bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Stress Wave Scattering at the Liquid-Particle Interface on Viscosity Calculation of Nanocolloidal Dispersions
Authors: Wang Tao , Wang Xinwei , Hong Haiping , Luo Zhongyang , Cen Kefa
Published: June 2009

In this work, extensive equilibrium molecular dynamics simulations are conducted to study the shear viscosity of nanocolloidal dispersion. Strong oscillation of the pressure tensor autocorrelation function is observed. The computational domain contains solvent of liquid argon at 143.4 K and spherical particles with volume fraction of 3%. By studying the effect of the particle size, particle density, and acoustic impedance, it is found for the first time that the stress wave scattering/reflecting at the liquid-particle interface due to acoustic mismatch plays a critical important role in the oscillation of pressure tensor autocorrelation function. The Brownian motion/vibration of solid particles is considered to have little effect on the oscillation of pressure tensor autocorrelation function curve except the frequency. And when the particle size is comparable with the wavelength of stress wave, the diffraction of stress wave happens at the interface that will also weaken the oscillation of pressure tensor autocorrelation function.
A New “Bounce-Normal” Boundary in DPD Calculations for the Reduction of Density Fluctuations
Authors: Moeendarbary Emadaldin , Lam K. Y. , Ng T. Y.
Published: June 2009

Dissipative Particle Dynamics (DPD) is a mesoscopic fluid modeling method, which facilitates the simulation of the statics and dynamics of complex fluid systems at physically interesting length and time scales. Currently, there are various applications of DPD, such as colloidal suspensions, multi-phase flow, rheology of polymer chains, DNA macromolecular suspension, etc., which employ this technique for their numerical simulation. The DPD technique is capable of modeling macroscopic properties of the bulk flow very well, but difficulties arise if the flows are confined through wall-bounded regions, or when different boundaries simultaneously exist in the simulation domain. These boundaries cause negative effects on the macroscopic temperature, density and velocity profiles, as well as the shear stress and pressure distributions. In particular, the interaction of DPD particles with solid boundaries causes large density fluctuations at the near wall regions. This density distortion leads to pronounced fluctuations in the pressure and shear stress, which are not actually present. To overcome these serious deficiencies, we introduce a new method in this work, which uses a combination of randomly distributed wall particles and a novel reflection adaptation at the wall. This new methodology is simple to implement and incurs no additional computational cost. More importantly, it does not cause any distortion in the macroscopic properties. This novel reflection adaptation is a novel version of the bounce back reflection, which we shall term the bounce-normal reflection. The most important characteristic of this method is that it reduces density fluctuations near the boundaries without affecting the velocity and temperature profiles. This new method is easily applicable to any wall-bounded problem with stationary boundaries and it has a very good consistency with macroscopic features. The eventual objective of this numerical development work is to investigate suspension flow through micro/nano channels of fluidic NEMS/MEMS devices, with applications to DNA and protein separation. These micro/nano channel devices, consisting of many entropic traps, are designed and fabricated for the separation of proteins and long DNA molecules.
Geometry Effect on the Electrokinetic Instability of the Electroosmotic Flow in Microfluidic Channels
Authors: Lam Yee Cheong , Tang Gongyue , Yan Deguang
Published: June 2009

To study the effect of geometry on electroosmotic flow in micro channels, we fabricated PDMS-glass microchannels of different designs, which have patterned channels with abrupt contraction of different sizes. Using fluorescent imaging technology, we demonstrated the effect of geometry on the instability of DC driven electroosmotic flow in microfluidic channels. For certain geometry and conductivity of the electrolyte solution (Sodium Bicarbonate), there is a threshold voltage for electroosmotic instability, exhibiting itself as “ripple”. Generally, the factors which affect the threshold voltage include channel width, channel geometry, and electrolyte conductivity. Narrower channel resulted in higher onset voltage. As conductivity of the electrolyte increases, the threshold voltage tends to increase. Early transition to unstable electroosmotic flow in microfluidic channels was observed under relatively low Re.
Numerical Study of Flow and Heat in Long Micro and Nano Channels
Authors: Darbandi Masoud , Vakilipour Shidvash
Published: June 2009

In this work, we extend a numerical tool capable of solving compressible and incompressible gas flows to study the momentum and heat transfer rates in micro/nano channels with high aspect ratio (L/H = 8000), where the compressibility effect is dominant. The constant heat flux thermal boundary condition is firstly applied at the wall. Next, the flow regime is extended to the early transition regime employing a high order slip velocity boundary condition based on the kinetic theory assumptions. The accuracy of the present results in the slip flow regimes is evaluated against other available theoretical and experimental results. The thermal and compressibility effects on the pressure and Knudsen number distribution are extensively studied along the channel at early transition regimes up to Kn = 0.5. Likely, this Knudsen is the highest one to be reached via applying the foregoing boundary conditions.
Influence of Thermal Loading on the Thermal Conductivity of Single-Wall Carbon Nanotube
Authors: Chen Bin-Hao , Jeng Ming-Shan , Tsau Fang-Hei
Published: June 2009

This investigation determines carbon nanotube thermal conductivity at heat flux ranging from 0.01 to 0.3 subject to different thermal loading of 5 ∼ 50 K/nm, using a non-equilibrium molecular dynamics simulation with true carbon potentials. The numerical model adopts Morse bending, a harmonic cosine and a torsion potential. The applied Nose´-Hoover thermostate describes atomic interactions taking place between the atoms. Hot and cold temperature reservoirs are respectively imposed on both computational domain sides to establish the temperature gradient along the carbon nanotube. Atoms at the interface exhibit transient behavior and undergo an exponential type decay with exerted temperature gradient. The thermal impact causes system fluctuation in the initial 3 ps leading to a transport region temperature as high as 600K. The thermal relaxation process reduces impact energy influence after 30 ps and leads to Maxwell’s distribution. Steady-state constant heat flux is observed after thermal equilibrium. Furthermore, the temperature curves show distinct high disturbance at initial time and linear distribution along the tube axial direction after steady-state. Results suggest that thermal conductivity value increases with increasing CNT subjected to thermal loading up to a temperature gradient of at least ∼ 41.3 K/Å representing thermal gradient convergence at heat conduction value 1258. Simulation results yield precise understanding of nano-scale transient heat transfer characteristics in a single-wall carbon nanotube.
Experimental Researches on Micro-Droplet Freezing on a Solid Surface Under Atmospheric Conditions: Part I
Authors: Liu Heyun , Ma Xiaohui
Published: June 2009

Atmospheric ice accretion on structures is a problem of fundamental importance to a number of industries. Examples of engineering problems caused by ice accretion involving aircraft, power transmission lines, telecommunication towers, electrical railway contact-wires, and other structures. Under atmospheric icing conditions two basic types of ice may form; rime or glaze. The supercooled micro-droplets in clouds are an important factor in icing. The objective of this study was to develop a new experimental method to investigate a single supercooled micro-droplet freezing process, in order to better understand the mechanism of rime or glaze ice accretion. The experimental device and principles are described in this paper. The experimental set has two small cold rooms, which is separated by a board with a central hole. A droplet with diameter of 15∼40 μm, temperature of 0∼−5°C was levitated in the cold air stream by electrostatic force. A CCD camera tracked its trace. The air temperature is from 0∼−10°C, the micro-droplet diameter is from 15∼40μm, and its temperature is from 0∼−5°C in the experimental study. This article focused on the experimental set and the experimental principles, and the next article will focus on the experimental data analysis.
The Effect of Temperature Jump on Microscale Heat Transfer: Slip Models and the Moment Methods
Authors: Myong Rho-Shin , Lee Dong-Ho , Lee Jin-Hee
Published: June 2009

The study of non-linear transport in gas flows associated with micro and nanodevices has emerged as an important topic in recent years. In the field of microscale heat transfer, convective heat transfer in slip-flow regimes in simple geometries like channels and tubes is a key problem. Constant-wall-temperature convective heat transfer in microscale tubes and channels has been studied recently using analytical solutions to an extended Graetz problem. In addition, much effort has been put into the development of computational models beyond the theory of linear constitutive relations for the analysis of microscale gas flow and heat transfer, since the Navier-Stokes-Fourier theory is not known to remain valid in the flow regimes of large Knudsen number. The objective of the present paper is to investigate microscale heat transfer where temperature jump is the dominant phenomena. The emphasis will be on the qualitative features of microscale heat transfer, for example, enhancement or reduction of heat transfer in microscale geometries. General features of computational models such as the full kinetic model and fluid dynamics model are also discussed.
Development of Miniature Loop Heat Pipes for the Thermal Control of Laptops
Authors: Mochizuki Masataka , Saito Yuji , Nguyen Thang , Nguyen Tien , Wuttijumnong Vijit , Horiuchi Yasuhiro , Tacomkang Roj , Singh Randeep , Akbarzadeh Aliakbar
Published: June 2009

Thermal management of laptops is becoming increasingly challenging task due to the high heat flux associated with the microprocessors and limited available space for the integration of the thermal control device inside the cabinet. In this paper, results from the investigation of two different designs of miniature Loop Heat Pipe (mLHP) for thermal control of compact electronic devices including notebooks have been discussed. Two prototypes of mLHP, one with a disk shaped evaporator of 30 mm in diameter and 10 mm thick, and the other with a rectangular shaped evaporator of 45×35 mm2 planar area and 5 mm thick, were designed to handle heat fluxes of up to 50 W/cm2. Total thermal resistance of these mLHPs lies in the range of 1 to 5 °C/W. In addition to this, two new designs of the mLHP pertaining to enhance the heat transfer inside the evaporation zone and to develop the loop evaporator with thickness as small as 3 mm are discussed. In conclusions, the designed mLHPs were able to satisfy the thermal and design requirements of the current laptop equipments and can be classified as potential candidates for cooling of the compact electronic devices with restricted space and high heat flux chipsets.
Experimental Study of Small Diffusion Flame Under Strong Electric Field
Authors: Yang Zeliang , Cheng Jing , Xu Tao , Gan Yunhua
Published: June 2009

The study of micro flame is very important for the design of an efficient, safe micro-burner. An experimental study of small jet diffusion flame was performed. Combustion tests under strong electric field were carried out in ceramic tube burners with the inner diameter of 1.0mm using liquid ethanol as fuel. The relationship between flame dimension and Reynolds number with the voltage was analyzed experimentally. The experiments were conducted with the voltage ranging from 0V to 10000V, and with different electrode spacing (3cm, 5cm, 7cm). The experimental results show that under electric field, the Reynolds number of ethanol vapor increases with increasing the voltage. And the Reynolds number may become instable when the electrode spacing is too large. A strong electric field can reduce soot emission from the flame.
Evaluation of Thermal Rectification at the Interface of Double-Layered Nanofilm by Molecular Dynamics Simulations
Authors: Yang Juekuan , Liu Zhenghua , Wang Yujuan , Chen Yunfei
Published: June 2009

The thermal rectification at the interface of double-layered nanofilm is investigated by molecular dynamics simulation. It is found that the interfacial thermal resistance is asymmetric, namely, it depends on the direction of heat flow across the interface. And at high temperature, the rectification of interfacial thermal resistance decreases with increasing temperature. The simulation results also demonstrated that the rectifying effects can not be interpreted only by temperature difference at interface.
Sessile Drop Evaporation on Surfaces of Various Wettability
Authors: Song Hyunsoo , Lee Yongku , Jin Songwan , Kim Ho-Young , Yoo Jung Yul
Published: June 2009

This work experimentally investigates the evaporation rates of water drops on surfaces of various wettability. By measuring the temporal evolutions of the drop radius and contact angle, we find the qualitative difference between the evaporation behavior on hydrophilic surfaces where the contact radius remains constant initially and that on the superhydrophobic surfaces where the contact angle remains constant. Also, the evaporation rate is observed to depend on the surface material although the currently available models assume that the rate is solely determined by the drop geometry. Although the theory to explain this dependence on the surface remains to be pursued by the future work, we give the empirical relations that can be used to predict the drop volume evolution for each surface.
Electrokinetic Properties and Their Effect on Thermal Conductivity of Nanofluids
Authors: Murshed S. M. Sohel , Leong Kai Choong , Yang Chun
Published: June 2009

This paper presents the effects of pH value, surfactant, and electrolyte concentration on zeta potential and effective thermal conductivity of nanofluids. The zeta potential of TiO2 (15 nm) nanoparticles in deionized water of different pH values was measured and the iso-electrical point was found to be between 4.9 and 5.2. The results show that the higher the electrolyte concentration, the smaller the particle zeta potential, which results in increased agglomeration of particles. The enhanced thermal conductivity of TiO2/deionized water-based nanofluids was found to decrease with increasing pH value or electrolyte concentration. For example, for 0.2 volume %, the thermal conductivity of this nanofluid was found to decrease from 5.5% to 2.5% when the pH value was increased from 3.4 to 9. A similar decrease (≈ 2%) in thermal conductivity was observed when NaCl concentration was increased from 0.01 mM to 10 mM. The presence of a surfactant showed considerably larger enhancement of the effective thermal conductivity compared to that of nanofluids without surfactant.
Multi-Spectral Techniques Applied for the Measurement of the Microscale Temperature Through Cooled Multiplier Tube in Photon Counting Mode
Authors: Pierre Thomas , Re´my Benjamin , Degiovanni Alain
Published: June 2009

The aim of this paper is to present a non-intrusive and optical method based on the classical thermal radiation laws for the measurement of microscale surface temperature. To overcome the diffraction limit, measurements are performed in the ultraviolet-visible range. According to the Planck’s law, emitting energy is low at these wavelengths and only a photonic flux can be measured through a cooled Photo-Multiplier Tube (PMT) and a photon-counting card. The photonic flux exhibits a random phenomenon that can be well-described through classical statistic laws such as Poisson or Normal distributions. We show in this paper that the signal we measure agrees well with these laws and that the surface temperature can be obtained either from the average or the standard-deviation of the Photonic flux. Multi-spectral techniques based on either physical and optical techniques like monochromatic filters and reflection/transmission diffraction gratings or digital techniques as a Multi-Channel Analyser (MCA) are proposed to get ride of the knowledge of the local surface emissivity. This is of a particular interest for the measurement of temperature in microscale applications. Finally, temperature measurements carried out on a specific High Temperature Blackbody developed in our laboratory are compared with those obtained through an infrared camera and allow to validate our facility and the presented measurement techniques.
Monte Carlo Simulation of Heat Pulse Propagation in Silicon Nanostructure
Authors: Lacroix David , Joulain Karl , Parent Gilles , Fumeron Sebastien
Published: June 2009

Heat transport in nanostructured material is a critical issue in the design of electronic devices. According to the size and the temperature, thermal properties of the considered medium can change. In order to appraise these modifications new simulation techniques based on phonon transport equation solution have been developed. In this field, the Monte Carlo method dedicated to phonon motion and collision modeling has prove to be efficient. In the present paper we propose a modified form of this method that takes into account short pulse heating in order to assess specific heat c and thermal conductivity k. The parameter assessment has been done using Laplace analysis of the Monte Carlo calculated temperature profiles at short and long times through asymptotic solutions fitting. These numerical tools have demonstrated to be efficient as c and k values obtained for bulk silicon at low and room temperatures confirmed it.
Molecular Dynamics Simulation of Thermal Conductivity of Single-Walled Carbon Nanotubes Filled With Water
Authors: Deng Li , Chen Yunfei , Qian Ruiming , Yang Juekuan , Bi Kedong
Published: June 2009

Thermal conductivities for single-walled carbon nanotubes (SWNTs) filled with water are calculated with non-equilibrium molecular dynamics (NEMD) simulation method. Simulation results demonstrate the thermal conduction for the tube filed with water is better than the pure nanotube at the same conditions. It is believed the translational movement of the water molecules along the tube axis helps carry energy from the hot bath to the heat sinks, which results in the increase of the thermal conductivities. In addition, with the introduction of the water molecules into the nanotube, the additional interaction between the carbon atoms and the water molecules provide extra channels for phonon transport, which further intensifies the energy transport along the nanotubes. The effects of the temperature variation and the tube length on the thermal conductivities are also analyzed in this paper.
Experimental Study on the Diffusion Flame Using Liquid Ethanol as Fuel in Mini-Scale
Authors: Yang Zeliang , Xu Tao , Gan Yunhua
Published: June 2009

At Present, the study of mini-scale motor is a very hot topic. The diffusion flame of liquid fuel in mini-scale is influenced by many factors, which should be studied further. In present experiments, three ceramic tubes were used as burners. The inner diameters of these ceramic tubes are 1.0mm, 0.6mm, and 0.4mm respectively. The combustion flame of liquid ethanol was visualized by high-accuracy camera, and the temperature field of the flame was measured by IR camera. And the effect of buoyancy on the flame was analyzed qualitatively. The measured images of the flame show that the whole flame is composed of gas area, diffusion area and reaction area. Experimental results show that there is a linear relationship between the flame height and the flow rate of liquid ethanol. The flame non-dimensional height is nearly directly proportional to Reynolds number. The critical non-dimensional flame heights are nearly the same when quench extinguishing appears for different burners with different inner diameters. Buoyancy causes flame elongated in the direction of height.
Effect of Axial Heat Conduction on Heat Transfer in Triangular Silicon Microchannels
Authors: Gan Yunhua , Yang Zeliang , Yang Cheng , Wang Shuangfeng
Published: June 2009

In order to study the fundamental physical mechanisms of fluid flow and heat transfer in microchannels, many effects should be taken into account. The effect of axial heat conduction on heat transfer in microchannels is one of the hottest topics. Experimental investigation was carried out to analyze heat transfer characteristics of methanol flowing through microchannel heat sink, which consists of 10 parallel triangular microchannels. Experimental results show that the temperature distribution on the heated wall do not change linearly along the channel, which mainly caused by the axial heat conduction in the wall. The ratio of the power due to the axial heat flux to the Joule heating in the heater, and the axial heat conduction number were used to evaluate the heat conduction effect in the wall. Many factors play important roles in the axial heat conduction, including geometric configuration, material of the wall, and Reynolds number. The experimental study reveals that the axial heat conduction effect is gradually strengthened as the Reynolds number decreases.
Near-Field Thermal Emission of Thin Films
Authors: Drevillon Je´re´mie , Ben-Abdallah Philippe
Published: June 2009

While the far-field thermal emission of thin film have been described in detail during the last five years [1–2], surprisingly, no systematic study on their near-field emission has been carried out so far. In this work we present an exact calculation of the local density of state of the non-radiative part of electromagnetic field close to the surface of thin metallic films. We describe the specific near-field properties of bounded structures. In particular it is found that this density closely depends on the thickness of films and can be significantly greater than that of near-field at the same distance from a semi-infinite medium. These results should have important consequences in the design of planar near-field thermal sources.
Experimental Study of Oscillating Flow Heat Transfer
Authors: Fan Angie , Fulmer Doreen , Hartenstine John
Published: June 2009

This paper presents an experimental study of the heat transfer characteristics of mechanically driven oscillating flows inside small diameter channels and at various frequencies and stroke lengths. Two important parameters were studied: the effective thermal conductivity between the heat source and sink, and the heat transfer coefficients in the heating and cooling regions. The test data were compared to theoretical correlations in the literature to assess their validity in the operating range of interest. Kurzweg’s correlation agreed reasonably well with the test data at low frequencies (1 Hz) and small amplitudes (7.6 cm). The highest effective thermal conductivity achieved during this study was more than 210,000 W/m-K. As a reference, pure copper and diamond materials have thermal conductivities around 400 W/m-K and 1,200 W/m-K, respectively. At low oscillating frequencies, the measured heat transfer coefficients in the heating region agreed reasonably well to Shin (1998)’s correlation. The correlation tends to under predict the heat transfer coefficient at higher frequencies. The experimental study investigated the effects of various frequencies and stroke length and demonstrated heat transfer coefficients in the heating region in excess of 34,000W/m2-K.
Shock Waves in Pulsed Laser Material Interaction: Internal Structure and Mass Penetration
Authors: Zhang Lijun , Wang Xinwei
Published: June 2009

This work pioneers the atomistic modeling of the shock wave in picosecond laser-material interaction by simulating the material that is irradiated with a picosecond laser pulse (11.3 ps FWHM) in a 0.25 MPa background gas. The dynamic structure and mutual mass penetration between the plume and background gas are investigated in detail. In the shock wave the compressed ambient gas region has a very uniform temperature distribution while the temperature decreases from the front of the plume to its end. The group velocity of atoms in the shock wave front is much smaller than the shock wave propagation speed and experiences a fast decay due to momentum exchange with the ambient gas. Strong decay of the shock wave front temperature and pressure is observed while its density features much slower attenuation. An effective mixing length is designed to quantitatively evaluate the mutual mass penetration between the plume and background gas. This effective mixing length grows at a rate of ∼ 60 m/s. This fast mixing/mass penetration is largely due to the strong relative movement between the plume and the background gas. The MD results agree well with the analytical solution in terms of relating various shock wave strengths.
Numerical Simulation of Two Phase Flow and Heat Transfer in CPL Condenser With Porous Wick
Authors: Liu ZhiChun , Liu Wei , Yang JinGuo
Published: June 2009

In the present paper, the Volume of Fluid (VOF) method is introduce to model the two phase flow and heat transfer with phase change in CPL condenser with porous element and the effect of different wall temperature and inlet vapor velocity to condensation phase change interface was studied with numerical simulation method. The results present that a fixed condensation physical interface can be formed on the face of porous wick, which can restrain, even eliminate the pressure oscillation. The reason of the effects are also analysed in this paper.
Dynamic Behavior of Nano-Particles on Bubble Interface
Authors: Zhang K. J. , Peng X. F.
Published: June 2009

The dynamic behavior of particles on a bubble interface contains axial and radial motions and is highly dependent upon the energy of this system. By introducing the concept of particle energy density (the average of the system energy based on particle number), four kinds of energy elements are estimated as critical elements, namely surface energy, line tension (an additive interaction at the contact line), electrostatic interaction and hydrophobic interaction (only for hydrophobic particle whose contact angle is greater than 64°). With consideration of different energy influences, the radial motion, vertical to bubble interface, and circumferential motion, parallel with bubble interface, are theoretically described and discussed.
Thermocapillary Actuation and Cycling of Liquid Plugs
Authors: Jiao Zhenjun , Nguyen Nam-Trung , Huang Xiaoyang
Published: June 2009

With the aim toward realizing polymerase chain reaction (PCR) of deoxyribonucleic acid (DNA) in plug-based capillary platforms, this paper reports the theoretical and experimental results of thermocapillary actuation for temperature cycling with an arbitrary ramping function. Two concepts were investigated: (a) actuation and spatial temperature cycling with three heaters and (b) actuation and temporal cycling with two heaters. The paper first describes the analytical models of both concepts. The model considers both the transient and coupling effect between heat transfer in the capillary wall and the surface tension driven movement of the plug. In the experiments, both temperature field and plug motion were measured and evaluated. The temperature field were captured by an infrared thermo tracer camera. The position of the plugs was automatically captured and evaluated with a CCD camera. Finally, analytical and experimental results are compared and discussed.
Study on Micro-Channels Design for Centrifugal Force Driven Micro-Fluid System
Authors: Chen Shia-Chung , Tseng Chia-Yen , Liaw Wei-Liang
Published: June 2009

Centrifugal force has been found to be an excellent method to control fluidic flow in biochips. Most micro-fluidics researchers already use the computer to simulate micro-fluidics flow behavior to save time and reduce mistakes. In this study, the overflow design accurately fixes the liquid volume with less than 5% error. Centrifugal force driven micro-fluidics system is designed with both simulation and experiment. The 3D simulations to utilize computational fluid dynamic software (CFD) to simulate the fluid flow and calculate burst frequency at different capillary switching and several dimension of micro-channels (300, 400, 500μm wide and each 200μm deep). For mercurochrome, the simulation results (384, 360, 348 rpm) shows burst frequency matches experimental results (468, 426, 402rpm) and accurately predicts measured trends followed the effect of channel width. This study also demonstrates the centrifugal application of an advanced computational fluidic dynamics model for the design and analysis of a centrifugal force driven micro-fluidics system.
A Pore Scale Model for the Transport Phenomena in the Catalyst Layer of a PEM Fuel Cell
Authors: Sui P. C. , Djilali N. , Wang Qianpu
Published: June 2009

In a proton exchange membrane fuel cell (PEMFC), the catalyst layer is a porous medium made of carbon-supported catalysts and solid electrolyte, and has a thickness in the order of 10 μm. Within this layer, complex transport phenomena take place: transport of charged species (H+, electrons and ionic radicals), non-charged species (gaseous H2O, O2, H2, N2 and liquid water) and heat transfer occur in their own pathways. Furthermore, phase change of water and physiochemical/electrochemical reactions also take place on phase boundaries. These transport process take place in an intertwined network of materials having characteristic length scale ranging from nano-meters to micro-meters. The objective of the present study is two-fold, i.e., to develop a rigorous theoretical framework based on which the transport in the micro-structural level can be modelled, and to construct a pore scale model that resolves the geometry of the phases (carbon, ionomer and gas pores) for which direct numerical simulation can be performed. The theoretical framework is developed by employing the volume-averaging techniques for multi-phase porous media. The complete set of the conservation equations for all species in all phases are derived and every interfacial transport is accounted. The problem of model closure on the terms in the transport equations is addressed by the pore-scale model reported in the present study. A 3-D pore-scale model is constructed by a solid model that consists of packing spherical carbon particles and simulated ionomer coating on these carbon aggregates. The index system of the pore-scale model allows easy identification of volumetric pathway, interfaces and triple phase boundaries. The transport of charged and non-charged species is simulated by solving the equations based on first principle in the entire representative element volume (REV) domain. The computational domain contains typically several million cells and a parallelized, iterative solver, GMRES, is employed to solve the coupled transport with complex geometries. Computational results based on the pore-scale model show that the effective transport properties of the species are strongly affected by the micro-structure, e.g. morphology and phase-connectivity. Further simulations and investigation on the coupling effects of the transport are underway. Combination of the proposed theoretical framework and pore-scale model will lay a foundation for the construction of multi-scale modelling of the PEMFC catalyst layer. On the one hand, the pore-scale model helps close the macroscopic volume-averaged equations in the framework. On the other hand, the pore-scale model provides a platform to include microscopic or atomistic simulations.
Experimental Study on the Thermal Physical Properties of a CNTS-Ammonia Binary Nanofluid
Authors: Ma Xuehu , Su Fengmin , Lan Zhong , Chen Jiabin
Published: June 2009

In this paper, carbon nanotubes—ammonia nanofluids (the binary nanofluids) have been prepared by two steps method. And the thermal conductivity, surface tension and kinetic viscosity of the binary nanofluid have been measured. The effects of the mass fraction of carbon nanotubes, the concentration of ammonia and temperature on the thermal physical properties of the binary nanofluid have been systematically studied. On the base, the effective thermal conductivities of the binary nanofluids have been calculated using the models in the literatures, and have been compared with the experimental values.
Alignment of MWCNTs in Polymer Composites by Dielectrophoresis
Authors: Wang Ming-Wen
Published: June 2009

Successful structural reinforcement of polymer matrices by carbon nanotube has been shown where significant improvement of mechanical properties was achieved at very low carbon nanotube loading. Due to the mechanical properties of aligned composites is better than random has been demonstrated; the conception is not easy to perform in carbon nanotube polymer composites via conventional techniques. Here, we report a novel operation to actively align and network multi-wall carbon nanotubes (MWCNTs) in a polymer matrix. In this process, MWCNTs were aligned via AC electric field induced dipolar interactions among the nanotubes in a viscous matrix followed by immobilization by curing polymerization under continued application of the anisotropic electric field. In situ SEM verified the electrostatic stabilization of the MWCNTs in the dispersion and the orientation and agglomeration caused by the dielectrophoretic force. Alignment of MWCNTs was controlled as a function of magnitude, frequency, and application time of the applied dielectrophoresis. In the present work, MWCNTs are not only aligned along the field, but also migrate laterally to form thick. The actively aligned MWCNTs amplify the flexural modulus and wear-resisting property.
Characterization of Temperature Dependence of Interfacial Tension and Viscosity of Nanofluid
Authors: Murshed S. M. Sohel , Nguyen Nam-Trung
Published: June 2009

Investigations on temperature dependence of surface tension, interfacial tension and viscosity a nanofluid are reported in this paper. Experimental results show that nanofluid having TiO2 nanoparticles (15 nm) in deionized water exhibit substantially smaller surface tension and oil-based interfacial tension than those of the base fluid (i.e. deionized water). These surface and interfacial tensions of this nanofluid were found to decrease almost linearly with increasing temperature. The Brownian motion of nanoparticles in base fluid was identified as a possible mechanism for reduced surface and interfacial tensions of nanofluid. The measured effective viscosity of nanofluid was found to be insignificantly higher than that of base fluid and it also decreases with increasing fluid temperature.
Investigation of Flow Resistance in Microchannels With a Sudden Contraction or Expansion Using DSMC
Authors: Yan Xiaohong , Wang Qiuwang , Suo Xiaona , Wu Yinin
Published: June 2009

Nitrogen gas flow in two-dimensional microchannels with a sudden contraction or expansion is simulated by direct simulation Monte Carlo (DSMC) method in both slip and transition regimes. The resistance coefficient is calculated and compared with that for conventional-scale channels. It is found that the resistance coefficient for microchannels differs from that for conventional-scale channels in two aspects. One is the resistance coefficient for microchannels is much smaller and the divergence between microchannels and conventional-scale channels increases with the increase of Knudsen number. For the cases considered in this paper, the coefficients for microchannels are about 5% to 61% of those for conventional-scale channels. On the other hand, the resistance coefficient for microchannels is more sensitive to the expansion ratio of channels, especially when the expansion ratio is less than 3.
Micro-PIV Study of Flow Inside Micro Channel With Surface Mounted Repeated Transverse Ribs
Authors: Asfer Mohammed , Panigrahi P. K.
Published: June 2009

Microchannels are an integral part of various micro devices i.e. micro chemical reactors, fuel cell and electronic packaging etc. There are two important criteria for design of microchannels. The first criterion is the minimization of pressure drop across the microchannel such that the parasitic power requirement is minimized during the operation of these devices. The second criterion is the mixing enhancement due to stronger wall-normal motion of the near wall fluid particles leading to reduction in velocity, temperature and concentration gradient at the wall. Wall surface roughness can be suitably modified for achieving the above objectives in macro-scale flows. Riblets on the channel surface are effective in reducing the frictional resistance for macro flow situations. Various ribs or turbulator geometries have proved to be effective for near wall mixing in heat exchangers and gas turbine blade cooling ducts. However, no systematic study on mixing enhancement due to flow past repeated micro ribs inside microchannel is available in literature. The present study presents the detailed flow past repeated micro-ribs inside a microchannel from micro-PIV (Particle Image Velocimetry) measurements. The microchannel made of Polydimethylsiloxane (PDMS) has been fabricated by PDMS replica moulding. The channel is 50μm × 120μm cross section. The width and height of ribs are 30μm and 10μm respectively. The pitch between the multiple ribs is equal to 100μm. The presence of flow separation at the edge of the rib with generation of vortex structure is not observed at the flow rate set in the present study contrary to the macro flow situations. However, the shear layer is deflected towards the wall with increase in wall-normal velocity indicating possible effectiveness of micro ribs for mixing enhancement in the microchannels.
Numerical Simulation and Optimization of Heat Transfer in Microchannel by Implementing Nonuniform Electrokinetic Forces
Authors: Esfahanian V. , Kowsary F. , Noroozi N. , Rezaei Barmi M.
Published: June 2009

The increasing power dissipation and decreasing dimensions of microelectronic devices have emphasized the demand for extremely efficient compact cooling technology. Microchannel heat sinks are of particular interest due to high rates of heat transfer, which have become known as one of the effective cooling technologies. In the present work, numerical simulation of incompressible flow in two dimensional microchannels by implementing nonuniform electrokinetic forces is performed using finite volume method. The velocity field and the heat transfer rate are influenced by the wall potential variations through the microchannel. Nondimensional parameters of heat transfer and fluid flows, Debay Huckel length, microchannel size and wall charge potential distribution, have major roles in this investigation. For fixed values of Reynolds number and microchannel size, the patterns of wall potentials are optimized to enhance the heat transfer rate. Velocity profiles are computed and temperature distribution and Nusselt number are obtained for uniform wall heat flux boundary condition. Average and local Nusselt numbers are illustrated for different wall potential configurations and Reynolds number. Velocity vectors and pressure drop are presented for different zeta potentials and Reynolds numbers. Finally, results of nonuniform electrical force are compared to uniform ones.
Spark Plasma Sintering of PbTe Thermoelectric Bulk Materials With Small Grains
Authors: Kuo Chia-Hung , Hwang Chii-Shyang , Ku Jie-Ren , Jeng Ming-Shan , Tsau Fang-Hei
Published: June 2009

PbTe is a conventional thermoelectric material for thermoelectric generator at intermediate temperature. Small grain size effect has been reported to improve PbTe ZT values (figure of merit). We report a combination process of attrition milling and spark plasma sintering (SPS) for preparing PbTe bulk materials with small grain sizes. The PbTe powders were milled by attrition under 600 rpm for 6–96 h and followed by SPS process under the sintering temperature of 573–773 K, the heating rate of 100 K/min, and the sintering pressure of 50 MPa. The powders and bulk materials as-prepared were then studied by X-ray diffraction patterns, scanning electron microscopy images, and transmission electron microscopy images. Transport properties of polycrystalline PbTe bulks were evaluated through temperature dependent thermal conductivity measurements.
Effects of Fluid Properties and Surface Parameters on Pool Boiling of Porous and Pin-Fin Surfaces
Authors: Chien Liang-Han , Lee Shu-Che , Wang Hon-Zen , Chen Shao-Wen
Published: June 2009

The present experimental study investigated the effect of heater size on pool boiling performance for various fluids. Water, methanol and FC-72 were tested at 50 and 70°C saturation temperature on a smooth surface, a porous surface, a pin-fin surface and a structured surface. The boiling test vessel has a 31 mm by 31 mm internal base area and 100 mm height. The sizes of the heating area are: 31×31, 12×12, 9×9, or 6×6 mm2. The test results of all the three fluids showed that boiling performance is independent on heater size for 31mm × 31mm, 12mm × 12 mm heaters, but the boiling heat transfer coefficients for the smooth surface having 6 mm × 6 mm heating area is approximately 70∼100% higher than those for the 12 mm × 12 mm heating area. The 0.2 mm thick square pin-fins, having 0.2 mm depth and 0.4 mm pitch, yields 2-to-3 folds enhancement of boiling performance in FC-72. For methanol and FC-72, the porous surface yields up to seven folds boiling enhancement as compared with the smooth surface. However, the enhancement ratio of the porous surface, having 0.15 mm average particle diameter, is only 2.3 for water. Boiling phenomena observation by a high speed video system showed that the bubble size depends on surface geometries and fluid properties.
Finite-Element Simulation on Electrothermal Effects for Immuno-Biosensors
Authors: Chang Jeng-Shian , Yang Chih-Kai , Chao Sheng D. , Wu Kuang-Chong
Published: June 2009

For a diffusion-limited protein, the diffusion boundary layer of the analyte formed on the reaction surface hinders the binding reaction from association and dissociation. With a non-uniform AC electric field, the electrothermal force generates a pair of stirring vortices to increase the transport of the analytes to the reaction surface and thus to enhance the association or dissociation of analyte-ligand complex. This work simulates a 2-dimensional full scale finite element analysis of the binding reaction kinetics of two commonly used proteins, CRP and IgG, by applying a non-uniform AC electric field. In addition to the electrothermal stirring effect, the blocking effect of the flow field due to the existence of the reaction surface at different positions of the micro-channel could cause different degrees of enhancement to the association and the dissociation. The largest enhancement is found at the position near the negative electrode. The initial slope of the curve of the analyte-ligand complex versus time can be raised up to 5.166 times for CRP and 1.934 times for IgG in association; and 3.744 times for CRP and 1.277 times for IgG in dissociation, respectively, with a field 15 Vrms peak-to-peak and operating frequency 100 kHz.
Semiconductor Thin Films Combined With Metallic Grating for Selective Improvement of Thermal Radiative Absorption/Emission
Authors: Fu Ceji , Tan Wenchang
Published: June 2009

We propose in this work a structure of semiconductor thin films combined with a one-dimensional metallic grating which allows for selective improvement of thermal radiative absorptivity (also emissivity) of the structure. We numerically demonstrate with a 2-D rigorous coupled-wave analysis (RCWA) algorithm that the proposed structure exhibits enhanced spectral absorptivity (for p-polarization) for photon energy slightly above the gap energy of the semiconductor (silicon in this work). The enhanced absorptivity is explained as due to excitations of surface polaritons (SPs) in the grating region, along with interactions of multiple-order diffracted waves in the semiconductor layer. Furthermore, the enhanced absorptivity of the structure can be achieved for a wide range of incidence angles so that it may have potential applications in energy conversion purposes.
Simultaneous Measurement of the Effective Thermal Conductivity and Effective Thermal Diffusivity of Nanofluids
Authors: Murshed S. M. Sohel , Leong Kai Choong , Yang Chun
Published: June 2009

A transient double hot-wire technique was developed for precise and simultaneous measurement of the effective thermal conductivity and effective thermal diffusivity of nanofluids. The measured effective thermal conductivities and effective thermal diffusivities of nanofluids were found to be higher than those of base fluids and they increase significantly with increasing volume fraction of nanoparticles. The increments of the thermal diffusivities were found to be slightly larger compared to the thermal conductivity values. For example, at 5% volumetric loading of TiO2 nanoparticles of 15 nm and 10 × 40 nm in ethylene glycol, the maximum increase in effective thermal conductivity was found to be 17% and 20%, whereas the maximum increase in effective thermal diffusivity was 25% and 29%, respectively. Besides particle volume fraction, particle material, particle size and the nature of the base fluid were found to have influence on the effective thermal conductivity and diffusivity of nanofluids. Based on the calibration results obtained for the base fluids the measurement error was estimated to be within 1.2 to 2%.
Use of Immersed Boundary Method for the Analysis of Electro-Osmotic Flow Generated by a Pair of Cylindrical Electrodes in a Micro-Cavity
Authors: Fernandes Dolfred V. , Kang Sangmo , Suh Yong K.
Published: June 2009

The bulk motion of an aqueous solution induced by the application of DC electric field is studied numerically. The physical model consists of a micro-cavity with two completely polarizable cylindrical electrodes. The electric double layer (EDL) model coupled with Navier-Stokes equations governing the electroosmotic flow has been described. The Nernst-Planck model uses two extra equations for the prediction of ion concentration. We employed IB (immersed boundary) technique for the implementation of boundary conditions and semi-implicit fractional-step method for solving the governing equations. A new method is described for implementing concentration boundary conditions on the electrodes. The bench mark problems, driven cavity flow and flow over a cylnder were used for the validation of our present code. The numerical results are compared with the analytical results obtained using Gouy-Chapman-Stern model for the one dimensional case. For the two dimensional case the flow field and the ionic concentration distributions obtained shows that the electoosmotic effect is predominant in the thin region around the electrode.