Enhanced heat transfer analysis of latent functionally thermal fluid

A physical model has been developed to analyze the enhanced heat transfer process of the latent functionally thermal fluid with microencapsulated phase‐change material. The problem is solved by the combination of the finite difference method and the moving heat source method. The calculated results reveal that putting the phase‐change microcapsules into the fluids can enhance the heat transfer capabilities of the mixture. The effects of capsule radius and concentration of particles are numerically predicted. The numerical results provide the theoretical basis for the application and design of the latent functionally thermal fluid. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(6): 383–392, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20025     

Research on the human thermal model with a poly‐segmented hand

A more integral human thermal model was built by combining the human thermal cylindrical model and the manual poly‐segment thermal model. Finite element methods (FEM) was used to define the body thermal model. It was in good agreement with the experimental results. The results show: the experimental results are consistent with the calculated value, when suitable blood flux is taken into consideration. The blood flux is in a certain range when the manual temperature is stable. Blood flux is the major factor in the manual temperature field. Body temperature and intake artery temperature have little effect on the hand temperature. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(2): 94–100, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20187     

High‐temperature stability in yttria‐stabilized zirconia

Yttria‐stabilized zirconia has been studied as a candidate standard reference material for the determination of thermal properties. This study evaluated the high‐temperature stability, which is important for yttria‐stabilized zirconia to be used for a standard reference material, using a common material of fine ceramics, Referceram ZR1 (zirconia). The high‐temperature stability was evaluated by measuring the change in the thermal diffusivity before and after heat treatment at temperatures between 200 ^°C and 1500 ^°C. No change in the thermal diffusivity was observed when the samples were treated at temperatures equal to or below 900 ^°C. However, it was revealed that the changes in the thermal diffusivity were caused by the transformation and separation of the crystal phase and the cracks that occur at grain boundaries when the samples were treated at temperatures equal to or above 1000 ^°C. From these results, we confirmed that Referceram ZR1 is sufficiently stable for use as a reference material at temperatures equal to or below 900 ^°C. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(2): 57–67, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20193     

Numerical simulation of high‐temperature phase change heat storage system

In this paper, numerical results pertaining to cyclic melting and freezing of an encapsulated phase‐change material (PCM) have been reported. The cyclic nature of the present problem is relevant to latent heat thermal energy storage system used to power solar Brayton engines in space. In particular, a physical and numerical model of the single‐tube phase change heat storage system was developed. A high‐temperature eutectic mixture of LiF‐CaF₂ was used as the PCM and dry air was used as the working fluid. Numerical results were compared with available experimental data. The trends were in close agreement. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 32–41, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10132     

Effects of a Nanofluid and Magnetic Field on the Thermal Efficiency of a Two‐Phase Closed Thermosyphon

Heat transfer of a CuO/water nanofluid in a two‐phase closed thermosyphon (TPCT) that is thermally enhanced by a magnetic field has been predicted by an optimized artificial neural network (ANN). The magnetic field strength, volume fraction of nanoparticles in water, and inlet power were used as input parameters and the thermal efficiency was used as the output parameters. The correlation coefficient (R² = 0.924), mean square error (MSE = 0.000340231), mean absolute error (MAE = 0.012410941), and normalized mean‐squared error (NMSE = 0.112417498) between the measured and predicted thermal efficiency by the ANN model were developed. The results were compared with experimental data and it was found that the thermal efficiency estimated by the multi‐layer perception neural network is accurate. In this study, a new approach for the auto‐design of neural networks, based on a genetic algorithm, has been used to predict collection output of a TPCT. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 42(7): 630–650, 2013; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21043     

Prediction of vortex penetration depth at thermal stratification by cavity flow in a branch pipe with closed end (effect of heat radiation condition on temperature fluctuations)

In a branch pipe with one closed end, the cavity flow penetrates into the branch pipe from the main loop and a thermal boundary layer occurs because the cavity flow is a hot fluid, but heat removal causes a colder fluid in the branch pipe. This thermal stratification may affect the structural integrity. Therefore, a pipe design standard to suppress thermal fatigue should be established. The pipe design standard consists of the maximum penetration depth L_sv and the minimum penetration depth L_sh. In order to establish an evaluation method for L_sh, a visualization test and a temperature fluctuation test were carried out. A theoretical formula for thermal stratification was introduced from the heat balance model. Then the model was used to obtain an empirical equation from the map of fluid temperature fluctuation. This method can predict the vortex penetration depth by cavity flow in horizontal branch pipes. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(1):38–55, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20135     

An application of thermal microscopes to the measurement of thermal effusivity of films

The application of thermal microscopes to the measurement of local thermal properties has drawn considerable scientific interest. We report on the application of a thermal microscope to the measurement of thermal effusivity for films comprising alumina deposited on a substrate, which were fabricated by an electrophoretic deposition method. The measured data was analyzed to consider the undulations on the sample surface The thermal effusivity of these samples was approximately 1×10³ Js^?0.5m^?2K^?1; this value is smaller than that for dense alumina because the alumina grain makes contact with a point. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20227     

Adhesion and detachment characteristics of a TBAB hydrate solid on a heat transfer surface (Effect of concentration of TBAB solutions)

In air‐conditioning systems, it is desirable that the liquid–solid phase change temperature of a cool energy storage material be approximately 10^°C, with respect to improving the coefficient of performance (COP). Moreover, a thermal storage material that forms slurry can realize a large heat capacity of the working fluids. A solid that adheres to the heat transfer surface forms a thermal resistance layer and significantly reduces the rate of cold storage; therefore, it is important to avoid the adhesion of a thick solid layer on the surface so as to realize efficient energy storage. Considering a harvest type cooling unit, the force required for removal of the solid phase from the heat transfer surface was investigated. Tetra‐n‐butylammonium bromide (TBAB) clathrate hydrate was used as a cold storage material and the effect of the TBAB solution concentration on the scraping force required to detach the adhered TBAB hydrate solid from the heat transfer surface was experimentally examined. The TBAB hydrate solids were broadly categorized into two types, and the scraping force required for removal of these two types of TBAB hydrate solid was different. The scraping force required for removal of the solid increased due to the effect of increasing the concentration of the TBAB solution. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20254     

板坯质量诊断模型重要钢种热物性参数的计算方法

对板坯质量在线诊断分析预测模型CISDI_SQDS ONLINER2011中耦合考虑的几个重要钢种热物性参数的冶金物理意义和数值计算方法进行了详细讨论,包括线性热膨胀系数、临界应变、临界应力和凝固潜热,模型投入实际运行后计算结果合理、计算过程可靠,表明模型在进行理论分析时所引入的热物性参数计算方法具有合理性和可靠性。     

Battery thermal models for hybrid vehicle simulations

This paper summarizes battery thermal modeling capabilities for: (1) an advanced vehicle simulator (ADVISOR); and (2) battery module and pack thermal design. The National Renewable Energy Laboratory’s (NREL’s) ADVISOR is developed in the Matlab/Simulink environment. There are several battery models in ADVISOR for various chemistry types. Each one of these models requires a thermal model to predict the temperature change that could affect battery performance parameters, such as resistance, capacity and state of charges. A lumped capacitance battery thermal model in the Matlab/Simulink environment was developed that included the ADVISOR battery performance models. For thermal evaluation and design of battery modules and packs, NREL has been using various computer aided engineering tools including commercial finite element analysis software. This paper will discuss the thermal ADVISOR battery model and its results, along with the results of finite element modeling that were presented at the workshop on “Development of Advanced Battery Engineering Models” in August 2001.     

Slip and micro flow characteristics near a wall of evaporating thin films in a micro channel

The microscopic liquid flow and heat transfer characteristics near the solid–liquid interface in the evaporating thin film region of a mini channel were investigated based on the augmented Young–Laplace equation and kinetic theory. A physical model using the boundary layer approximation and a constant slip length was developed to obtain the solid–liquid interfacial thermal resistances and interfacial temperatures. The results show that the ordered micro layer and micro flow near the wall reduce the effective liquid superheat and the liquid pressure difference mainly due to the reduced capillary pressure gradient. The solid–liquid interfacial thermal resistances and U‐shaped temperature drops tend to reduce the thin film spreading and heat transfer. The effects of the solid–liquid interfacial thermal resistances on the thin film evaporation outweigh the effects of the thermal conductivity enhancement due to the liquid ordering. The concepts of the micro flow and ordered adsorbed flowing micro layer are clarified to express the Kapitza resistance analytically in terms of the slip length and micro layer thickness. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; 39(7): 460–474, 2010; Published online 3 June 2010 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20310     

Convective‐radiative fins with simultaneous variation of thermal conductivity,heat transfer coefficient,and surface emissivity with temperature

This paper is a numerical study of thermal performance of a convective‐radiative fin with simultaneous variation of thermal conductivity, heat transfer coefficient, and surface emissivity with temperature. The convective heat transfer is assumed to be a power function of the local temperature between the fin and the ambient which allows simulation of different convection mechanisms such as natural convection (laminar and turbulent), boiling, etc. The thermal conductivity and the surface emissivity are treated as linear functions of the local temperature between the fin and the ambient which provide a satisfactory representation of the thermal property variations of most fin materials. The thermal performance is governed by seven parameters, namely, convection–conduction parameter Nc, radiation–conduction parameter Nr, thermal conductivity parameter A, emissivity parameter B, the exponent n associated with convective heat transfer coefficient, and the two temperature ratios, θ_a and θ_s, that characterize the temperatures of convection and radiation sinks. The effect of these parameters on the temperature distribution and fin heat transfer rate are illustrated and the results interpreted in physical terms. Compared with the constant properties model, the fin heat transfer rate can be underestimated or overestimated considerably depending on the values of the governing parameters. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20408     

A reversible thermal panel for spacecraft thermal control (evaluation of effectiveness and reliability of new autonomous thermal control device)

The current trend in spacecraft design toward high‐density packing of the payload electronics and increased waste heat flux will require the development of lightweight high thermal conductive materials and innovative thermal control techniques. Additionally, new challenging missions will require new techniques to adapt to a variety of environmental conditions. In this study, a new passive thermal control device—a Reversible Thermal Panel (RTP) based on high thermal conductive graphite sheets and shape memory alloy—is proposed. The RTP changes its function reversibly from radiator to solar absorber by deploying/stowing a reversible fin. Thermal analyses were conducted to predict the fundamental thermal performance of the RTP. A breadboard model of the RTP based on the graphite sheets and aluminum alloys was fabricated, and the thermal vacuum test was conducted with the reversible fin deployed and stowed. The desired variability in thermal performance of the RTP due to deploying or stowing the reversible fin was demonstrated. Comparison of the predicted results with test results indicated a good agreement. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(5): 350–367, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20060     

Numerical investigation of mixing length on performance of lobed forced mixer nozzles

Geometric models of a lobed mixer nozzle with variation of mixing length are created and corresponding flow fields are simulated using a steady Reynolds Averaged Navier–Stokes (RANS) equation with realizable k–? turbulence model. The numerical simulation results show that the mixing length of the nozzle has a relatively great influence on the development of streamwise vortices and the effective range for streamwise vortices to intensify mixing is about 0.5D distance from the lobe trailing edge. The change of mixing length has little effect on the thermal mixing efficiency. As for the total pressure recovery coefficient, the shorter the mixing length, the sharper the total pressure recovery coefficient curve. On the nozzle exit section, the thermal mixing efficiency increases at first and then slightly declines, and the total pressure recovery efficiency changes little as the mixing length increases. In addition, the thrust coefficient has some small relationship with the mixing length. The thrust coefficient increases in waves as the mixing length increases and the difference between the maximum and the minimum values is slightly less than 1% when the mixing length increases from 0.4D to D. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library wileyonlinelibrary.com/journal/htj . DOI 10.1002/htj.20343     

Thermal conductivity of carbon nanotube nanofluid—Experimental and theoretical study

In this work, thermal conductivity of carbon nanotubes' (CNTs) nanofluid is studied both experimentally and theoretically. CNT nanofluids were stabilized using gum arabic (GA). The concentration of CNTs was varied from 0.01–0.1 wt% while the concentration of GA was varied from 1–2.5 wt%, respectively. The effect of particle volume fraction and temperature on the thermal conductivity enhancement of the nanofluids was also studied. A simple thermal conductivity model which demonstrates the effect of diameter and aspect ratio of the CNTs and takes into account the effect of temperature on thermal conductivity enhancement is presented. Good agreement between experimental and estimated values proves that the proposed model can provide precise prediction of the thermal conductivity of fluid containing CNTs. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20405     

THERMAL STRESS ANALYSIS OF THERMOVISCOELASTIC HOLLOW CYLINDER WITH TEMPERATURE-DEPENDENT THERMAL PROPERTIES

ABSTRACT

We analyzed the thermal stress on a thermoviscoelastic hollow cylinder with temperature-dependent thermal properties with the finite difference method. It was gradually heated at the inner surface and the outer surface was kept at the initial temperature. The cylinder material was thermorheologically simple and had a temperature-dependent coefficient of linear thermal expansion, thermal conductivity, and thermal diffusivity (and/or specific heat). A bisphenol A–type epoxy resin was chosen as the thermoviscoelastic material of the cylinder for numerical analysis. Based on these results, we discuss the effects of thermoviscoelasticity and temperature-dependent thermal properties on the stress field.     

Performance of a double pass solar air collector

Double pass counter flow solar air collector with porous material in the second air passage is one of the important and attractive design improvement that has been proposed to improve the thermal performance. This paper presents theoretical and experimental analysis of double pass solar air collector with and without porous material. A mathematical model has been developed based on volumetric heat transfer coefficient. Effects of various parameters on the thermal performance and pressure drop characteristics have been discussed. Comparison of results reveals that the thermal efficiency of double pass solar air collector with porous absorbing material is 20-25% and 30-35% higher than that of double pass solar air collector without porous absorbing material and single pass collector respectively.     

Diagnostic technique for degradation of engineering materials by measurement of thermophysical properties

The thermal conductivity of the surface layer of engineering materials changes in the early stages of material degradation due to the appearance of micro‐cracks. A new method for evaluating and assessing the degree of degradation of a material using this change is proposed herein. The influence of the micro‐cracked layer on the temperature response as measured by a thermophysical handy tester was theoretically examined. By defining a thermal degradation parameter, the amount of degradation of various materials was evaluated. In order to verify this theory, fatigue tests using metallic materials were conducted. Comparisons before and after the fatigue tests were then made to establish the correlation between the density of micro‐cracks and the decrease in thermal conductivity. As a result, it was ascertained that a thermal degradation parameter can easily be estimated from a temperature response curve obtained using the tester. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 501–512, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20178