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.     

THERMAL RADIATION IN A DISCRETELY HEATED IRREGULAR GEOMETRY USING THE MONTE-CARLO,FINITE VOLUME,AND MODIFIED DISCRETE ORDINATES INTERPOLATION METHOD

This article presents a comparison of results obtained by the Monte-Carlo, finite volume, and modified discrete ordinates interpolation method for an absorbing, emitting, and isotropically scattering medium that is surrounded by discretely heated irregular surfaces. The medium is maintained at an isothermal condition or in radiative equilibrium. While the results of the Monte-Carlo method present the benchmark solutions within a statistical limit, the comparative results by the other methods provide a relative accuracy in the given geometry. In particular, the ray effects stemming from a discrete set of angular discretization for flux methods are found to be pronounced in the case of localized heating.     

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。     

基于半不变量及最大熵原理的静态电压稳定概率分析

针对含不确定出力的静态电压稳定分析问题,提出一种基于半不变量及最大熵原理的静态电压稳定概率分析方法.首先,应用拉丁超立方采样法随机生成若干场景后利用K-均值算法进行场景聚类,获取每簇场景的高阶矩和半不变量等数字特征,进而建立最大熵模型拟合负荷裕度的概率分布曲线,将每一簇的分布曲线加权后求得总的分布函数;最后,在IEEE...     

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.     

SOLUTION OF THE INVERSE PROBLEMS IN HEAT TRANSFER AND THERMAL STRESS ANALYSIS

The stable method of soloing inverse problems in heat transfer and inverse problems concerning thermal stress control in elements of complex shapes is presented. Based on the discrete form of Duhamel's integral and on so-called future time steps, a simple and accurate method of solving one- and multi-dimensional inverse problems is developed. Two examples illustrating the application of the described method are presented.     

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.     

某型汽轮机转子热应力的计算和分析

本文运用有限元子结构法和有限元-边界元联合计算法,在IBM微型机上计算和分析了某舰用主机全锻转子冷态紧急启动时的热应力变化情况,得到了一些有益的初步结论。     

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。     

黄土高原典型塬区冬小麦田地表辐射平衡各分量特征分析

利用2006年4～7月份实际观测资料,分析了黄土高原典型塬区冬小麦生长过程中不同天气条件下的地表辐射平衡各分量特征,并对比分析了冬小麦下垫面与裸土下垫面下的辐射平衡特征,发现:在不同天气条件下辐射平衡日变化特征有很大变化,冬小麦下垫面地面向上长波辐射晴天、阴天、降水天时依次减小,到达峰值时间约滞后总辐射峰值到达时间1h.大气向下长波辐射与地表向上长波辐射恰恰相反,晴天时量值最小,基本稳定在300W·m-2,阴天和降水天时依次增大.小麦田与裸地相比,地表向上短波辐射比裸地小;地表向上长波辐射白天裸地大于小麦地,夜间相反;净辐射白天小麦地略大于裸地,夜间则裸地大于小麦地.在辐射平衡中地表长波贡献最大,大气长波辐射、总辐射和净辐射依次减小,反射辐射最小.植被覆盖度高时,土壤植被系统吸收的总辐射和净辐射也高.     

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.     

基于室内均匀辐射场的西北农村节能建筑热工设计

针对西北气候条件和当地农村建筑特点,利用室外综合温度作为围护结构热工分析的边界条件,提出了基于室内均匀辐射场的农村节能建筑围护结构热工设计方法,并对利用该方法与常规方法设计的围护结构内表面温度进行了对比分析.结果表明,该方法能有效降低室内不对称辐射温度差异,并能作为同类地区农村建筑围护结构设计的理论参考.     

兆瓦级风力机结构系统可靠性分析的样本分数阶矩最大熵方法

以兆瓦级风力机塔架和叶片极限载荷的概率外推模型为基础,结合载荷动态响应峰值的独立同分布假设和三参数威布尔模型,外推获取了正常湍流和极端湍流强度条件下风力机关键部件长期服役载荷概率分布;进一步通过无量纲极值统计量定义系统失效的结构可靠性状态函数,结合样本分数阶矩和最大熵理论提出兆瓦级风力机关键部件结构可靠性分析的数值方法,对比湍流模型对兆瓦级风力机关键部件结构失效概率的影响。计算结果表明：样本分数阶矩最大熵方法能有效重构结构可靠性状态函数的概率分布;基于无量纲极值统计量的系统可靠性建模方法能有效表征风力机关键部件耦合相关失效问题,结合该文方法可获得系统失效概率的准确预测结果;湍流模型对风力机结构失效概率影响较大,难以预先判定何种模型将得到结构失效概率的保守预估结果,需结合IEC 61400-1标准中的设计载荷工况细致分析后才能确定。     

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。