bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo Simulation of Thermal Conductivities of Silicon Nanowires
Authors: Chen Yunfei , Li Deyu , Lukes Jennifer R. , Ni Zhonghua
Published: April 2009

One-dimensional (1D) materials such as various kinds of nanowires and nanotubes have attracted considerable attention due to their potential applications in electronic and energy conversion devices. The thermal transport phenomena in these nanowires and nanotubes could be significantly different from that in bulk material due to boundary scattering, phonon dispersion relation change, and quantum confinement. It is very important to understand the thermal transport phenomena in these materials so that we can apply them in the thermal design of microelectronic, photonic, and energy conversion devices. While intensive experimental efforts are being carried out to investigate the thermal transport in nanowires and nanotube, an accurate numerical prediction can help the understanding of phonon scattering mechanisms, which is of fundamental theoretical significance. A Monte Carlo simulation was developed and applied to investigate phonon transport in single crystalline Si nanowires. The Phonon-phonon Normal (N) and Umklapp (U) scattering processes were modeled with a genetic algorithm to satisfy both the energy and the momentum conservation. The scattering rates of N and U scattering processes were given from the first perturbation theory. Ballistic phonon transport was modeled with the code and the numerical results fit the theoretical prediction very well. The thermal conductivity of bulk Si was then simulated and good agreement was achieved with the experimental data. Si nanowire thermal conductivity was then studied and compared with some recent experimental results. In order to study the confinement effects on phonon transport in nanowires, two different phonon dispersions, one based on bulk Si and the other solved from the elastic wave theory for nanowires, were adopted in the simulation. The discrepancy from the simulations based on different phonon dispersions increases as the nanowire diameter decreases, which suggests that the confinement effect is significant when the nanowire diameter goes down to tens nanometer range. It was found that the U scattering probability engaged in Si nanowires was increased from that in bulk Si due to the decrease of the frequency gap between different modes and the reduced phonon group velocity. Simulation results suggest that the dispersion relation for nanowire solved from the elasticity theory should be used to evaluate nanowire thermal conductivity as the nanowire diameter reduced to tens nanometer.
Experiment of a Novel Capillary-Pumped Loop With a Semi-Arc Porous Evaporator in 1U Simulation Cabinet
Authors: Lin Hung-Wen , Wu Ming-Chieh , Lin Wei-Keng
Published: April 2009

The CPL is a high efficiency two-phase heat transfer device using the phase change of working fluid to transport heat from evaporator to condenser; it is a cyclic circulation pumped by capillary force. Since CPL does not need any other mechanical force such as pump, it might be used to do the thermal management of high power electronic component on spacecraft. This study presents a novel CPL with a semi-arc porous evaporator in 1U server on the ground with a horizontal position and scale down the whole device to the miniature size (range from mm to cm). This miniature-CPL is made of aluminum with the shape of evaporator and the porous structure using a semi-arc instead of square and cylinder structure. Testing results show that the miniature-CPL could remove heat 45W in steady state and keep the heat source temperature about 73°C.
Operating Ranges of the Planar Loop Heat Pipe Under Non-Vacuum Conditions
Authors: Suh Junwoo , Shuja Ahmed , Medis Praveen , Parimi Srinivas , Gerner Frank M. , Henderson H. Thurman
Published: April 2009

As the trend of high throughput in small packages continues, the heat dissipation becomes a very critical design issue in electronic devices and spacecrafts. The two phase loop heat pipe utilizes the latent heat of working fluid. It consists of an evaporator, compensation chamber, condenser, and liquid and vapor line. The primary wick used as a core part to circulate the working fluid is located in the evaporator. The planar loop heat pipe uses coherent porous silicon (CPS) wick as opposed to the conventional cylindrical configuration, which uses a sintered amorphous metal wick. The clear evaporator machined from Pyrex glass and transparent silicone tubes were utilized to monitor the complex phenomena which occur in the evaporator. Tests were conducted under the non-vacuum condition without a secondary wick. DI-water was used as a working fluid. Like an open loop test previously conducted, there was an operating range in which the liquid could be properly pumped from the compensation chamber to the vapor line under the pumping motion. In this device, more than 6 Watts could be convected from the evaporator to the ambient. Therefore circulation was not observed until powers greater than 6 Watts. There was a circulation of working fluid occurring due to energy transport within the loop when the input power was from 7.94 Watts to 17.6 Watts. The quantity of heat transportation to the loop was calculated by acquiring the empirical heat transfer coefficient. From this calculation it was found that, roughly, 12.1 Watts was transported to the loop and 5.51 Watts was convected to the ambient from the evaporator itself when the applied power was 15.27 Watts.   This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.
Phase Transition of Argon in a Nanocavity
Authors: Schoen Philipp A. E. , Poulikakos Dimos
Published: April 2009

We performed molecular dynamics simulations of argon liquid enclosed in an infinitely extended channel made out of platinum atoms. It was found that for small temperatures the van der Waals forces at the liquid-substrate interface are increased. Using this fact and the nature of argon, that this liquid thermally contracts if cooled, phase transition of liquid to vapor could also be achieved in this nanocavity of constant volume. However, the phase diagram is altered significantly compared to bulk argon.
The Influence of Thermal Expansion Flow on Droplet Evaporation
Authors: Shusser Michael
Published: April 2009

It has been demonstrated recently that it follows from conservation of mass that unsteady temperature fields create flow in an incompressible fluid with a temperature-dependent density even in the absence of gravity. The paper studies the influence of thermal expansion flow on spherically symmetric evaporation of an isolated droplet. A model problem of a droplet evaporating at a constant rate is first considered. In this idealized situation one can use the assumption of a thin thermal boundary layer to solve analytically the unsteady moving-boundary heat conduction problem to find the temperature field inside the droplet both with and without the thermal expansion flow. Next evaporation of a fuel droplet in a diesel engine is studied numerically. The heat diffusion equation is solved in the liquid phase while the standard quasi-steady model is used for the gas phase. The results of the calculation show that for high ambient temperatures the influence of the thermal expansion flow can be considerable.
Simulations of Dimensional Effects in Solid Oxide Fuel Cells
Authors: Hasan A. B. Mahmud , Ekkad Srinath V. , Mensah Patrick
Published: April 2009

Two dimensional simulation models for a solid oxide fuel cell (SOFC) including gas channels have been studied using the commercial software FEMLAB which has a multi-physics approach. The models include full coupling between the mass balances at the anode and cathode, the balance of the ionic current carried by the oxide ion, and a balance of electronic current with an assumption of isothermal conditions. Three cross-section areas of 1cm × 1cm, 3cm × 3cm, and 5 cm × 5cm were chosen to study the dimensional effect. An additional 1cm × 1cm channel with rectangular protrusions was also studied to investigate geometry effects. Simulation results show expected result of increases in fuel cell properties like convective flux, electric potential with increasing cell flow area. The smaller fuel cell surface is altered by adding rectangular protrusions to increase surface area. The presence of rectangular protrusions also show increased current density production compared to a wall without protrusions. It is expected that the proposed simulations can be used to significantly help the design and operation of a SOFC in practical applications.
Film Cooling Measurements for Novel Hole Configurations
Authors: Lu Yiping , Faucheaux David , Ekkad Srinath V.
Published: April 2009

Film cooling performance for a row of cylindrical holes can be. The effect of the slot exit area and shape is investigated. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single test transient IR thermography technique. The study is performed at a single mainstream Reynolds number based on free-stream velocity and film hole diameter of 7150 at three different coolant-to-mainstream blowing ratios of 0.5, 1.0, and 1.5. Two designs with a crescent shaped exit and a slot exit are considered. The results show that the crescent shaped exits provide significantly higher film cooling effectiveness than the cylindrical hole exit at all blowing ratios. The converging slot exit provides similar effectiveness as the crescent for higher blowing ratios. However, the crescent shape also enhances heat transfer coefficients significantly. Overall effectiveness for both crescent and converging slot exits are clearly superior to the standard cylindrical hole.
Three-Dimensional Diffusion of Carbon Monoxide in an Alveolar Capillary
Authors: Merrikh Ali A. , Lage Jose´ L.
Published: April 2009

Theoretical capillary gas exchange models available in the literature are limited to either two-dimensional or axisymmetric configurations. The present study investigates shortcomings of a two-dimensional capillary CO-gas transfer model by comparing it to a more realistic three-dimensional configuration. Numerical results from both models, compared to measurements obtained experimentally via the rebreathing technique, demonstrate that CO-diffusion model results are strongly affected when a third dimension is included. The effect of including a third dimension is to increase the gas exchange from the alveolar region into the RBCs, due to a larger gas diffusion surface within the capillary, leading to a significant increase in the resulting CO diffusing capacity. The results also show a better agreement between the experimental results and the numerical results obtained with the three-dimensional model than with the two-dimensional model. Finally, the strong sensitivity of the results vis-a`-vis the domain lengths (keeping the capillary volume constant) highlight the importance of considering the three-dimensional capillary morphology very carefully.
Numerical Simulation of ‘Piston Effect’ in Supercritical Carbon Dioxide
Authors: Lei Zhiheng , Farouk Bakhtier , Oran Elaine S.
Published: April 2009

Piston effect is an interesting mechanism of heat transfer in supercritical fluids. It is the result of propagation and damping of a thermoacoustic waves in supercritical fluids. The generation and propagation of a thermoacoustic wave in supercritical carbon dioxide is investigated by solving the fully compressible form of the Navier-Stokes equations. Two infinite plates filled with supercritical carbon dioxide, where the right walls is thermally insulated, is considered as the computational domain. A thermally induced pressure wave is generated by heating the left wall. The wave repeatedly traverses the length between the left and right walls, and its amplitude eventually damps out due to the viscous and thermal losses within the fluid, thus heating the bulk fluid. Results are presented in this paper that shows the heating up of the bulk fluid due to piston effect.
Fluid Flow and Heat Transfer in a Composite Trapezoidal Microchannel
Authors: Rahman Muhammad M. , Shevade Shantanu S.
Published: April 2009

The study considered the analysis of heat transfer in a composite channel of trapezoidal cross-section fabricated by etching a silicon <100> wafer and bonding that with a slab of gadolinium. Gadolinium is a magnetic material that exhibits high temperature rise during adiabatic magnetization around its transition temperature of 295K. Heat was generated in the substrate by the application of magnetic field. The conjugate heat transfer scenario where part of generated heat is directly dissipated to the working fluid from gadolinium whereas part is conducted through the silicon structure and reaches the working fluid was studied. Water, ammonia, and FC-77 were studied as the possible working fluids. This kind of heat exchanger is being developed for a micro-scale refrigeration system that works with magnetic heating and cooling principle. Equations governing the conservation of mass, momentum, and energy were solved in the fluid region. In the solid region, heat conduction equation was solved. The volumetric heat generation rate due to magnetic heating was included in the gadolinium portion of the composite channel. A grid independence study was carried out to choose the optimum number of elements to mesh the channel geometry and surrounding structure. A thorough investigation for velocity and temperature distribution was performed by varying channel aspect ratio, Reynolds number, and the magnetic field. The thickness of gadolinium slab, spacing between channels in the heat exchanger, and fluid flow rate were varied. To check the validity of simulation, the results were compared with existing results for single material channels. It was found that the peripheral average heat transfer coefficient and Nusselt number is larger near the entrance and decreases downstream because of the development of the thermal boundary layer. With the increase in Reynolds number, the outlet temperature decreased and the average heat transfer coefficient increased.
Axial Development of Vertical Upward Bubbly Flow in a Minipipe
Authors: Hazuku Tatsuya , Tamura Naohisa , Fukamachi Norihiro , Takamasa Tomoji , Hibiki Takashi , Ishii Mamoru
Published: April 2009

Accurate prediction of the interfacial area concentration is essential to successful development of the interfacial transfer terms in the two-fluid model. Mechanistic modeling of the interfacial area concentration entirely relies on accurate local flow measurements over extensive flow conditions and channel geometries. From this point of view, accurate measurements of flow parameters such as void fraction, interfacial area concentration, gas velocity, bubble Sauter mean diameter, and bubble number density were performed by the image processing method at five axial locations in vertical upward bubbly flows using a 1.02 mm-diameter pipe. The frictional pressure loss was also measured by a differential pressure cell. In the experiment, the superficial liquid velocity and the void fraction ranged from 1.02 m/s to 4.89 m/s and from 0.980% to 24.6%, respectively. The obtained data give near complete information on the time-averaged local hydrodynamic parameters of two-phase flow. These data can be used for the development of reliable constitutive relations which reflect the true transfer mechanisms in two-phase flow. As the first step to understand the flow characteristics in mini-channels, the applicability of the existing drift-flux model, interfacial area correlation, and frictional pressure correlation was examined by the data obtained in the mini-channel.
Convection and Stability in a Magnetic Fluid Saturated Rotating Porous Layer
Authors: Saravanan S. , Yamaguchi H.
Published: April 2009

The influence of magnetic field on the onset of centrifugal convection in a magnetic fluid filled differentially heated porous layer is studied theoretically using linear stability analysis. The resulting eigenvalue problem is solved using the Galerkin technique. The critical centrifugal Rayleigh number, the critical wavenumber and the eigenfunctions corresponding to two-dimensional flow pattern at the threshold are calculated. It is found that the magnetic field has a destabilizing effect and can be suitably adjusted depending on particle magnetization to enhance convection. This phenomenon can be utilized to increase the efficiency of heat transfer devices.
Constructal Theory of the Minimum Requirements for Forced Convection Cooling Solutions
Authors: Walsh E. J. , Grimes R.
Published: April 2009

The advances in microfluidics and microelectronics bring with them the need to provide new cooling solutions for many applications. A number of technologies are under development for forced convection cooling at the microscale. This short paper, through the new constructal theory of Bejan, presents the minimum velocity requirements of any such technology to be truly useful in a new design. Thus the theory presented in this paper, should form the first step in the design process of any new forced cooling technology for mini-micro scale applications. Furthermore, it is demonstrated that the use of the constructal theory provides a heightened level of understanding to the problem of forced convection, while simultaneously deriving the empirical correlations proposed in the literature over the past number of decades.
Acoustic Filtering Procedure in Supercritical Fluids: Application to Thermal Instabilities
Authors: Amiroudine S. , Ambari A. , Boutrouft K.
Published: April 2009

An acoustic filtering procedure [1] has been applied to fluids above their critical point in order to decrease computational costs. The Navier-Stokes equations coupled with the energy and linearized state equations have been considered in order to solve problems related to thermal instabilities in such fluids. In the vicinity of the critical point, we showed that the Schwartchild criterion dominates over the classical one in the Rayleigh-Be´nard configuration. The numerical calculation by finite volume methods considered here, in comparison with the experiments of Meyer & Kogan [2], has given an interpretation of the unexpected oscillations at the convection onset. We have also studied the stability of two layers (hot and cold) of the same supercritical fluid (SCF) in an unstable configuration. The analogy with the stability of two miscible fluids has been clearly identified and stabilisation has been observed by reducing the height of the lower layer in this Rayleigh-Taylor like configuration.
Characterisation of Stability Criteria for Pressure Driven Flows in Small Length Scale Fluidic Devices
Authors: Walsh P. A. , Davies M. R. D. , Dalton T.
Published: April 2009

This paper develops criteria for the prediction of two distinct instabilities in microflows, one isothermal, the other with heat transfer. The engineering objective is to transport droplets that act as micro-reactors and are carried through various processes in a carrier fluid to prepare sample reactants or complete a chemical reaction. The requirement is that the carrier fluid flow be stable so that droplet trajectories can be accurately controlled. The popular two-dimensional microfluidic geometry of three streamlines merging at a junction is chosen for this analysis. A dimensional analysis of the governing flow-field and boundary conditions is undertaken to derive the non-dimensional groups upon which the flow characteristics of the junction are dependent. The emerging parameters are the Grasshof number (Gr) and Reynolds numbers (Re) of both inlet streams. Experimental flow visualisation images are used to determine the relationship between these scaling groups for both isothermal flow and buoyancy opposing mixed convection. The experimental range of inlet Re’s is from 1 to 100. It is found that the ratio of the inlet Re’s is sufficient to describe isothermal flows and that a parameter referred to as W* (the product of the Richardson number (Ri) and Re of the centreline stream) provides a good correlation for buoyancy opposing mixed convection. Inertia dominated flow regimes are seen to exist for W* values below approximately 2 and re-circulation zones are observed when W* is increased above this value. It was also observed that buckling flow was attainable at a critical Re of 65 for isothermal flow and that this critical Re is significantly reduced as W* is increased. An analogy is drawn from the results between the flow studied in this paper and that of cross flow over bluff objects such as a cylinder. Finally, based on the results of this work a design envelope is developed for predicting the stability of scaled models of the fluidic junction.
Numerical Calculations of Heat Conduction Between Soot Aggregates and the Surrounding Gas in the Free-Molecular Regime Using the DSMC Method
Authors: Liu Fengshan , Yang Min , Snelling David R. , Smallwood Gregory J.
Published: April 2009

Numerical calculations were conducted to calculate the heat conduction rate between soot (carbon) aggregates of different sizes and the surrounding gas in the free-molecular regime using the direct simulation Monte Carlo method. This method is based on simulation of the trajectories of individual molecules and calculation of the heat transfer at each of the molecule/molecule collisions and the molecule/particle collisions. Soot aggregates of known fractal dimension and pre-factor are first numerically generated using a cluster-cluster aggregation algorithm. Effect of incomplete thermal accommodation was accounted for by employing the Maxwell gas-surface interaction model. Gas collisions were treated using the simple hard sphere model. Numerical results were obtained for aggregate sizes between 10 and 228 primary particles and the thermal accommodation coefficient between 0.1 and 1. A simple scaling for the heat transfer equivalent sphere diameter was also presented for incorporation into a laser-induced incandescence model.
Silicon Thin Film Thermal Conductivity in Ballistic and Diffusive Regimes Predicted by Molecular Dynamics
Authors: Gomes Carlos J. , Madrid Marcela , Goicochea Javier V. , Amon Cristina H.
Published: April 2009

The thermal conductivity of silicon thin films is predicted in the directions parallel and perpendicular to the film surfaces (in-plane and out-of-plane, respectively) using equilibrium molecular dynamics, the Green-Kubo relationship and the Stillinger-Weber interatomic potential. Film thicknesses range from 2 to 220 nm and temperatures from 300 to 1000 K. In this range of temperatures, the relation between the phonon mean free path (Λ) and the film thickness (ds) spans from the ballistic regime (≫ ds) to the diffusive, bulk-like regime (≪ ds). We show that equilibrium molecular dynamics and the Green-Kubo relationship can be applied to the study of the thermal conductivity of thin films in the ballistic, transitional and diffusive regimes. When the film is thin enough, the thermal conductivity becomes orthotropic and decreases with decreasing film thickness as a consequence of the scattering of phonons with the film boundaries. The in-plane thermal conductivity follows the trend observed experimentally at 300 K. In the ballistic limit, in accordance with the kinetic theory, the predicted out-of-plane thermal conductivity varies linearly with the film thickness and is temperature-independent for temperatures near or above Debye’s temperature.   This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.
Buoyancy-Driven Flow in Saturated Porous Media
Authors: Sakamoto Hitoshi , Kulacki Francis A.
Published: April 2009

Buoyancy-driven convection on a vertical, constant heat flux surface that bounds a fluid-saturated porous medium is experimentally studied with a primary focus on developing steady-state heat transfer correlations for porous media comprising different particulate solid with water being the interstitial fluid. Results show that heat transfer coefficients can be adequately determined via a Darcy-based model, and our results confirm a correlation proposed by Bejan [1]. It is speculated that the reason that the Darcy model works well in the present case is that the porous medium has a lower effective Prandtl number near the wall than in the bulk medium. The factors that contribute to this effects include the thinning of the boundary layer near the wall and an increase of effective thermal conductivity.
The Effect of a Small Patch of Realistic Surface Roughness on the Boundary Layer of an Accelerating Flow Over a Flat Plate Model With Elevated Freestream Turbulence
Authors: McIlroy Hugh M. , Budwig Ralph S. , McEligot Donald M.
Published: April 2009

Results are presented of extensive boundary layer measurements taken over a flat, smooth plate model representing the features of the front one-third of a turbine blade and over the model with a small embedded strip of realistic rough surface. The model included elevated freestream turbulence and an accelerating freestream in order to simulate conditions on the suction side of a rotor blade in the first stage of a high-pressure turbine; however, the study was limited by the relatively small size of the patch of realistic surface roughness. The realistic rough surface was developed by scaling actual turbine blade surface data provided by U. S. Air Force Research Laboratory. The results indicate that bypass transition occurred very early in the flow over the model and that the boundary layer remained unstable (transitional) throughout the entire length of the test plate. Results from the rough patch study indicate the boundary layer thickness and momentum thickness Reynolds numbers increased over the rough patch and the shape factor increased over the rough patch but then decreased downstream of the patch. It was also found that flow downstream of the patch experienced a gradual re-transition to laminar-like behavior but in less time and distance than in the smooth plate case. Additionally, the rough patch caused a significant increase in streamwise turbulence intensity and normal turbulence intensity over the rough patch and downstream of the patch. The skin friction coefficient over the rough patch increased by nearly 2.5 times the smooth plate value. Finally, the rough patch caused the Reynolds shear stresses to increase in the region close the plate surface.
A Survey of Important Auxiliary Equations for Selected Compact Heat Exchanger
Authors: Carranza Richard G.
Published: April 2009

Important auxiliary equations are presented that are typically used in compact heat exchanger research. These relationships are presented only for selected compact heat exchangers — bare pipe, helically finned pipe, plate finned pipe, spined pipe, and plate exchangers. The equations primarily address issues relating to heat exchanger geometry, surface area to volume ratio, and fin efficiency. Furthermore, they are organized in a systematic manner and consolidated in one central location for easy reference.
Thermal Hydraulics of an ADSS With Sharp 180° Turn and Straight Flow Guide Using Finite Element Method
Authors: Prakash K. Arul , Kumar B. V. Rathish , Biswas G.
Published: April 2009

ADSS (Accelerator Driven Sub-critical System) nuclear reactors are expected to play a significant role in nuclear power generation in future due to their ability to utilize Thorium as the nuclear fuel, operating in sub-critical conditions and to transmute radioactive nuclear wastes. Numerical investigation of fluid flow and heat transfer characteristics of an ADSS has been accomplished using a finite element method based on Streamline Upwind Petrov-Galerkin (SUPG) technique. The time-dependent governing equations for conservation of mass, momentum and energy are solved. The simulations have been carried out to predict the heat transfer in the spallation regime. At the first place, laminar regime of the flow is considered for the ADSS geometry. The Reynolds number of interest were varied over a specified range.
Fokker-Planck Approach to Photon-Electron Interactions in Dielectrics During Ultrafast-Pulse Laser Ablation
Authors: Lee Seong Hyuk , Kang Kwan Gu , Lee Joon Sik , Choi Young Ki , Park Seungho
Published: April 2009

This article investigates numerically the electronic transport characteristics in fused silica (SiO2) irradiated by ultra-short pulsed lasers. The non-local type of Fokker-Planck equation which accounts for avalanche ionization, three-body recombination, and multiphoton ionization is used to describe the ultra-short pulsed laser-induced damage phenomena in the energy-position space. It is observed that the recombination plays an important role in determining the ablation depth, and also contributes to reduce substantially the rate of increase in electron number density when the electron density exceeds a certain threshold. With very intense laser irradiation, a strong absorption of laser energy takes place and an initially transparent solid is converted to a metallic state. Full ionization is achieved at intensities above threshold and all further laser energy is deposited within a thin skin depth. The absorption length is on the order of a wavelength at very high laser fluence, and it becomes thinner as laser fluence is larger. It is because the absorbed energy is no longer consumed for multiphoton ionization, but rather leads to the drastic increase in the absorption coefficient because of the Joule heating.
Passive Cooling of Heat Generating Cylinders in Parallel Channels
Authors: Nir G. , Dubovsky V. , Weiss Y. , Aharon J. , Ziskind G. , Letan R.
Published: April 2009

The objective of this paper is to study cooling by induced convection of vertical cylinders in parallel channels. The system has four vertical channels of circular cross-section, connected to shared entrance and exit horizontal ducts. One to three heat-generating cylinders are mounted inside each channel. The cylinder is 50mm in diameter and 100 mm high. It is heated from inside by a cartridge heater, installed in a drill along the cylinder axis and connected to an external power source. The study aims at finding the preferable configuration in which the maximum and mean temperature of the cylinders would be the lowest. Among the factors explored, there are various channel diameters, non-equal number of cylinders in the channels, different power inputs to the cylinders, and different entrance-exit configurations of the ducts. A laboratory-scale model is studied both experimentally and numerically. Temperature measurements are performed at various locations by fine thermocouples using a multi-channel data acquisition unit. Numerical simulations are performed for the velocity and temperature fields in the system, using the Fluent 6.0 software, accounting for both convection and radiation. The cylinders are modeled as they were built in reality: they have a heat-generating core and conducting body. Comparison of the experimental results with the numerical predictions is presented and discussed.
Experimental Investigations Into Mixed Convection About a Horizontal Cylinder: Part B — Fluid Dynamics Using Particle Image Velocimetry
Authors: Egan Vanessa , Dalton Tara , Davies Mark R. D.
Published: April 2009

The second of two papers investigating mixed convection about a cylinder in the buoyancy opposing cross flow regime. The experimental configuration is essentially the same as that used in part A, [1]. Particle image velocimetry (PIV) is used to measure the fluid flow at Reynolds numbers in the range of 32 to 89 and Rayleigh numbers of 1.7E+04 and 2.4E+04. Fluid flow results are shown in the form of instantaneous vector plots with corresponding flow visualisation images, streamline plots and velocity vector plots. The presence of the opposing flow created an unsteady flow field about the cylinder at lower Reynolds numbers while increasing the Reynolds number resulted in the development of a steady flow field tending towards that of a forced convection flow. An increase in Rayleigh number led to an increase the instability of the flow field. The velocity field results were found to compliment the heat transfer investigation detailed in part A, [1].
Analysis of Compact Heat Exchangers as an Intercooler in PEMFC Systems
Authors: Ito Takamasa , Yuan Jinliang , Sunde´n Bengt
Published: April 2009

In Proton Exchange Membrane Fuel Cell (PEMFC) systems, an intercooler contributes to proper thermal management of the reactant air into the cathode. However, the structure/performance of it is not clear yet. In this paper, two case studies concerning an intercooler in a 100 kW PEMFC system are carried out. Plate-fin and tube-fin heat exchangers are analyzed as the intercooler, in terms of volume, pressure drop and weight. The ε-NTU method is used. As general characteristics of the intercooler, the volume is increased with system operating pressure, while the pressure drop is decreased due to the volume expansion. The plate-fin intercooler contributes to the weight reduction of the system because aluminum is used. However, at the high operating pressure, space consumption is large. The tube-fin intercooler contributes to the volume reduction because the coolant is a liquid. However, the usage of stainless steel contributes to weight increase. The tube-fin intercooler in aluminum may contribute to space and weight reduction. However, a liquid coolant, which is proper for the PEMFC stack cooling without corrosion of aluminum, is needed.
Multi-Scale Full-Spectrum k-Distribution Method for Radiative Transfer in Inhomogeneous Gas Mixtures With Wall Emission
Authors: Wang Liangyu , Modest Michael F.
Published: April 2009

The multi-scale full-spectrum k-distribution (MSFSK) method has become a promising method for radiative heat transfer in inhomogeneous media. In this paper an original distribution scheme is proposed to extend the MSFSK’s ability in dealing with boundary wall emission. This scheme pursues the overlap concept of the MSFSK method and requires no changes in the original MSFSK formulation. A boundary emission overlap coefficient is introduced and two approaches of evaluating the coefficient are outlined. The distribution scheme is evaluated and the two approaches are compared by conducting sample calculations for radiative heat transfer in strongly inhomogeneous media using both the MSFSK method and the line-by-line method.
A Review of Heat Transfer in Contemporary Optical Fiber Technology
Authors: Yablon Andrew D.
Published: April 2009

Several recent technological breakthroughs have led to a renaissance of interest in optical fibers, which are now widely used for applications as diverse as telecommunications, medicine, and sensing. Contemporary optical fiber technology is inherently multidisciplinary, inter-relating fields as diverse as glass science, mechanical engineering, and optics. This paper reviews several aspects of silica optical fiber technology in which thermal transport plays a critical role. Future research directions are discussed.
Molecular Dynamics Simulation Study of Convective Heat Transport in a Nanoscale Channel
Authors: Han Minsub , Lee Joon Sik , Park Seungho
Published: April 2009

We study the convective heat transport phenomenon of liquid in a nanoscale straight channel by performing the non-equilibrium molecular dynamics simulation (NEMD). Fundamental heat transfer phenomenon distinctive in a nanoscale is reported by Han and Lee [Phys. Rev. E 70, 061205 (2004)]. It is the significant heat transfer in the upstream direction even in the absence of temperature gradient in the direction. A planar Poiseuille flow is considered in the simulation, where liquid argon in a straight channel of Pt atoms is driven by a gravity-like body force. The intermolecular force and plane peculiar velocity mainly induces the heat transfer, which becomes significant when the velocity gradient is sizable in the range of the intermolecular force. While the simulation results agree well with the prediction by Navier-Stokes (NS) equation and Fourier’s law, the heat transfer remains of a significant amount. The heat transfer depends on the velocity gradient in a nonlinear fashion, which can not be accounted for the third order generalized Fourier’s law in the literature.
Experimental Study on Turbulent Flow and Thermal Mixing in T-Junction Modeled After Automobile HVAC Unit
Authors: Asano Hideo , Hirota Masafumi , Nakayama Hiroshi , Asano Taichi , Hirayama Shunsaku
Published: April 2009

This paper reports the experimental results on turbulent mixing of hot and cold airflows in a T-junction, which simulates the HVAC unit used in an automobile air conditioning system. The flow entering the main channel from the branch is separated at the edge of the T-junction and forms a large separation bubble. Longitudinal vortices are formed around this separation bubble. In spite of such a complex flow field, the mean temperature of the mixed flow shows quite uniform distributions in the spanwise direction of the channel. It has been also found that the strong turbulence produced around the separation bubble does not contribute effectively to the thermal mixing of hot and cold airflows.
Modeling of Steam Explosion in Partially Flooded PWR Reactor Cavity
Authors: Koncˇar Bosˇtjan , Leskovar Matjazˇ , Cizelj Leon
Published: April 2009

When the hot molten core comes into contact with the water in the reactor cavity a steam explosion can occur. The steam explosion might be triggered during some scenarios of severe nuclear reactor accidents, when extremely hot molten nuclear fuel interacts with the coolant water. A highly energetic steam explosion in a nuclear power plant could cause the containment failure and the release of radioactive fission products to the environment. The purpose of the performed analysis is to provide a first estimation of the expected pressure loadings on the typical PWR cavity structures during a steam explosion. To achieve this, the fit-for-purpose steam explosion model is proposed, followed by a Computational Fluid Dynamics (CFD) analysis. In the present work two steam explosion scenarios in the partially flooded Pressurized Water Reactor (PWR) cavity were simulated with the general purpose code CFX-5 [1] to estimate pressure loadings on cavity walls.
Drag Force and Heat Transfer Corrections for a Confined Circular Cylinder in Uniform and Poiseuille Flow
Authors: Champmartin Ste´phane , Ambari Abdelhak
Published: April 2009

The modifications to the drag force and to the heat transfer on an infinite circular cylinder induced by the presence of two bounding walls are numerically investigated in the case where the cylinder is located midway between two parallel planes in uniform motion and in Poiseuille flow. At very low Reynolds and Pe´clet numbers, we show that the solutions are of the “Stokes type” and we give new expressions of the drag force and of the heat flux corrections, valid for a very wide range of confinements extending from a weak interaction to the lubrication regime.
Stability of Smoke Layer Under Sprinkler Water Spray
Authors: Zhang C. F. , Huo R. , Li Y. Z. , Chow W. K.
Published: April 2009

Stability of smoke layer induced by a fire will be studied in this paper. A series of full-scale burning tests were carried out to study the interaction of the hot smoke layer with a sprinkler water spray. A new rig was built to give a smoke layer. The results show that the stability of a smoke layer will be disturbed by the discharged water spray. The operating pressure of the sprinkler system is a key point. When the smoke layer lost its stability, the dragging force acting on the smoke layer was found to be less than the buoyancy.
Adaptive High Precision Temperature Control of a Thermo-Mechanical Test Bench for Turbine Blades
Authors: Naderhirn Michael , del Re Luigi
Published: April 2009

Material testing is an essential element in the technological development of airplane turbines. The blades are subject to very high stresses, both thermal and mechanical. To assess material suitability, thermo-mechanical test benches are used where a typical thermo-mechanical load is applied to the test rigs. The sensitivity of the fatigue assessment is very high which requires a very high control precision over a large temperature range and a fast adaptation to different setups. This paper discusses two approaches where the first is a self tuning inverse control loop. The second approach is a data based control method where identification routines are used to dynamically compute the approximated system inverse and use this information to automatically tune the control system for saw-tooth profiles. It is shown that a simple two-zone thermal model describes the behavior of the probe and is then used to derive the inverse model and a control strategy. Finally experimental results of a typical test cycle are presented.
Experiments and Analysis of Interaction Among Multiple Fires in Equidistant Fire Arrays
Authors: Satoh Kohyu , Liu Naian , Zhu Ji Ping , Yang K. T.
Published: April 2009

The study of dynamics of multiple fires is important to gain a physical insight of the conditions under which destructive phenomena could result in city fires such those caused by earthquakes. Particularly, heavy populated cities such as Tokyo are highly vulnerable. Unfortunately, previous studies on multiple fires and their dynamics are rather limited. An extensive experimental study has been carried out to examine the fire interactions among freestanding equidistant multiple fires in square arrays, to supplement the authors’ previous related studies. Four square arrays, namely, 5×5, 9×9, 15×15 and 17×17, with various inter-fuel pan distances were treated. The burnout time (BOT) from ignition at every fire in the array was experimentally recorded and expressed as multiples of the BOT of a single free-standing fire as a reference. Since the BOT at any fire location in an array is inversely proportional to an average burning rate (BR) at that location, the local BR can then be directly inferred, and their comparisons thus indications of the physical interactions as affected by the fire location, inter-fuel pan distance and size of the fire array. It is shown that all these parameters play remarkable roles in the interactions among multiple fires in square fire arrays.
A Three-Dimensional Numerical Modelling of Atmospheric Pool Boiling by the Coupled Map Lattice Method
Authors: Gupta A. , Ghoshdastidar P. S.
Published: April 2009

In the present paper, the characteristic atmospheric pool boiling curve is qualitatively reproduced for water on a temperature controlled thin copper strip having comparable length and breadth by the coupled map lattice (CML) method using a three-dimensional boiling field model. The basic objective of the work is to improve the prediction of the critical heat flux (CHF) with respect to the 2D CML model of Ghoshdastidar et al. [10]. The work models saturated pool boiling of water at 1 bar on a large (much larger than the minimum wavelength of 2D Taylor waves) and thin horizontal copper strip. The pool height is 0.7 mm, indicating thin film boiling. In the present model, it is assumed that boiling is governed by (a) nucleation from cavities on a heated surface, (b) thermal diffusion, (c) bubble rising motion and associated convection, (d) phase change and (e) Taylor instability. The changes with respect to 2D model are primarily with respect to 3D modelling of thermal diffusion and 2D distribution of nucleating cavity sizes. The predicted CHF is 1.57 MW/m2 as compared to the actual value of 1.3 MW/m2 and 0.36 MW/m2 predicted by the 2D CML model of Ghoshdastidar et al. [10]. It can be said that for the first time a coupled map lattice method which is essentially qualitative in nature has been able to predict the CHF of saturated pool boiling of water at 1 bar very close to the actual value.
Strategy for Innovation in Heat Exchanger Design: Computational Approach Combined With Experimental Tests Leads to Performance Improvement
Authors: Colombo Emanuela , Macchi Ennio , Merlo Umberto
Published: April 2009

The paper describes the novel design strategy adopted by LU-VE Contardo (a major European manufacturer of air condenser, evaporators and liquid coolers) for the selection of the finned coil geometry, in both the development and the innovation processes of new products. The search of the optimal shape of the louvered fins used in modern, state-of-the-art heat exchangers requires an in-depth analysis of the flow regime crossing the fins and of the interactions between the velocity and the temperature field, nowadays possible by using Computational Fluid Dynamics. The numerical approach, when properly managed and integrated with the experimental activity for validating the results, provides an essential support for a rationale selection of the de-sign features of advanced heat exchangers. The presented results demonstrate how the adopted design strategy was successful in achieving a substantial enhancement of the heat transfer characteristics of the fins. Indeed, as demonstrated by the experimental campaign, when these novel, optimized fin shapes are applied to the design of the new series of industrial evaporators, they yield considerable results in terms of increase of the evaporator’s capacity.
Prandtl Number Dependent Natural Convection in a Rectangular Pool With Internal Heat Sources
Authors: Lee Seung D. , Son Hyoung M. , Suh Kune Y. , Rempe Joy L. , Cheung F. Bill , Kim Sang B.
Published: April 2009

Natural convection plays an important role in determining the thermal load from debris accumulated in the reactor vessel lower head during a severe accident in nuclear power plants. The natural convection heat transfer involving internal heat generation is generally represented by the modified Rayleigh number, Ra’, which quantifies the internal heat source and hence the strength of the buoyancy force. The SIGMA RP (Simulant Internal Gravitated Material Apparatus Rectangular Pool) tests are concerned with high Ra’ turbulent natural convection in a rectangular pool. The internal heating was realized by cable-type heaters in the test to simulate uniform heat generation. Ra’ was found to be in the range of 1×109–1×1012. Experiments were conducted using air with the Prandtl number, Pr, of about 0.7 as the working fluid. The test results confirmed feasibility of the direct heating method to simulate uniform volumetric heat generation in the pool.
Three-Dimensional Analysis on the Performance of the Thermoelectric Micro-Cooler
Authors: Lee Kong Hoon , Kim Ook Joong
Published: April 2009

Three-dimensional numerical analysis has been carried out using the FEMLAB software package to figure out the performance of the thermoelectric micro-cooler. A small-size and column-type thermoelectric cooler is considered and Bi2Te3 and Sb2Te3 are selected as the n- and p-type thermoelectric materials, respectively. The thickness of the thermoelectric element considered is 5 to 20 μm and the thickness affects the performance of the cooler. The effect of parameters such as the temperature difference, the current, and the thickness of the thermoelectric element on the performance of the cooler has also been investigated. The coefficient of performance (COP) is the primary factor to evaluate the performance of the cooler and the COP varies with the parameters. The COP has the maximum value at a certain current and the value decreases with the temperature difference or the thickness. The predicted results also show that the performance can be improved for thick thermoelectric element at the small temperature difference and small current.