USE OF THE DISCRETE MAXIMUM PRINCIPLE IN FINITE-ELEMENT ANALYSIS OF COMBINED CONDUCTION AND RADIATION IN NONPARTICIPATING MEDIA

Since closed-form solutions are virtually nonexistent, the finite-element method (FEM) is frequently used to solve the combined conduction and radiation problem in nonparticipating media. In this article a Gakerkin finite-element formulation is developed for the problem. However, under certain conditions, usually when the radiative component dominates the conduction component, the finite-element solutions generated by this formulation display anomalous behaviors associated with a violation of the discrete maximum principle (DMP). In this article the DMP is used as a guideline to improve the finite-element results, and the effectiveness of this approach is demonstrated through some examples.     

近地倾斜轨道航天器在轨热辐射分析

根据航天器轨道动力学和热辐射理论，建立了运行于近地倾斜轨道的圆柱形航天器外热流数学模型；用数值方法对外热流和等效热沉温度进行求解，计算出瞬态工况及周期平均工况下的航天器表面辐射热流及辐射散热器和防辐射涂层表面等效热沉温度的波动。结果表明，航天器受到的辐射热流除了随航天器绕地球公转变化外，还随其轨道面的旋转在更长的周期上产生更大的变化；对柱面上的不同位置受到的辐射作了比较，发现辐射散热器布置在顶部时其等效热沉温度最低。     

A DISCONTINUOUS FINITE-ELEMENT FORMULATION FOR INTERNAL RADIATION PROBLEMS

This article presents a discontinuous finite-element formulation for internal thermal radiation problems. In contrast to the conventional finite-element formulation, the discontinuous algorithm permits the discontinuity of field variables across the internal interelement boundaries and is useful for integral-differential equations describing thermal radiation in absorbing/scattering media. Mathematical formulation and numerical implementation are given. The convergence rate and local mesh adaptivity are discussed. Two approaches for coupling of the discontinuous and conventional methods for mixed heat transfer calculations are presented. Numerical results are given for internal radiation and combined conduction/radiation problems and are compared with analytical solutions whenever available.     

EFFECTS OF THERMAL RADIATION ON THE SOUND WAVE PROPAGATION IN A GAS-PARTICLE TWO-PHASE MEDIUM

A study is made of the sound wave propagation through a radiating gas medium that contains solid particles in suspension. The relaxation models are introduced to describe the temporal momentum and thermal nonequilibrium interactions between gas and particles. The gray gas differential approximation is used for radiation. It is found that the radiation induces the attenuation mode, the position of which is varied with the absorption coefficient in addition to the immovable mode by suspended particles. The attenuation because of radiation is greatly influenced by the absorption coefficient of the radiative medium while the dispersion remains almost unchanged. As the absorption coefficient increases, the attenuation mode because of radiation shifts to the higher frequency zone. This radiation effect is significantly reduced as the particle mass loading increases, since the convection becomes much more dominant.     

GEOMETRICALLY LINEAR ANALYSIS OF THE THERMAL STRESSES IN ONE-SIDED COMPOSITE REPAIRS

This article presents an explicit solution of the thermal stresses in a one-sided composite patch repair, with the patch being orthotropic in both mechanical and thermal properties. The emphasis is placed on the analysis of the bending deformation of the reinforced structure resulting from thermal loading. The reinforced region is represented by an equivalent inclusion that undergoes a combination of in-plane extension and out-of-plane extension. Explicit formulae are derived for the thermal expansion coefficients as well as the bending stiffness and the in-plane extensional stiffness of the equivalent inclusion. The Eshelby inclusion analogy is first extended by postulating uniform bending curvatures and mean strains throughout the inclusion. The correctness of this conjecture is then proved by the satisfaction of force, bending moment, and displacement continuity conditions. Explicit solutions are finally derived for the bending and membrane stresses resulting from thermal loading of an isotropic plate reinforced by a circular patch. It is shown that the present solutions correlate well with the numerical results obtained using fully three-dimensional finite element analysis.     

ROLE OF ABSORPTION COEFFICIENT IN THE FREQUENCY DEPENDENCE OF THE THERMAL STRESSES IN A PARTIALLY ABSORBING PLATE SUBJECTED TO CYCLIC THERMAL RADIATION: A BRIEF NOTE

The frequency dependence of the thermal stresses in a partially absorbing plate subjected to cyclic thermal radiation is shown to be a function of optical thick ness.     

ASSESSMENT OF A DIFFERENTIAL TOTAL ABSORPTIVITY SOLUTION TO THE RADIATIVE TRANSFER EQUATION AS APPLIED IN THE DISCRETE TRANSFER RADIATION MODEL

Abstract

A differential total absorptivity (DTA) solution to the radiative transfer equation is assessed for application in the discrete transfer radiation model (DTRM). The new solution technique treats the source temperature dependence of adsorption explicitly, without the need for spectral integration. Predictions are presented for the radiative intensity across single lines of sight, and for the volumetric source variations in a full DTRM calculation between solid walls. DTA exhibits superior performance relative to a differential total transmissivity solution and the weighted sum of gray gases solution. Additionally, gray gas solutions and a homogeneous isothermal path solution are shown to be unsatisfactory.     

墙体储能用定形相变石蜡储热性能的实验研究

实验配制并研究了以高密度聚乙烯为支撑材料的定形相变石蜡.所使用的石蜡为由液体石蜡分别与46#石蜡、48#石蜡配制的两种适于墙体中使用低熔点混合物,相变温度分别为26.6、25.5℃,相变潜热较大.利用DSC实验研究了两种石蜡混合物分别与高密度聚乙烯按40%～90%比例配制成的定形相变材料的相变温度、相变潜热、均匀性和稳定性.定形相变材料在低温区的相变温度与石蜡的相变温度基本一致,在26～28℃之间,定形材料相变潜热近似等于石蜡的相变潜热与石蜡百分含量的乘积.定形相变材料中石蜡的最佳含量为70%,此时相变潜热约为100J/g.     

地板辐射采暖空间温度场的数值模拟

根据地板采暖的特点，建立数学模型，采用有限差分法，模拟地板辐射采暖中地板温度场的变化关系。通过与实测结果进行比较和调整，可有效地控制供暖温度，取得了满意的结果，为工程设计提供比较准确的方法。     

COMBINED RADIATION AND LAMINAR MIXED CONVECTION IN THE THERMAL ENTRANCE REGION OF HORIZONTAL ISOTHERMAL RECTANGULAR CHANNELS

The interaction of thermal radiation with laminar mixed convection for a gray fluid in the thermal entrance region of a horizontal isothermally heated rectangular channel is numerically investigated. The vorticity-velocity formulation of the Navier-Stokes equation and the integral formulation for radiation solved by finite-element nodal approximation are employed. The effects of radiation and convection on local Nusselt number, the development of bulk temperature, and the friction factor are examined. Secondary flow induced by the buoyancy effects leads to a significant enhancement in heat transfer in the entrance region. The result shows that the existence of secondary flow causes fluctuations in local Nusselt number and this phenomenon is reduced by the effect of thermal radiation and a large aspect ratio.     

THERMAL STRESSES IN A PARTIALLY ABSORBING FLAT PLATE SYMMETRICALLY HEATED BY THERMAL RADIATION AND COOLED BY CONVECTION

The stresses due to “thermal trapping” of internally absorbed radiation in solid materials are analyzed for a flat plate symmetrically heated by radiation and cooled by convection for finite values of the heat transfer coefficient. The thermal-stress state consists of an initial transient followed by a steady-state stress at t -> ∞ and involves a reversal in the sign of the stresses. The maximum value of the transient stress increases with optical thickness, whereas the maximum value of the steady-state stress occurs at an optical thickness μa = 1·3 with zero stresses at μa. = 0 and ∞.     

A FAST MONTE CARLO SCHEME FOR THERMAL RADIATION IN SEMICONDUCTOR PROCESSING APPLICATIONS

Thermal radiation is the most effective way for rapid thermal processing (RTP) and rapid thermal chemical vapor deposition (RTCVD) of wafers. It is well known in the semiconductor equipment design community that the Monte Carlo method for radiation is the only method that can accurately model radiative transport in RTP and RTCVD reactors. However, it has often been argued that it is expensive and difficult to use as a commercial design tool. In this article, a fast Monte Carlo scheme is presented. The basic algorithm is the classical surface-to-surface ray-tracing algorithm. In addition, a modified form of the binary spatial partitioning (BSP) algorithm is implemented to speed up ray tracing by at least a factor of 3. The results demonstrate a high level of accuracy with fairly low computational cost.     

熔融盐斜温层蓄热的热特性研究

对熔融盐高温斜温层蓄热过程进行较深入的理论与实验研究,建立熔融盐单相流体斜温层蓄热的瞬态热分析模型,模型考虑熔融盐的变物性。利用Fluent软件,通过求解N-S方程与能量方程,对熔融盐单相流体斜温层蓄热罐在各工况条件下的传热蓄热过程进行数值模拟。研究时间进程、初始条件以及结构尺寸等对蓄热性能的影响。结果表明:斜温层的厚度随时间的推移而增加,达到一定厚度后增加量趋缓;流体进口流速、长径比等是影响有效蓄热容量的主要因素,当进口速度为0.001m/s级、长径比为2∶1时,将减少斜温层厚度。     

太阳辐射下不同地表覆盖物的热反应及对城市热环境的影响

该文利用红外热像仪等设备研究了建筑物表面、绿地、柏油路面、水泥砖地面等城市地表覆盖物对太阳辐射的热反应，指出在同一天中，柏油路面温度最高，水泥砖和建筑物表面次之，绿地温度最低。通过分析各种覆盖物在城市中所占比例，得出建筑物对城市热环境影响较大的结论，并提出了改善城市热环境的几种方法。     

太阳空气集热器与热水器的热性能分析和测试的差别

总结和推导了常用太阳空气集热器的有用得热的数学表达式,分析了ＵＬ,Ｆ‘,ＦＲ的数学表达式与热水器的异同,指出了测试和设计空气集热器时应注意的有关问题。     

ANALYSIS OF TWO-PHASE RADIATION IN THERMALLY DEVELOPING POISEUILLE FLOW

Combined conductive and radiative heat transfer in a thermally developing two-phase Poiseuille flow in a cylindrical duct is studied here. A two-phase radiative transfer equation (RTE) considering radiation by both gas and particles is taken into account. A complexform of nonlinear integrodifferential RTE is solved by the discrete ordinates method (DOM, or so called SN method) in axisymmetric geometry. After such validation, namely, the solution in a two-dimensional channel flow between two flat plates is compared with that solved by the zone method, the program is then applied to fully developed gas-particle two-phase flow in a cylindrical duct. A parametric study is performed for gas and particle absorption coefficients, particle number density, particle emissivity, and wall emissivity. The results show a significant effect of two-phase radiation on the thermal characteristics. However, in all cases, it was found that conduction is predominant near the wall.     

一种高密度辐射计的设计及性能分析

分析了辐照投射角、测量桥的传热特性、结构和材料等因素对一种高密度辐射计测量性能的影响.结果表明:铜制辐射计能完成10kW/m2～15MW/m2范围内的辐照测量,响应时间不超过5s,输出温差与投射辐射的密度值成正比;较长的测量桥适用于辐照密度低、分辨率要求高的场合,较短的测量桥则适用于辐照值较高的场合;用热扩散系数高的材料制造的辐射计,其测量上限值较高,而用热扩散系数低的材料制造的辐射计,则有较高的分辨率和较低的测量下限值.     

并联PID型水轮机调节器控制规律分析

以功率调节品质为目标，利用ＣＡＡ技术对并联ＰＩＤ型水轮机调节器不同结构进行研究分析，得出一些有用的结论。     

槽式太阳能热发电站中集热管摆动支撑的结构分析

在槽式太阳能热发电站中,集热管作为镜场的核心部件,其热性能及其支撑装置的结构可靠性直接决定了整个槽式太阳能热发电站的热效率和经济成本。通过对槽式太阳能热发电站中集热管支撑装置中几种摆动支撑结构的优劣进行对比,选出了最优结构形式,并计算分析了集热管运行过程中摆动支撑的受力情况,以期为集热管摆动支撑的进一步创新设计提供借鉴。     

ANALYSIS ON TRANSIENT THERMAL STRESSES IN AN ANNULAR FIN

Transient thermal stresses are an important consideration in production processes involving large temperature changes. Recently, thermal stresses have also become significant in design problems related to microelectronic devices through their effects on material properties and system parameters. To calculate the thermal stresses, three kinds of methods are available. The first is the analytical method, in which the elastic theory is used to find the exact solution. The second approach consists of some kind of approximate technique, such as a perturbation procedure. The third method is the use of a numerical process, such as a finite-difference or a finite-element method.

This article investigates the transient thermal stresses in an annular fin with its base subjected to a heat flux of a decayed exponential function of time. In order to obtain the solution of the governing equation, which is a partial differential equation, the following procedures of analysis are used.

1. Normalize the governing partial differential equation subject to appropriate initial and boundary conditions.

2. Take the Laplace transform of the resulting equation with respect to time.

3. Utilize the exponential-like solutions introduced by Keller and Keller to solve the transformed system.

4. Achieve the inverse Laplace transform by means of complex contour integration and the residue theorem.

5. Substitute the temperature distribution function into the governing equation of thermal stresses. Then use Simpson's rule to obtain the thermal stress distribution as a function of time and position of the fin.