bluuebunny/crossref_metadata_2025 huggingface dataset:
Forced Convection Film Boiling Heat Transfer From Single Horizontal Cylinders in Saturated and Subcooled Liquids: Part 2—Experimental Data for Subcooled Liquids and Its Correlation
Authors: Liu Qiusheng , Shiotsu Masahiro , Sakurai Akira , Fukuda Katsuya
Published: June 2010

Forced convection film boiling heat transfer from a horizontal cylinder in water and Freon-113 flowing upward perpendicular to the cylinder under subcooled conditions was measured for the flow velocities from 0 to 1 m/s at the system pressures ranging from 100 to 500 kPa: the platinum horizontal cylinders with diameters ranging from 0.7 to 5 mm were used as the test heaters. The film boiling heat transfer coefficients were obtained for the surface superheats from about 800 K for water and from about 400 K for Freon-113 down to minimum film boiling surface superheats. These heat transfer coefficients increase with the increase in flow velocity, liquid subcooling, system pressure, and with the decrease in cylinder diameter. A correlation for subcooled forced convection film boiling heat transfer was presented, which can describe the experimental data obtained within ±20% for the flow velocities below 0.7 m/s, and within −30% to +20% for the higher flow velocities. The correlation also predicted well the data by Shigechi (1983), Motte and Bromley (1957), and Sankaran and Witte (1990) obtained for the larger diameter cylinders and higher flow velocities in various liquids at the pressures of near atmospheric. The Shigechi’s data were in the range from about −20% to +15%, the data of Motte and Bromley were about ±30%, and the data of Sankaran and Witte were within +20% of the curves given by the corresponding predicted values.
Natural Convection Flow in Vertical Channel Due to Ramped Wall Temperature at One Boundary
Authors: Marneni Narahari , Raghavan Vijay R.
Published: June 2010

An exact solution to the problem of unsteady natural convective flow of a viscous and incompressible fluid in a vertical parallel plate channel due to ramp heating at one boundary is presented. The temperature at one of the channel plates increases linearly over a certain time period and then remains constant while that at the other plate is maintained at the initial fluid temperature. The Laplace transform technique has been used to obtain the expressions for the velocity and temperature fields by solving the dimensionless governing partial differential equations under appropriate boundary conditions. The influence of the physical parameters on the velocity field, the temperature field, rate of heat transfer, skin-friction and volume flow rate of the fluid are analyzed systematically. The shear stress at the plate with ramped temperature boundary condition is significantly higher than that at the other plate because of the steeper velocity profiles in the vicinity. The Nusselt number at the plate with ramped temperature is much higher than that at the other plate indicating that much of the energy released from the plate because of its increasing temperature with time is convected out by the fluid before it reaches the second plate. The natural convection due to ramp heating has also been compared with the baseline case of flow with constant temperature.
Protection of an Aerowindow, One Scheme to Enhance the Cavity Efficiency of a Solid Particle Solar Receiver
Authors: Tan Taide , Chen Yitung
Published: June 2010

The existing open aperture of a Solid Particle Solar Receiver (SPSR) lowers the cavity efficiency by convection heat transfer. Aerowindows have the potential of increasing the efficiency of an SPSR. Aerodynamic windows consist of a transparent gas stream, which is injected from an air jet, across the receiver aperture to isolate its interior from the surrounding atmosphere. In the present paper, the influences of an aerowindow are investigated numerically on the cavity efficiency, particle exit temperature, and particle distribution of an SPSR. Different injection velocities, injection temperatures and injection directions of an air jet have been studied in order to form an efficient aerowindow. The numerical results provide a reference idea to enhance the performance in the conceptual design of an SPSR.
Analysis of Microscale Heat Transfer and Ultrafast Thermoelasticity in a Multi-Layered Metal Film
Authors: Tsai Tsung-Wen , Lee Yung-Ming , Liao Yang-Hsu
Published: June 2010

The micro-scale heat transfer and ultrafast thermoelasticity of a gold-chromium film subjected to ultra-short pulse laser heating is investigated. To predicate the thermal response accurately, the ballistic motion and hot electron diffusion are adopted in the laser source term. The ultrafast thermoelasticity (UTE) model with the modified laser heat source is applied to solve ultrafast thermoelastic behaviors inside a two-layered thin-film and the effect of the contact conductance on the thermo-elastic fields is included in the analysis. It is found that the excessive concentration stress appears at the interface due to the contact conductance effect. Therefore, the mechanical failure or damage may occur at the interface during the very early stage of the heating process even though the thermal resistance is extremely small (as small as 10−7 m2K/W).
Natural Convection Heat Transfer From a Horizontal Circular Cylinder With Constant Surface Heat Flux
Authors: Neagu Maria
Published: June 2010

This work analyses the natural convective heat transfer from a horizontal circular cylinder with constant surface heat flux using radial equally spaced exterior low conductivity baffles. The dimensionless governing equations in the stream function–vorticity formulation were solved using the finite difference method. The influence of the system parameters, the number of baffles and the baffle height, on the average Nusselt number is presented.
An Interaction of Natural Convective Heat Transfer From Two Adjacent Isothermal Narrow Vertical and Inclined Flat Plates
Authors: Kalendar Abdulrahim , Oosthuizen Patrick H. , Kalandar Bader
Published: June 2010

Natural convective heat transfer from a two narrow adjacent rectangular isothermal flat plates of the same size embedded in a plane adiabatic surface, the adiabatic surface being in the same plane as the surfaces of the heated plates, has been numerically investigated. The two plates have the same surface temperature and they are aligned with each other but are separated form each other by a relatively small gap. Results for the case where the plates are vertical and where they are inclined at positive or negative angles to the vertical have been obtained. It has been assumed that the fluid properties are constant except for the density change with temperature which gives rise to the buoyancy forces, this having been treated using the Boussinesq approach. It has also been assumed that the flow is symmetrical about the vertical center plane between the two plates. The solution has been obtained by numerically solving the full three-dimensional form of governing equations, these equations being written in dimensionless form. The solution was obtained using the commercial finite volume method based cfd code, FLUENT. The solution has the Rayleigh number, the dimensionless plate width, the angle of inclination, the dimensionless gap between two flat plates, and the Prandtl number as parameters. Results have only been obtained for a Prandtl number of 0.7 Results have been obtained for Rayleigh numbers between 103 and 107 for plate width-to-height ratios of between 0.15 and 0.6, for gap between the adjacent edges to plate height ratios of between 0 and 0.2, for angles of inclination between +45° and −45°.
Effect of Configuration on the Radiative Properties of High Density Fiber Composites
Authors: Lee Siu-Chun
Published: June 2010

The influence of the geometric arrangement of fiber bundles on the radiative properties of high density woven fiber composites are examined in this paper. Of particular interest is the effect of the polar orientation of fiber bundles on the angular variation of the extinction and scattering coefficients and scattering phase function. The configuration effect is examined by numerical analyses on four types of cross-ply composites with fiber bundles at specific polar inclinations. The numerical analyses utilized the theoretical model that accounts for dependent scattering within, and uncorrelated scattering between, the dense fiber bundles. The extinction and scattering coefficients and scattering phase function are shown to depend strongly on the spatial orientation of the fiber bundles. These results indicate the feasibility of customizing the radiative properties and thus radiative transport by tailoring the geometric configuration of the fiber bundles.
Finite Element Technique for Phase-Change Heat Conduction Problem
Authors: Nallathambi Ashok Kumar , Specht Eckehard , Bertram Albrecht
Published: June 2010

Liquid-solid phase transition is accompanied by a latent heat release, both in isothermal and non-isothermal phase transformations. The latent heat and the discontinuous phase change function increase the difficulty of obtaining a solution for the Fourier heat conduction equation. Celentano et al. [Int. J. Numer. Meth. Eng. 37(20), 1994] proposed a temperature-based finite element model for solving multidimensional transient heat conduction involving phase change. The present work addresses the computational aspects of the Celentano et al. model. The importance of a line search algorithm for improving the convergence of the Newton-Raphson iterations are explained in detail. While performing the iterations in this kind of fixed domain methods, the phase front moves back and forth fictitiously. The introduced phase change matrix handles the latent effect efficiently. The phase fractions are evaluated at the integration points instead of the nodal points. Several numerical examples are presented and the benefits and difficulties of the solution technique are elaborately discussed.
Cooling Enhancement of Radiators Using Dimples and Delta Winglets
Authors: Guntaka Anuradha , Gokhale Manoj , Dey Subhrajit , Tamma Bhaskar , Somani Ankit
Published: June 2010

Cooled exhaust gas recirculation and lower intake manifold temperature (post compressor) are used to meet emission regulations for a turbocharged intercooled diesel engine. This places a significant demand on the cooling load and space constraint on the radiator of the engine. A typical radiator is a cross-flow fin-tube heat exchanger with coolant water flowing inside the tube and ambient air taking out heat from the fin and tube surfaces. The major resistance to heat transfer in this configuration is offered by the air-side heat transfer co-efficient. The current study focuses on enhancing convective cooling rates on air side in a typical radiator which helps in taking additional load of EGR cooling with minimal increase in space and radiator fan power. Published literature clearly indicates that specific geometrical structures such as delta winglets and dimples, when placed in a convective flow path, act as vortex generators. This ability helps in disturbing/disrupting a steady thermal boundary layer, resulting in enhanced convective heat transfer. Detailed CFD simulations have been carried out to study the individual and combined effect of dimples and delta winglets on the heat transfer rates in a typical radiator geometry. Delta winglets on the fins indicated significant heat transfer enhancement but with increased pressure drop. Dimples on the tubes also led to enhanced heat transfer rates, but with a comparatively lesser increase in the pressure drop. A combination of delta winglets on the fins and dimples on the tubes increased the heat transfer rates substantially (+40%) with a minimal increase in pressure drop compared to the baseline case.
Visualization and Thermal Resistance Measurement for the Sintered Mesh-Wick Evaporator in Operating Flat-Plate Heat Pipes
Authors: Liou Jhan-Hong , Chang Chia-Wei , Chao Chi , Wong Shwin-Chung
Published: June 2010

This work presents visualization and measurement of the evaporation resistance for operating flat-plate heat pipes with sintered multi-layer copper-mesh wick. A glass plate was adopted as the top wall for visualization. The multi-layer copper-mesh wick was sintered on the copper bottom plate. With different combinations of 100 and 200 mesh screens, the wick thickness ranged from 0.26 mm to 0.8 mm. Uniform heating was applied to the base plate near one end with a heated surface of 1.1×1.1 cm2. At the other end was a cooling water jacket. At various water charges, the evaporation resistances were measured with evaporation behavior visualized for heat fluxes of 16–160 W/cm2. Quiescent surface evaporation without nucleate boiling was observed for all test conditions. With heat flux increased, the water film receded and the evaporation resistance reduced. The minimum evaporation resistances were found when a thin water film was sustained in the bottom mesh layer. With heat flux further increased, partial dry-out appeared with dry patches in the bottom mesh holes, first at the upstream end of the heated area and then expanded across the evaporator. The evaporation resistance rose in response to the appearance and expansion of partial dry-out. When the fine 200 mesh screen was used as the bottom layer, its thinner thickness and stronger capillarity led to smaller minimum evaporation resistances.
Thermal Modeling of the Human Eye as a Porous Structure
Authors: Shafahi Maryam , Vafai Kambiz
Published: June 2010

Human eye is one of the most sensitive parts of the body when exposed to radiation effects. Since there is no barrier (such as skin) to protect the eye against the absorption of the external thermal waves, radiation can readily interact with cornea. On the other hand, lack of blood flow in the interior part of the eye makes it more vulnerable compared to other organs even in the case of weak heat interaction. Further, blood flow circulation alone cannot establish thermal equilibrium between the eye and body organs effectively. There are limitations in measuring human eye temperature profile experimentally due to the required invasive procedures in monitoring the inner layers. Therefore, there is a need to develop an accurate model to represent the eye structure and energy transport through it. Thermal modeling of the eye is important to investigate the effect of external heat sources as well as in predicting the abnormalities within the eye. Modeling of heat transport through the human eye has been the subject of interest for years, but the application of porous media models in this field is new and will be one of the themes of this study. In this work, iris/sclear is considered as a porous medium and energy transport is modeled using the tissue local thermal equilibrium equations. The eye is assumed to include six different parts: cornea, anterior chamber, posterior chamber, iris/sclera, lens and vitreous. A two-dimensional finite element simulation will be performed. Results are shown in terms of transient corneal surface temperature, isothermal lines in different regions and local temperature of pupillary axis. Effects of external radiation sources, convection coefficient of the surrounding air, blood temperature, blood convection coefficient and ambient temperature on different regions of the eye are also investigated.
Experimental Study on Two-Phase Heat Transfer of FC-72 in Microchannels Heat Sink
Authors: Hu Xiao , Lin Guiping , Zhang Hongxing
Published: June 2010

A closed-loop two-phase microchannels cooling system using a micro-gear pump was built in this paper. The microchannels heat sink was made of oxygen-free copper, and 14 parallel microchannels with the dimension of 0.8mm(W)×1.5mm(D)×20mm(L) were formed by electric spark drilling followed by linear cutting which separated the channels from each other. The heat transfer performance was evaluated by the fluid temperature, the pressure drop across the micro-channels and the volumetric flow rate. Experiments were performed with refrigerant FC-72 which spanned the following conditions: initial pressure of Pin = 73 kPa, mass velocity of G = 94 – 333 kg/m2s, outlet quality of xe,out = 0 – superheat and heat flux of q″= 25–140 W/cm2. The result showed that, the maximum heat flux achieved 96 W/cm2, as the heating surface temperature was kept below 85 °C and critical heat flux occurred in the condition of low flow rate. Average two-phase heat transfer coefficients increased with the heat flux at low mass flux (G = 94 and 180 kg/m2s) and all heat fluxes, high mass flux (G = 333 kg/m2s) and all heat fluxes, and moderate mass fluxes (G = 224kg/m2s) under low and moderate heat fluxes (q″<110 W/cm2 for G = 224 kg/m2s), which was a feature of nucleate boiling mechanism. Pressure drop through microchannels heat sink was found to be below 4kPa.
Molecular Dynamics Simulations of the Thermal Conductivity of Bismuth Telluride Using Two-Body Interatomic Potentials
Authors: Qiu Bo , Ruan Xiulin
Published: June 2010

Two-body interatomic potentials in the Morse potential form have been developed for bismuth telluride, and the potentials are used in molecular dynamics (MD) simulations to predict the thermal conductivity. The density functional theory with local density approximations is first used to calculate the total energies for many artificially distorted Bi2Te3 configurations to produce the energy surface. Then by fitting to this energy surface and other experimental data, the Morse potential form is parameterized. The fitted empirical interatomic potentials are shown to reproduce the elastic and phonon data well. Molecular dynamics simulations are then performed to predict the thermal conductivity of bulk Bi2Te3 at different temperatures, and the results agree with experimental data well.
Experimental Demonstrations and Optimal Design Conditions of Snow-Melting System Using Geothermal and Solar Energy
Authors: Nagai Niro , Miyamoto Shigenobu , Tsuda Toru , Yamahata Shinya
Published: June 2010

The authors have been proposed and developed snow-melting system using geothermal and solar energy. In summer, solar heat is stored into underground from road surface to underground piles. In winter, the underground heat is utilized to melt snow on the road surface. This system was applied to parking lots and bridges of relatively small scale (less than 1000 m2). Numerical simulation program was also developed to predict temperature field of the system and to evaluate system performance. This program was verified by experimental data only for relatively small scale test area. In addition, appropriate design conditions, such as pile diameter, length and number, can not be easily estimated when road surface area and ability (average heat flux) of snow-melting are given. This paper aims to demonstrate the system for relatively large scale (larger than 1000 m2), and to obtain optimal design conditions of the system at given road surface area and ability. The snow-melting system using geothermal and solar energy was applied to a parking lot and a bridge of large scale. Both sites were under practical use which means cars were sometimes parked and run over the bridge. Obtained experimental data of temperature field of the system and snow melting situation show that numerical simulation program predicted system performance and temperature field adequately even though the program contains several simplifications. To discuss the optimal design conditions, numerical simulation was conducted by changing the following parameters: diameter, length, number and pitch of piles, pitch and diameter of heat dissipation pipe, flow rate of circulating water, road surface area. All these parameters are considered to affect system performance. The simulation results revealed that pile surface area determined by diameter, length and number of piles is the dominant parameter for deciding snow-melting ability. Namely, when road surface and snow-melting ability are given, necessary pile surface area can be obtained from the simulation results, and system design of piles becomes possible with considering cost for embedding piles.
The Application of Snapshot POD Method in Characterization and Analysis of Numerically Simulated Fire-Induced Flows
Authors: Yang Dong , Huo Ran , Hu Longhua , Xu Xiaoyuan , Tang Fei , Zhu Shi , Jiang Yaqiang
Published: June 2010

Snapshot proper orthogonal decomposition (POD) was performed on the analysis of dominant structures of fire-induced flows. The data for POD analysis was obtained from large eddy simulation (LES). Identification and analysis of dominant flow patterns have been carried out for two important types of fire-induced flows, including vertical plumes induced by pool fire and fire-induced horizontal channel flows. The essential features or energetic motions of these fire-induced flows were identified by combination of desired orders of POD modes. For the fire plumes, the counter-rotating vortex tubes were identified as the most dominant flow patterns. It is revealed that the oscillation dynamics of fire plume were related to the vortices near these vortex tubes. A larger number of small-scale structures and more structure scales were found in the fire plumes with higher Reynolds number (or higher heat release rate). For fire-induced horizontal channel flows, both the energy fractions and the structure patterns associated with POD modes depend more strongly on Reynolds number than those for fire plumes. The energy fractions contained within the most energetic modes significantly decrease with the increase of Reynolds number (or extraction flow rate) for fire-induced channel flows. It is found that the locations of strong vortices areas identified by POD mode are higher than the interface heights estimated by Janssens’ method, especially at the positions where counter flow mixing is strong.
A Numerical Procedure for the Evaluation of the Engine Head-Cylinder Group Cooling Effectiveness
Authors: Antonelli Marco , Martorano Luigi , Simi Alessio , Di Palma Stefano , Carapellucci Carlo
Published: June 2010

The purpose of this paper is to investigate the thermal flows and heat transfer phenomena occurring in the cooling circuit of a high specific power engine and to suggest a valid method to evaluate its effectiveness in keeping the temperature below a safety limit even in the highest thermal power points. This is a first work showing the analysis of the cooling circuit of a small single-cylinder, four-stroke, high power density gasoline engine carried out with a numerical three-dimensional CFD analysis by means of a CFD conjugate simulation, whose boundary conditions have been taken from a validated one-dimensional fluid dynamic engine model. Once its validity has been assessed by the comparison between the simulation results and data collected by literature and experiments, the interest for this procedure relies on the fact that heat fluxes are directly calculated by the CFD code through the knowledge of gas temperatures and convective heat transfer coefficients. Hence an arbitrary, a priori subdivision of the total heat flux released by fuel combustion into heat converted into mechanical work, heat released to the cooling system, heat rejected to the exhaust, etc. can be avoided; at the same time, the model provides the proper distribution of the heat rejected to the various surfaces constituting the water jackets. The evaluation of the effectiveness of the cooling system is then directly performed in terms of temperature distribution. By this way, once the engine has been designed from a fluid dynamic and mechanical point of view, the effectiveness of the cooling system can be immediately verified through the application of the described procedure. This study takes into consideration the evaluation of average and instantaneous heat transfer coefficient and in-cylinder gas temperature through the use of a validated 1D CFD model, the analysis of the temperature field by means of a conjugate heat transfer simulation of the whole head and cylinder group and an example of the application of this procedure for the evaluation of a simple modification of the cooling system.
Reduced Basis Methods and a Posteriori Error Estimators for Heat Transfer Problems
Authors: Rozza G. , Nguyen C. N. , Patera A. T. , Deparis S.
Published: June 2010

This paper focuses on the parametric study of steady and unsteady forced and natural convection problems by the certified reduced basis method. These problems are characterized by an input-output relationship in which given an input parameter vector — material properties, boundary conditions and sources, and geometry — we would like to compute certain outputs of engineering interest — heat fluxes and average temperatures. The certified reduced basis method provides both (i) a very inexpensive yet accurate output prediction, and (ii) a rigorous bound for the error in the reduced basis prediction relative to an underlying expensive high-fidelity finite element discretization. The feasibility and efficiency of the method is demonstrated for three natural convection model problems: a scalar steady forced convection problem in a rectangular channel is characterized by two parameters — Pe´clet number and the aspect ratio of the channel — and an output — the average temperature over the domain; a steady natural convection problem in a laterally heated cavity is characterized by three parameters — Grashof and Prandtl numbers, and the aspect ratio of the cavity — and an output — the inverse of the Nusselt number; and an unsteady natural convection problem in a laterally heated cavity is characterized by two parameters — Grashof and Prandtl numbers — and a time-dependent output — the average of the horizontal velocity over a specified area of the cavity.
System Instability of Evaporative Micro-Channels
Authors: Lee Hee Joon , Yao Shi-Chune
Published: June 2010

During the operation of parallel evaporative micro-channels, system instability may occur in terms of cyclical fluctuations at a long period. This is due to the co-existence of the liquid phase flow at high mass flux and the two-phase flow at a lower mass flux among different parallel channels under the same total pressure drop. For a system at constant flow-rate pumping, with a pressure regulating tank, and a constant heating pre-heater, the system may experience severe alternations between these two states of boiling and non-boiling with a period of minutes. This cyclical system instability has been modeled. In the model, the existence of the liquid phase flow happens at conditions of inlet subcooling and low surface heat flux that the boiling inception is hard to occur. Accordingly, the system instability criteria are established in terms of a System binary states parameter S and a non-dimensional surface heat flux, which is normalized with the boiling incipient heat flux. This model has been validated experimentally.
Processing and Thermal Conductivity of Carbon Nanotube-Reinforced Nickel Matrix Composites
Authors: Hwang J. Y. , Singh A. R. P. , Banerjee R. , Tiley J. , Choi T. Y.
Published: June 2010

Multi-wall carbon nanotube (MWCNT)-reinforced nickel composites have been manufactured in a bulk form by using a laser deposition technique, commercially known as the laser engineered net shaping (LENS™) process. These nanocomposites have been characterized in detail by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and high-resolution TEM study has also been conducted on these nanocomposites to characterize the nanotube/metal matrix interface. In addition, the thermal conductivities of Ni/CNT composites deposited by the LENS™ process have been measured by using Fourier Law of conduction in vacuum. The measurement did not show enhancement of thermal properties, which is caused by the inherent formation of voids and carbide formed during the LENS™ process.
CRAC Heat Exchanger Response to Step Change in Chilled Water Flowrate
Authors: Shields Shawn P. , Joshi Yogendra K. , Patterson Michael , Meakins Michael
Published: June 2010

This paper presents experimental data showing the response of a computer room air conditioning unit (CRAC) to chilled water (CHW) pump restart. The data are offered to improve and develop modeling of cooling equipment restart events following data center power failure. There are estimates that power failures will increase and limits on availability will affect data center operations at more than 90 percent of all companies over the next five years. Because providing backup power to cooling equipment increases data center first cost, it is important to have accurate models for cooling events and processes following power failure that help predict server inlet temperatures during the transient phase caused by a power failure. Since power density of computing equipment continues to rise, the temperature rise of air within the data center has been predicted to rise more quickly to an unacceptable level, increasing concern. Accurate models of CRAC response to pump restart can aid in data center cooling design, backup power infrastructure provisioning, and even compute equipment selection by predicting the air supply temperature after the generator provides power to the chilled water pump. Previous transient models include zonal models with large time scales and CFD/HT models with boundary conditions developed for steady state. These models can be improved by comparison with experimental data. The experiment consists of measuring the response of the CRAC heat exchanger to the step change in CHW flow rate upon pump restart. Inlet and outlet temperatures of both CHW and air were measured, as well CHW flow rate. A point measurement of air at the CRAC fan outlet was also taken to verify that airflow remained relatively constant. Outlet temperatures from the CRAC follow a first order response curve; it is found that the CRAC under consideration has fan outlet temperature time constant of 10 seconds. A delay of 20 seconds is observed between the fan outlet temperature response and the CHW return temperature response.
Heat Transfer at an Evaporating Front in Porous Media
Authors: Pakala V. K. Chaitanya , Plumb O. A.
Published: June 2010

Evaporating fronts propagate through porous media during drying processes, underground coal gasification, geothermal energy production from hot dry rock, and around nuclear waste repositories. Present work will focus on the one-dimensional heat transfer at the interface between vapor saturated porous matrix and water saturated porous region and evaluate the conditions for which various approximations yield an accurate representation of front velocity. An implicit finite difference scheme is utilized to simulate the propagation of an evaporating front in a porous medium saturated with water and undergoing the phase change process. The assumption of local thermal equilibrium (LTE) which results in a one-equation model and a simple two-equation model that does not assume LTE are examined by comparison with a quasi-analytic numerical model. We consider the case for low Reynolds number, hence Nusselt number is assumed constant. Results illustrate that the one-equation model does not yield accurate results even if the length scale for diffusion in the solid phase is relatively small. The one-equation model predicts faster front propagation than the two-equation model. It is illustrated that the one-equation model yields satisfactory results only when thermophysical properties characterized by the volume weighted ratio of thermal diffusivities is reduced to an order of magnitude less than those for the applications of interest. In addition, consistent with the established “rule of thumb”, for Biot < 0.1, the traditional two-equation model which makes the lumped capacitance assumption for the solid phase compares well with a two-equation model that more accurately predicts the time dependent diffusion in the solid phase using Duhamel’s theorem.
Numerical Study on Thermal Characteristics in Fluidized Beds Using DEM-CFD
Authors: Tian Fengguo , Zhang Mingchuan , Wu Bin , Fu Dong
Published: June 2010

This work used a pseudo three-dimensional discrete element method (DEM) to study the way gas supply patterns affect the thermodynamics characteristics in fluidized beds. During the simulations, gas-to-particle and particle-to-particle heat transfers were considered. Results indicate that there is a lateral temperature gradient of particles in fluidized bed using horizontal air distributor incorporated with even gas supply; nevertheless, in the case of inclined air distributor together with uneven gas supply, it takes a short time for particle phase to achieve a homogenous temperature field. Hydrodynamics analysis and comparison of solid fluxes between the two cases reveal that the bubbles’ lateral movement are reduced due to even gas supply, and the particle-to-gas heat transfer is localized; however, there is an global circulating solids stream in the bed with uneven gas supply, which is thought to expand the particles’ movement range and enhance the thermal transport performance of the fluidizing system.
Thermal Conductivity of Cubic and Hexagonal Mesoporous Silica Thin Films
Authors: Coquil Thomas , Hutchinson Neal , Pilon Laurent , Richman Erik , Tolbert Sarah
Published: June 2010

This paper reports the cross-plane thermal conductivity of highly ordered cubic and hexagonal templated mesoporous amorphous silica thin films synthesized by evaporation-induced self-assembly process. Cubic and hexagonal films featured spherical and cylindrical pores and average porosity of 25% and 45%, respectively. The pore diameter ranged from 3 to 18 nm and film thickness from 80 to 540 nm while the average wall thickness varied from 3 to 12 nm. The thermal conductivity was measured at room temperature using the 3ω method. The experimental setup and the associated analysis were validated by comparing the thermal conductivity measurements with data reported in the literature for the silicon substrate and for high quality thermal oxide thin films with thickness ranging from 100 to 500 nm. The cross-plane thermal conductivity of the synthesized mesoporous silica thin films does not show strong dependence on pore size, wall thickness, or film thickness. This is due to the fact that heat is mainly carried by very localized non propagating vibrational modes. The average thermal conductivity for the cubic mesoporous silica films was 0.30 ± 0.02 W/mK, while it was 0.20 ± 0.01 W/mK for the hexagonal films. This corresponds to a reduction of 79% and 86% from bulk fused silica at room temperature.
Simulation of Focused Radiation Propagation and Transient Heat Transfer in Turbid Tissues
Authors: Jiao Jian , Guo Zhixiong
Published: June 2010

Thermal analysis of a cylindrical tissue subject to a train of ultrashort pulse irradiations was made by developing a combined time-dependent radiation and conduction bio-heat transfer model. Ultrashort pulsed radiation transport in the cylindrical tissue is simulated using the transient discrete ordinate method. Treatment of focused beam is introduced. The model skin tissue is stratified as three layers with different optical, thermal and physiological properties. Comparisons between the collimated irradiation and focused beam are conducted. The effect of pulse train is investigated.
Simulation of Thermal Transport in a Nanocomposite Blow Mold
Authors: Alam M. K. , Klein P. , Garg D.
Published: June 2010

In this study, the thermal transport issues for a nanocomposite material used in the blow molding process are addressed in the fabrication of the composite mold. For low production cycles, there is a significant interest in using lower cost composite molds to replace the expensive traditional metal molds used for making polymer parts by the blow molding process. A critical issue in using a polymer matrix composite as an alternative to a metal mold is the large difference in the thermal transport property. The composite mold design must integrate enhanced cooling so that the product can cool sufficiently within each cycle time. Nanocomposites that use carbon nanofiber offer improvements in thermal and mechanical properties; therefore they are potential candidates for making molds for polymer products. This paper describes the design of the cooling system for a nanocomposite blow mold using numerical simulations; and the processing steps by which the design is incorporated in the fabrication of the mold.
Contribution of D-Band Electrons to Ballistic Electron Transport and Interfacial Scattering During Electron-Phonon Nonequilibrium in Thin Metal Films
Authors: Hopkins Patrick E.
Published: June 2010

Electron-interface scattering during electron-phonon nonequilibrium in thin films creates another pathway for electron system energy loss as characteristic lengths of thin films continue to decrease. As power densities in nanodevices increase, excitations of electrons from sub-conduction-band energy levels will become more probable. These sub-conduction-band electronic excitations significantly affect the material’s thermophysical properties. In this work, the effects of d-band electronic excitations are considered in electron energy transfer processes in thin metal films. In thin films with thicknesses less than the electron mean free path, ballistic electron transport leads to electron-interface scattering. The ballistic component of electron transport, leading to electron-interface scattering, is studied by a ballistic-diffusive approximation of the Boltzmann Transport Equation. The effects of d-band excitations on electron-interface energy transfer is analyzed during electron-phonon nonequilibrium after short pulsed laser heating in thin films.
A Method to Achieve Robust Aerodynamics and Enhancement of Updraft in Natural Draft Dry Cooling Towers
Authors: Chu Christopher C. M. , Rahman Md. Mizanur
Published: June 2010

A method to stabilize the draft through natural cooling towers is introduced. Natural draught dry cooling towers are widely used in arid regions of the world for the power industry especially those employing nuclear reactors. Their presence has become iconic of the process industry for their dominance of the landscape. These towers control the overall efficiency of power plants, and with the ongoing energy crisis it is desirable to raise efficiency by stabilising the draught through the tower. Energy comsumption is a substantial part of the overall cost of plant operation, and therefore even with a conservative 5 per cent improvement is feasible. It has been noted by some researchers like Baer, Ernst and Wurz (1980) that cooling towers do experience unstable flow with breezes. This phenomenon can be explained by Jo¨rg and Scorer (1967) to occur even in a still ambience with cold air inflow down into the tower shell from exit. Jo¨rg and Scorer (1967) developed a correlation to predict cold inflow to a glass tube for various fluids in a laboratory. By using their formula, it is found that under typical exit bulk velocities, of 3–5 m/s or below, cold air is liable to ‘sink’ into the shell, even in a quiescent surrounding. Indeed this phenomenon was demonstrated in the laboratory using a duct of size 457 × 457 mm2 of a heat exchanger by employing a smoke generator to detect that cold air did flow into the duct rather than the hot air filling the entire cross sectional area of the duct exit. A device was applied by Chu (1986) to prevent this cold air from sinking into the duct and enhance the stability and quantity of the updraft. In this paper, for the first time data obtained from a 700 × 700 mm2 cross-sectional flow area model air-cooled heat exchanger are presented that proves the air flow rate enhancement due to this device. It is hoped that more tests can be conducted to optimize the design for application in boiler chimneys and natural draught dry cooling towers.
Performance Improvement in Pulsating Heat Pipes Using a Self-Rewetting Fluid
Authors: Fumoto K. , Kawaji M.
Published: June 2010

New experimental results have been obtained on the enhancement of heat transport by a pulsating heat pipe (PHP) using a self-rewetting fluid. Self-rewetting fluids have a property that the surface tension increases with temperature unlike other common liquids. The increasing surface tension at a higher temperature could cause the liquid to be drawn towards a heated surface if a dry spot appears, and improving boiling heat transfer. In the present experiments, 1-butanol was added to water at a concentration of less than 1 wt% to make the self-rewetting fluid. A pulsating heat pipe made from an extruded multi-port tube was partially filled with the butanol-water mixture and tested for its heat transport capability at different input power levels. The experiments showed that the maximum heat transport capability was enhanced by a factor of four when the maximum heater temperature was limited to 120 °C. Thus, the use of a self-rewetting fluid in a PHP has been shown to be highly effective in improving the heat transport capability of pulsating heat pipes.
Numerical Simulation of Fluid Transport in a Single Micro-Nano Pore of Porous Membrane
Authors: Chu Yang , Lu Junfeng , Lu Wen-Qiang
Published: June 2010

In this paper, Smoothed Particle Hydrodynamics (SPH) and Molecular Dynamics (MD) methods are used to respectively simulate the flow and mass transfer phenomena inside a pore at microns scale, and then at nanons scale. The trans-membrane filtration velocities acquired using SPH method are in good agreement with the results calculated by simplified K-K equation when the pore’s diameter of the membrane is between 1 μm and 200 μm. The simulation accuracy is also discussed in the paper. However, the MD numerical results reveal that the micro-scale effect dominates and could not be neglected near the wall surface inside a nano sized pore. In this size, slip boundary appears at the wall surfaces. This situation is different from the flow in macro-scale that the velocity at the wall could be considered as zero. The MD numerical results also reveal the variational nature of the viscosity with the applied body force. Finally, we find that the ultra-filtration velocity acquired by MD simulation is different from the results estimated from the simplified K-K equation in nano-scale.
Heat Transfer in Encased Graphene
Authors: Chen Zhen , Jang Wanyoung , Bao Wenzhong , Lau Chun Ning , Dames Chris
Published: June 2010

Experimentally understanding the heat transfer in graphene (sheets of graphite a few atoms thick) is important for fundamental physics as well as device applications. In particular, measurements of the heat flow through graphene encased by oxide layers are essential for future graphene-based nanoelectronics, interconnects, and thermal management structures. Here we use a “heat spreader method” to study the heat dissipation performance of encased graphene. Measurements show enhanced heat spreading by a graphene layer as compared to control samples without graphene. At room temperature, the in-plane thermal conductivity of encased graphene sheets of thickness 2 nm and 5 nm is measured to be ∼150 W/m-K, more than one order of magnitude smaller than a published report for a freely-suspended graphene sheet [A. A. Balandin et al., Nano Lett. 8, 902], as well as bulk graphite. We also used a differential 3ω method to measure the thermal contact resistance between graphene and SiO2, finding a value around 10−8 m2-K/W at room temperature. Possible reasons for the unexpectedly low thermal conductivity are also discussed.
Semi Analytical Prediction of Automobile Body Temperature Distribution in the Top Coat Paint Oven
Authors: Hanafizadeh P. , Sajadi B. , Saidi M. H. , Khalkhali H. , Taherraftar M.
Published: June 2010

Automotive industry frequently needs to test new products, according to different production parameters, in order to determine the actual thermal behavior of bodies before mass production is implemented. Numerical simulation of these processes can reduce the very expensive and time consuming experimental procedures. For the drying and hardening process of the top paint applied in the coating process, the body temperature must be raised according to the paint manufacturer regulations. Consequently, prediction of temperature distribution of the car body during various zones of ovens is very vital in the design and performance analysis of the paint dryers. In this research, a novel semi-analytical approach has been used to predict the body temperature variation during the curing process. Considering the energy balance for the body, a set of differential equation has been extracted, depending on the oven zone. These equations can be solved numerically to find the transient temperature profile of the car body. Some parameters in these equations have been achieved by experimental procedure. The results show that the present model predictions are in a good agreement with the experimental data. Therefore, the developed model has a reasonable accuracy and can be used as an efficient robust approach to distinguish overall thermal behavior of the body. These techniques can be used to optimize the design of curing paint oven.
The Role of Aperture in the Extraction of Functional Components From Plant Materials
Authors: Di Qianqian , Yang Junhong , Zhao Jun , Wang Liqiu
Published: June 2010

The extraction of functional components from natural plant is one of important processing in the development and further practical application of plant product. Microwave assisted extraction (MAE) has been widely used in the extraction of many samples for its unique heating mechanism, short extraction time and high yield of extract. Astragalus slice is a special and typical plant porous media. We describe an approach by scanning electronic microscope (SEM) to characterize the trachea and aperture of Astragalus slices irradiated 20 min by microwave at 600 W and 900 W, with the aim to analyze the effect of the microwave power on the extraction yield by SEM and discuss further the relationship between the microstructure characteristics of sample and mechanism on mass transfer in micro-scale. It is found that extract with the 20 min irradiation of microwave at 600 W is higher than that at 900 W because the apertures on the trachea wall remain open at 600 W, but shrink at 900 W. Moreover, we confirm the important role of the aperture in the extraction of plant materials. Therefore, this study has significant meanings to develop the optimized extraction technology of plant porous media and maintain or improve the quality of extract.
Comparison of Steady and Unsteady RANS Heat Transfer Simulations of Hub and Endwall of a Turbine Blade Passage
Authors: Ameri Ali A. , El-Gabry Lamyaa A.
Published: June 2010

The necessity of performing an unsteady simulation for the purpose of predicting the heat transfer on the endwall surfaces of a turbine passage is addressed. This is measured by the difference between the two solutions obtained from a steady simulation and time average of an unsteady simulation. The heat transfer coefficient (Nusselt number) based on the adiabatic wall temperature is used as the basis of the comparison. As there is no film cooling in the proposed case, a heat transfer coefficient so computed should be a better measure of such difference than say, wall heat flux. Results show that the effect of unsteadiness due to wake passage on the pressures and recovery temperatures on both the hub and casing is negligible. Heat transfer on the endwalls, however, is affected by the unsteady wake; the time-average results yield higher heat transfer; in some regions up to 15% higher. The results for the endwall heat transfer were compared to results in the open literature and are found to be comparable.