Analytical transient phase change heat transfer model of wearable electronics with a thermal protection substrate

As thermal protection substrates for wearable electronics, functional soft composites made of polymer materials embedded with phase change materials and metal layers demonstrate unique capabilities for the thermal protection of human skin. Here,we develop an analytical transient phase change heat transfer model to investigate the thermal performance of a wearable electronic device with a thermal protection substrate.The model is validated by experiments and the finite element analysis(FEA). The ...     

岩土介质非稳态热固结耦合问题的热源函数法

考虑耦合效应的饱和土体热固结问题控制方程,利用Fourier变换、Laplace变换给出其在变换域上的解,将初始温度场分布视为虚拟的热源或者将热源等价为特定的初始温度分布,利用热源函数法给出瞬时线热源非稳态温度场、应力场和位移场的解析求解方法,通过在时间域和空间域上进行积分,给出有初始温度场分布以及有分布内热源存在且热源强度随时间变化条件下的热固结问题计算方法。对一无限大物体内存在有平面矩形域热源情况下周围介质的温度、孔隙水压力以及位移等的变化特征进行分析。研究表明,热源函数法可有效地求解一系列复杂情况下的热固结问题。     

Free Convection Heat Transfer From A Sphere In A Porous Medium Using A Thermal Non-equilibrium Model

This work studies the free convection heat transfer from a sphere with constant wall temperature embedded in a fluid-saturated porous medium using a thermal non-equilibrium model. The governing equations are transformed into boundary-layer partial differential equations by the coordinate transform, and the obtained governing equations are then solved by the cubic spline collocation method. The temperature distributions for fluid and solid phases are shown for different values of the porosity scaled thermal conductivity ratio, the interphase heat transfer parameter, and the streamwise coordinate. The effects of the porosity scaled thermal conductivity ratio and the interphase heat transfer parameter between solid and fluid phases on the local Nusselt numbers for fluid and solid phases are examined. Results show the local Nusset number for the porous medium can be increased by increasing the porosity scaled thermal conductivity ratio. Moreover, the thermal non-equilibrium effect is more significant for low values of the porosity scaled thermal conductivity ratio or the interphase heat transfer parameter.     

Stochastic transient analysis of thermal stresses in solids by explicit time-domain method

Stochastic heat conduction and thermal stress analysis of structures has received considerable attention in recent years. The propagation of uncertain thermal environments will lead to stochastic variations in temperature fields and thermal stresses. Therefore, it is reasonable to consider the variability of thermal environments while conducting thermal analysis. However, for ambient thermal excitations, only stationary random processes have been investigated thus far. In this study, the highly efficient explicit time-domain method(ETDM) is proposed for the analysis of non-stationary stochastic transient heat conduction and thermal stress problems. The explicit time-domain expressions of thermal responses are first constructed for a thermoelastic body. Then the statistical moments of thermal displacements and stresses can be directly obtained based on the explicit expressions of thermal responses. A numerical example involving non-stationary stochastic internal heat generation rate is investigated. The accuracy and efficiency of the proposed method are validated by comparison with the Monte-Carlo simulation.     

Heat transfer mechanism of miniature loop heat pipe with water-copper nanofluid: thermodynamics model and experimental study

In order to ensure the normal work of electronic product, the thermal management is of key importance. Miniature loop heat pipe (mLHP) is a promising device of heat transfer for electronic products. Cu-water nanofluid with different concentration is used as working material in mLHP. Experiments are conducted to investigate its heat transfer performance. The heat flux owing to thermal diffusion is calculated. It is found that this heat flux and the boiling temperature are non-monotonic function of concentration of nanoparticle. Turning concentration appears at about 1.5 wt%. Differential equation of thermal diffusion produced by micro movement of nanoparticle is established in this paper. Average speed formula for nanoparticles is derived and slope of the curve of phase equilibrium is obtained. Based on the theoretical research in this paper, enhanced heat transfer mechanism of nanofluid is analyzed. The facts that heat flux owing to thermal diffusion and boiling temperature are all associated with nanoparticle concentration are also well explained with the aid of the derived theory in this paper.     

Nonsimilar Boundary Layer Analysis of Free Convection Heat Transfer Over a Vertical Cylinder in Bidisperse Porous Media

This work presents a boundary layer analysis for the free convection heat transfer from a vertical cylinder in bidisperse porous media with constant wall temperature. A boundary layer analysis and the two-velocity two-temperature formulation are used to derive the nonsimilar governing equations. The transformed governing equations are solved by the cubic spline collocation method to yield computationally efficient numerical solutions. The effects of inter-phase heat transfer parameter, modified thermal conductivity ratio, and permeability ratio on the heat transfer and flow characteristics are studied. Results show that an increase in the modified thermal conductivity ratio and the permeability ratio can effectively enhance the free convection heat transfer of the vertical cylinder in a bidisperse porous medium. Moreover, the thermal nonequilibrium effects are strong for low values of the inter-phase heat transfer parameter.     

L- and U-shaped heat pipes thermal modules with twin fans for cooling of electronic system under variable heat source areas

This study utilizes a versatile superposition method with thermal resistance network analysis to design and experiment on a thermal module with embedded six L-shaped or two U-shaped heat pipes and plate fins under different fan speeds and heat source areas. This type of heat pipes-heat sink module successively transfer heat capacity from a heat source to the heat pipes, the heat sink and their surroundings, and are suitable for cooling electronic systems via forced convection mechanism. The thermal resistances contain all major components from the thermal interface through the heat pipes and fins. Thermal performance testing shows that the lowest thermal resistances of the representative L- and U-shaped heat pipes-heat sink thermal modules are respectively 0.25 and 0.17 °C/W under twin fans of 3,000 RPM and 30 × 30 mm² heat sources. The result of this work is a useful thermal management method to facilitate rapid analysis.     

Forced convective heat transfer in porous medium of wire screen meshes

The hydrodynamic and heat transfer characteristics of a porous medium consisting of 20 wire screen meshes are examined theoretically and experimentally. The hydrodynamic experiments are conducted for the range of Reynolds number based on mean velocity and wire diameter from 1.5 to 12. The Ergun's constants and thermal dispersion coefficients are calculated in this range. Nusselt number variation is determined in both thermally developing and fully developed flows by the help of forced convection heat transfer experiments conducted for the uniform heat flux boundary condition. Correlation functions of Nusselt number in the range of fully developed and thermally developing, and of thermal entrance length are obtained from experimental data. Solutions of momentum and energy equations simulating the experimental model are obtained numerically with variable porosity and the anticipated thermal dispersion coefficients. The thermal dispersion coefficients well-adjusted to the experimental data are determined by numerical solution of the energy equation. Received on 22 November 1996     

Transient thermal cracking associated with non-classical heat conduction in cylindrical coordinate system

This paper studies the fracture behavior of a thermoelastic cylinder subjected to a sudden temperature change on its outer surface within the framework of non-classical heat conduction.The heat conduction equation is solved by separation of variable technique.Closed form solution for the temperature field and the associated thermal stress are established.The critical parameter governing the level of the transient thermal stress is identified.Exact expression for the transient stress intensity factor is obtained for a crack in the cylinder.The difference between the non-classical solutions and the classical solution are discussed.It is found that the traditional classical heat conduction considerably underestimates the transient thermal stress and thermal stress intensity factor.     

十次对称二维准晶中的热应力分析

推导了当考虑热效应时十次对称二维准晶体平面应变问题的通解表示．作为应用，采用所获得的通解首先得到了十次对称二维准晶体中的一个点热源所引起的声子场和相位子场，给出了点热源所引起的声子场和相位子场应力分量的解析表达式；接着获得了在均匀热流作用下十次对称二维准晶体中-绝缘椭圆孔洞所引起的热应力问题的弹性解答，给出了沿椭圆边界环向应力分布的解析表达式；当椭圆的短轴趋于零时，则获得了裂纹问题的解答，给出了应力强度因子、裂纹表面张开位移及能量释放率的解析表达式；推导了在任意热载荷作用下裂尖附近的渐近场．     

Thermal analyses of LiCoO2 lithium-ion battery during oven tests

A three dimensional thermal abuse model for graphite/LiPF₆/LiCoO₂ batteries is established particularly for oven tests. To investigate the influence of heat release condition and oven temperature on battery thermal behaviors, we perform a series of simulations with respect to a unit cell during oven thermal abuses of various oven temperatures and under various heat release conditions. Simulation results enable detailed analyses to thermal behaviors of batteries. It is found that during oven thermal abuse processes that do not get into thermal runaway, the negative electrode is the maximum heat generation rate zone; during oven thermal abuse processes that do get into thermal runaway, the positive electrode is the maximum heat generation rate zone. The positive-solvent reaction is found to be the major heat generation source causing thermal runaway. It is also found that the heat release condition and the oven temperature are combined to dictate thermal behaviors of the battery. The critical oven temperature that causes thermal runaway rises if the heat release condition is better and the critical heat release coefficient that can effectively restrain the occurrence of thermal runaway increases with the increase of oven temperature.     

HEAT-PIPE-COOLED THERMAL PROTECTION FOR THE LEADING EDGE OF THE WING

针对高超声速飞行器飞行时翼前缘存在着严重的气动加热问题,为了保证翼前缘的尖锐外形,提出疏导式热防护结构,利用内置高温热管结构为翼前缘提供热防护。采用数值模拟和电弧风洞试验的方法对翼前缘疏导式结构进行了分析,得到翼前缘内置高温热管具有的防热效果。数值模拟结果表明在一定热环境条件下,翼前缘驻点温度下降了304K,尾部最低温度升高了130K,实现了热流从高温区到低温区的疏导,减弱了翼前缘的热载荷,强化了翼前缘的热防护能力。通过电弧风洞试验可以获得相同的热防护结果,并且在一定飞行条件下高温热管可以自适应启动,验证了数值模拟方法的准确性以及翼前缘内置高温热管疏导式热防护结构的可行性。     

Dynamics of Sutterby fluid flow due to a spinning stretching disk with non-Fourier/Fick heat and mass flux models

The magnetohydrodynamic Sutterby fluid flow instigated by a spinning stretchable disk is modeled in this study. The Stefan blowing and heat and mass flux aspects are incorporated in the thermal phenomenon. The conventional models for heat and mass flux, i.e., Fourier and Fick models, are modified using the Cattaneo-Christov(CC)model for the more accurate modeling of the process. The boundary layer equations that govern this problem are solved using the apt similarity variables. The subsequent system of equations is tackled by the Runge-Kutta-Fehlberg(RKF) scheme. The graphical visualizations of the results are discussed with the physical significance. The rates of mass and heat transmission are evaluated for the augmentation in the pertinent parameters. The Stefan blowing leads to more species diffusion which in turn increases the concentration field of the fluid. The external magnetism is observed to decrease the velocity field. Also,more thermal relaxation leads to a lower thermal field which is due to the increased time required to transfer the heat among fluid particles. The heat transport is enhanced by the stretching of the rotating disk.     

疏导式热防护在翼前缘中的应用探索

针对高超声速飞行器飞行时翼前缘存在着严重的气动加热问题,为了保证翼前缘的尖锐外形,提出疏导式热防护结构,利用内置高温热管结构为翼前缘提供热防护。采用数值模拟和电弧风洞试验的方法对翼前缘疏导式结构进行了分析,得到翼前缘内置高温热管具有的防热效果。数值模拟结果表明在一定热环境条件下,翼前缘驻点温度下降了304K,尾部最低温度升高了130K,实现了热流从高温区到低温区的疏导,减弱了翼前缘的热载荷,强化了翼前缘的热防护能力。通过电弧风洞试验可以获得相同的热防护结果,并且在一定飞行条件下高温热管可以自适应启动,验证了数值模拟方法的准确性以及翼前缘内置高温热管疏导式热防护结构的可行性。     

竖向裂缝对功能梯度板的热响应影响

采用条单元法分析了竖向裂缝对功能梯度板的热响应影响。功能梯度板的材料属性沿板厚方向连续线性变化,温度变化热源作用于带竖向裂缝的功能梯度板上。通过研究得出了裂缝距离热源不同位置、不同长度裂缝及不同频率温度变化热源等状态时,功能梯度板上表面的热响应情况。采用条单元法研究带竖向裂缝的功能梯度板的热响应问题,为带裂缝的功能梯度板的热研究提供了一定的指导。     

Effects of thermal dispersion on heat transfer in cross-flow tubular heat exchangers

Effects of thermal dispersion on heat transfer and temperature field within cross-flow tubular heat exchangers are investigated both analytically and numerically, exploiting the volume averaging theory in porous media. Thermal dispersion caused by fluid mixing due to the presence of the obstacles plays an important role in enhancing heat transfer. Therefore, it must be taken into account for accurate estimations of the exit temperature and total heat transfer rate. It is shown that the thermal dispersion coefficient is inversely proportional to the interstitial heat transfer coefficient. The present analysis reveals that conventional estimations without consideration of the thermal dispersion result in errors in the fluid temperature development and underestimation of the total heat transfer rate.     

A fitting, simple and versatile window program (HSHPTM) design using lumped parameters and one-dimensional thermal resistance models

A heat sink-heat pipes thermal module (HSHPTM) applies U-type or L-type heat pipes to transferring the total heat capacity from the heat source to the based plate and fins successfully, and then dissipates heat flow into the surrounding air. This article utilizes Visual Basic commercial software to develop a window program named HSHPTM V1.0 for proper design of the heat sink-heat pipes thermal module. The computing core of the HSHPTM program employs the theoretical thermal resistance analytical approach with iterative convergence stated in this study to obtain a numerical solution. The results show that this calculated error comparison with experimental results is within ±5 %. The embedded U-type heat pipes carry 46 and 63 percentages of the total dissipated heat capacity for one-pair and two-pairs embedded U-type heat sink-heat pipes thermal modules, and dissipate 87.2 % heat flow for six embedded L-type heat pipes, respectively. There has a benefit for HSHPTM V1.0 of rapidly and capably calculating the thermal performance of a heat sink-heat pipes thermal module installed with a processor horizontally by inputting simple and lumped parameters.     

短脉冲激光加热分数阶导热及其热应力研究

短脉冲激光加热引起材料内部复杂的传热过程及热变形,现有的以Fourier定律或Cattaneo-Vernotte松弛方程结合弹性理论为框架建立起来热应力理论在刻画其热物理过程存在严重缺陷.本文基于分数阶微积分理论,以半空间为研究对象,建立了分数阶Cattaneo热传导方程和相应的热应力方程,给出了问题的初始条件和边界条件,采用拉普拉斯变换方法,给出了非高斯时间分布激光热源辐射下温度场和热应力场的解析解,研究了短脉冲激光加热的温度场及热应力场的热物理行为.数值计算中,首先对理论解进行数值验证,然后取分数阶变量p=0.5研究温度场和热应力场的变化特点及激光参数对温度和热应力的影响,最后数值计算分数阶参数对温度和热应力场的影响.计算结果表明,分数阶Cattaneo传热方程和热应力方程描述的温度和热应力任然具有波动特性,与经典的Fourier传热模型和标准的Cattaneo传热模型相比,分数阶阶次越大,热波波速越小,热波波动性越明显;反之,则热波波速越大,热扩散性越强.激光加热和冷却的速度越快,温度上升和下降的速度越快,压应力和拉应力交替变化越快,温度变化幅值越小,热应力幅值影响不明显.     

Influence of thermophysical properties on the thermal stability of insulated composite superconducting tape

The effect of thermal conductivity and heat capacity on thermal stability of Nb-Ti tape superconductor stabilized with copper and subjected to transient thermal disturbance, was numerically investigated. The problem was solved by using the three- dimensional heat conduction equation. The results show that the anisotropy of thermophysical properties of the superconductor have significant effect on the thermal stability. It is found that the thermal stability of the tape is improved by increasing the heat capacity and decreasing the thermal conductivity. The best limits for anisotrpy factors α _k and α _c are (1.0; 1.5) and (2.0; 2.5), respectively.     

Experimental investigation of plastic finned-tube heat exchangers,with emphasis on material thermal conductivity

In this paper, two modified types of polypropylene (PP) with high thermal conductivity up to 2.3 W/m K and 16.5 W/m K are used to manufacture the finned-tube heat exchangers, which are prospected to be used in liquid desiccant air conditioning, heat recovery, water source heat pump, sea water desalination, etc. A third plastic heat exchanger is also manufactured with ordinary PP for validation and comparison. Experiments are carried out to determine the thermal performance of the plastic heat exchangers. It is found that the plastic finned-tube heat exchanger with thermal conductivity of 16.5 W/m K can achieve overall heat transfer coefficient of 34 W/m² K. The experimental results are compared with calculation and they agree well with each other. Finally, the effect of material thermal conductivity on heat exchanger thermal performance is studied in detail. The results show that there is a threshold value of material thermal conductivity. Below this value improving thermal conductivity can considerably improve the heat exchanger performance while over this value improving thermal conductivity contributes very little to performance enhancement. For the finned-tube heat exchanger designed in this paper, when the plastic thermal conductivity can reach over 15 W/m K, it can achieve more than 95% of the titanium heat exchanger performance and 84% of the aluminum or copper heat exchanger performance with the same dimension.