bluuebunny/crossref_metadata_2025 huggingface dataset:
Computation of Nitrogen Oxides in Radiant Porous Burner Flows
Authors: Tong Timothy W. , Abou-Ellail Mohsen M. , Li Yuan , Beshay Karam R.
Published: July 2009

The present paper is concerned with the numerical computation of flow, heat transfer and chemical reactions in porous burners. One of the important features of porous burners is their presumed low levels of nitrogen oxides. In the present work, the computed NOx is compared with similar conventional premixed burners and measured nitrogen oxides in porous burners. In order to accurately compute the nitrogen oxides levels in porous burners, both prompt and thermal NOx mechanisms are included. In the present work, the porous burner species mass fraction source terms are computed from an ‘extended’ reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e. the porosity is not uniform over the entire domain. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to ensure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. Finite-difference equations are solved iteratively for velocity components, pressure correction, gas enthalpy, species mass fractions and solid matrix temperature. The grid used to solve the solid energy equation is extended inside the zero-porosity solid annular wall of the burner porous disk. This was found useful for computing the solid wall temperature with high accuracy. A two-dimensional, discrete-ordinate, model is used for the computation of thermal radiation emitted from the solid matrix. The porous burner uses a premixed CH4-air mixture, while its radiating characteristics are studied numerically under equivalence ratio ranging from 0.5 to 0.8. Twenty-one species are included, involving 55 chemical reactions. The computed solid wall temperature profiles are compared with experimental data of similar porous burners. The obtained agreement is fairly good. The present numerical results show that as the equivalent ratio decreases, the reaction zone moves downstream. Moreover, as the flame speed increases, the NOx mole fraction increases. Some reacting species, such as H2O, CO2 and H2 increase steadily inside the reaction zone; they stay appreciable in the combustion products. However, unstable products, such as HO2, H2O2 and CH3, first increase in the preheating region of the reaction zone; they are then consumed in the remaining part of the reaction zone. The numerical results show that most of the formed NOx is composed of nitric oxide. The velocity and temperature profiles were accurately predicted using a grid of 80×80 while the nitrogen oxides were computed accurately utilizing a finer grid of 160×160.
Immersion Cooling of Power Electronics in Segregated Hydrofluoroether Liquids
Authors: Barnes Cindy M. , Tuma Phil E.
Published: July 2009

Passive two-phase immersion cooling with dielectric liquids is a well established method of cooling thyristor type power semiconductors. However, the capabilities of this method for cooling high heat flux power semiconductor devices such as insulated gate bipolar transistors (IGBTs) have not been thoroughly explored. This work quantifies the junction-to-fluid thermal resistance of IGBTs soldered to boilers and immersed in the segregated hydrofluoroether liquid C3F7OCH3, one of a class of new dielectric liquids with a low Global Warming Potential. The boilers were square copper heat spreaders with a microporous metallic boiling enhancement coating applied to the wetted surfaces. Bare 0.54cm2 IGBT die were soldered to experimentally-optimized boilers and immersed in saturated C3F7OCH3 liquid at atmospheric pressure. Boiler temperature measurements showed a peak boiler-to-fluid heat transfer coefficient of ∼14W/cm2-K at a die level heat flux of 255 W/cm2. Direct junction temperature measurements yielded junction-to-fluid resistivities of 0.15°C/(W/cm2) at this heat flux and 0.20°C/(W/cm2) at 400 W/cm2, the maximum heat flux studied. These results, coupled with published data for air cooled condensers, show that passive two-phase cooling of power modules may provide junction-to-ambient thermal resistances approximately 25% those of conventional air cooled modules and junction-to-fluid resistances 30%–50% of conventional liquid cooled modules, and similar to emerging direct substrate liquid cooling schemes.
Experimental and Numerical Determination of Thermal Radiative Properties of ZnO Particulate Media
Authors: Coray Patrick , Lipin´ski Wojciech , Steinfeld Aldo
Published: July 2009

The radiative characteristics of packed beds of ZnO particles, applied in the design of high-temperature solar thermochemical reactors, were investigated experimentally. ZnO samples of varying thickness were exposed to a continuous beam of near monochromatic thermal radiation in the 0.5–1 μm wavelength range. The overall transmitted fraction measured as a function of sample thickness s obeys an exponential trend exp(–As), with the fit parameter A ranging from (4000 ± 100) m−1 at 555 nm to (2100 ± 100) m−1 at 1 μm. In the forward directions, the measured intensity distribution is approximately isotropic, whereas in the backward directions it is well approximated by a Henyey–Greenstein equation with asymmetry factor g ≈ − 0.4 at 555 nm and g ≈ − 0.1 at 1 μm. A Monte Carlo ray-tracing model of the experimental set-up is employed to extract the extinction coefficient and the scattering albedo for the case of non-grey absorbing-scattering medium.
Impact of Thermophoresis on Carbon Nanotube Growth by Chemical Vapor Deposition
Authors: Lysaght Andrew C. , Chiu Wilson K. S.
Published: July 2009

Thermophoretic effect on the growth of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) has been investigated using a fully coupled gas-phase and surface chemistry model. This reactor-scale model employs conservation of mass, momentum, species, and energy equations to describe the evolution of hydrogen and hydrocarbon feed streams as they undergo thermal transport and chemical reactions within the CVD reactor. The resulting CNT growth rates on individual catalytic iron nanoparticles located on the reactor wall is predicted by the model as well as steady state velocity, temperature, and concentration fields within the reactor volume and concentrations of species adsorbed onto the nanoparticle surfaces. The effect of thermophoresis on volumetric concentration fields and surface species adsorption for deposition occurring in differing reactor boundary and flow conditions has been investigated to understand the impacts on CNT growth. This investigation is useful in order to optimize reactor design and boundary conditions to promote optimal CNT deposition rates.
Numerical Simulation of Reactive Turbulent Flows Over Bluff Body Flame Holders: A Parametric Study
Authors: Holder Richard , Lin Cheng-Xian
Published: July 2009

In this paper, a numerical parametric study has been conducted to examine the effects of varying both inlet turbulent intensity and angle of attack for a bluff body flame holder (V-gutter) in a channel. The geometry used was based on previous experiments. The inlet turbulent intensity was varied from 2% to 100% while the angle of attack of the V-gutter was varied from −20° to 20°. The turbulent flow was modeled with a RANS-based realizable k-ε turbulence model. The combustion setup used was premixed propane-air combustion with an equivalence ratio of 0.6. The combustion-turbulence interaction is simulated with an eddy-dissipation model. Calculations were carried out using a finite volume based solver, and all equations were solved using the second order upwind method. Results indicate that increasing the inlet turbulent intensity and V-gutter angle of attack will result in an increase not only in the size but also in magnitude of the downstream high turbulence areas with vortexes.
Numerical Simulation of Laminar Confined Impinging Jet in a Composite Channel
Authors: de Lemos Marcelo J. S.
Published: July 2009

This work shows numerical results for a jet impinging onto a flat plane covered with a layer of a porous material. Porosity of the porous layer is varied in order to analyze its effect on the local distribution of Nu. Macroscopic equations for mass and momentum ae obtained based on the volume-average concept. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure-velocity coupling. Results indicate that inclusion of a porous layer decreases the peak in Nu avoiding excessive heating or cooling near the stagnation region.
Mathematical Modeling of Flow and Heat Transfer in a Moving Bed Reactor
Authors: de Lemos Marcelo J. S.
Published: July 2009

This paper shows a proposition of a set of transport equations and their boundary conditions for solving problems involving flow and heat transfer in a moving bed equipment. The reactor is seen as a porous matrix in which the solid phase is moving. Additional drag terms appearing the momentum equation are a function of the relative velocity between the fluid and solid phases. Turbulence equations are also influenced by the speed of the solid phase. Results show the decrease for turbulent kinetic energy as the solid speed approaches the fluid speed. Heat transfer rate between phases is also damped as the solid speed increases.
The Developmental Research of Regeneration Ladle Heater With Gas Backflow Technology
Authors: Zou Linjiang , Cheng Lin
Published: July 2009

By adopting a gas jet principle in developmental metal smelting research, the combustion, heating effect, and the flow distribution of a new ladle configuration can be simulated. In this paper, using universal fluid software, the relation between the structure, the flow distribution, and the temperature distribution is examined. The study results indicate that a new ladle heater, in which gas jet technology and high-temperature air combustion technology are adopted, can improve the bake effect and the temperature uniformity of the ladle lining effectively. These methods can also cut down on pollution emissions.
Local Wall Temperature Measurements in Microchannel Flow Boiling Using Infrared Thermography
Authors: Krebs Daniel , Narayanan Vinod , Liburdy James , Pence Deborah
Published: July 2009

Quantitative measurement of channel wall temperature in two-phase microchannel flows using infrared thermography is discussed. In particular, the top channel wall temperature of a branching microchannel silicon heat sink is presented. Quantitative temperature measurements require several considerations in the design and fabrication of the heat sink and test facility, in calibration, and in data analysis; each of these are discussed. Temperature measurements, with a spatial resolution of 10 μm, are presented over an 18 mm × 18 mm area of the heat sink during flow boiling.
Temperature and Concentration Simulations in the Methanol Steam Reformer
Authors: Chen Yen-Cho , Chein Rei-Yu , Chen Li-Chun
Published: July 2009

The methanol steam reforming plays an important role for hydrogen supply to the proton membrane exchange fuel cell in the portable power applications. The catalyst coating on the walls of channels is often used in the fabrication of the reactors in the reformer to minimize the pressure loss. In this study, the temperature and concentration fields in the reactors for the methanol steam reforming were investigated numerically. The methanol conversion is usually used to evaluate the performance of the reformer. The effects of the inlet gas temperature in the heat supply channel and inlet velocity in the reforming channel on the performance of the methanol steam reforming are presented.
Simulation and Optimization of Drying of Wood Chips With Superheated Steam in a Rotary Kiln
Authors: Ghoshdastidar P. S. , Agarwal Ankit
Published: July 2009

The present work reports a computer simulation and optimization study of heat transfer in a rotary kiln used for drying and preheating of wood chips with superheated steam at 1 bar. A rotary kiln employed for drying and preheating of wet solids consists of a refractory lined cylindrical shell mounted at a slight incline from the horizontal plane. The kiln is very slowly rotated about its longitudinal axis. Wet solids are fed into the upper end of the cylinder and during the process, are dried and heated by the countercurrent flow of the hot gas. Finally, it is transferred to the lower end where it reaches the desired temperature and discharged. The heat transfer model includes radiation exchange among hot gas, refractory wall and the solid surface, transient conduction in the refractory wall, and mass and energy balances of the hot gas and the solids. A finite-difference based computational heat transfer approach is used. A Univariate Search method has been used to obtain minimum kiln length with respect to various kiln operating parameters subject to a constraint on the inlet gas temperature. The parametric study lent a good insight into the physics of the drying process in a rotary kiln. The optimization study reveals that for an economical design of a rotary kiln low rotational speed, small inclination angle and medium gas flow rate is required.
Numerical Prediction of the Temperature Distribution Within a Human Eye During Laser Surgery
Authors: Kushwaha Sandeep Singh , Ghoshdastidar P. S.
Published: July 2009

In this paper a computational heat transfer model for prediction of the temperature distribution within the human eye during laser surgery is presented. The heat transfer within a tissue is described by the classic Pennes bioheat transfer equation. The intraocular temperature distribution is calculated using finite-difference method. Two types of computational domain have been considered: (i) rectangular parallelepiped and (ii) cylindrical. The eye is modeled as a composite layered structure consisting of four different ocular tissues, namely, cornea, aqueous, lens and vitreous. It is assumed that the eye is symmetrical about the pupillary axis. The absorption probability of ocular tissue is modulated based on the Lambert-Beer’s law to reproduce the exponential attenuation of the laser light with depth within a biomaterial. The heat flow is modeled as transient and three-dimensional for rectangular parallelepiped geometry and two-dimensional (axi-symmetric) for the cylindrical geometry. The results indicate that for the insulation condition imposed on the periphery of the eye the model based on rectangular parallelepiped geometry of the eye at no laser power and at the initial temperature of 25°C predicts temperature closer to in-vitro experimental measurements reported in literature whereas the model based on cylindrical geometry predicts higher temperature. The opposite is true (that is, lower temperature is predicted by the model based on cylindrical geometry) for high laser heat flux (2000 W/m2) and higher initial temperature (37°C). This study also presents changes in eye temperature subjected to intermittent laser source used in laser surgery techniques such as PRK and LASIK. A comparison of the results based on three different boundary conditions such as convection (hb = 10 W/m2K), constant temperature (37°C) and insulation on the eye periphery reveals that the model based on insulation condition predicts results closer to that of in-vitro experiment at no laser power and initial temperature of 25°C whereas at a laser power of 200 W/m2 and at the initial temperature of 37°C insulation boundary condition produces highest temperature followed by that produced by convection and constant temperature conditions. The heat transfer is one-dimensional for the insulated eye periphery whereas multi-dimensional heat flow takes place when the circumferential boundary condition is either convective or isothermal.
A Novel Finite Difference Method for Flow Calculation on Colocated Grids
Authors: Xia Z. Z. , Zhang P. , Wang R. Z.
Published: July 2009

A new finite difference method, which removes the need for staggered grids in fluid dynamic computation, is presented. Pressure checker boarding is prevented through a dual-velocity scheme that incorporates the influence of pressure on velocity gradients. A supplementary velocity resulting from the discrete divergence of pressure gradient, together with the main velocity driven by the discretized pressure first-order gradient, is introduced for the discretization of continuity equation. The method in which linear algebraic equations are solved using incomplete LU factorization, removes the pressure-correction equation, and was applied to rectangle duct flow and natural convection in a cubic cavity. These numerical solutions are in excellent agreement with the analytical solutions and those of the algorithm on staggered grids. The new method is shown to be superior in convergence compared to the original one on staggered grids.
Transient Heat Transfer From Single Horizontal Heaters in Forced Flow of Helium Gas at Exponentially Increasing Heat Inputs
Authors: Liu Qiusheng , Fukuda Katsuya , Shibahara Makoto
Published: July 2009

Forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input to a horizontal cylinder and a plate (ribbon) was experimentally and theoretically studied. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than around 1 s. The dependence of transient heat transfer on the gas flowing velocity becomes weaker when the period becomes very shorter. However, the gas temperature in this study shows little influence on the heat transfer coefficient. Empirical correlations for quasi-steady-state heat transfer and transient heat transfer were obtained based on the experimental data. In the theoretical study, transient heat transfer was numerically solved based on a turbulent flow model. The values of numerical solution for surface temperature and heat flux were compared and discussed with authors’ experimental data. It was clarified that the surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. The temperature distribution near the heater becomes larger as the surface temperature increases. The values of numerical solution for surface temperature and heat flux agree well with the experimental data for the cylinder diameter of 1 mm. However, the heat fluxes show some differences from the experimental values for the cylinder diameters of 0.7 mm and 2.0 mm. And for the numerical solution for a plate, the values of numerical solutions for surface temperature and heat flux at the velocity of 6 m/s agree well with the experimental data, though they show some differences at other velocities.
Single Impinging Slot Jet on a Concave Surface: Inlet/Outlet Influence
Authors: Brizzi L.-E. , Hoang T. K. D. , Dorignac E. , Fe´not M.
Published: July 2009

An experimental study of fluid flow and heat transfer has been carried out for a single slot jet impinging on a concave surface. Experiments have been conducted with nozzle exit Reynolds number (Reb = 3200), dimensionless impinging height (H/b = 7) and relative curvature (Dc/b = 5) to determine the heat transfer coefficients under a constant heat flux condition by infrared thermography (≈ 500 samples). The distributions of mean velocity and velocity fluctuation on the concave surface have been measured by using classical Particle Image Velocimetry (≈ 5000 samples). The presence of an oscillatory impinging jet around three different positions has been observed. This type of behavior modifies considerably the impinging jet structure and the turbulence values which results in a modification the cooling efficiency. In order to use this type of jet for the optimization of the cooling of a concave surface, we propose to study, in this present study, the effect of Inlet/Outlet condition (riblets and obturator). The results underline the strong effect of outlet condition (e varies from 0 to 10 mm) for a single slot impingement jet on a concave surface. The flow is characterized by an oscillatory jet when the exit is totally opened (e = 10 mm) while the jet is stabilized at the opposite part for the closed exist (e = 0 mm). Therefore, the oscillatory impinging jet generated a uniform and symmetric heat transfer area while the stabilized impinging jet causes a dissymmetrical heat transfer.
Experimental Study on Double-Diffusive Convection During Solidification of NH4Cl-H2O Hypereutectic Solution in Cylinder Cavity
Authors: Bai Bofeng , Lu Jun , Zhang Lei , Li Heng
Published: July 2009

In order to reveal the law of double-diffusive convection of multi-compound solution in cylindrical cavity, experimental study on solidification of NH4Cl-H2O hypereutectic solution has been performed by using particle image velocimetry (PIV). The influencing factors of flow patterns and intensity are also analyzed. The results show that: 1) There are two approximately symmetric main convection cells in the liquid which are down along the sidewall and up along the center of the cylindrical cavity. Meanwhile, there are also two secondary cells on the bottom corner of cylindrical cavity, which flow in contrary direction to that of the main ones; 2) Due to the release of water during the solidification process, solute layers and diffusive interface are developed in the liquid and will be disappeared in the end; 3) The cooling temperature and the initial concentration have significantly effects on the flow velocity, solute layers and diffusive interface.
Thermal and Visual Observation of Water and Acetone Oscillating Heat Pipes
Authors: Wilson C. , Borgmeyer B. , Winholtz R. A. , Ma H. B. , Jacobson D. , Hussey D.
Published: July 2009

Identical oscillating heat pipes (OHP) charged separately with water and acetone were observed thermally and visually at varying condenser temperatures and heat inputs. Neutron radiography allowed visualization of hydrogen rich liquid water and acetone within the copper OHP. Four identical 6 turn OHPs were constructed; two as open loop and two close loop. One set of open and close loop OHP was charged with acetone and an open and close loop OHP were charged with water at fill ratios of 50%. The OHPs were also instrumented with 24 thermocouples, heated with a strip heater and cooled with heavy water. Both thermal and visual data were collected simultaneously. The experiments show that at low heat flux and condenser temperature, the acetone OHP performed better than the water OHP. Using neutron radiography, the acetone OHP was seen to have higher fluid velocity than the water OHP. The fluid flow pattern was also more consistent throughout the entire close loop acetone OHP including full circulation of the fluid. The close loop water OHP never fully circulates through the turn connecting the two sides. Lastly, neutron radiography shows that the open loop OHPs not only have less fluid motion at the side turns, the interior turns also have reduced fluid motion compared to the close loop OHPs.
An Efficient Software Architecture for Automated Coupling of Convection and Thermal Radiation Tools
Authors: Rauch Christian , Ho¨rmann Thomas , Jagsch Sebastian , Almbauer Raimund
Published: July 2009

Much attention has been paid recently by research and development engineers on performing multi-physics calculations. One way to do this is to couple commercial tools for examining complex systems. Since the proposal of an software architecture for coupling programs as published in a previous paper significant changes have led to an improved performance for large-scale industrial applications. This architecture is being described and as a proof of concept a simulation is being conducted by coupling two commercial solvers. The speed-up of the new system is being presented. The simulation results are then compared with measurements of surface temperatures of an exhaust system of an actual sports utilities vehicle (SUV) and conclusions are being drawn. The proposed architecture is easily adaptable to various programs as it is implemented in C++ and changes for a specific code can be restricted to a view classes.
Spectral Radiation Properties of a Turbulent Ethylene Pool Fire
Authors: Biswas Kaushik , Zheng Yuan , Gore Jay
Published: July 2009

In the present work, line-of-sight spectral radiation intensities (Iλ) were measured in a 7.1 cm ethylene (C2H4) buoyant diffusion flame, designed to mimic pool fires. Various time series statistics were calculated using the radiation data. Both soot and gaseous species had significant radiation emissions, emphasizing the need for spectrally-resolved radiation measurements. Significant fluctuations were observed in the radiation intensities from the fire, especially at higher elevations and near the flame edges. In addition, root-mean-square (rms) and probability density functions (PDF) of Iλ indicated higher fluctuations in soot compared to gaseous species. Autocorrelations of Iλ showed periodic oscillations due to the puffing phenomenon typically seen in pool fires. The observed oscillation frequencies ranged from 7.47 to 7.86 Hz and are in excellent agreement with empirical correlations based on past data. Characteristic frequencies of these oscillations were also reflected in the power spectral densities (PSD) of Iλ. Based on the measured autocorrelations of Iλ, it was observed that the integral time scales decrease with increasing height above the burner exit, which is expected since mean velocities increase with height due to combustion-induced buoyancy in pool fires and buoyant flames.
Transient Plane Source Method for Thermal Property Measurements of Metal Hydrides
Authors: Flueckiger Scott , Zheng Yuan , Pourpoint Timothe´e
Published: July 2009

Metal hydrides are promising hydrogen storage materials with potential for practical use in a passenger car. To be a viable hydrogen storage option, metal hydride heat transfer behavior must be well understood and accounted for. As such, the thermal properties of the metal hydride are measured and compiled to assess this behavior. These properties include thermal conductivity, specific heat, and thermal diffusivity. The transient plane source (TPS) method was selected primarily due to a high level of versatility, including customization for high pressure hydrogen environments. To perform this measurement, a TPS 2500 S thermal property analyzer by the Hot Disk Company was employed. To understand the measurement and analysis process of the TPS method, two different sample materials were evaluated at ambient conditions. These samples included a stainless steel pellet and an inactivated (non-pyrophoric) metal hydride pellet. Thermal conductivity and thermal diffusivity of these samples were measured using the TPS method. The thermal property measurements are compared to the data available in the literature (stainless steel) and the data obtained using laser flash method (metal hydride). The improvements needed to successfully implement the TPS method are discussed in detail.
Thermally Induced Flow Structures in Aircraft Wing Compartments
Authors: Newport D. , Egan V. , Aguanno M. , Lacarac V. , Estebe B. , Murer Y.
Published: July 2009

The use of composite material in modern commercial aircraft has increased significantly in recent years. The very low conductivity relative to Aluminium of composite materials means that the thermal environment experienced in an aircraft, during flight and on the apron, are significantly altered. The heat transfer mechanism is complex: natural and mixed convection flows established in compartments. This paper presents the thermally induced flow structures under representative conditions for a rectangular cavity representative of wing boxes and horizontal tail planes. The paper highlights the sensitivity to boundary conditions, the effect of structural stiffeners. The results indicate it may be possible to incorporate the effect of stringers and heating from above into existing correlations.
Effects of Channel Height and Bulk Temperature Considerations on Heat Transfer Coefficient of Wetted Surfaces in a Single Inline Row Impingement Channel
Authors: Ricklick Mark , Kersten Stephanie , Krishnan V. , Kapat J. S.
Published: July 2009

High performance turbine airfoils are typically cooled with a combination of internal cooling channels and impingement/film cooling. In such applications, the jets impinge against a target surface, and then exit along the channel formed by the jet plate, target plate, and side walls. Local convection coefficients are the result of both the jet impact, as well as the channel flow produced from the exiting jets. Numerous studies have explored the effects of jet array and channel configurations on both target and jet plate heat transfer coefficients. However, little work has been done in examining effects on the channel side walls, which may be a major contributor to heat transfer in real world applications. This paper examines the local and averaged effects of channel height and on heat transfer coefficients, with special attention given to the channel side walls. The effects on heat transfer results due to bulk temperature variations were also investigated. High resolution local heat transfer coefficient distributions on target and side wall surfaces were measured using temperature sensitive paint and recorded via a scientific grade charge-coupled device (CCD) camera. Streamwise pressure distributions for both the target and side walls was recorded and used to explain heat transfer trends. Results are presented for average jet based Reynolds numbers between 17,000 and 45,000. All experiments were carried out on a large scale single row, 15 hole impingement channel, with X/D of 5, Y/D of 4, and Z/D of 1, 3 and 5. The results obtained from this investigation will aid in the validation of predictive tools and development of physics-based models.
Turbulent Natural Convection in a Differentially Heated Vertical Channel
Authors: Shiri Abolfazl , George William K.
Published: July 2009

The turbulence natural convection boundary layer inside a infinite vertical channel with differentially heated walls is analyzed based on a similarity solution methodology. The differences between mean temperature and velocity profiles in a boundary layer along a vertical flat plate and in a channel flow, make it necessary to introduce new sets of scaling parameters. In the limit as H* → ∞, two distinctive parts are considered: an outer region which dominates the core of the flow and inner constant heat flux region close to the walls. The proper inner scaling velocity is showed to be determined by the outer parameters due to momentum integral. The theory is contrasted with the one suggested by George & Capp (1), the deficiencies of which are identified.
Plume Rise Calculations Using a Control Volume Approach and the Damped Spring Oscillator Analogy
Authors: Brown Alexander L. , Bixler Nathan E.
Published: July 2009

The PUFF code was originally written and designed to calculate the rise of a large detonation or deflagration non-continuous plume (puff) in the atmosphere. It is based on a buoyant spherical control volume approximation. The theory for the model is updated and presented. The model has been observed to result in what are believed to be unrealistic plume elevation oscillations as the plume approaches the terminal elevation. Recognizing a similarity between the equations for a classical damped spring oscillator and the present model, the plume rise model can be analyzed by evaluating equivalent spring constants and damping functions. Such an analysis suggests a buoyant plume in the atmosphere is significantly under-damped, explaining the occurrence of the oscillations in the model. Based on lessons learned from the analogy evaluations and guided by comparisons with early plume rise data, a set of assumptions is proposed to address the excessive oscillations found in the predicted plume near the terminal elevation, and to improve the robustness of the predictions. This is done while retaining the basic context of the present model formulation. The propriety of the present formulation is evaluated. The revised model fits the vast majority of the existing data to +/− 25%, which is considered reasonable given the present model form. Further validation efforts would be advisable, but are impeded by a lack of quality existing datasets.
Thermal Transport in Finite-Sized Nanocomposites
Authors: Singh Dhruv , Murthy Jayathi Y. , Fisher Timothy S.
Published: July 2009

We report finite volume simulations of the phonon Boltzmann Transport Equation (BTE) for heat conduction in periodic nanowire composites. Models for phonon transport across heterogeneous interfaces are developed, and simulations are performed over a wide range of Knudsen numbers. Conditions are identified under which the thermal conductivity of the composite material is less than the bulk thermal conductivity of the individual host materials and under which the alloy limit of thermal conductivity is recovered. We also compute the length scale needed to achieve bulk behavior in nanoscale composites. The results of this study are expected to inform and improve applications such as thermoelectric devices and flexible macroelectronics.
Investigation of Loop Heat Pipe Startup Using Liquid Core Visualization
Authors: d’Entremont B. P. , Ochterbeck J. M.
Published: July 2009

In this investigation, a Loop Heat Pipe (LHP) evaporator has been studied using a borescope inserted through the compensation chamber into the liquid core. This minimally intrusive technique allows liquid/vapor interactions to be observed throughout the liquid core and compensation chamber. A low conductivity ceramic was used for the wick and ammonia as the working fluid. Results indicate that buoyancy driven flows, both two-phase and single-phase, play essential roles in evacuating excess heat from the core, which explains the several differences in performance between horizontal and vertical orientations of the evaporator. This study also found no discernable effect of the pre-start fill level of the compensation chamber on thermal performance during startup at moderate and high heat loads.
Forced Convection Film Boiling Heat Transfer From Single Horizontal Cylinders in Saturated and Subcooled Liquids: Part 1 — Experimental Data and Its Correlation for Saturated Liquids
Authors: Liu Qiusheng , Fukuda Katsuya
Published: July 2009

Forced convection film boiling heat transfer on a horizontal cylinder in saturated water and Freon-113 flowing upward perpendicular to the cylinder was measured for the flow velocities ranging from zero to 1 m/s at the system pressures ranging from 100 to 500 kPa: the platinum cylinders with the diameters ranging from 0.7 to 5 mm were used as the test cylinder heaters. The existing correlation for forced convection film boiling heat transfer given by Bromley et al. could not well describe the experimental data obtained, especially those for the higher pressures. The forced convection film boiling heat transfer correlation including the radiation contribution from the cylinders with various diameters for saturation conditions was developed based on forced convection two-phase laminar boundary layer film boiling model and the experimental data obtained. The experimental data agreed with the corresponding values derived from the correlation within ±15% for the flow velocities below 0.7 m/s, and within −30% to +15% for higher flow velocities. It was confirmed that the experimental data obtained by Bromley et al. for the horizontal carbon cylinders with the diameters ranging from 9.83 to 16.2 mm and with the significant radiation effect from the cylinder surfaces in various liquids for the various flow velocities up to 4.4 m/s at an atmospheric pressure agreed with the corresponding values derived from the new correlation within ±20%.
Atomic-Scale Three-Dimensional Phononic Crystals With a Lower Thermal Conductivity Than the Einstein Limit of Bulk Silicon
Authors: Gillet Jean-Numa , Chalopin Yann , Volz Sebastian
Published: July 2009

Extensive research about superlattices with a very low thermal conductivity was performed to design thermoelectric materials. Indeed, the thermoelectric figure of merit ZT varies with the inverse of the thermal conductivity but is directly proportional to the power factor. Unfortunately, as nanowires, superlattices reduce heat transfer in only one main direction. Moreover, they often show dislocations owing to lattice mismatches. Therefore, fabrication of nanomaterials with a ZT larger than the alloy limit usually fails with the superlattices. Self-assembly is a major epitaxial technology to fabricate ultradense arrays of germaniums quantum dots (QD) in a silicon matrix for many promising electronic and photonic applications as quantum computing. We theoretically demonstrate that high-density three-dimensional (3-D) periodic arrays of small self-assembled Ge nanoparticles (i.e. the QDs), with a size of some nanometers, in Si can show a very low thermal conductivity in the three spatial directions. This property can be considered to design thermoelectric devices, which are compatible with the complementary metal-oxide-semiconductor (CMOS) technologies. To obtain a computationally manageable model of these nanomaterials, we simulate their thermal behavior with atomic-scale 3-D phononic crystals. A phononic-crystal period (supercell) consists of diamond-like Si cells. At each supercell center, we substitute Si atoms by Ge atoms in a given number of cells to form a box-like Ge nanoparticle. The phononic-crystal dispersion curves, which are computed by classical lattice dynamics, are flat compared to those of bulk Si. In an example phononic crystal, the thermal conductivity can be reduced below the value of only 0.95 W/mK or by a factor of at least 165 compared to bulk silicon at 300 K. Close to the melting point of silicon, we obtain a larger decrease of the thermal conductivity below the value of 0.5 W/mK, which is twice smaller than the classical Einstein Limit of single crystalline Si. In this paper, we use an incoherent-scattering approach for the nanoparticles. Therefore, we expect an even larger decrease of the phononic-crystal thermal conductivity when multiple-scattering effects, as multiple reflections and diffusions of the phonons between the Ge nanoparticles, will be considered in a more realistic model. As a consequence of our simulations, a large ZT could be achieved in 3-D ultradense self-assembled Ge nanoparticle arrays in Si. Indeed, these nanomaterials with a very small thermal conductivity are crystalline semiconductors with a power factor that can be optimized by doping using CMOS-compatible technologies, which is not possible with other recently-proposed nanomaterials.
Measurement of Near-Field Thermal Radiation Between Two Closely-Spaced Glass Plates
Authors: Hu Lu , Narayanaswamy Arvind , Chen Xiaoyuan , Chen Gang
Published: July 2009

At a finite temperature, electrons and ions in any matter are under constant thermal agitation, acting as the random current source for thermal emission. The thermally-excited electromagnetic waves have two forms: the propagating modes that can leave the surface of the emitter and radiate freely into the space, and the non-propagating modes (evanescent modes) that do not radiate. The contribution from the propagating modes, or the far-field radiation modes, to the radiative heat flux is well-known and its maximum is governed by Planck’s law of blackbody radiation. The non-propagating modes do not propagate and thus do not carry energy in the direction normal to the surface, unless a second surface is brought close to the first to enable photon tunneling. The contribution from the non-propagating modes to radiative heat flux is the near-field radiative flux.
Thermal Wave Based Measurement of Liquid Thermal Conductivities
Authors: Wang Zhefu , Peterson Richard B.
Published: July 2009

This work develops an experimental technique capable of determining thermal conductivity of liquids with application to nanofluids. A periodic current passing through a thin stainless steel strip generates a periodic Joule heating source and an infrared detector measures the temperature response at the front surface of the stainless steel strip. An open chamber is machined out of a delrin plate with the stainless steel strip acting as the sealing cover. This resulting closed chamber contains the test liquid. The phase and magnitude of the temperature response were measured using a lock-in amplifier at various frequencies from 22 to 502 Hz. A one-dimensional, two-layered transient heat conduction model was developed to predict the temperature response on the front surface of the stainless steel strip. This temperature response, including phase and magnitude, is a function of the thermal properties of the liquid. The phase information shows high sensitivity to thermal properties of the liquid layer and is employed to match experimental data to find thermal conductivities. The measured thermal conductivities of water and ethylene glycol agree well with data from the literature and support the validity of this measurement technique. An aqueous fluid consisting of gold nanoparticles was tested. Anomalous thermal conductivity enhancement was observed. Our measurement results also show a divergence of thermal transport behavior between nanofluids and pure liquids. This suggests the need to carefully examine the role of measurement techniques in the study of nanofluid heat transfer phenomena.
A Combined Experimental and Numerical Study of Heat Transfer Characteristics for Methane/Air Flame Impinging Normally on a Flat Surface
Authors: Chander Subhash , Ray Anjan
Published: July 2009

An experimental and numerical study has been conducted to determine the heat transfer characteristics for laminar methane/air flame impinging on a flat surface. A commercial numerical code (FLUENT) was used to simulate the laminar premixed flame. Simulation results were compared with the experimental results and there was good agreement between the results. The purpose of simulation is to understand the impinging flame structure and the chemical physical combustion process. Further, simulation results are presented to define the reasons for a sharp peak in radial heat flux distribution when the inner reaction zone was intercepted by the plate. Here, it has been observed that the resultant effect of peak in radial velocity, axial velocity and velocity magnitude along with peak in the temperature and temperature gradient caused that sharp peak in heat flux value in the radial direction.
Large Eddy Simulation of the Heat Transfer Due to Swirling and Non-Swirling Jet Impingement
Authors: Uddin Naseem , Neumann S. O. , Weigand B.
Published: July 2009

Turbulent impinging jet is a complex flow phenomenon involving free jet, impingement and subsequent wall jet development zones; this makes it a difficult test case for the evaluation of new turbulence models. The complexity of the jet impingement can be further amplified by the addition of the swirl. In this paper, results of Large Eddy Simulations (LES) of swirling and non-swirling impinging jet are presented. The Reynolds number of the jet based on bulk axial velocity is 23000 and target-to-wall distance (H/D) is two. The Swirl numbers (S) of the jet are 0,0.2, 0.47. In swirling jets, the heat transfer at the geometric stagnation zone deteriorates due to the formation of conical recirculation zone. It is found numerically that the addition of swirl does not give any improvement for the over all heat transfer at the target wall. The LES predictions are validated by available experimental data.
Transient Modeling of Direct Internal Reforming Planar Solid Oxide Fuel Cells
Authors: Colpan C. Ozgur , Dincer Ibrahim , Hamdullahpur Feridun
Published: July 2009

In this paper, a 2-D cross section of a single cell of a direct internal reforming planar SOFC is taken and a heat transfer model is developed. For this purpose, the cell is divided into five control volumes: anode interconnect, fuel channel, positive/electrolyte/negative (PEN) structure, air channel and cathode interconnect. Mass and energy balances are solved in these control volumes (2-D in solid structures, 1-D in gas channels). Ohmic, activation and concentration polarizations, convection effects in rectangular ducts and surface-to-surface radiation effects are also taken into account. For the numerical solution, fully implicit finite difference scheme is chosen. The input parameters of the model are inlet temperature and gas composition of air and fuel channel, pressure of the cell, cell voltage, mass flow rate at the fuel channel inlet, excess air coefficient and cell geometry. The model also gives temperature and current density distributions within the cell as output.
Modeling of Subcontinuum Thermal Transport Across Semiconductor-Gas Interfaces
Authors: Singh Dhruv , Guo Xiaohui , Alexeenko Alina , Murthy Jayathi Y. , Fisher Timothy S.
Published: July 2009

A physically rigorous computational algorithm is developed and applied to calculate sub-continuum thermal transport in structures containing semiconductor-gas interfaces. The solution is based on a finite volume discretization of the Boltzmann equation for gas molecules (in the gas phase) and phonons (in the semiconductor). A partial equilibrium is assumed between gas molecules and phonons at the interface of the two media, and the degree of this equilibrium is determined by the accommodation coefficients of gas molecules and phonons on either side of the interface. Energy balance is imposed to obtain a value of the interface temperature. The problem of heat transfer between two parallel plates is investigated. A range of transport regimes is studied, varying from ballistic phonon transport and free molecular flow to continuum heat transfer in both gas and solid. In particular, the thermal interface resistance (or temperature slip) at a gas-solid interface is extracted in the mesoscopic regime where a solution of the Boltzmann equation is necessary. This modeling approach is expected to find applications in the study of heat conduction through microparticle beds, gas flows in microchannel heat sinks and in determining gas gap conductance in thermal interface materials.
An Active Learning Approach for Teaching Undergraduate Heat Transfer
Authors: Pate Michael B.
Published: July 2009

A novel method for teaching undergraduate heat transfer by using an active learning approach is presented. The method focuses on minimizing lecture time and maximizing student engagement. Learning is achieved by forming small groups of two to three students who then work on in-class graded assignments.