medium in the simulation. The Forchheimer-Brinkman-extended Darcy equation is used to describe the fluid flow and the two-equation model with thermal dispersion is utilized for heat transfer. Typical results for the temperature distributions of the fin and fluid phase are presented for various values of the inertial force parameter. It is found that the fin temperature distribution is practically not sensitive to the inertial effect, while the fluid temperature distribution and the total thermal resistance change significantly due to the inertial force effect. In general, the effect of fluid inertia is to reduce the total thermal resistance and the temperature difference between the fin and the fluid phase. The total thermal resistances obtained from the present model with inertial effect match well with the available experimental results, whereas the thermal resistance is overestimated as the inertial effect is neglected.
Investigating the Effect of Solid Boundaries on the Gas Molecular Mean-Free-Path
Authors: Arlemark Erik J. , Reese Jason M.
Published: September 2010

A key parameter for micro-gas-flows, the mean free path, is investigated in this paper. The mean free path is used in various models for predicting micro gas flows, both in the governing equations and their boundary conditions. The conventional definition of the mean free path is based on the assumption that only binary collisions occur and is commonly described using the macroscopic quantities density, viscosity and temperature. In this paper we compare the prediction by this definition of the mean free paths for helium, neon and argon gases under standard temperature and pressure conditions, with the mean free paths achieved by measurements of individual molecules using the numerical simulation technique of molecular dynamics. Our simulation using molecular dynamics consists of a cube with six periodic boundary conditions, allowing us to simulate an unconfined gas “package”. Although, the size of this package is important, since its impact on computational cost is considerable, it is also important to have enough simulated molecules to average data from. We find that the molecular dynamics method using 20520 simulated molecules yields results that are within 1% accuracy from the conventional definition of the mean free paths for neon and argon and within 2.5% for helium. We can also conclude that the normal approximation of only considering binary collisions is seemingly adequate for these gases under standard temperature and pressure conditions. We introduce a single planar wall and two parallel planar walls to the simulated gas of neon and record the mean free paths at various distances to the walls. It is found that the mean free paths affected by molecular collisions with the walls corresponds well with theoretical models up to Knudsen numbers of 0.2.