bluuebunny/crossref_metadata_2025 huggingface dataset:
Pulsating Electroosmotic Flow and Wall Block Mixing in Microchannels
Authors: Tang G. H. , Gu X. J. , Barber R. W. , Emerson D. R. , Zhang Y. H. , Reese J. M.
Published: June 2009

Understanding electroosmotic flow in microchannels is of both fundamental and practical significance for the design and optimization of various microfluidic devices to control fluid motion. Electroosmotic flows in microfluidic systems are restricted to the low Reynolds number regime, and mixing in these systems becomes problematic due to negligible inertial effects. To enhance the species mixing effect, the current study presents a numerical investigation of steady-state electroosmotic flow mixing in smooth microchannels, channels patterned with surface blocks, channels patterned with heterogeneous surfaces, as well as pulsating electroosmotic flow. The lattice Boltzmann equations, which recover the nonlinear Poisson-Boltzmann equation, the Navier-Stokes equation including the external force term, and the diffusion equation, were solved to obtain the electric potential distribution in the electrolyte, the velocity field, and the species concentration distribution, respectively. The simulation results confirm that wall blocks, heterogeneous surfaces, and electroosmotic pulsating flow can all change the flow pattern and enhance mixing in microfluidic systems. In addition, it is shown that pulsating flow provides the most promising method for enhancing the mixing efficiency.
Energy Streamlines in Near-Field Thermal Radiation
Authors: Lee B. J. , Zhang Z. M.
Published: June 2009

In the present paper, we investigate the energy propagation direction in near-field thermal radiation between two semi-infinite surfaces separated by a vacuum gap. Based on the fluctuational electrodynamics, we demonstrate in detail that Poynting vectors for each parallel wavevector component (β) are decoupled due to the randomness of thermal radiation. The results reveal that the spectral radiative energy is transferred in infinite directions. By separately tracing the Poynting vector for each given β, the energy propagation direction in the vacuum gap is visualized. Depending on β values, there exist considerable lateral shifts of the energy streamline. The range of β values dominantly contributing to the spectral energy flux is identified for different wavelengths. Furthermore, the effect of surface polaritons on the lateral shift is also discussed.
Fabrication of Silicon Microchannel With Integrated Heater and Temperature Sensors for Flow Boiling Studies
Authors: Gedupudi S. , Cummins G. P. , Lin H. , Walton A. J. , Sefiane K. , Karayiannis T. G. , Kenning D. B. R.
Published: June 2009

Two-phase microchannel heat sinks are a promising solution to meet the requirements for cooling electronic components with high-density heat dissipation. However, their design requires a thorough understanding of flow boiling and pressure drop in microchannels. The channels described in this paper have been fabricated in silicon, with rectangular cross-sections ranging in hydraulic diameter between 0.62 and 0.1 mm, for studies of boiling in single channels. To facilitate visualisation, the top of each channel is covered with Pyrex 7740, predrilled for fluid inlet and outlet connections. Integrated tantalum resistors are located uniformly along the bottom of the channel for temperature sensing. Tantalum pentoxide and PECVD silicon dioxide (which also conformally coats the channel walls) are used to electrically insulate the sensor from any liquid in the channel. The heater is an integrated aluminium serpentine track on the back of the bottom wafer. The channel is etched down to the sensors on the bonded bottom silicon wafer using the Bosch process. The objective related to the development of these silicon microchannels is to achieve heat fluxes of 2 MW m−2 with low, near-uniform wall superheat (by means of bubble triggering and artificial nucleation sites). Experiments will be carried out with mass fluxes varying from 100 to 500 kg m−2 s−1, using de-ionized water and an organic fluid as the working fluids.
Numerical Modeling of Micro Fin Arrays Using Slip Flow and Temperature Jump Boundary Conditions
Authors: Sobhan C. B. , Ameen Muhsin M. , Abraham Praveen P.
Published: June 2009

A numerical investigation of natural convection heat transfer from a rectangular fin array of microscale dimensions, where a “down and up” flow pattern occurs, is carried out. The stream function vorticity formulation is used in the analysis and the governing equations of the transient two dimensional field are solved using an explicit finite difference scheme. The dimensions of the domain are such that the problem falls under the slip flow regime. The non continuum effects are modeled through Maxwell’s velocity slip and Smoluchowski’s temperature jump boundary conditions. The steady state velocity and temperature distributions in the field are obtained by marching through the transient state. The average heat transfer coefficient and the Nusselt Number are calculated. The influence of the fin spacing, fin height and operating pressure on the performance of the fin array is studied through parametric studies and some conclusions are drawn regarding the significance of non continuum effects in the micro scale dimensions considered.
Numerical Investigation of Flow and Heat Transfer in Microchannels
Authors: Deniz Emrah , Uralcan I. Yalcin
Published: June 2009

Mini and microchannel applications have become an important and attractive research area during the past decades. For micro systems design purposes, numerical and experimental studies have been conducted on flow and heat transfer characteristics of mini and microchannels and various friction factor and Nusselt number correlations have been proposed. Some researchers have tried to apply conventional tube correlations to mini and micro channels, rather than deriving new correlations. In this study, using commercial CFD software, flow and heat transfer characteristics in laminar and turbulent flow through circular channels are analyzed numerically. The applicability of conventional correlations in calculating the friction factor and Nusselt number is investigated. It is concluded that, in laminar regime conventional correlations can be used to calculate the friction factor for the channel sizes considered. In turbulent regime, however, numerical results for friction factor yielded greater values than those calculated by the conventional correlations. Numerical Nusselt numbers are found to be closer to the conventional values in laminar and turbulent regimes. In turbulent regime, on the other hand, Nusselt number values calculated with the microchannel correlations are determined to be greater than the numerical results and the values calculated with the conventional correlations.
Multiscale Modeling of Heat Transfer in a Thermal Sensing Atomic Force Microscope
Authors: Masters Nathan D. , Ye Wenjing
Published: June 2009

Thermal Sensing Atomic Force Microscope (TSAFM) systems utilize heat transfer in continuum and low-speed rarefied gas flow regimes to perform topological surface scans. Spanning flow regimes complicates the modeling required to analyze scan data. In this paper we present a hybrid Octant Flux Splitting IP-DSMC/BEM model of the steady state heat transfer from a TSFAM cantilever and compare with the uncorrected continuum solution as well as a macro model developed for this problem.
Phonon Thermal Boundary Conductance in 1-D Chains: Comparison Between the Semiclassical and Nonequilibrium Green’s Function Formalisms
Authors: Hopkins Patrick E. , Norris Pamela M. , Tsegaye Mikiyas S. , Ghosh Avik
Published: June 2009

Thermal boundary conductance (hBD) across solid interfaces is a limiting factor in many applications, especially as characteristic length scales of devices are decreasing to the nanoscale. Accurate prediction of this interfacial energy transfer, however, is difficult. The most widely used model for hBD prediction is the diffuse mismatch model (DMM), which considers the transmission of the incident phonon intensity across the interface for the case of thermal equilibrium between the two materials adjacent to the interface. This semi-classical model is limited by the equilibrium assumption, and breaks down for the case of two similar materials comprising the interface. Recently, the Nonequilibrium Green’s Function (NEGF) formalism has been extended to phonon transport. The NEGF formalism is rooted in nonequilibrium transport theory, making it ideal to study energy transfer applications, especially in nanosystems where the concept of thermal equilibrium breaks down due to the small dimensions of the transport regions. The purpose of this paper is to derive, from first principles, the NEGF formalism of thermal conductance, and compare the assumptions of this formalism to the semi-classical DMM assumptions. The NEGF formalism is derived for two simple 1-D examples, and the 1-D conductance and phonon transmission is calculated for a 1-D atomic chains of Si and compared to DMM calculations and experimental data.
Numerical Simulation of a Fluidic Oscillator for Water Removal Enhancement in PEM Fuel Cell
Authors: Tseng S. H. , Yang Y. T. , Allen J. , Yang S. L.
Published: June 2009

The fluidic oscillator is one solution to the problems of water management of the Polymer Electrolyte Membrane Fuel Cell (PEMFC). In low temperature fuel cells such as the PEMFC, liquid water is produced as a byproduct. At low gas flow rates, the water produced has a tendency to plug the reactant supply channels. When the channels are plugged, reactant supply to the catalyst sites is prohibited and results in fuel cell failure. One proposed method of removing a water drop is to oscillate the drop near its natural frequency in order to free it from the substrate. A variety of methods for oscillating the drop are available including mechanical vibrations, acoustic vibrations and flow pulsations. It is the latter method, i.e., use of the fluidic oscillator that is investigated in this study. The purpose of this paper is to use the experimental and numerical techniques to study and simulate a fluidic oscillator so that detail of flow physics inside the oscillator can be realized. Method on how to use the flow field solution to determine the oscillator response frequency is also described. Numerically results of oscillator frequency will be compared with the available experimental data.
Future of NEMS
Authors: Bharath Sudharsan , Balaji M. , Sai Krishna G. R.
Published: June 2009

Nanorobots are quintessential NEMS (nanoelectromechanical systems) and raise all the important issues that must be addressed in NEMS design: sensing, actuation, control, communications, power, and interfacing across spatial scales and between the organic/inorganic and biotic/abiotic realms. Nanorobots are expected to have evolutionary applications in such areas as environmental monitoring and health care. This paper begins by discussing nanorobot construction, which is still at an embryonic stage. The emphasis is on nanomachines, an area which has seen a spate of rapid progress over the last few years. Nanoactuators will be essential components of future NEMS. The paper’s focus then changes to nanoassembly by manipulation with scanning probe microscopes (SPMs), which is a relatively well established process for prototyping nanosystems. Prototyping of nanodevices and systems is important for design validation, parameter optimization and sensitivity studies. Nanomanipulation also has applications in repair and modification of nanostructures built by other means. High throughput SPM manipulation may be achieved by using multi tip arrays. Experimental results are presented which show that interactive SPM manipulation can be used to accurately and reliably position molecular-sized components. These can then be linked by chemical means to form subassemblies, which in turn can be further manipulated. Applications in building wires, single electron transistors and nanowaveguides are presented.
Investigation of Various Material Properties in Polymer/Clay Nanocomposites
Authors: Bharath Sudharsan , Prem Kumar R.
Published: June 2009

Understanding the structure/property relations in polymer/clay nanocomposites is of great importance in designing materials with desired properties. Along these lines, a critical overview is attempted on the physical and molecular origins of material properties enhancements in polymer/clay hybrid nanocomposites. A comparative discussion of mechanical, thermal, optical, and ammability properties across various polymers focuses on those properties that are universally improved. In general, such properties originate from the nature of the layered inorganic fillers and from their nano-dispersion in a polymer. In contrast, other properties are determined by the particular/distinctive interactions between a specific polymer with the filler; such attributes can not be transfered from one polymer system to another. We shall try to distinguish between these two classes of properties, and provide some insight into which properties can be improved concurrently across a wide range of polymers.
Effect of Channel Geometry on the Dynamics of a Water Droplet in a Microchannel
Authors: Zhu Xun , Sui P. C. , Djilali Ned
Published: June 2009

The objective of the present study is to investigate the effects of the microchannel geometry on the dynamic behaviour of liquid water emerging from a pore into a microchannel of a cross gas flow. The flow characteristics are resolved using the volume-of-fluid (VOF) method in conjunction with an interface tracking technique. A microchannel with dimensions of a typical proton exchange membrane fuel cell (PEMFC) gas channel (a square cross section of 250 μm in width) and a pore of 50 μm in diameter on the bottom wall is adopted as the baseline case. Simulations for microchannels of different cross sections, including trapezoid, upside-down trapezoid, triangle, rectangle, and rectangle with a arch bottom wall, are performed and the results are compared with the baseline case. The evolution of liquid water includes stages identified as emergence, growth, deformation, detachment, and remove. The simulations show that the cross section of the microchannel has significant impacts on the dynamics of the water droplet. The detachment time and diameter and the remove time of the water droplet are found to be in this order: triangle < trapezoid < rectangle with arch bottom wall < rectangle < upside-down trapezoid. The present study will advance our understanding in the transport of liquid water in a PEMFC where water is produced in the catalyst layer and flows through the pores of the porous electrode to the gas channel.
Convective Heat Transfer Coefficient and Pressure Drop of Al2O3-Water Nanofluid in a Single-Phase Microchannel for Cooling of High Heat Output Devices
Authors: Valencia Guillermo E. , Ramos Miguel A. , Bula Antono J.
Published: June 2009

The paper describes an experimental procedure performed to obtain the convective heat transfer coefficient of Al2O3 nanofluid working as cooling fluid under turbulent regimen through arrays of aluminum microchannel heat sink having a diameter of 1.2 mm. Experimental Nusselt number correlation as a function of the volume fractions, Reynolds, Peclet and Prandtl numbers for a constant heat flux boundary condition is presented. The correlation for Nusselt number has a good agreement with experimental data and can be used to predict heat transfer coefficient for this specific nanofluid, water/Al2O3. Furthermore, the pressure drop is also analyzed considering the different nanoparticles concentration.
Measurements of Thermal Conductivity of Thin Films by 3-Omega Method
Authors: Tanaka Saburo , Takiishi Makoto , Miyazaki Koji , Tsukamoto Hiroshi
Published: June 2009

In this study, the thermal conductivity of a bismuth-telluride (Bi2Te3) thin film is measured at room temperature by using the 3ω method [1, 2]. The 3ω method for thin films uses a single metal-line as both the heater and thermometer. An alternating-current driving current at angular frequency ω heats the surface of the sample at a frequency of 2ω. Since the resistance of a metal increases with temperature, temperature oscillations produce an oscillation of the electrical resistance at a frequency of 2ω. Consequently, the voltage drop across the metal line has a small component at 3ω that can be used to measure the temperature oscillations and the thermal response of the sample with a Bismuth-telluride thin film. Differential amplifiers are used to subtract the ω component of the voltage for the measurement of the small 3ω signal as shown in Fig.1. The amplified 3ω component and the attenuated reference voltage are acquired to a personal computer through a 16bit DAC card. The bismuth-telluride thin films are manufactured by flash evaporation and coating. The narrow Aluminum lines for the 3ω method are made by vacuum deposition through metal masks. The measured temperature oscillation ΔT versus ln(ω) yields the thermal conductivity of the substrates. The thermal conductivity of the glass is measured as 0.97 W/(m·K), and that of alumina is 15.4 W/(m·K). These results agree well with the reference data. The thermal conductivities of Bismuth-telluride thin films are calculated from the measured thermal resistances of the films. The measured value for a flash evaporated Bismuth-telluride is 1.20 W/(m·K), and the coated film is 0.51 W/(m·K).
Whole Chip Temperature Measurements Using Thin-Film PDMS/Rhodamine B for Microfluidic Chip Design
Authors: Samy Razim , Glawdel Tomasz , Ren Carolyn
Published: June 2009

A novel method for in-situ temperature measurements of microfluidic devices using thin-film poly(dimethylsiloxane) (PDMS) saturated with Rhodamine B dye is reported. Rhodamine B, a dye with temperature dependent fluorescent intensity, is frequently injected into the working fluid for on-chip temperature field visualization of glass and silicon based microfluidic devices. However, such a visualization method results in unreliable temperature measurements due to high absorption and adsorption for polymeric devices such as PDMS. Thus, an inexpensive temperature measurement technique is developed in which a thin PDMS layer (∼30 μm) is fabricated and submersed for several days into a Rhodamine B solution. To prevent backward diffusion of the dye into the working fluid during operation, a glass barrier (∼150 μm) is bonded between the thin film and the PDMS mold containing the microchannel design of interest. Temperature measurements are made by utilizing standard method of measuring changes in the normalized fluorescent intensity. For verification purposes, a new calibration curve is developed and the thin film is tested with a microchannel subjected to joule heating. The resulting temperature field along the axial direction of the channel for different applied powers compares well with numerical simulations. Analysis of dye intensities before and after experiments provides temperature deviation estimates due to photobleaching. Errors in temperature measurement due to the film thickness are discussed.
Gas Micro-Flow Modelling Based on Regularized 13-Moment-Equations
Authors: Torrilhon Manuel , Struchtrup Henning
Published: June 2009

We summarize our recent contributions to the development of macroscopic transport equations for gas micro-flows. A combination of the Chapman-Enskog expansion and Grad’s moment method in kinetic theory of gases yields the Regularized 13-Moment-Equations (R13 equations). These equations overcome deficiencies of Grad’s equations or Burnett models. They are asymptotically of super-Burnett order, i.e., of third order in the Knudsen number and linearly stable for all wave frequencies. In addition, a complete set of boundary conditions can derived from the accommodation boundary conditions of the Boltzmann equations. Mathematically, more boundary conditions are required and they can be derived from the R13 system itself through coherence relations. We present micro-channel and shock wave simulations to prove that R13 is a reliable and efficient continuum model for micro-flows of gases with moderate Knudsen numbers.
A Molecular Dynamics Study on Effects of Nanostructural Clearances at an Interface on Thermal Resistance
Authors: Shibahara Masahiko , Inoue Kosuke , Kobayashi Kiyomori
Published: June 2009

The classical molecular dynamics simulation was conducted in order to clarify the effects of structural clearances in nanometer scale on thermal resistance at a liquid-solid interface. A liquid molecular region confined between the solid walls, of which the interparticle potential was Lennard-Jones type, was employed as a calculation system. The solid walls consisted of three atomic layers where the temperature of the middle layer was controlled by the Langevin method. Heat flux in the system was calculated numerically by integrating the forces that acted on the temperature controlled atoms by the Langevin method. The temperature jump between the solid wall and the liquid molecular region was calculated numerically. The thermal resistance at a liquid-solid interface was calculated numerically with changing the surface structural clearances in nanometer scale. Temperature gradient and liquid density were also changed as calculation parameters. With changing the surface structural clearances from 0nm to 2.5nm the thermal resistance at the interface once decreased and became the minimum value when the structural clearances were between 0.6 to 1.0 nm. The thermal resistance between the solid and the liquid increased when the structural clearances were more than 1.0nm. With the increase of the liquid density the thermal resistance between the solid and the liquid substantially decreased regardless of the temperature gradient and the surface structures in nanometer scale.
Frictional Pressure Drops During Vapour-Liquid Flow in Minichannels: Experimental Data and Modelling
Authors: Cavallini Alberto , Del Col Davide , Matkovic Marko , Rossetto Luisa
Published: June 2009

Condensation in minichannels is widely used in air-cooled condensers for the automotive and air-conditioning industry, in heat pipes and other applications for cooling electronics. The knowledge of pressure drops in such small channels is important in order to optimize heat transfer surfaces. Most of the available experimental work refers to measurements obtained within multiport smooth extruded tubes and deal with the average values over the number of parallel channels. In this context, the present authors have set up a new test apparatus for heat transfer and fluid flow studies in single minichannels. This paper presents new experimental frictional pressure gradient data, relative to single-phase flow and adiabatic two-phase flow of R134a inside a single horizontal mini tube, with 0.96 mm inner diameter and with not-negligible surface roughness. The new all-liquid and all-vapour data are successfully compared against predictions of single-phase flow models. Also the two-phase flow data are compared against Cavallini et al. [1, 2] model, valid for adiabatic flow or for flow during condensation of halogenated refrigerants inside smooth minichannels. Surface roughness effects on the liquid-vapour flow are discussed. In this respect, the friction factor in the proposed model is modified, in order to take into consideration also effects due to wall roughness.
A Streaming Flow Based Lab-on-Chip Platform Technology
Authors: Zhang Zongqin , Fadl Ahmed , Liu Chang , Meyer Donna
Published: June 2009

Numerous studies on microfluidics diagnostic devices have been published in the last decade. Although the first generation of Lab-on-chip (LOC) devices was functional in 1999, some of the promises of microfluidics (integration of all functions on a chip and the commercialization of truly handheld microfluidic instruments) have yet to be fulfilled. The major challenges of LOC technology include cost–effective pumping, function integration, multiple detection, and system miniaturization. In this paper, we propose a novel and simple streaming-based LOC technology that may have potential to directly address these challenges. The phenomenon of the flow streaming is found in zero-mean velocity oscillating flows in a wide range of channel geometries. Although there is no net flow (zero-mean velocity) across the channel, a 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: 1) to transport, mix and separate particles/molecules/bacterium/cells entrained in flows; 2) to perform multi-channel/generation micro-array sample distributions; and 3) to achieve function integrations and biomarker detections. Mechanisms of using flow streaming to achieve the various LOC functions are described. Preliminary results are presented to demonstrate the potential of this technology for LOC applications.
Non-Fourier Heat Conduction Phenomena Applied Different Temperature and Heat Flux Pulses on Boundary
Authors: Lu Wen-Qiang , Fan Qing-Mei
Published: June 2009

A new numerical method [1], which combines the dual reciprocity boundary element method with Laplace transforms, has been used to solve ultrafast heat conduction problems. By this method, the time micro scale heat transfer problems applied different extreme high frequency temperature and heat flux pulses (the width of a single pulse is less than 10−12 s) on the boundary are simulated in this paper. Numerical results open out some phenomena of non-Fourier heat conduction. “Thermal accumulation (TA)” as a typical phenomenon of non-Fourier heat conduction takes on different characteristics under different pulsed conditions. The pulse time-width has important effect on the non-Fourier characteristics for single pulse, while different pulse periods for seriate pulse make obvious different non-Fourier characteristics.
Surface Plasmons in Light Interaction With Metallic Nanostructures and Applications
Authors: Han Li-Hsin , Battula Arvind , Chen Shaochen
Published: June 2009

In this invited paper, we present the surface plasmons effect as a result of light interaction with metallic nanostructures such as a gold nanosphere or an array of gold nano-slits. Numerical simulation using a finite difference time domain method coupled with Drude model revealed the light enhancement near the gold nanostructures due to the surface plasmon effect. Experimentally, we demonstrated such effect in laser deformation of a polymer shell coated with gold nanospheres. We also employed such enhanced light field for nanoscale lithography.
Heat Transfer Behavior of Aqueous Suspensions of Nano-Diamonds in Turbulent Pipe Flow
Authors: Torii Shuichi , Yang Wen-Jie
Published: June 2009

This paper aims to study the convective heat transfer behavior of aqueous suspensions of nano-diamond particles flowing through a horizontal tube heated under constant heat flux condition. Consideration is given to the effect of particle concentration and Reynolds number on heat transfer enhancement. It is found that (i) significant enhancement of heat transfer performance due to suspension of nano-diamond particles in the circular tube flow is observed in comparison with pure water as the working fluid, (ii) the enhancement is intensified with an increase in the Reynolds number and the nano-diamond concentration, and (iii) substantial amplification of heat transfer performance is not attributed purely to the enhancement of thermal conductivity due to suspension of nano-diamond particles.
Microdfluidic Based 3-Dimensional Cell Culture Platform
Authors: Huang Song-Bin , Wu Min-Hsien , Cui Zhanfeng , Cui Zheng , Lee Gwo-Bin
Published: June 2009

This study reports a new perfusion-based, micro three-dimensional (3-D) cell culture platform for drug testing using enabling microfluidic technologies. In this work, a perfusion-based, micro 3-D cell culture platform is designed and is fabricated based on SU-8 lithography and polydimethylsiloxane (PDMS) replication processes. One of the key features of the system is that the incorporation of a multiple medium pumping mechanism, consisting of 15 membrane-based pneumatic micropumps with serpentine-shape (S-shape) layout, coupled with a pneumatic tank, into the micro 3-D cell culture platform to provide efficient and economical culture medium delivery. Moreover, a “smart cell/agarose (scaffold) loading mechanism” was proposed, allowing the cell/3-D scaffold loading process in one step and avoiding too much laborious works and manual error. The results show that in all of the 15 S-shape pneumatic micropumps studied, the medium delivery mechanism is able to provide a uniform flow output ranging from 5.5 to 131 μl/hr depending on the applied pulsation frequency of the micropumps. In addition, the cell/agarose (scaffold) loading mechanism was proved to be able to perform sample loading tasks precisely and accurately in all of the 15 microbioreactors integrated. Furthermore, anti-cancer drug testing was successfully demonstrated using the proposed culture platform and fluorescent microscopic observation. As a whole, because of miniaturization, not only does this perfusion 3-D cell culture platform provide a homogenous and steady cell culture environment, but it also reduces the need for human intervention. Moreover, due to the integrated pumping of the medium and the cell/agarose (scaffold) loading mechanisms, time efficient and economical research work can be achieved. These characteristics are found particularly useful for high-precision and high-throughput 3-D cell culture-based drug testing.
Thermal Conductivity Measurements on Polysilicon Microbridges Using the 3-Omega Technique
Authors: Hopkins Patrick E. , Phinney Leslie M.
Published: June 2009

The thermal properties of microelectromechanical systems (MEMS) devices are governed by the structure and composition of the constituent materials as well as the geometrical design. With the continued reduction of the characteristic sizes of these devices, experimental determination of the thermal properties becomes more difficult. In this study, the thermal conductivity of polycrystalline silicon (polysilicon) microbridges are measured with the transient 3ω technique and compared to measurements on the same structures using a steady state joule heating technique. The microbridges with lengths from 200 microns to 500 microns were designed and fabricated using the Sandia National Laboratories SUMMiT™ V surface micromachining process. The differences between the two measurements, which arise from the geometry of the test structures, are explained by bond pad heating and thermal boundary resistance effects.
Convective Boiling of R-22 in a Flat Aluminum Multi-Minichannel Tube With Dh = 1.4 mm
Authors: Kim Nae-Hyun , Oh Wang-Kyu , Ham Jung-Ho , Kim Do-Young , Shin Tae-Ryong
Published: June 2009

Convective boiling heat transfer coefficients of R-22 were obtained in a flat extruded aluminum tube with Dh = 1.41 mm. The test range covered mass flux from 100 to 600 kg/m2 s, heat flux from 5 to 15 kW/m2 and saturation temperature from 5°C to 15°C. The heat transfer coefficient curve shows a decreasing trend after a certain quality (critical quality). The critical quality decreases as the heat flux increases, and as the mass flux decreases. The early dryout at a high heat flux results in a unique ‘cross-over’ of the heat transfer coefficient curves. The heat transfer coefficient increases as the mass flux increases. At a low quality region, however, the effect of mass flux is not prominent. The heat transfer coefficient increases as the saturation temperature increases. The effect of saturation temperature, however, diminishes as the heat flux decreases. Both the Shah and the Kandlikar correlations underpredict the low mass flux and overpredict the high mass flux data.
Experimental Study of Flow Critical Heat Flux in Low Concentration Water-Based Nanofluids
Authors: Kim Sung Joong , McKrell Tom , Buongiorno Jacopo , Hu Lin-Wen
Published: June 2009

Nanofluids are known as dispersions of nano-scale particles in solvents. Recent reviews of pool boiling experiments using nanofluids have shown that they have greatly enhanced critical heat flux (CHF). In many practical heat transfer applications, however, it is flow boiling that is of particular importance. Therefore, an experimental study was performed to verify whether or not a nanofluid can indeed enhance the CHF in the flow boiling condition. The nanofluid used in this work was a dispersion of aluminum oxide particles in water at very low concentration (≤0.1 v%). CHF was measured in a flow loop with a stainless steel grade 316 tubular test section of 5.54 mm inner diameter and 100 mm long. The test section was designed to provide a maximum heat flux of about 9.0 MW/m2, delivered by two direct current power supplies connected in parallel. More than 40 tests were conducted at three different mass fluxes of 1,500, 2,000, and 2,500 kg/m2sec while the fluid outlet temperature was limited not to exceed the saturation temperature at 0.1 MPa. The experimental results show that the CHF could be enhanced by as much as 45%. Additionally, surface inspection using Scanning Electron Microscopy reveals that the surface morphology of the test heater has been altered during the nanofluid boiling, which, in turn, provides valuable clues for explaining the CHF enhancement.
Study of Heat Transfer in Microsystems by Infrared Technique
Authors: Hetsroni G. , Mosyak A. , Rozenblit R.
Published: June 2009

The problem of dissipating high heat fluxes has received much attention due to its importance in such applications as cooling of electronic equipment, micro heat exchangers and many others. The most effective way of cooling is pumping liquid inside these devices through microchannels or porous media. Advantages and disadvantages using the Infrared Technique for the study of the heat transfer in microsystems are analyzed. The generalized model for analysis using IR technique for the temperature measurement in microdevices is suggested. The model is illustrated by various examples of heat transfer determination using image analysis as the heater temperature as well direct measurement of the liquid temperature.
The Characteristics of Discharge Coefficient of Nozzles With Throat Diameter Decreasing
Authors: Li C. H. , Peng X. F. , Wang C.
Published: June 2009

Referring to regulation of ISO 9300, the discharge coefficients of 2mm, 5mm and 10mm nozzles with 2.5°, 4.0° and 6.0° diffuser angle were measured, respectively. Through comparisons among experimental results, simulation results and prediction results of empirical equations, it was clear that the discharge coefficients were the same for the nozzles with the same throat diameter and were in good agreement with the results of the empirical equations for 10mm nozzles, while that changed with different diffuser angle for other two sets of nozzles. The influence of throat diameter, surface roughness and entrance contour on discharge coefficient was analyzed one by one. The results showed the large out-of-roundness of entrance contour might be the most important reason resulted in the experimental results.
Augmentation and Optimization of Heat Transfer in Reciprocating Flows Through Small-Scaled Channels Partially Filled With a Porous Medium
Authors: Shokouhmand H. , Haibbi K. , Mosahebi A.
Published: June 2009

This paper presents the effect of introducing a porous medium on the heat transfer by a reciprocating flow in a small-scaled two-dimensional channel. The channel is discretely heated from above and is insulated in the bottom. In this ideal geometry, a fully developed reciprocating flow is established by a sinusoidal pressure gradient. In side boundaries, velocity and temperature are assumed to be periodic. A certain volume of this channel is occupied by a porous medium which is shown to be an effecting tool for heat transfer augmentation. The results of this study are declared by some dimensionless parameters such as Womersley number (α), Darcy number (Da), porosity (ε), Prandtl number (Pr), Pressure gradient amplitude (A) and dimensionless characteristic lengths in the channel and porous medium. Heat transfer enhancement in presence of a porous medium is investigated in this study. Despite decreasing the fluid velocity in porous media, a high conductivity of such media can effectively enhance the heat transfer rate and because of these two contradictory effects, an optimum depth for the porous region is expected. For this purpose, at first the momentum equations of the domain are solved analytically (Brinkmann-extended Darcy model is used for porous region) and on the other hand the energy equation is investigated and solved numerically using Alternating Direction Implicit (ADI) method. The enhancing effect and optimization criteria are discussed in result section.
Hardening of the Porcine Thoracic Aorta Subjected to Freezing and Thawing: Experimental Evaluation and Mathematical Modeling
Authors: Yamada Hiroshi , Yasuno Kousuke , Fujisaki Kensuke , Ishiguro Hiroshi
Published: June 2009

Identifying changes in the mechanical behavior of blood vessels subjected to freezing and thawing, such as occur with cryopreservation, are of key importance. Excising pairs of fresh ring specimens from identical porcine thoracic aortas (n = 8 for each cooling rate), we carried out uniaxial tensile loading and unloading tests over the physiological stress range (first and second tests) and performed a loading test until the breaking point within the range of a load cell (third test). After the first test, one specimen of the pair was frozen at −80°C at a cooling rate of −1°C or −50°C/min and thawed, while the other was held at 5°C as a control. At both cooling rates, for the specimens subjected to freezing, the ratios of the tangential modulus in the stress-strain curve (between 130 and 150 kPa) in the second test to that in the first test differed significantly (p < 0.01) from the respective ratios of the control specimens. We formulated a mathematical model of the stress–strain relationship considering elastic and collagen fibers and an incompressible fluid phase. We evaluated the working hypothesis that collagen fibers reduce their extensibility either by hardening as a mechanical change or by shortening as a geometric change. We attributed this response to the formation of dehydration-induced cross-linking in collagen molecules at the microscopic level.
Numerical Analysis of Automobile Environment During the Cooling Period in Summer
Authors: Lee Kum-Bae , Kader Md. Faisal , Youn Young-Muk
Published: June 2009

Understanding the temperature and humidity profiles inside a vehicle when the A/C is on during summer is of paramount practical significance to maintain comfortable environment. In this paper, the airflow patterns, temperature and humidity distributions are predicted utilizing a three-dimensional finite volume numerical method and measured experimentally inside the automobile. Numerical analyses of the three-dimensional geometry predict a detailed description of fluid flow patterns. The velocity vectors from the side instrument panel registers impinge on the front occupant’s chest to head then travel forward and finally reach to the rear parcel shelf area. The undesirable returning flow passing by the driver’s head and neck has been observed. The velocity vectors from the central instrument panel registers travel directly to the rear compartment. A recirculation has been observed near the occupant’s knee area of the front compartment and rear compartment. The Temperature comes down to a comfortable range almost linearly at the initial stage. After that no considerable change has been observed. A good agreement has been found between the numerical and experimental results.
Numerical Simulation and Experimental Analysis on the Crystal Growth of Excimer-Laser Crystallization of a-Si Film
Authors: Chao Long-Sun , Chen Yu-Ru , Peng Hsiun-Chang
Published: June 2009

In this work, the excimer-laser-induced crystallization of amorphous silicon (a-Si) films was investigated numerically and experimentally. The basic structure is an a-Si film on a glass substrate. This study had investigated the effects of irradiating energy density on the grain size and structure by scanning electron microscopy (SEM). In the surface microstructure analysis of the laser-irradiated area, the critical fluences (full-melt threshold, FMT) between the partial melting and complete melting regimes can be found by applying scanning electron microscopy. An efficient two-dimensional numerical model is carried out to predict the critical fluences (FMT) and the transient temperature distribution during the laser processing. Numerical analysis of the temperature profile showed that a temperature drop occurred at the center of melted zone immediately after laser irradiation. From the analysis of temperature responses, the FMT obtained from the simulation results of the proposed model agree fairly well with those from the experimental data reported in the literature and acquired in this research. Furthermore, the grain growth of the poly-Si was studied by the grain observation of the cross section and its corresponding numerical simulation. The cross-sectional grain structure of the resulting poly-Si film was observed with different laser intensities. The grain sizes decreased with increasing irradiating energy intensity in the partial melting regime. From the surface observation, the grain distribution was uniform and most of the grain has a single crystallographic direction. The average grain size had the biggest value at FMT. But some super large grains occurred and combined with more than one crystallographic direction when the film obtained sufficiently high energy intensities that was closed or over the FMT. The grain distribution was not uniform. The super large gain was around the small grain size. The modified cellular automation method (MCA) was used to simulate the grain growth two-dimensionally and explain the grain development during the solidification process. The grain morphology of the numerical simulation was satisfied with the experimental observation. From the analysis of the grain growth, this model was able to simulate the undercooling effect and grain growth phenomenon and fitted for the experimental grain observation in the excimer-laser-induced crystallization.
A Thermopneumatic Valveless Micropump With PDMS-Based Nozzle/Diffuser Structure for Microfluidic System
Authors: Yang Lung-Jieh , Lin Tzu-Yuan , Ou Yu-Cheng
Published: June 2009

A thermopneumatic valveless micropump with a PDMS-based nozzle/diffuser structure was firstly designed and realized herein by stacking three layers of PDMS on a glass slide. Unlike the conventional peristaltic pumping configuration, the new structure of the micropump consists of only one set of heater on the glass slide, a thermopneumatic actuation chamber, and an actuation diaphragm. Additionally, it includes a flowing channel with nozzle/diffuser structure and inlet/outlet ports. In this valveless microchannel, fluid is driven by asymmetric flow resistance produced from the nozzle and diffuser configuration. The actuation diaphragm between the gas-pneumatic chamber and the flowing channel can bend up and down due to the gas expansion as well as the thermal buckling of the PDMS diaphragm imposed from the heating in the gas-pneumatic actuation chamber.
Nano Fluids and Critical Heat Flux
Authors: Golubovic Mihajlo , Madhawa Hettiarachchi H. D. , Worek William M.
Published: June 2009

In recent years nanofluids have been attracting significant attention in the heat transfer research community. These fluids are obtained by suspending nanoparticles having sizes between 1 and 100 nm in regular fluids. It was found by several researchers that the thermal conductivity of these fluids can be significantly increased when compared to the same fluids without nanoparticles. Also, it was found that pool boiling critical heat flux increases in nanofluids. In this paper, our objective is to evaluate the impact of different nanoparticle characteristics including particle concentration, size and type on critical heat flux experimentally at saturated conditions. As result, this work will document our experimental findings about pool boiling critical heat flux in different nanofluids. In addition, we will identify reasons behind the increase in the critical heat flux and present possible approaches for analytical modeling of critical heat flux in nanofluids at saturated conditions.
Heat Transfer Enhancement in Microchannels Incorporating Slanted Grooves
Authors: Lee Poh-Seng , Teo Chiang-Juay
Published: June 2009

The ever-increasing density, speed, and power consumption of microelectronics has led to a rapid increase in the heat fluxes which need to be dissipated in order to ensure their stable and reliable operation. The shrinking dimensions of electronics devices, in parallel, have imposed severe space constraints on the volume available for the cooling solution, defining the need for innovative and highly effective compact cooling techniques. Microchannel heat sinks have the potential to satisfy these requirements. However, significant temperature variations across the chip persist for conventional single-pass parallel flow microchannel heat sinks since the heat transfer performance deteriorates in the flow direction in microchannels as the boundary layers thicken and the coolant heats up. To accommodate higher heat fluxes, enhanced microchannel designs are needed. The present work presents an idea to enhance the single-phase convective heat transfer in microchannels. The proposed technique is passive, and does not require additional energy to be expended to enhance the heat transfer. The idea incorporates the generation of a spanwise or secondary flow to enhance mixing and hence decrease fluid temperature gradients across the microchannel. Slanted grooves can be created on the microchannel wall to induce the flow to twist and rotate thus introducing an additional component to the otherwise laminar flow in the microchannel. Numerical results are presented to demonstrate the effectiveness of such an enhanced microchannel heat sink. The heat transfer was found to increase by up to 12% without incurring substantial additional pressure drops.
The Influence of Molecular Orientation on the Ability of Water to Enter a Carbon Nanotube at Different Temperatures
Authors: Cannon James , Moore Natalie , Hess Ortwin
Published: June 2009

Temperature plays a significant role in determining the dynamics of flow on the nanoscale. This is particularly important with carbon nanotubes, which are likely to form an integral part of future nanofluidic and biological devices. We demonstrate through first-principles density-functional theory (DFT) that the orientation of the individual water molecules plays a significant role in determining the temperatures and energies at which water is able to enter the nanotube. This has a number of implications for the flow of water through the nanotube at different temperatures and densities, particularly when considering low-density water vapour.