Simultaneous transfer of heat and mass occurring in casting process

Studied in this paper is simultaneous transfer of heat and water vapor which takes place in a green sand mold in a very short period of time after pouring a molten metal in its cavity. Governing equations describing heat and mass transfer in a mold are solved by finite difference method and the results are compared with the actually measured values to examine the validity of the calculated results. The effect of thermal properties and permeability of the mold, the amount of water contained, the heating temperature (i.e., temperature of casting metal) and other factors on the heat transfer rate at the interface between the molten metal and the mold, the pressure rise in the mold and the development of dried zone around the casting are investigated to propose some empirical relations available for predicting those transfer phenomena by using dimensionless parameters presented.     

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.     

Analytical solution of time fractional Cattaneo heat equation for finite slab under pulse heat flux

A fractional Cattaneo model is derived for studying the heat transfer in a finite slab irradiated by a short pulse laser. The analytical solutions for the fractional Cattaneo model, the classical Cattaneo-Vernotte model, and the Fourier model are obtained with finite Fourier and Laplace transforms. The effects of the fractional order parameter and the relaxation time on the temperature fields in the finite slab are investigated. The results show that the larger the fractional order parameter, the slower the thermal wave. Moreover, the higher the relaxation time, the slower the heat flux propagates. By comparing the fractional order Cattaneo model with the classical Cattaneo-Vernotte and Fourier models, it can be found that the heat flux predicted using the fractional Cattaneo model always transports from the high temperature to the low one, which is in accord with the second law of thermodynamics. However, the classical Cattaneo-Vernotte model shows that the unphysical heat flux sometimes transports from the low temperature to the high one.     

Development of a modular zeolite-water heat pump

The working pair zeolite-water has very good characteristics for the heat pump application. It is non-poisonous, non-flammable and low-corrosive so that the use of a zeolite-water heat pump in the large field of domestic heating is very promising. The poor heat and mass transfer of the zeolite has to be considered by an appropriate design of the adsorber heat exchanger. Compact zeolite layers directly linked with the heat exchanger enable a high specific thermal output (thermal output related to the mass of zeolite) which is the main shortcoming of these machines. Additionally the coefficient of performance (COP) can be improved significantly by a modular design of the machine consisting of six to eight heat pump modules. Due to the periodical operating mode which is required by the zeolite-water pair the single module is built up in a simple way without any moving parts. The different modules, each of them operating in another phase of the sorption cycle, are connected in series by a heat transfer medium circuit so that a continuous thermal output together with high COP is achieved by this zeolite-water heat pump. First experimental investigations focus on the layout of the different components of the heat pump, e.g. the single module, the adsorber/desorber and the evaporator/condenser. The paper will present the design of these components as well as the design of the entire modular machine. Furthermore there will be a theoretical discussion of the COPs of the modular heat pump depending on the ambient temperature, on the number of modules and on the heating system. Received on 12 November 1998     

THE DIRECTION DEPENDENCE OF SURFACE WAVE SPEED AT THE SURFACE OF MAGNETO-ELECTRO-ELASTIC HALF-SPACE

The direction dependence of surface wave speed and the influence of electrically and magnetically short/open circuit are investigated in this paper. First, the elastic, piezoelectric and piezomagnetic coefficients in the considered ordinate system are obtained by Bonde transformation from those in the crystal axis ordinate system. Then, the secular equation of surface wave is derived from the free traction condition on the surface of half space with consideration of short/open circuit case. Some numerical examples are given. The direction dependence of surface wave speed and the influences of short/open circuit are shown graphically and discussed based on the numerical results.     

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 ...     

Computer diagnosis system for thermal analysis of engine piston

This study presents a finite element heat-transfer model for the prediction of piston temperature distributions in a real time operation engine. The thermal boundary conditions are specified adequately in the model for various operations. The time-mean, area-averaged gas temperature and heat transfer coefficient could be obtained from the engine simulation. The ambient tremperatures along piston ring belt and skirt were estimated with a formula and the corresponding heat transfer coefficients were evaluated through equivalent thermal circuit analysis. The model is also calibrated and the predicted and measured temperatures compared for the operating engine. The predictions are reasonable, providing confidence in the use of the simulation model.     

Thermal analysis of annular fins with temperature-dependent thermal properties

The thermal analysis of the annular rectangular profile fins with variable thermal properties is investigated by using the homotopy analysis method （HAM）. The thermal conductivity and heat transfer coefficient are assumed to vary with a linear and power-law function of temperature, respectively. The effects of the thermal-geometric fin parameter and the thermal conductivity parameter variations on the temperature distribution and fin efficiency are investigated for different heat transfer modes. Results from the HAM are compared with numerical results of the finite difference method （FDM）. It can be seen that the variation of dimensionless parameters has a significant effect on the temperature distribution and fin efficiency.     

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.     

Thermal-transient effects on pressure measurements

A set of experiments were performed on a flush-mounted strain-gage pressure transducer to determine the nature of erroneous responses induced by two types of thermal transients. The thermal transients studied were a constant step increase in transducer-diaphragm surface temperature, and a constant step increase in diaphragm-surface heat flux. The responses of the transducer to changes in ambient temperature and to temperature gradients in the transducer housing at steady state were investigated also. Analytic models for the transducer were solved under the experimental conditions and compared with the experimental results for the first two transients. These models predicted the response of the transducer quite well for about the first 25 percent of the transient period. Beyond this short time, the transducer behavior was drastically different from the models.     

Comparative study on performance of a zigzag printed circuit heat exchanger with various channel shapes and configurations

Comparative study has been performed with various channel cross-sectional shapes and channel configurations of a zigzag printed circuit heat exchanger (PCHE), which has been considered as a heat exchanging device for the gas turbine based generation systems. Three-dimensional Reynolds-averaged Navier–Stokes equations and heat transfer equations are solved to analyze conjugate heat transfer in the zigzag channels. The shear stress transport model with a low Reynolds number wall treatment is used as a turbulence closure. The global Nusselt number, Colburn j-factor, effectiveness, and friction factor are used to estimate the thermal–hydraulic performance of the PCHE. Four different shapes of channel cross section (semicircular, rectangular, trapezoidal, and circular) and four different channel configurations are tested to determine their effects on thermal–hydraulic performance. The rectangular channel shows the best thermal performance but the worst hydraulic performance, while the circular channel shows the worst thermal performance. The Colburn j-factor and friction factor are found to be inversely proportional to the Reynolds number in cold channels, while the effectiveness and global Nusselt number are proportional to the Reynolds number.     

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.     

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。     

Experimental investigation of flow and heat transfer for the ethanol-water solution and FC-72 in rectangular microchannels

Rapid development of super scale integration circuit (IC) provides unprecedented challenge to thermal control for aviation electronic equipments. To solve the problem of cooling electronic chips and devices for aircraft avionics, this paper experimentally investigated the characteristics of single-phase forced convection heat transfer and flow resistance in rectangular microchannels with two liquid coolants. One was 30% of ethanol–water solution, the most commonly used coolant in aviation. The other was FC-72, the latest coolant for electronic equipments. Based on the experimental data collected and those available in the open literature, comparisons and analyses were carried out to evaluate the influences of liquid velocity, supercooling temperature, microchannel structures and wall temperature etc. on the heat transfer behaviors. And the correlations of flow resistance and heat transfer characteristics were provided for the ethanol–water solution and FC-72 respectively. The results indicate transition from laminar to turbulent flow occurs at the Reynolds number of 750–1,250 for FC-72, and the behaviors of flow and heat transfer in rectangular microchannels strongly depend on the kind of coolant and geometric configuration of microchannels.     

热局部非平衡下一维半无限长饱和多孔柱体的温度场分析

对于端部受温度载荷的一维半无限长多孔介质柱体,给出了热局部非平衡下固相和流相温度场在Laplace变换域中的解析表达式.对于冲击温度载荷的情况,获得了温度场在短时间内的Laplace逆变换渐近解析解.数值分析了流、固两相热扩散系数之比以及热交换系数对固相和流相温度场的影响,比较了热局部非平衡下加权温度与热局部平衡下温度之间的差别.     

Analytical simulation in heat storage systems

Packed bed heat exchangers for thermal energy storage systems are investigated by means of two phase heat transfer models. The paper is mainly aimed at deriving analytical solutions to the thermal balance equations relevant to different kinds of packed beds, taking into account the roles played by heat capacity and conduction effects. The results are shortly discussed and some graphs are shown for situations typical of various operational modes.     

含水层热量输运中自然热对流和水-岩热交换作用的研究

本文对含水层热量输运过程中存在的自然热对流和水-岩热交换作用进行了深入研究, 目的是为了弄清两种作用对热量输运的影响。首先针对普通水流方程和热量输运方程的局限性, 给出了能够描述这两种作用的新的数学方程, 并通过建立一个非线性三维含水层热量输运模型加以实现。然后利用上海第二承压含水层的群井储能试验资料, 根据不同的条件进行计算。计算结果与实测数据的对比分析表明, 自然热对流通常对含水层的热量输运有明显作用, 定量研究时不应忽略；水-岩热交换作用持续时间短, 对整个热量输运过程影响较小。     

用于低温风洞的新颖制冷方法

描述了用于低温风洞的新颖制冷系统,利用热交换器回收排气冷量预冷压缩空气,然后再用热分离器将其降至深低温作风洞气源．原理性实验结果证实新制冷方法的可行性．讨论了新制冷方法产生的有一定压力的低温空气作引射气源,引射驱动回流型风洞的特性．其制冷方法与现有低温风洞喷雾液氮制冷相比,由于仅需压缩空气而无需液氮,造价更便宜．更由于能量利用合理,效率高,因而运行成本可显著降低．     

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.      

Thermo-Mechanical Model and Thermal Analysis of Hollow Cylinder Planetary Roller Screw Mechanism

Planetary roller screw mechanism (PRSM) is widely used for rapid and precise motion translation from rotary into linear motion due to its high stiffness and high position accuracy. However, a high speed PRSM drive system naturally generates significant amount of frictional heat at the contact interfaces, which causes thermal deformation and thermal error and reduces motion accuracy. Preload is usually applied to remove the axial backlash of the PRSM for achieving high accuracy and great stiffness. However, more frictional heat is produced by such preload. On the other hand, larger numbers of angular contact bearings are needed to support the heavy axial load. The friction heat generated in support bearings has also to be investigated. In order to estimate the thermal distribution and thermal error of the hollow cylinder PRSM, a thermo-mechanical model based on finite element method (FEM) is developed, where heat generation from the two main sources of the PRSM, the parameters calculation of heat transfer coefficient and other thermal boundary conditions were studied. The presented model is proven capable of investigating temperature distribution, thermal error, and cooling performances of coolants of the PRSM system.