bluuebunny/crossref_metadata_2025 huggingface dataset:
Joule Heating Induced Temperature Gradient Focusing in a Microfluidic Channel With a Sudden Change in Cross Section
Authors: Tang Gong Yue , Yang Chun
Published: June 2009

Temperature gradient focusing is a recently developed technique for spatially focusing and separating ionic analytes in microchannels. The temperature gradient required for temperature gradient focusing can be generated either by an imposed temperature gradient or by Joule heating resulted from an applied electric field that also drives buffer flow. In this study, a numerical model describing the Joule heating induced temperature development and temperature gradient focusing is developed. The model consists of a set of governing equations including the Poisson-Boltzmann equation, the Laplace equation, the Navier-Stokes equations, the energy equations and the mass transport equation. As the thermophysical and electrical properties including the liquid dielectric constant, viscosity and electric conductivity are temperature-dependent, these governing equations are coupled, and therefore the coupled governing equations are solved numerically by using a CFD based numerical method. The numerical simulations agree well with the experimental results, suggesting that the valid mathematical model presented in this study.
The Development of the Micro-Generator on the Substrate Based Thin Film
Authors: Kurosaki Jun-Ichiro , Tanaka Saburo , Miyazaki Koji , Tsukamoto Hiroshi
Published: June 2009

We fabricated bismuth-telluride based thin films and their in-plane thermoelectric micro-generators (4mm×4mm) on a glass substrate by using the flash evaporation method through shadow masks. We prepared fine powders of Bi2.0Te2.7Se0.3 (n-type) and Bi0.4Te3.0Sb1.6 (p-type). The shadow masks are fabricated by standard micro-fabrication processes such as nitridation of silicon, dry etching and wet etching. The output voltages of micro-generators are lower than that of a thermoelectric generator based on bulk materials. The main reason is because the temperature difference between cool and hot junctions of the micro-generator is small compared to a thermoelectric generator based on bulk materials. In this study, the micro-generators were fabricated on a silicon nitride substrate based thin film. By fabricating the micro-generator on the thin film substrate, a large temperature difference between cool and hot junctions is obtained due to the thin film effect and the heat radiation to air of the thin film substrate. At the silicon nitride substrate based thin films, the thermal conductivity is significantly reduced by 1.2 W/ (m K). The thin film substrate is prepared by applying the fabrication processes used for shadow masks. The silicon nitride substrate based thin film is fabricated by nitridation of silicon and then back etching the silicon wafer. The fabricated substrate thickness is 2.5 μm and 4.5 μm (4 mmx4 mm). The temperature between cool and hot junctions is measured by using the noncontact thermometer which senses the far-infrared radiation. The output voltage of the micro-generator based thin film is measured by giving a temperature difference by heating the bottom of the silicon nitride substrate based thin film.
Molecular Dynamics Based Analysis of Nucleation and Surface Energy of Droplets in Supersaturated Vapors of Methane and Ethane
Authors: Vrabec Jadran , Horsch Martin , Hasse Hans
Published: June 2009

Molecular dynamics (MD) simulations are applied for studying homogeneous nucleation during condensation of supersaturated vapors of methane and ethane. Nucleation processes are characterized by the nucleation rate, i.e. the number of stable droplets produced per volume and time. Nucleation rates from simulations are compared to the classical nucleation theory (CNT) and a model that introduces a size dependence of the specific surface energy. CNT is found to agree well with the simulation results, deviations are throughout lower than three orders of magnitude.
Study on Condensation Behavior in Two-Phase Flow Through a Microchannel
Authors: Takeuchi Genki , Fujiwara Akiko , Abe Yutaka , Suzuki Yutaka
Published: June 2009

It is requested to develop a small and high performance heat exchanger for small size energy equipments such as fuel cells and CO2 heat pumps, et.al... In author’s previous studies, a high pressure resistant microchannel layers stacked heat exchanger has been developed. The heat exchanger is manufactured by diffusion bond technique. It can be used under high pressure condition larger than 15 MPa. Due to the high pressure resistance, the device can be applied for high flow rate condition with boiling and condensation. The objectives of the present study are to estimate the heat transfer performance of the heat exchanger and to investigate the thermal hydraulic behavior in the microchannel. The flow pattern in a glass capillary tube is observed by fabricating visualization system. As the results, it is measured that the present device attained high heat transfer quantity of approximately 7000 W on steam condensation despite the weight is only 230 g. The measurement results clarified that the device achieves very high heat transfer rate of hundreds times larger than that of the existing heat exchanger. Furthermore, visualization experiment with single glass pipe is conducted to clarify the flow condensation behavior in the microchannel. In the experiment, the microchannel of Pyrex glass is surrounded by the subcooling water. The flow patterns can visualized from the side of the microchannel. Flow patterns observations are conducted for various inlet pressure and temperatures of the subcooling water. It is observed that the continuous flow transition from annular and injection flow to slug-bubble flow in the microchannel. The reason of large heat transfer rate per unit volume is discussed as relating to narrow interval of each microchannels and small thermal resistance.
Application of a High-Order Macroscopic Approach to Force-Driven Poiseuille Flow in the Slip and Transition Regimes
Authors: Mizzi Simon , Gu Xiao-Jun , Emerson David R. , Barber Robert W. , Reese Jason M.
Published: June 2009

In this paper various extended macroscopic models are described and applied to force-driven Poiseuille flow. In particular, details are given for the regularized Grad 13- and 20-moment equations. Extended macroscopic models have, until recently, been limited by the uncertainity surrounding the prescription of boundary conditions on solid-walls. The gas-solid wall interaction plays an important role in describing the dynamics of confined gaseous flows. This problem is tackled in the context of the moment equations whereby the simplified Maxwell microscopic formalism is used to derive boundary conditions for a given moment equation set. The proposed governing equations and boundary conditions are applied to force-driven Poiseuille flow where anomalous thermal behavior is observed as the Knudsen number increases. Results are compared to DSMC data and it is established that the proposed extended macroscopic models can capture this non-intuitive behavior. However, the models show some quantitative disparity in representing this behavior. It is proposed that this is addressed by development of a consistent theory of molecular collision geometries in the extended hydro-dynamic model or by the utilization of more extended moment sets.
Molecular Dynamics Modeling of the Effect of Thermal Interface Material on Thermal Contact Conductance
Authors: Jayadeep U. B. , Krishna Sabareesh R. , Nirmal R. , Rijin K. V. , Sobhan C. B.
Published: June 2009

Thermal contact conductance is used to indicate the resistance offered by a contact interface to the flow of heat. When an interface material is applied as nano-layered coatings on super-finished contacting surfaces, the possibility of size effects necessitates the use of a discrete computation method for its analysis. Hence, a methodology is proposed which utilizes Molecular Dynamics (MD) simulations to obtain the size affected thermal conductivity of the interfacial layer, which in turn characterizes the thermal contact conductance behavior. Molecular Dynamics codes have been developed, making use of Sutton-Chen many-body potential, suitable for metallic materials. The model includes the asperities at the contact interface, assuming the asperities to be of a simplified geometry. The paper also presents the validation of the codes developed, and parametric studies on the effect of temperature, number of asperities and the material used for thermal interface coating on the size-affected interfacial conductivity.
Analytical and Experimental Studies of Diffuser/Nozzle Valve-Less Micro-Pump
Authors: Tanaka Seiichi , Moriyama Shun , Tsukamoto Hiroshi , Miyazaki Koji
Published: June 2009

A valve-less micro-pump was realized with just one diffuser/nozzle element. The pressure-loss in a nozzle is lower than that in a diffuser, and therefore one-way flow may be realized in the nozzle direction. The frequency characteristics and the pump characteristics are measured. Dimensionless numbers are introduced to rearrange the measured data and to understand the physical mechanisms of the micro-pump. Simplified analysis was done for unsteady operation of the pump by considering the channel geometries and pressure-loss coefficients based on Bernoulli’s theorem. The calculated pump characteristics agreed with the measured ones. Numerical calculations were made using the commercial CFD (computational fluid dynamics) code CFX. The calculated flow patterns showed differences between the diffuser and nozzle directions.
An Experimental Investigation of the Boiling Performance of Water-Based Nanofluids
Authors: Sajith V. , Madhusoodanan M. R. , Sobhan C. B.
Published: June 2009

The addition of nanoparticles modifies various thermophysical properties of water such as the thermal conductivity, viscosity and the surface tension. The present work experimentally investigates the influence of the addition of nanoparticles on the boiling performance of water. The effects of the volume fraction of the nanoparticles, the sonication time used in nanofluid preparation by ultrasonic shaking, and the time allowed for sedimentation after preparation, on the boiling performance, have been investigated. The nanofluids used in the present study are stabilized suspensions of aluminum oxide and copper oxide of size ranging from 30 to 40 nm in water. The boiling curve was plotted for the base fluid and the nanofluids using a suitable experimental set up by means of a power-controlled experimentation. Comparison of experimental results with pure water indicated a considerable increase in the burn out heat flux in the nanofluids, and also gave the variation of the burn out heat flux with respect to the volume fraction of nanoparticles in the suspension. For all volume fractions of nanoparticles, the heat transfer coefficients in the natural convection and the nucleate boiling regimes were observed to be lower than in pure water, which implies a reduction in the boiling performance. A slight increase in the boiling point of water was observed with the addition of nanoparticles. The burn out heat flux was found to increase with the agitation time in preparing the nanofluids, but this enhancement decreased on sedimentation of nanoparticles. Experiments were conducted using heating elements of different thicknesses, and the effect of the heater size on the burn out heat flux was found to be diminishing in the nanofluids, with an increasing particle concentration.
Multicomponent Energy Conserving Dissipative Particle Dynamics: A General Framework for Mesoscopic Heat Transfer Applications
Authors: Chaudhri Anuj , Lukes Jennifer R.
Published: June 2009

The energy conserving formulation of the Dissipative Particle Dynamics (DPD) mesoscale method for multicomponent systems is analyzed thoroughly. A new framework is established by identifying the dimensionless groups using general scaling factors. When the scaling factors are chosen based on the solvent in a multicomponent system, the reduced system of equations can easily be solved computationally. Simulation results are presented for one dimensional transient and steady-state heat conduction in a random DPD solid, which compare well with existing published and analytical solutions. This model is extended to two dimensions and shows excellent agreement with the analytical solution.
Heat Pulse Propagation in Silicon Nanostructures by Solving Phonon Transport Equation
Authors: Terris Damian , Joulain Karl , Lemonnier Denis
Published: June 2009

The temperature evolution prediction of silicon nanofilms and nanowires can be useful to safeguard high technology systems of its deterioration. The simulation of a level and a pulse in these nanostructures is then made with Boltzmann Transport Equation (BTE) resolution using the single time approximation. The Discrete Ordinate (DO) method helps to numerate the angle space. BTE is written in cylindrical coordinates which corresponds to wires. Therefore, the cylindrical plane is considered as an isotropic scattering to mimic a nanowire and then, as a specular reflexion (which conserve z momentum) to simulate a nanofilm. Using the axisymmetry done with a specular reflexion, the cylinder is two dimensionally discretized with a regular rectangular mesh.
Modeling of Heat Transfer in Nanoscale Multilayer Solid-State Structures
Authors: Wu Shih-Kuo , Chou Ya-Wen
Published: June 2009

Modeling of heat transfer in nanoscale multilayer solid-state structures is presented in this article to seek a potential design of thermoelectric materials. The phonon radiative heat conduction equation is used to describe the heat transport behavior in nanoscale multilayer solid-state structures and the diffuse mismatch model is utilized to simulate the interface condition between two dissimilar materials. In this paper, the thermal conductivity of thin film superlattices, nano wire superlattices and nano tube superlattices were calculated. Then, size effects on the performance of thermoelectric micro coolers were examined in detail. The results show that the effective thermal conductivity of thermoelectric materials in superlattice structures decreases as the layer thickness decreases. In addition, the thermal conductivities of nano wire and nano tube superlattices are less than that of thin film superlattices when they have the same layer thickness. It is noted that the restriction on the radial direction not only decreases the thermal conductivity in radial direction but also in axial direction. Thus, nano wire and nano tube superlattices are potential materials for high performance thermoelectric devices.
Convective Heat Transfer Performance of Water and FC-72 in an One Side Heated Rectangular Channel Having Pin-Fins
Authors: Chien Liang-Han , Pei S.-Y. , Wu T.-Y.
Published: June 2009

This study investigates the convective heat transfer performance of two fluids (water and FC-72) in a one side heated rectangular channel of 20mm in width and 2mm in height. The heated side has either a smooth surface or a pin-finned surface. The inlet fluid temperature was maintained at 30°C. The total length of the test channel was 113 mm, with a heated length of 25mm. The flow rate varied between 80 and 960 ml/min, and the heat flux was between 18 and 98 W/cm2. Single phase convection was the dominant heat transfer mechanism in the present water tests, and the performance was mainly controlled by flow rate. Contrarily, the heat flux was the major factor for the heat transfer performance in FC-72 as a result of the dominant boiling effect. At a fixed flow rate, the pin-finned surface yielded up to 30% higher heat transfer coefficient and greater critical heat flux than those of a smooth surface. The convective heat transfer coefficient of FC-72 was greater than water at low flow rates (80∼160 ml/min) and heat fluxes between 18 and 35 W/cm2. However, the heat transfer performance of water was superior to FC-72 at high flow rates.
Dynamical Behavior of Water Inside a Capped Single-Walled Carbon Nanotube
Authors: Lu Jian-Ming , Wu Chun-Yi , Hung Cheng-Shiu , Chien Wen-Tung , Li Wang-Long , Hwang Chi-Chuan , Wang Yun-Che
Published: June 2009

Dynamical behavior of water confined inside a capped single-walled carbon nanotube (SWCNT) is investigated at different temperatures via the Molecular Dynamics (MD) Simulation method. Water in a SWCNT behaves in the fashion of random walk and increases amplitudes with temperature. Moreover, the SWCNT’s tip vibrates more significantly as temperature increases. The water molecules embedded nanotubes exhibit less thermal noise amplitude, indicating increases in effective stiffness in the water-nanotube composite. Further, the vibrational amplitude of a water-embedded SWCNT’s tip is more noticeable during the initial transient state at the beginning of our MD simulations, and gradually decays, and reaches a steady state, as MD simulation time increases. The variation of vibrating amplitude of the SWCNT’s tip increases linearly as temperature increases when no water is present inside. The tip vibration exhibits the largest amplitude when temperature is at the boiling point of water. Moreover, the tip vibration increases monotonically as temperature increases, providing information to estimate the effective Young’s modulus of the water-nanotube composite. The diffusion pathways of water inside a SWCNT are also studied in terms of temperature changes.
Cooling Dynamics of a Gold Nanoparticle in a Host Medium Under Ultrafast Laser Pulse Excitation: A Ballistic-Diffusive Approach
Authors: Rashidi-Huyeh Majid , Volz Sebastian , Palpant Bruno
Published: June 2009

We present a numerical model allowing to determine the electron and lattice temperature dynamics in a gold nanoparticle under subpicosecond pulsed excitation, as well as that of the surrounding medium. For this, we have used the electron-phonon coupling equation in the particle with a source term linked with the laser pulse, and the ballistic-diffusive equations for heat conduction in the host medium. Our results show that the heat transfer rate from the particle to the matrix is significantly smaller than the prediction of Fourier’s law. Consequently, the particle temperature rise is much larger and its cooling dynamics is much slower than that obtained using Fourier’s law, which is attributed to the nonlocal and nonequilibrium heat conduction in the vicinity of the nanoparticle. These results are expected to be of great importance for interpreting pump-probe experiments performed on single nanoparticles or nanocomposite media.
The Effect of Porous Material on the Improvement of the Micro-Chip Heat Dissipation
Authors: Huang Chyouhwu B. , Chen Hung-Shyong , Wu Szu-Ming
Published: June 2009

Heat dissipation is a very important subject when dealing with industrial application especially in modern semiconductor related applications. Several techniques have been developed to solve the heat generated problem, such as heat dissipation device in IC packaging, high heat conductivity materials, heat tube, force convection, etc. Porous material is used in this study. Porous material is known to have large interior surface, therefore, with proper force convection; it can easily carry heat away. Micro porous ceramic (porous size: 490 μm) is attached to uninterruptible power supply (UPS) power chips. The increase of the heat dissipation rate improves UPS performance. Heat transfer properties comparisons for power chip with and without micro porous materials attached are studies. Also, heat transfer rate under different fan speeds (force convection) is studied. The results show that, heat transfer increases with the use of micro porous materials, the effectiveness ranges between 2–22%. Also, the heat transfer rate varies with air flow rate, the increase of heat transfer is about 4–6%. The dust effect was also performed; experimental results show that heat transfer rate will not be affected by the accumulated dust if a micro porous material is applied.
Microfluidic Cell Heating Characterized by 3-ω Measurements
Authors: Barilero Thomas , Le Saux Thomas , Julien Ludovic , Croquette Vincent , Chapuis Pierre-Olivier , Volz Se´bastian , Philippe Gwendal , Guilet Ste´phane , Gosse Charlie
Published: June 2009

Ion beam etching (IBE) was used to microfabricate resistive heaters in indium-tin-oxide (ITO). The device was then closed with a microfluidic chamber and its thermal behavior was investigated using the 3ω method. Experiments and finite element model (FEM) simulations both satisfactorily agreed with a simple one-dimensional model for heat diffusion.
FDTD Study of the Surface Waves Detection in Apertureless Scanning Near-Field Microscopy
Authors: Parent G. , Fumeron S. , Lacroix D.
Published: June 2009

Recent studies have shown the importance of surface waves in heat transfer near interfaces. The scanning near field optical microscopy (SNOM) provides an experimental tool to investigate the thermal electromagnetic field near surfaces. In this work, we present a three dimensional model of SNOM devices. This model is based on the finite-difference-time domain (FDTD) method associated to a near to far field transformation. Near field and far field scattered by a silicon tetrahedral tip and by a pecfectly conducting one are presented and discussed.
Molecular Dynamics Simulation of Water and Ion Profiles Near Charged (100) and (111) Silicon Surfaces
Authors: Xu Dongyan , Li Deyu , Leng Yongsheng , Chen Yunfei
Published: June 2009

Fundamental studies in nanofluidics have attracted significant attention in the past decade since the success of nanofluidic devices depends on a thorough understanding of the fluidic, ionic, and molecular behaviors in highly confined nano-environments. In this work, molecular dynamics simulations of the effect of surface charge densities on the ion and water distribution in the near wall region has been performed for both (100) and (111) silicon surfaces. We demonstrate that surface charges not only interact with mobile ions in the electrolyte, but also interact with water molecules due to their polarizability and hence influence the orientation of water molecules close to the charged surface. It is shown that as the surface charge density increases, water molecules within ∼ 5 Å from the (100) silicon surface can evolve from one layer into two layers and meanwhile, the orientation of water molecules is more aligned instead of randomly distributed. However, no extra water layer is observed near a (111) silicon surface even under a surface charge density of as high as −0.2034 C/m2. The above phenomenon may be related to the different surface atom densities of (100) and (111) silicon surfaces.
Ultrasensitive Micro/Nanoscale Resistive Pulse Sensors by Integrating Fluidic Devices With MOSFETs
Authors: Xu Dongyan , Kang Yuejun , Li Dongqing , Li Deyu , Sridhar Manoj , Hmelo Anthony B. , Feldman Leonard C.
Published: June 2009

Nanofluidic sensors have been developed over the past decade and demonstrated the capability of sensing single DNA molecules. One class of nanofluidic devices is based on the resistive pulse sensing technique and a modulation of the baseline ionic current can be observed when molecules are translocated through the sensing nanopore or nanochannel. In this scheme, the ionic current modulation is approximately the same as the channel resistance modulation, requiring the channel size be comparable to the molecules to be detected. In this paper, we present a new sensing scheme to detect the translocation of particles through a fluidic channel, which amplifies the resistance modulation by 40–80 times. The device connects the gate of a MOSFET with a fluidic circuit and monitors the drain current modulation of the MOSFET to detect particles, instead of directly monitoring the ionic current through the fluidic channel. The minimum volume ratio detected is 0.006%, about ten times smaller than the lowest detectable volume ratio reported in the literature by using the resistive pulse sensing technique. Although at current stage the device is only fabricated at microscale level, we envision that the same scheme can be applied in nanofluidic devices for single molecule detection.
Velocity Measurement of In Vitro Blood Flow in Microchip Cultured Endothelial Cells
Authors: Segawa Naoki , Sugii Yasuhiko
Published: June 2009

In order to investigate vascular diseases such as cause of atherosclerosis and myocardial infarction, relationships of endothelial cells (ECs) covered with surface blood vessels and blood flow stimulation have been experimentally studied. In the study, a blood vessel model for in vitro experiment made from polydimethylsiloxane (PDMS) microchip with a straight microchannel with 400 μm width and 100 μm depth was developed. By optimizing cells cultured condition such as the liquid introduction method and the surface coating for enhancement of cell attachment on the microchannel wall, cell culture method in the microchip were developed. Velocity distributions on the ECs surface in the blood vessel model were measured using micro PIV technique. Measured velocity vectors on the ECs surface were fluctuated caused by the three dimensional effect of the cell shape.
Experimental Study of Laminar Convective Heat Transfer and Viscous Pressure Loss of Alumina-Water Nanofluid
Authors: Rea Ulzie , McKrell Tom , Hu Lin-Wen , Buongiorno Jacopo
Published: June 2009

Laminar convective heat transfer and viscous pressure loss were investigated for alumina-water nanofluid in a flow loop with a vertical heated tube. The experimental results are in good agreement with traditional model predictions for laminar flow, if the loading- and temperature-dependent thermophysical properties are utilized. No abnormal heat transfer enhancement was observed. The heat transfer coefficients in the entrance region and in the fully-developed region are estimated to increase by 17% and 27%, respectively, for alumina nanofluid at 6 vol%. Measured pressure loss of the nanofluid is within 20% of theory. It is concluded that the nanofluid laminar convective heat transfer and viscous pressure loss behavior can be predicted by existing models as long as the correct mixture properties are used. This finding is consistent with our previous observation for alumina nanofluid tested in the fully-developed turbulent flow regime.
Quantitative Scanning Thermal Microscopy With Double Scan Technique
Authors: Kim Kyeongtae , Jaung Seungpil , Chung Jaehoon , Won Jongbo , Kwon Ohmyoung , Lee Joon Sik , Park Seungho , Choi Young Ki
Published: June 2009

Scanning Thermal Microscope (SThM) is known as a tool of the highest spatial resolution in measuring local temperature and thermophysical properties. However, despite the highest spatial resolution of SThM, its usefulness has been limited because of the difficulties related to the quantitative interpretation of the measured data. We suggest a double scan technique that can make an advantage of heat transfer through the tip-sample contact by subtraction of air conduction signal obtained from ‘lift mode’ scan. The thermal signal obtained by the new method is free from the influence of air conduction. This, in turn, allowed the quantitative profiling of the sample temperature.
Bead-Based Miniature Microfluidic Systems for Rapid RNA Extraction and Reverse Transcription
Authors: Lien Kang-Yi , Liu Chien-Ju , Shin Jyh-Wei , Chang Tsuey-Yu , Lee Gwo-Bin
Published: June 2009

The current study presents a new integrated microfluidic chip for rapid ribonucleic acid (RNA) purification, extraction and reverse transcription (RT) in an automatic fashion. The miniature system consists of two individual functional devices including a two-way microfluidic control module and a magnetic field/temperature control module. The functional microfluidic control module can perform pumping, mixing, purification and concentration of the RNA samples by incorporating with the magnetic bio-separator consisting of 2-dimension twisted microcoils. Notably, the magnetic bio-separators are developed either to perform the separation of magnetic beads or to control the temperature field for the subsequent RT process. Experimental results showed that the total RNA was successfully purified and extracted by the magnetic beads, and the subsequent RT process of RNA was completed automatically. As a whole, the developed system may provide a powerful platform for biomedical application and biological analysis.
A Broad Range of Phonon Mean Free Paths Is Important for Heat Conduction
Authors: Dames C.
Published: June 2009

The thermal conductivity is modeled with a spectral form of kinetic theory k=13∫CωνLdω(1) where ω is the angular frequency, Cω is the specific heat per unit frequency, ν = ∂ω/∂q is the group velocity, and L is the effective mean free path (MFP) which combines bulk and boundary scattering using Matthiessen’s rule: Cω=ħωDOS∂f/∂T(2)L−1=Lbulk−1+Lboundary−1.(3) Here q is the wavevector, DOS is the density of states (acoustic modes only), T is the temperature, and f is the Bose-Einstein distribution function.
Effect of pH and Chemical Surfactant on Thermal Conductivity Enhancement of Cu-H2O Nanofluids
Authors: Li Xinfang , Zhu Dongsheng , Wang Xianju , Wang Nan , Wang Zhengdong , Tu Shandong
Published: June 2009

Nanofluids have been attractive for the last few years with the enormous potential to improve the efficiency of heat transfer fluids. This work focuses on the effect of pH and sodium dodecylbenzenesulfonate (SDBS) surfactant on the thermal conductivity of nanofluids.The thermal conductivity was measured by a Hot Disk Thermal Constants Anlyser. The results showed that the thermal conductivity enhancements of Cu–H2O nanofluids are highly dependent on the weight fraction of nanoparticle, pH values and SDBS surfactant concentration of nano-suspensions. The Cu–H2O nanofluids with an ounce of Cu have noticeably higher thermal conductivity than the base fluid without nanoparticles, For Cu nanoparticles at a weight fraction of 0.001 (0.1 wt %), thermal conductivity was enhanced by up to 10.7%, with an optimal pH value and SDBS concentration for the highest thermal conductivity. Therefore, the combined treatment with both the pH and chemical surfactant is recommended to improve the thermal conductivity for practical applications of nanofluid.
Natural Convection With Porous Medium in a Narrow Horizontal Cylindrical Annulus With Constant Volumetric Heat Flux
Authors: Mansour Kamyar
Published: June 2009

We consider the two-dimensional problem of steady natural convection in a narrow Horizontal Cylindrical annulus filled with porous medium due to a constant temperature variation on the outer and adiabatic conditions at the inner boundaries with constant volumetric heat flux The solution is expanded in powers of a single combined similarity parameter, which is the product of the Gap ratio to the power of two, and Rayleigh number. The series is extended by means of symbolic calculation up to 28 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is small, but Pade approximation leads our results to be good even for much higher value of the similarity parameter.
Numerical Investigation of Controlling Mixing in Coaxial Jets Using Synthetic Jet Actuator Arrays
Authors: Liu Yanming , Wang Baoguo , Liu Shuyan
Published: June 2009

A Numerical Simulation has been conducted to investigate the mixing of coaxial jets with zero-mass-flux synthetic jet actuators, two arrays of which are placed side by side around coaxial jets and in the exit plane of the outer primary jet. Both the temperature field of the primary jet and mixing processes are studied at given temperature ratio. High-frequency forcing leads to increases in the radial extent of the jet and turbulent intensity in the near-field and increased turbulent dissipation. During the analysis, the influence of adjacent synthetic jets is also considered. The comparison is made between the control effectiveness of two arrays of actuators and that of an array of actuators. The results show both of the actuator’s configurations can result in the significant mixing enhancement, but the former exhibits more desirable.
A Concept of Pumpless Convective Micro/Micro Channel Cooling Technology
Authors: Zhang Zongqin , Sun Hongwei , Fadl Ahmed , Faghri Mohammed
Published: June 2009

The sustained drive for faster and smaller micro electronic devices has led to a considerable increase in power density. The ability to effectively pump and enhance heat transfer in micro/mini channels is of immense technological importance. The micro channel heat exchanger has great advantages for high heat flux applications due to their high surface-to-volume ratio. Unfortunately, the small dimension of the micro channel leads to a large pressure drop and low Reynolds flow, which is usually associated with the low heat transfer coefficient. Therefore, forced convection micro heat exchangers require advanced micro pumping and heat transfer enhancement technologies. Using oscillatory flow to enhance the convective heat transfer coefficients in micro/mini channels is one of many new concepts and methodologies that have been proposed. In this paper, we propose a novel and simple streaming-based micro/mini channel cooling technology. The phenomenon of the flow streaming are found in zero-mean velocity oscillating flows in a wide range of channel geometries. Although there is no mass flow (zero-mean velocity) passing through the channel, the discrepancy in velocity profiles between the forward flow and backward flow causes fluid particles near the walls to drift toward one end while fluid particles near the centerline drift to the other end. We hypothesize that the unique characteristics of flow streaming could be used to achieve the convective cooling. The advantages of the streaming based convective cooling technique includes enhanced heat transfer coefficient, pumpless, and cost-effective. Preliminary results of scaling analysis and computer simulations are presented to demonstrate the potential of the stream based technology for micro cooling applications.
Influence of SDBS on Dispersive Stability of Al2O3 Nano-Suspenions
Authors: Zhu Dongsheng , Wang Xianju , Li Xinfang
Published: June 2009

In this paper, the influence of sodium dodecylbenzenesulfonate (SDBS) on the dispersive behavior of Al2O3 nanoparticles in water are investigated by measuring zeta potential and hydrodynamic sizes of the nanoparticles in suspension at different pHs and different concentration of SDBS. The experimental results show that at different pHs the magnitude of zeta potential has very homologous relation with particle size, the higher magnitude of zeta potential values are, the smaller of the hydrodynamic sizes are, and the better dispersive stability of the nano-suspension is, and that an optimizing pH value of the nano-suspension can induce the highest magnitude zeta potential, then result in the best dispersion stability of the nano-suspension. It is also found that concentrations of SDBS can significantly affect the values of zeta potential and hydrodynamic sizes of the nanoparticles by electrostatic repulsions. Similarity, optimizing concentrations of SDBS can also lead to the highest enhancement of the dispersive stability of the Al2O3 suspension and the optimizing concentration for SDBS is 0.09% in 0.1% Al2O3 nano-suspensions, which have the best disperse results.
Thermal Properties of Thin Metallic Layer Deposited on Insulators: Effect of the Interface on the Independent Determination of the Thermal Diffusivity and Conductivity of the Layer
Authors: Fournier Danie`le , Roger Jean Paul , Fretigny Christian
Published: June 2009

Lateral heat diffusion thermoreflectance is a very powerful tool for determining directly the thermal diffusivity of layered structures. To do that, experimental data are fitted with the help of a heat diffusion model in which the ratio between the thermal conductivity k and the thermal diffusivity D of each layer is fixed, and the thermal properties of the substrate are known. We have shown in a previous work that it is possible to determine independently the thermal diffusivity and the thermal conductivity of a metallic layer deposited on an insulator, by taking into consideration all the data obtained at different modulation frequencies. Moreover, it is well known that to prevent a lack of adhesion of a gold film deposited on substrates like silica, an intermediate very thin (Cr or Ti) layer is deposited to assure a good thermal contact. We extend our previous work: the asymptotic behaviour determination of the surface temperature wave at large distances from the modulated point heat source for one layer deposited on the substrate to the two layers model. In this case (very thin adhesion coating whose thermal properties and thickness are known), it can be establish that the thermal diffusivity and the thermal conductivity of the top layer can still be determined independently. It is interesting to underline that the calculus can also be extended to the case of a thermal contact resistance which has often to be taken into account between two solids. We call thermal resistance a very thin layer exhibiting a very low thermal conductivity. In this case, the three parameters we have to determine are the thermal conductivity and the thermal diffusivity of the layer and the thermal resistance. We will show that, in this case, the thermal conductivity of the layer is always obtained independently of a bound of the couple thermal resistance – thermal diffusivity, the thermal diffusivity being under bounded and the thermal resistance lower bounded. Experimental results on thin gold layers deposited on silica with and without adhesion layers are presented to illustrate the method. Discussions on the accuracy will also be presented.
Independent Determination of the Thermal Diffusivity and Conductivity of a Thin Metallic Layer Deposited on Silica
Authors: Fretigny Christian , Roger Jean Paul , Liu Li , Fournier Danie`le
Published: June 2009

It is well known that the thermal parameters of materials confined in thin layers may significantly differ from their bulk value. Lateral heat diffusion thermoreflectance experiment is a very powerful tool for determining directly the thermal diffusivity of bulk materials and of layered structure. Nevertheless, in the latter case, experimental data are fitted with the help of a heat diffusion model in which the layer thermal conductivity and thermal diffusivity are taken together into consideration. In this paper, we show that both parameters can be determined independently, in the case of a thermal conductive layer deposited on a thermal insulator, with a careful analysis of the amplitude and the phase of the lateral temperature field associated to a point source.
Developing Flow With Combined Forced and Free Convection of Nanofluids for Horizontal Semicircular Ducts With the Flat Wall at the Vertical Position
Authors: El Hasadi Yousef M. F.
Published: June 2009

This study is concerned with the numerical investigation of the developing laminar mixed convection of a nanofluid which consists of water- γAl2O3 in a horizontal semicircular duct with the flat wall at a vertical position. The governing momentum and energy equations are solved numerically using a marching technique with the finite control volume approach following the SIMPLER algorithm. The properties of the nanofluid have been simulated by using a well known models and correlations from the literature. Results are obtained for the thermal boundary condition of uniform heat input axially with uniform wall temperature circumferentially, at any cross section (H1 boundary condition), different values of particles volume concentration and for two values of Grashof number 104 and 106. These results include the velocity and temperature distributions at different axial locations, axial distributions of local Nusselt number, and local average wall friction factor. It was observed that increasing the nanoparticles concentrations at low Gr, has a negligible effect on the developing of Nusselt number and friction factor. However, at high Gr it was found that increasing the particle volume concentration increases the Nusselt number in the developing and fully developed regions and reduces the friction factor at the developing and fully developed regions, if it is compared to the results obtained from the results obtained from the pure water at the same flow conditions. As an example for the case of Grashof number equals to 106 and particles volume concentration equals to 0.1, the enhancement of the Nusselt number and the reduction of wall friction factor at the fully developed region, are 17.5% and 6.4% respectively, if it is compared to that of the base fluid (water), at the same flow conditions.
Mechanisms of Convective Heat Transfer of Nanofluids
Authors: Wen Dongsheng
Published: June 2009

Research on nanofluids has progressed rapidly since its enhanced thermal conductivity was identified about a decade ago. Much evidence shows that the enhancement of convective heat transfer is much higher than that of thermal conductivity only. The mechanism of such enhancement, however, is still unclear. This work reviews the mechanisms of convective heat transfer of nanofluids in a single channel, and identifies two most likely mechanisms: the modification of effective properties and the migration of nanoparticles under flow conditions. A numerical simulation based on a combined Euler and Lagrange method is investigated in this work to illustrate the feature of nanoparticle migration and the drawback of the effective property approach. The motion of discrete nanoparticles is determined by the Lagrangian trajectory method based on the Newton’s second law that includes influence of the body force, various hydrodynamic forces, and the Brownian and thermophoresis forces. The coupling of discrete particles with continuous flow is realized through the modification of the source term of the continuous equation. It concludes that the two-phase flow nature of nanofluids, especially the nanoparticle migration and the resultant non-uniform particle and effective property profile, needs to be considered to properly model the convective heat transfer.
Asymptotic and Exact Analysis for Constructal Optimization of Microchannel Heat Sink
Authors: Asgari Omid , Saidi Mohammad Hassan
Published: June 2009

Microchannel are at the fore front of today’s cooling technologies. They are widely being considered for cooling of electronic devices and in micro heat exchanger systems due to their ease of manufacture. One issue which arises in the use of microchannels is related to the small length scale of the channel or channel cross-section. In this work, the maximum heat transfer and the optimum geometry for a given pressure loss have been calculated for forced convective heat transfer in microchannels of various cross-section having finite volume for laminar flow conditions. Solutions are presented for 10 different channel cross sections, namely parallel plate channel, circular duct, rectangular channel, elliptical duct, polygonal ducts, equilateral triangular duct, isosceles triangular duct, right triangular duct, rhombic duct and trapezoidal duct. The model is only a function of Prandtl number and geometrical parameters of the cross-section, i.e., area and perimeter. This solution is performed with two exact and approximate methods. Finally, in addition to comparison and discussion about these two methods, validation of the relationship is provided using results from the open literature.
Multichannel Effect of Condensation Flow and Heat Transfer in Horizontally Positioned Silicon Microchannels With the Channel Depth Down to Thirty Microns
Authors: Zhang Wei , Xu Jinliang
Published: June 2009

Multichannel effect is an important issue in micro condensers. In the present paper, we fabricated a silicon chip, consisting of three rectangular microchannels, with each width of 800 microns but depth down to 30 microns. Saturated vapor of water enters the entrance of microchannels which were horizontally positioned. There are two types of flow patterns: (1) the annular flow at high inlet pressures; (2) the quasi-stable bubble slug in the microchannel upstream followed by the isolated bubbles downstream, at moderate or low inlet pressures. For the second flow pattern, the downstream isolated bubble is formed by breaking up: (1) the single vapor thread connecting the upstream bubble slug and the detaching bubble; (2) the dual vapor threads synchronously in the center microchannel; (3) the dual vapor threads non-synchronously in the side microchannels. The flow patterns in the two side channels are symmetry against the center microchannel. The upstream bubble slug is shorter in the side channels than in the center channel. The temperature gradient across the chip width direction accounts for the non-synchronously break-up of the dual vapor threads connecting the bubble slug and the detaching bubble, in the side channels, indicating the multichannel effects.
Nano-Scale Transport Phenomena and Thermal Effect of the PEMFC Electrolyte
Authors: Cheng Chin-Hsien
Published: June 2009

This paper employed molecular dynamics (MD) simulation to investigate the transport phenomena and thermal effect at nano-scale inside fuel cell electrolyte. The material of the electrolyte was chosen to be Nafion® which is the most commonly used material for proton exchange membrane fuel cell (PEMFC). The transport of protons inside the electrolyte is one of the major issues that influencing the fuel cell performance. The structure of the Nafion® includes carbon-fluorine back bones and side chains (with SO3− attached at the end). Simulation results show that the transport of protons was confined to some specific regions. These specific regions (hydrophilic phase region) consist of water molecules, protons and sulfonated acid groups. Different hydration levels (3, 61.25, 9 and 15.375 H2O/SO3−) was also studied to test the sensitivity of the electrolyte water content on proton conduction. Higher water content shows greater proton mobility due to the larger water cluster size and more water clusters. The influence of the temperatures (333K, 343K and 353K) on proton mobility was due to different sizes of hydrophilic phase regions. Diffusion coefficients at various operation conditions were also evaluated and showed satisfactory agreement with the published experimental data.
Optimization of Heat Transfer Rate in the Rectangular Microchannel of Different Aspect Ratios With Constant Cross Sectional Area
Authors: Kowsary F. , Noroozi N. , Rezaei Barmi M.
Published: June 2009

The increased power dissipation and reduced dimensions of microelectronics devices have emphasized the need for highly efficient compact cooling technologies. Microchannel heat sinks are of particular interest due to the very high rates of heat transfer they enable in conjunction with greatly reduced heat sink length scales and coolant mass flow rate. Therefore, in the present work, optimization of laminar convective heat transfer in the microchannel heat sinks is investigated for uniform heat flux and different cross sectional areas of different aspect ratios. Three-dimensional numerical simulations of general form of energy equation were performed to predict Nusselt number in the laminar flow regime. Using these results, an optimum forced convective heat transfer coefficient was computed for several cross sectional areas and Reynolds numbers, utilizing the univariable search method. Different aspect ratios have different influences on Nusselt number in thermally developing and fully developed regions for different cross sectional areas and Reynolds numbers. There exists an optimum Nusselt number for each Reynolds number and cross sectional area by varying aspect ratio. Thus, optimized state is computed and related graphs are presented.