bluuebunny/crossref_metadata_2025 huggingface dataset:
Collapse Prediction During Hollow Optical Fiber Fabrication
Authors: Chakravarthy Srinath S. , Chiu Wilson K. S.
Published: April 2009

A numerical study of the drawing of hollow capillary rods over a range of draw speeds and feed speeds, with application to the drawing of photonic crystal fibers is presented. In this study the axisymmetric Navier-Stokes equations are solved numerically using the finite element method. In the numerical study inertia, gravity, surface tension and internal pressurization of the capillary is included and the effect of each of these parameters on the neck down profile is presented. The free surface which defines the neck down profile, is not assumed but is determined using a surface force balance. The results of isothermal and non-isothermal analysis results are compared with experimental, numerical and analytical results from the literature for validation. In the non-isothermal analysis, arbitrary temperature profiles are assumed to represent the temperature distribution in the neck down region and the effect of different temperature distributions on the neck down profile is discussed. The effect of the draw parameters (draw speed and draw down ratio) and the temperature distribution on the collapse of the capillaries is examined. A qualitative discussion on the onset of collapse of the capillary is presented and the effect of the draw parameters on the draw tension is discussed. For isothermal cases, the feed speed dominated the determination of collapse, while the draw speed had a relatively small effect on the collapse. It was found that collapse can be avoided with higher feed speeds and lower temperatures, while the draw tension can be minimized at lower feed speeds and higher temperatures.
The Effect of Reynolds Number on the Aerodynamic Performance of Micro Radial Flow Fans
Authors: Hanly K. , Grimes R. , Walsh E. , Rodgers B. , Punch J.
Published: April 2009

Elevated heat dissipation and simultaneous reductions in package sizes are well documented for a range of electronics systems. The problem is heightened in portable systems where the space available for the implementation of an active cooling methodology is limited and conventional cooling products are too large. Using micro scale radial flow fans is a potential solution. However, little is known about the aerodynamic effects of reducing the fan scale and therefore Reynolds number to the extent required for typical portable electronic applications. This paper investigates this issue, by quantifying the reduction in aerodynamic performance which accompanies the reductions in scale. To do this, geometrically similar radial flow fans were fabricated with diameters ranging from 80 to 10mm. Measurements of the rotors’ geometries are presented, showing a high degree of geometric similarity between the fans. The aerodynamic performance of each of the fans was measured. Non-dimensional performance of each of the larger fans were almost identical, while the performance plot of the smallest fan differed significantly from the others. The paper tentatively concludes that a fundamental change in flow phenomena has emerged in the smallest scale fan which has altered its aerodynamic characteristics.
Coupling a Discrete Transfer Method to a View Factors Technique for Radiative Heat Transfer Modeling in Industrial Furnaces
Authors: Joumani Youssef , Mougin Guillaume , Ammouri Fouad , Till Marc
Published: April 2009

Air Liquide has been involved in the design of industrial furnaces (glass melting, reheating, aluminum, …) for several years. Thanks to that experience, known-how and expertise in modeling such applications have been developed. Dedicated simulation tools — 0D for global heat and mass balance, 1D for the prediction of longitudinal temperature profiles and 3D for detailed analysis — have been built. Each of them is very helpful when used relevantly and offers numerous opportunities at each step of the design of a furnace. In such kind of applications, the temperature levels are very high (up to 2500 K). As a consequence it is very crucial to simulate the radiative heat transfer as accurately as possible. This requires the use of a radiation model that can take into account complex geometries, non-isothermal media and various gas mixture compositions. Very often, three-dimensional simulations are necessary and reduction to smaller dimension problems is difficult or inadequate. The present paper introduces a new radiation model for computing two-dimensionally radiative heat transfer in an industrial furnace with a piecewise distributed load. To reduce the three-dimensional problem to two dimensions, the method consists in coupling the 2D radiation transport equation to a boundary condition based on view factors through an imaginary plane to homogenize the radiative behavior of the load surface. A solution procedure using the discrete transfer method associated to a weighted-sum-of-gray-gases database to deal with absorption and emission of a CO2-H2O mixture is proposed. Simulation results are finally compared to an analytical formula and then to a full-3D approach taking into account participating media, non-isothermal and gray walls. All tests show that this model can be used to simulate industrial configurations with a good accuracy.
In-Situ Diagnostics of Combustion Parameters Using Laser-Induced Breakdown Spectroscopy
Authors: Lee T.-W. , Hegde N. , Han I.
Published: April 2009

Laser-Induced breakdown spectroscopy is a powerful diagnostic method that can be applied to combustion systems for measurements of fuel-air ratios, fuel composition, and temperature. In this study, we specifically show the application of LIBS for in-situ flame temperature measurements at conditions up to moderately sooting conditions (equivalence ratio of 15 for methane-air flames). Comparisons with existing data obtained from thermocouple exhibit excellent agreement in weakly sooting flames and good agreement at higher equivalence ratios. The deviation at higher equivalence ratios is attributed to signal trapping due to soot and precursor species and also due to increase in hydrogen spectral signal at these conditions. These are factors that can be circumvented and do not preclude application of LIBS to highly sooting flames.
Soret, Dufour and Heat by Chemical Reaction Effects in a Chemical Vapor Deposition Optical Fiber Coating Process
Authors: Huang Wei , Chiu Wilson K. S.
Published: April 2009

In this paper, we studied the effect of thermal diffusion (Soret effect), the heat flux due to species concentration gradient (Dufour effect) and the heat by chemical reactions in a chemical vapor deposition (CVD) process used to hermetically coat optical fibers. Using a previously developed mass and heat transfer model to investigate the transport phenomena in this process, the Soret and Dufour effects are compared to ordinary mass diffusion. The thermal conductivity, molecular diffusivity and thermal diffusivity are calculated using a multi-component model. The contribution of heat of chemical reactions to overall heat transfer in the CVD is also discussed. Soret effect and heat by chemical reactions are found to be very important in this process, and their effect is related to operating conditions such as draw speed and optical fiber inlet temperature.
A Low Profile Thermal Management Device for High Power Processors Using Enhanced Flow Boiling Techniques and Perfluorocarbon Fluids
Authors: Schwarzkopf John D. , Tilton Charles L. , Crowe Clayton T. , Li Ben Q.
Published: April 2009

It is advantageous to the electronic industry to remove in excess of 150W from a processor die with a case-to-ambient thermal resistance < 0.2 ° C/W. Although this is achievable with air cooled thermal solutions, the form factor is unwieldy and as the ambient temperature rises, the processor throttles the performance. Alternatively, thermal solutions containing water are trivial to implement but potential leaks can pose a threat to the electronic suite. The present solution is non-trivial; it utilizes a dielectric perfluorocarbon fluid (PF5050) and a combination of spray cooling and mini-channel flow boiling techniques with a particular focus on superficial quality at the onset of the channels. The idea behind mixing the two techniques is to augment the heat transfer coefficient by driving the fluid into an annular state upon entry of the mini-channels while retaining a low profile package. A macro-channel boiling heat transfer correlation was used and results show that it predicts the average surface temperature to within 10% of the experimental data. Quasi-one-dimensional numerical models were developed to predict vapor entrainment and two-phase pressure drop. Experimental results show that in excess of 250W can be removed from a 1.24cm × 1.24cm die without removing the integrated heat sink (IHS), with a case-to-ambient thermal resistance of <0.2° C/W at 150W. The packaged size is 5.6cm long by 3.1cm wide by 1.6cm tall, thus allowing the end user to adapt the technology into blade style or 1U style servers with low risk.
Simulation of Heat Transfer With Uncertainty
Authors: Wan Xiaoliang , Karniadakis George Em
Published: April 2009

In this work we model uncertainty propagation in heat transfer via the multi-element generalized polynomial chaos (ME-gPC) method by focusing on the stochastic heat-transfer enhancement in a two-dimensional grooved channel. Based on the global energy balance, a similar technique as in [1] is employed to develop proper numerical schemes for the fully developed flow. The velocity and temperature field are expressed by the ME-gPC. Using a Galerkin projection, we reduce the stochastic PDE system to a high-dimensional deterministic one, which is solved by the spectral/hp element method. A time, spaced-averaged Nusselt number with error bars is obtained at different excitation frequencies. The probability density function (PDF) of the Nusselt number is also presented.
Three Dimensional Numerical Modelling of Staggered Tube Bundle Turbulent Crossflow in Duct
Authors: Ramezanpour Ahad , Shirvani Hassan , Rahmani Ramin , Mirzaee Iraj
Published: April 2009

A numerical study has been conducted to investigate the three dimensional (3D) staggered tube bundle turbulent cross flow confined between two parallel flat plates using RNG k-ε model and standard wall function utilizing commercial code FLUENT. The maximum Reynolds numbers of 1000, 5000, and 50000 and the distance between plates of H = 3, 5, 10, 15, and 20 mm have been considered. The arrangement of the staggered tube bundle is fix with geometrical characteristics of Sn/D = 1.5 and Sp/D = 1.2 which has been found optimum in previous two-dimensional studies. The constant temperature of 360K on tubes, constant inlet flow and plates’ temperature of 300K have been set as the boundary conditions. The global Nusselt number, friction factor for the dissimilar Reynolds numbers, distance between plates, local Nusselt number and different angles on first and third tubes have been evaluated.
Validation of 3-D Electromagnetic-Thermal Model for Microwave Food Processing
Authors: Lee H. K. , Li B. Q. , Huo Y. , Tang J.
Published: April 2009

A numerical model was developed to study the electromagnetic field and thermal phenomena during microwave heating food processing. Three-dimensional finite difference time domain and finite element integration is presented for the analysis of the electromagnetic field distribution and temperature distribution. Experimental measurements validated 3-D model of microwave food processing for the electromagnetic and thermal phenomena. The extensive numerical simulations were used to study effect of various processing parameters on the heating patterns in food package in a 915 MHz microwave applicator. Computed results are presented for the electric field distribution, power absorption and temperature profile for the food sterilization processing in Microwave-Circulated Water Combination (MCWC) heating system.
The Discontinuous Finite Element Method for Polycrystalline Grain Growth With Convection
Authors: Ai X. , Shu Y. , Li Ben Q.
Published: April 2009

In this paper, a numerical study of the convection effect on polycrystalline grain growth is performed. The coupled two-dimensional polycrystalline phase field model, energy equation and Navier-Stokes equations are solved, which is based on the discontinuous Galerkin finite element method. The numerical algorithm is validated, and the effect of the external convection flow is examined for growth of grains with different orientation.
Simulation of Steady State Surface Temperature Response of a Novel Micro Loop Heat Pipe to Various Heat Loads
Authors: Ghajar M. , Darabi J.
Published: April 2009

Numerical investigations have been performed to simulate a novel Micro Loop Heat Pipe (MLHP) under steady state conditions. For most electronics, the maximum working temperature is an important design factor; therefore an accurate estimation of this temperature is crucial. The model predicts the steady state temperature distribution at the surface of the heat source as a function of applied heat loads. This code builds upon a previous code developed by the authors [1], and utilizes a hybridizing of an Alternating Direction Implicit (ADI) Computational Fluid Dynamics (CFD) code and relevant thermodynamic equations. Using this simulation tool, the minimum required compensation chamber cavity has been calculated and checked for various operating temperature ranges. Additionally, the design of the MLHP has been improved by evaluating the effects of the geometric feature variations. Considering the fabrication limitations, some of the optimized geometry dimensions were found to be a groove wall thickness of 2um, a groove width of 7um, a wicking structure length of 500μm, and a vapor line width of 2mm.
Comparison of Scattering Rates and Thermal Conductivity in Diamond Using Dispersion Curve Data
Authors: Kalisik Todd , Majumdar Pradip , Shafer John
Published: April 2009

The understanding of the mechanism of thermal energy transfer in thin films ranging in thicknesses from micro-scale to nano-scale is becoming very important. Thin films must be modeled at the atomic level and this entails treating the heat transfer as vibrations in a crystal lattice. The concept of phonons can be used to model the vibrational energy of the crystal. Phonon scattering rates and thermal conductivity are investigated for Cubic C (diamond). Boundary scattering, Umklapp processes, and Normal processes are the mechanisms considered for heat flow resistance. The normal processes are included due to there indirect effect on resistance (through phonon redistribution). Three symmetry directions [001], [110], [111], and three polarizations for each direction in the first Brillouin zone are considered. The main purpose of the paper is to study the effect of the curvature of the phonon dispersion curves when computing the phonon scattering rates and thermal conductivity. A comparison of thermal conductivity for each polarization and symmetry direction is made between a continuum model, a linear curve fit and a polynomial curve fit of dispersion data. A comparison is also made between the scattering rates for each polarization, symmetry direction as well as the group velocity for each.
Heat Transfer With Thermoacoustic Self-Circulating Heat Exchangers
Authors: Backhaus S. , Swift G. W.
Published: April 2009

An asymmetrical constriction in a pipe functions as an imperfect gas diode for acoustic oscillations of gas in the pipe. One or more gas diodes in a loop of pipe create substantial steady flow, which can carry substantial heat between a remote heat exchanger and a thermoacoustic or Stirling engine or refrigerator; the loop’s flow is driven directly by the oscillations in the engine or refrigerator itself. This invention gives Stirling and thermoacoustic devices unprecedented flexibility, and may lead to Stirling engines of unprecedented power. We have built two of these self-circulating heat exchangers, one for fundamental tests and the other as a demonstration of practical levels of heat transfer. Measurements of flow and heat transfer are in factor-of-two agreement with either of two simple calculation methods. One calculation method treats the oscillating and steady flows as independent and simply superimposed, except in the gas diodes. The other method accounts for the interaction between the oscillating and steady flow with the quasi-steady approximation. The mutual influence of superimposed turbulent oscillating and steady flows is a theoretical challenge.
Thermal and Electrochemical Three Dimensional CFD Model of a Planar Solid Oxide Electrolysis Cell
Authors: Hawkes Grant , O’Brien Jim , Stoots Carl , Herring Steve , Shahnam Mehrdad
Published: April 2009

A three-dimensional computational fluid dynamics (CFD) model has been created to model high-temperature steam electrolysis in a planar solid oxide electrolysis cell (SOEC). The model represents a single cell, as it would exist in an electrolysis stack. Details of the model geometry are specific to a stack that was fabricated by Ceramatec, Inc. and tested at the Idaho National Laboratory. Mass, momentum, energy, and species conservation and transport are provided via the core features of the commercial CFD code FLUENT. A solid-oxide fuel cell (SOFC) model adds the electrochemical reactions and loss mechanisms and computation of the electric field throughout the cell. The FLUENT SOFC user-defined subroutine was modified for this work to allow for operation in the SOEC mode. Model results provide detailed profiles of temperature, Nernst potential, operating potential, anode-side gas composition, cathode-side gas composition, current density and hydrogen production over a range of stack operating conditions. Mean model results are shown to compare favorably with experimental results obtained from an actual ten-cell stack tested at INL.
CFD Thermal Analysis and Optimization of Motor Cooling Fin Design
Authors: Chen Ya-Chi , Chen Bing-Chung , Chen Chung-Lung , Dong Jimmy Q.
Published: April 2009

This study is focused on improving cooling performance of the housing fin for Total Enclosed Fan Cooled (TEFC) motors. We conducted a sensitivity study on the motor housing fin to determine key design parameters and developed an optimization procedure. The goal is to use the optimizer to achieve an efficient design process for optimal fin design under specified operating conditions. Response Surface Methodology (RSM) was constructed out of the numerical data with multi-quadratics (MQ) as basis functions to predict the response. The RSM, in conjunction with generic optimization methods, was used to find the optimal fin design in the parametric design space. The parameter database was non-dimensionalized so that the optimizer can be applied to various motor frame sizes. Compared with the original fin design, in some cases the optimal fin configuration reduces thermal resistance to heat convection from the fin surface by more than 50%.
Computer Modeling of Liquid Droplet Impact on Heat Transfer During Spray Cooling
Authors: Selvam R. Panneer , Bhaskara Sandya , Balda Juan C. , Barlow Fred , Elshabini Aicha
Published: April 2009

Spray cooling is a high flux heat removal technique considered for systems dissipating high power within small areas such as advanced lasers. Recently Selvam and Ponnappan (2004 & 2005) identified the importance of modeling heat transfer in a thin liquid film on a hot surface at the micro level and illustrated how this micro level modeling could help to improve the macro level spray cooling. The goal of this research is to advance the theoretical understanding of spray cooling to enable efficient system level hardware designs. Two-phase flow modeling is done using the level set method to identify the interface of vapor and liquid. The modifications made to the incompressible Navier-Stokes equations to consider surface tension and phase change are presented. The equations are solved using the finite difference method. The effect of liquid droplet impact on a 40 μm thick liquid film containing vapor bubble and the consequent heat removal is explained with a sequence of temperature vs. time contours. From that, the importance of fast transient conduction in the liquid film leading to high heat flux in a short time is illustrated. The optimum positioning of the droplet with respect to the vapor bubble for effective heat removal is also systematically investigated. This information is expected to help in proper positioning of the droplet in three-dimensional modeling.
Heat Transfer Measurements in a First Stage Nozzle Cascade Having Endwall Contouring, Leakage and Assembly Features
Authors: Piggush J. D. , Simon T. W.
Published: April 2009

This work supports new gas turbine designs for improved performance by evaluating sealing flow effects in a cascade representative of a contoured first stage stator passage. Contouring accelerates the flow, reducing the thickness of the endwall inlet boundary layer to the turbine stage and reducing the strength of secondary flows within the passage. Injected flows, used to seal gaps and cool surfaces, may affect endwall boundary layers, increase secondary flows and possibly create additional vortex structures in the passage. The present paper documents injected flow effects on the endwall heat transfer within a passage with one contoured and one straight endwall. The paper discusses heat transfer distributions measured with different leakage flow rates. In particular, leakage is from the gap between the combustor and turbine sections and from the gap at the assembly joint on the vane platform between two vanes.
Numerical Study of the Effective Thermal Conductivity of Nanofluids
Authors: Shukla Ratnesh K. , Dhir Vijay K.
Published: April 2009

Nanofluids, that is liquids containing nanometer sized metallic or non-metallic solid nanoparticles show an increase in thermal conductivity compared to that of the base liquid. In this paper we present numerical results obtained from Molecular Dynamics Simulations of a solid-liquid system comprising of Lennard-Jones atoms used to study the liquid layering on solid nanoparticles. It is found that close to the solid surface the liquid atoms form ordered layers which display higher thermal conductivity compared to the bulk liquid. We also present a model for thermal conductivity of nanofluids based on the theory of Brownian motion of a free particle and show that the thermal conductivity of the nanofluid predicted from the model agrees qualitatively with the experimental observations.
Upper Bounds to Carbon Nanotube Thermal Conduction and Entropy Flow
Authors: Mingo N. , Broido D. A.
Published: April 2009

Quantum upper bounds to thermal conductance are computed for zigzag single walled carbon nanotubes of different diameters. Upper bounds to energy and entropy flow are also computed. The results impose a stringent limit on the maximum values that any experiment or simulation could obtain. Some previous theoretical simulations have violated the theoretical maxima.
Effects of a Wire Mesh on Droplet Size and Velocity Distributions of Cryogenic Sprays
Authors: Franco Walfre , Vu Henry , Aguilar Guillermo
Published: April 2009

Cryogen spray cooling allows removing large amounts of heat in short periods of time. Previous investigations have shown that stainless steel wire meshes placed between spray nozzle and target surfaces effectively increase uniformity in lateral heat extraction while reducing cooling efficiency. In this study we used phase Doppler particle analysis to assess the effect of a wire mesh on the diameter and velocity radial distributions of cryogen droplets and, consequently, on the surface heat transfer. We measured at 40 mm downstream and various radial locations within steady state spray cones from two different nozzles without and with a mesh at two mesh-nozzle distances. Results show that with the mesh, droplets lose velocity and increase in size. The closer to the center of the spray cone the droplets are, the larger the loss in velocity. As a consequence, the radial velocity profile is more uniform relative to that without a mesh.
The Effect of Roughness on the Impact Dynamics and Heat Transfer of Cryogen Droplets Impinging Onto Indented Skin Phantoms
Authors: Liu Jie , Franco Walfre , Aguilar Guillermo
Published: April 2009

Laser dermatological surgery (LDS) is the preferred therapeutic modality for various dermatoses, including port wine stain (PWS) birthmarks. LDS is commonly used in conjunction with cryogen spray cooling, which is an auxiliary procedure that pre-cools the superficial skin layer (epidermis) prior to laser irradiation to avoid non-specific and excessive epidermal heating. Clinical observations show that skin indents markedly during spray deposition due to the large momentum of cryogen droplets. Furthermore, the human skin surface is far from smooth. Therefore, with the objective to provide some insight into the interaction between cryogen sprays and the rough and deformable human skin surface, the impingement dynamics and heat transfer induced by single cryogen droplets falling on rough and indented skin phantoms are present in this paper. Epoxy skin phantoms with a constant semispherical indentation of depth and radius of 2.44 mm and 6.34 mm, respectively, were used to simulate indented skin. Each phantom had a different surface roughnesses varying from 0.5 μm to 50μm. The experiments were carried out within a pressurized chamber to control or eliminate droplet evaporation. A high-speed camera and the temperature sensors placed on the upper surface of the skin phantoms were synchronized to record the impact dynamics and temperature changes as cryogen droplets fell on them. The results show that the surface roughness affects the impact dynamics and heat transfer during single droplet impingement. As the surface roughness (Ra) increasing, the heat flux decrease.
Study of Thermal Energy Transport Between Hydrogen Gas Molecules and a Single-Wall Carbon Nanotube Using Molecular Dynamics Simulations
Authors: Solomon Jose´ E. , Kapat Jay , Kumar Ranganathan , Srivastava Deepak
Published: April 2009

The focus of the current research is the investigation and characterization of the energy transport between a (10,10) single-wall carbon nanotube (SWCNT) and surrounding molecular hydrogen gas using molecular dynamics (MD) simulations. The MD simulations use Tersoff-Brenner hydrocarbon potential for C-C, C-H, and H-H bonding interactions and the conventional Lennard-Jones potential for soft-sphere gas-CNT collision dynamics of H-H and H-C non-bonding van der Waals interactions. A simulation cell with periodic boundary conditions is created for 1200 carbon atoms in an armchair nanotube configuration and three distinct gas densities corresponding to 252, 500, and 1000 H2 molecules in the same volume. The MD simulation runs are performed with time steps of 0.1 fs and the total simulation times of 40 ps. The simulations are initialized by setting the gas species and CNT at two different temperatures. Initial gas temperatures range from 2000K to 4000K, whereas the carbon nanotube is held at 300K. After the equilibrium temperatures of the CNT and the gas molecules are achieved, the external constraints to maintain the temperature are removed and the thermal energy transport between the two is studied. The kinetic energy exchange between the nanotube and the surrounding gas is monitored to study thermal energy transport over the duration of the simulation. A parameter is proposed, the coefficient of thermal energy transfer (CTET), to characterize the thermal transport properties of the modeled systems based on parameters governing the transport process, thus mimicking the conventional heat transfer coefficient. Values for CTET vary directly with gas density and range from 50 MW/m2K to 250MW/m2K, showing that gas density has a significant impact with higher density corresponding to higher collision rates and higher rates of energy transfer. In contrast, the gas temperature has a lower impact on CTET, with colder gas providing in certain cases a slightly lower value for the coefficient. In order to validate the MD simulations, the time-series data of molecular vibrations of the CNT is converted to a vibrational frequency spectrum through FFT. The characteristic frequencies obtained on the spectra of isolated SWCNT and H2 simulations are compared against the known natural frequencies of the CNT phonon modes and vibrational modes of H2 molecules. The comparison is quite favorable.
Study on the Heat Transfer in the Water Pool Type Reactor Cavity Cooling System of the Very High Temperature Gas Cooled Reactor
Authors: Cho H. K. , Seo D. U. , Kim M. O. , Park G. C.
Published: April 2009

In the HTGR (High Temperature Gas Cooled Reactor), the Reactor Cavity Cooling System (RCCS) is equipped to remove the heat transferred from the reactor vessel to the structure of the containment. The function of the RCCS is to dissipate the heat from the reactor cavity during normal operation including shutdown. The system also removes the decay heat during the loss of forced convection (LOFC) accident. A new concept of the water pool type RCCS was proposed at Seoul National University. The system mainly consists of two parts, water pool located between the containment and reactor vessel and five trains of air cooling system installed in the water pool. In normal operations, the heat loss from the reactor vessel is transferred into the water pool via cavity and it is removed by the forced convection of air flowing through the cooling pipes. During the LOFC accident, the after heat is passively removed by the water tank without the forced convection of air and the RCCS water pool is designed to provide sufficient passive cooling capacity of the after heat removal for three days. In the present study, experiments and numerical calculations using CFX5.7 for the water pool and cooling pipe were performed to investigate the heat transfer characteristics and evaluate the heat transfer coefficient model of the MARS-GCR (Multi-dimensional Analysis of Reactor Safety for Gas Cooled Reactor Analysis) which was developed for the safety analysis of the gas cooled reactor. From the results of the experiments and CFX calculations, heat transfer coefficients inside the cooling pipe were calculated and those were used for the assessment for the heat transfer coefficient model of the MARS-GCR.
Polymeric Hollow Fiber Heat Exchangers (PHFHEs): A New Type of Compact Heat Exchanger for Lower Temperature Applications
Authors: Zarkadas Dimitrios M. , Li Baoan , Sirkar Kamalesh K.
Published: April 2009

Plastic heat exchangers are characterized by an inferior thermal performance compared to their metal counterparts. Therefore, their usage is mainly limited to handling corrosive media or when ultra high purity is required, e.g., pharmaceutical industry. Polymeric Hollow Fiber Heat Exchangers (PHFHEs) have recently been proposed [1] as a new type of heat exchanger that can overcome these constraints and offer the same or better thermal performance than metallic shell and tube or plate heat exchangers while occupying a much smaller volume. In this paper we report our results for heat transfer in PHFHEs with both parallel and cross flow in the shell side of the device. Fibers made of polypropylene (PP) and polyetheretherketone (PEEK) were tested. In addition, steam condensation studies in PHFHEs are reported for the first time. The overall heat transfer coefficients achieved for water-water and water-brine systems are as high as 1400 Wm−2K−1. These values are higher than any value reported for plastic heat exchangers and comparable with commonly acceptable design values for metal shell and tube heat exchangers. Similar coefficients were obtained for steam condensation. Polymeric hollow fiber heat exchangers can also achieve high thermal effectiveness, large number of transfer units (NTU) and very small height of a transfer unit (HTU), if properly rated. If designed like commercial membrane contactors, they can achieve up to 12 transfer units in a single device, not longer than 60–70 cm! In addition, the conductance per unit volume PHFHEs achieved was up to one order of magnitude higher compared to metal heat transfer equipment. This superior thermal performance is also accompanied by considerably lower pressure drops. Therefore, the operation of PHFHEs will be characterized by a low operating cost. Combined with the much lower cost, lower weight and elimination of metal contamination polymer materials offer, it is obvious that PHFHEs constitute a potential substitute for metal heat exchangers on both thermal performance and economical grounds. Possible application fields include the food, pharmaceutical and biomedical industries as well as applications where corrosion resistant, light and very efficient devices are required, i.e., desalination, solar and offshore heat transfer applications.
Measurements of Thermoelectric Properties of Bismuth Telluride Nanowires
Authors: Zhou Jianhua , Shi Li , Jin Chuangui , Li Xiaoguang
Published: April 2009

Theoretical calculations have predicted that nanowire materials may have enhanced thermoelectric figure of merit compared to their bulk counterparts due to classical and quantum size effects. We have measured the thermoelectric properties of bismuth telluride nanowires deposited using an electrochemical deposition method in porous anodized alumina templates with the average pore size of about 60 nm. Transmission electron microscopy results of these nanowires showed that the nanowires were single crystalline with a composition of 54% Te and 46% Bi and the thickness of the surface oxide layer was in the range of 5–10 nm. The thermal conductance and Seebeck coefficient of the nanowires were measured using a microfabricated device that consists of two suspended membranes, across which the nanowire sample was placed. The obtained Seebeck coefficient of a bundle consisting of two 100 nm bismuth telluride nanowires increased with increasing temperature from 160 K to 360 K, and the room temperature value was 260 μV/K, which was 60% higher than the bulk value. The thermal conductance of the sample also increased with increasing temperature from 25 K to 360 K. Current design of the microdevice does not allow for four-probe electrical resistance measurement of the nanowire. We have measured the four-probe electrical resistance of a 57 nm diameter and a 43 nm diameter bismuth telluride nanowires from the same template, and found that the room-temperature electrical conductivity of the nanowires was close to the bulk value and showed much weaker temperature dependence than bulk electrical conductivity.
An Analytical Model to Simulate the Automotive Cooling System
Authors: Samadiani Emad , Kakaee Amirhasan
Published: April 2009

In this paper the effect of simultaneous operation of the automotive radiator, fan, engine, and cooler system on radiator coolant inlet temperature is studied. For this purpose, an analytical model is developed to simulate the automotive cooling system. First, the heat transfer rate and air pressure drop in radiators with louvered corrugated fins and flat tubes are modeled and compared with wind tunnel test results over six radiator cores. Also, the air flow path is simulated. Two preexisting codes are used to simulate the automotive engine and cooler system. Then a code is developed in order to investigate the effect of different parameters related to each component of the cooling system on cooling water inlet temperature. In the end, the effect of ambient temperature, cooling water flow rate, and fin pitch on cooling water temperature at different speeds of a sample automobile is examined.
Numerical Simulation of Transient Natural Convection in a Channel-Chimney System
Authors: Andreozzi Assunta , Buonomo Bernardo , Manca Oronzio
Published: April 2009

In the present numerical investigation, a transient numerical analysis for natural convection in air, between two vertical parallel plates (channel), heated at uniform heat flux, with adiabatic parallel plates downstream (chimney), is carried out by means of the finite volume method. The analyzed transient problem is two-dimensional and laminar. Results are presented in terms of wall temperature, mass flow rate and air velocity profiles. They are given at different Rayleigh number and expansion ratios (chimney gap/channel gap) for a fixed channel aspect ratio (channel height/channel gap) equal to 10 and extension ratio (channel-chimney height/channel height) equal to 2.0. Wall temperature profiles vs time show the presence of overshoots and undershoots. The comparison among the maximum wall temperatures shows that the simple channel is the most critical configuration at steady state condition, but the best configuration during the transient heating at the first overshoot. Velocity profiles in the chimney allow for identification of some different fluid dynamic behaviors such as the vortex in lower corner and the cold inflow in the chimney. According to the temperature profiles, average Nusselt number profiles as a function of time show minimum and maximum values and oscillations before the steady state.
Lattice Boltzmann Simulation of Flow and Heat Transfer Characteristics in a Symmetric Bifurcation
Authors: Xing K. Q. , Tao Y.-X.
Published: April 2009

The lattice Boltzmann method (LBM) originates from the discrete kinetic theory and has been applied for simulation of various kinds of fluid flows under different conditions. In this paper, a passive-scalar-based thermal lattice Boltzmann model is employed to simulate the steady flow in a symmetric bifurcation channel under constant wall heat flux boundary conditions. The bifurcation effects on the heat transfer and fluid flow are thoroughly investigated under different Reynolds numbers, wall heat fluxes and bifurcation angles. The results are compared with the commercial software output. A useful discussion about how to transfer from lattice units to actual physical units is also presented.
Spray Cooling Trajectory Angle Impact Upon Heat Flux Using a Straight Finned Enhanced Surface
Authors: Silk Eric A. , Kim Jungho , Kiger Ken
Published: April 2009

Experiments were conducted to study the effects of spray trajectory angles on heat flux for flat and enhanced surface spray cooling. The surface enhancement consisted of straight fins machined on the top surface of a copper heater block. Spray cooling curves were obtained with the straight fin surface aligned both parallel (axial) and perpendicular (transverse) to the spray axis. Measurements were also obtained on a flat surface heater block for comparison purposes. Each copper block had a cross-sectional area of 2.0 cm2. A 2×2 nozzle array was used with PF-5060 as the working fluid. Thermal performance data was obtained under nominally degassed (chamber pressure of 41.4 kPa) conditions. Results show that the highest CHF in all cases was attained for a trajectory angle of 30° from the surface normal. Also, straight finned surfaces can enhance critical heat flux (CHF) as much as 75% (heat flux value of 140 W/cm2) relative to the vertical spray orientation for the analogous flat surface case.   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.
Simulation of Heat, Mass, and Momentum Transfer Within Building Interiors
Authors: Wang Xiuling , Pepper Darrell W.
Published: April 2009

An hp finite element-based model has been developed to calculate heat, mass, and momentum transfer within rooms and building interiors. The hp-adaptive methodology is based on both mesh enrichment (h-adaptation) and spectral order incensement (p-adaptation) in an effort to produce accurate results with the least computational cost. A Lagrangian Particle Transport (LPT) technique is coupled with the adaptive scheme to simulate mass transport. The model is particularly amenable for depicting the transport of contaminants associated with indoor air quality. The hp-adaptive algorithm is validated using natural convection in a square enclosure. The model is subsequently applied to the simulation of momentum, heat, and mass transport within building interiors: air and temperature distribution patterns are presented along with potential pathways of a powder dispersing within an office.
Effective Optical Properties of Nanoporous Silicon
Authors: Braun Matt , Pilon Laurent
Published: April 2009

Nanoporous materials consist of nanosize voids embedded in a solid matrix. The pores can be closed or open and have various shapes and sizes. Their applications range from optical and optoelectronics devices to biosensors. In order to effectively utilize and characterize nanoporous media for these various applications, models that describe their effective optical properties are necessary. Numerous effective medium models have been proposed. However, validations of these models against experimental data are often contradictory and inconclusive. This issue was numerically investigated by solving the two-dimensional Maxwell’s equations in absorbing nanoporous silicon thin-films. All interfaces are assumed to be optically smooth and characteristic pore size is much smaller than the wavelength of incident radiation so electromagnetic wave scattering by pores can be safely neglected. The envelope method was then used to retrieve the effective index of refraction and absorption index from the computed transmittance. The numerical results agree very well for both the index of refraction and the absorption index with a recent model obtained by applying the Volume Averaging Theory (VAT) to the Maxwell’s equations. However, commonly used models such as the Maxwell-Garnett, Bruggeman, parallel, and series models systematically and sometimes significantly underpredict the numerical results.
Cryogenic Chilldown Model for Stratified Flow Inside a Pipe
Authors: Liao Jun , Yuan Kun , Mei Renwei , Klausner James F. , Chung Jacob
Published: April 2009

A pseudo-steady model has been developed to predict the chilldown history of the pipe wall temperature in horizontal transport pipelines for cryogenic fluids. A new film boiling heat transfer model is developed by incorporating the stratified flow structure for cryogenic chilldown. A modified nucleate boiling heat transfer correlation for the cryogenic chilldown process inside a horizontal pipe is proposed. The efficacy of the correlations is assessed by comparing the model predictions with measured values of wall temperature in several azimuthal positions in a well controlled experiment by Chung et al. (2004). The computed pipe wall temperature histories match well with the measured results. The present model captures important features of thermal interaction between the pipe wall and the cryogenic fluid, provides a simple and robust platform for predicting the pipe wall chilldown history in a long horizontal pipe at relatively low computational cost, and builds a foundation to incorporate the two-phase hydrodynamic interaction in the chilldown process.
A Study on Numerical Instability of Inviscid Two-Fluid Model Near Ill-Posedness Condition
Authors: Liao Jun , Mei Renwei , Klausner James F.
Published: April 2009

The two-fluid model is widely used in studying gas-liquid flow inside pipelines because it can qualitatively predict the flow field at low computational cost. However, the two-fluid model becomes ill-posed when the slip velocity exceeds a critical value, and computations can be quite unstable before flow reaches the unstable condition. In this study computational stability of various convection schemes for the two-fluid model is analyzed. A pressure correction algorithm for inviscid flow is carefully implemented to minimize its effect on numerical stability. Von Neumann stability analysis for the wave growth rates by using the 1st order upwind, 2nd order upwind, QUICK, and the central difference schemes shows that the central difference scheme is more accurate and more stable than the other schemes. The 2nd order upwind scheme is much more susceptible to instability at long waves than the 1st order upwind and inaccurate for short waves. The instability associated with ill-posedness of the two-fluid model is significantly different from the instability of the discretized two-fluid model. Excellent agreement is obtained between the computed and predicted wave growth rates. The connection between the ill-posedness of the two-fluid model and the numerical stability of the algorithm used to implement the inviscid two-fluid model is elucidated.
Modeling Single-Phase Counter-Current Natural Convection Heat Removal in Horizontal Heated Channel Connected to Vertical Piping
Authors: Gulshani P. , Huynh H. M.
Published: April 2009

This paper develops a simple mathematical model to examine the heat transfer phenomena in a single-phase counter-current subcooled water flow in a volumetrically heated horizontal channel connected to an unheated vertical pipe at each end as shown in Figure 1. In Figure 1, the heated horizontal channel and the vertical pipes connected to it are initially filled with subcooled water up to a certain height in the vertical pipes. The vertical pipes can have horizontal runs. The piping arrangement in the model with horizontal fuel (i.e., heated) channels and vertical feeder pipes is relevant to a reactor such as the Canadian Deuterium Uranium (CANDU) reactor. The single-phase water flow condition considered in the model is relevant to CANDU in a shutdown, maintenance state where the main heat-transport-circuit pumps are shutoff and the shutdown-cooling pumps are or become unavailable. Under such postulated loss-of shutdown-cooling pump scenario, it is desirable to know whether the fuel fission-product decay heat can be adequately removed by single-phase subcooled water natural-circulation flow before the water in the fuel channels begins to boil. Boiling and the resulting two-phase conditions, condensation and changes in the buoyancy forces induce intermittent flow in the channel causing intermittent limited fuel heatup Ref [1–3]. Unlike counter-current flow of gas and liquid, counter-current flow of liquids, particularly the same miscible unequal-temperature liquids and in the geometry considered in this paper has not been studied either theoretically or experimentally to the authors’ knowledge.
Aerogel for Microsystems Thermal Insulation: System Design and Process Development
Authors: Smith Brian , Romero David , Agonafer Damena , Gu Jason , Amon Cristina H.
Published: April 2009

Extreme miniaturization in the microelectronics component market along with the emergence of system-on-chip applications has driven interest in correspondingly small-scale thermal management designs requiring novel material systems. This paper concentrates on aerogel, which is an amorphous, nanoporous dielectric oxide fabricated through a sol-gel process. Its extremely high porosity leads to very low thermal conductivity and dielectric constants. Significant research has been devoted to its electrical properties; however, there are several emerging applications that can leverage the thermal characteristics as well. Two promising applications are investigated in this paper: a monolithically integrated infrared sensor that requires thermal isolation between sensor and silicon substrate, and an ultra-miniature crystal oscillator device which demands thermal insulation of the crystal for low-power operation. This paper identifies the potential benefits of aerogel in these applications through system modeling, demonstrates aerogel’s compatibility with standard low-cost microfabrication techniques, and presents results of thermal testing of aerogel films compared with other microelectronics insulators and available data in the literature. The goal is to explore system thermal design using aerogel while demonstrating its feasibility through experimentation. The combination of numerical simulations, Bayesian surrogate modeling, and process development helps to refine candidate aerogel applications and allow the designer to explore thermal designs which have not previously been possible in large-scale microelectronics system production.   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.