关于离散热传导物理模型的探讨

本文从热传导离散物理模型上论证了集中质量热容矩阵模型可以在离散点上满足热量守恒定律，采用它就可以避免许多热传导时间积分中的不合理现象。几个算例表明了该模型具有良好的精度。     

二阶非定常多宗量热传导反问题的正则解

引入Bregman距离函数及其加权函数作为正则项，应用Tikhonov正则 化方法，对二阶非定常多宗量热传导反问题进行求解. 利用测量信息和计算信息构造最小二 乘函数，将多宗量反演识别问题转化为一个优化问题. 空间上采用8节点等参元进行离散， 时域上采用时域精细算法进行离散，建立了二阶非定常多宗量热传导问题的有限元正/反演数 值模型. 该模型不仅考虑了非均质和参数分布的影响，而且也便于正反演问题的敏度分析， 可对导热系数和边界条件等宗量进行有效的单一和组合识别. 给出了相关的数值验证，对信 息测量误差以及不同正则项的计算效率作了探讨. 数值结果表明，该方法能够对二阶非定常 多宗量热传导反问题进行有效的求解，并具有较高的计算精度.     

共轭梯度法求解非线性多宗量稳态传热反问题

应用共轭梯度法求解非线性多宗量稳态热传导反问题。采用八节点的等参单元在空间上进行离散,建立了便于敏度分析的非线性正演和反演的有限元模型,可直接求导进行敏度分析。给出了相关的数值验证,对测量误差及测点数目的影响作了初步探讨,结果表明,采用的算法能够对非线性稳态热传导中导热系数和边界条件联合反问题进行有效的求解,并具有较高精度。     

计及热传导影响对长杆弹侵彻陶瓷靶的数值分析

采用有限元方法离散瞬态热传导方程,编写成侵彻过程热传导计算模块,并将之嵌入已有的冲击动力学程序中,然后运用于长杆弹在900~1 800 m/s着速范围内侵彻AD95陶瓷靶的数值分析,得到了符合物理事实的计算图像,所得的计算结果比采用传统的绝热模型得到的计算结果更符合实验结果。探讨了计及热传导效应对长杆弹侵彻AD95陶瓷靶数值模拟的影响:着速在900~1 350 m/s范围内时,计及热传导的数值计算所得侵深小于绝热模型计算结果;着速在在1 350~1 450 m/s范围内时,两种模型计算侵深接近;着速在在1 450~1 800 m/s范围内时,热传导模型计算侵深大于绝热模型计算结果。     

带源参数的二维热传导反问题的无网格方法

利用无网格有限点法求解带源参数的二维热传导反问题，推导了相应的离散方程. 与 其它基于网格的方法相比，有限点法采用移动最小二乘法构造形函数，只需要节点信息，不 需要划分网格，用配点法离散控制方程，可以直接施加边界条件，不需要在区域内部求积分. 用有限点法求解二维热传导反问题具有数值实现简单、计算量小、可以任意布置节点等优点. 最后通过算例验证了该方法的有效性.     

材料热传导系数随温度变化函数的反演方法

材料热传导系数的反演是一类典型的热传导逆问题。针对材料热传导系数随温度变化的情况,本文将材料的热传导系数值按温度区间分段离散,建立了通过材料边界点的温度测量来反演各温度区间热传导系数值的遗传算法和伴随方程法。通过典型算例分析和考虑测量噪声、系统噪声的反演计算结果分析表明：所建立的两种反演算法都是可行有效的,受测量随机噪...     

粘性及热传导对于爆轰波的影响

对在满足化学当量比的氢氧混合气体中传播的一维和二维连续旋转爆轰波进行了数值模拟,以此检验粘性和热传导对爆轰波发展和结构的影响。模拟分别基于NS和Euler控制方程,采用二步化学反应模型,对流项采用5阶MPWENO格式求解,时间方向采用3阶TVD Runge-Kutta法,粘性项采用中心差分格式进行离散。结果表明:粘性和热传导不会对爆轰波流场的基本流场结构产生影响;在具体数值上,粘性和热传导的影响值在爆轰波、斜激波、接触间断等速度或温度剧烈变化处相对较大,但总体上其影响量均比爆轰波流场的步进值小三个量级。因此,在没有壁面效应的一维爆轰和二维连续旋转爆轰波流场中,粘性和热传导项作为很小的扰动存在,对爆轰波的流场结构和数值大小基本不会产生影响。     

带源参数热传导问题的基于滑动Kriging插值的MLPG法

利用基于滑动Kriging插值的无网格局部Petrov-Galerkin(MLPG)法来求解带源参数的二维热传导问题,推导了相应的离散方程。由于滑动Kriging插值法构造的形函数满足Kronecker Delta特性,因此可以直接施加本质边界条件。在离散过程中采用Heaviside分段函数作为局部弱形式的权函数,时间域则通过向后差分法进行离散,这一处理过程中刚度矩阵只涉及到边界积分,而没有涉及到区域积分。最后通过算例验证了本方法的有效性。     

基于状态空间理论的功能梯度圆球瞬态热传导分析

基于状态空间理论研究功能梯度圆球的球对称瞬态热传导问题。根据热传导方程和热流密度的定义,取温度场和热流密度为系统的状态向量,通过将圆球分层和在时域内应用差分格式对控制方程进行离散,建立了系统的状态方程,给出了功能梯度圆球瞬态热传导问题的半解析解。算例分析表明:本文解不但结果正确、计算效率高,而且适用于材料参数沿径向任意梯度变化的圆球瞬态热传导分析。     

热传导问题的比例边界等几何分析

提出比例边界等几何分析SBIGA(Scaled Boundary IsoGeometric Analysis)方法来求解热传导问题。SBIGA兼具比例边界有限元和等几何分析的优势,特别适用于求解包含无限域和奇异物理场的问题。该方法造型十分方便,在径向具有半解析性质,仅需在计算域边界上用NURBS基函数自然离散,为实现CAD/CAE无缝融合提供了新的途径,大大节约前处理和计算耗时。此外,SBIGA无需进一步与CAD系统数据交换就可以保型细分。三个基准算例证明了其在热传导分析中的有效性。与传统比例边界有限元相比,SBIGA模型消除了几何模型误差,并显示出更高的计算精度和收敛速度。     

Heat transfer by conduction,natural convection and radiation across a rectangular cellular structure

This paper describes results on the effects of wall conduction and radiation heat exchange among surfaces on laminar natural convection heat transfer in a two-dimensional rectangular cavity modelling a cellular structure. Parametric heat transfer calculations have been performed, and numerical results are presented in graphical and tabular form. Local and average Nusselt numbers along the cavity walls are reported for a range of parameters of physical interest. The findings suggest that the local or the average Nusselt number is one of many parameters that control conjugate heat transfer problems. The results indicate that natural convection heat transfer in the cavity is reduced by heat conduction in the walls and radiation exchange among surfaces. The results obtaibed for the total heat transfer rate through the system using the two-dimensional model are compared with those based on a one-dimensional model.     

Turbulent mixed convection heat transfer to liquid sodium

The influences of buoyancy on turbulent heat transfer to a liquid metal flowing in a vertical pipe are considered. A theoretical model is presented which provides a criterion for the conditions under which such influences become significant and which predicts the impairment of heat transfer for upward flow and enhancement for downward flow. The variation with Peclet number of the maximum impairment of heat transfer and conditions under which it occurs are established. A generalization of the model leading to an equation for the entire mixed convection region is proposed. From this an equation for turbulent free convection to liquid metals is obtained.     

Local heat transfer in the first baffle compartment of the shell-and-tube heat exchangers for staggered tube arrangement

A mass transfer measuring method based on absorption, chemical and coupled colour reaction is used to visualize and determine the shell side local heat transfer in the first baffle compartment of shell-and-tube heat exchangers with segmental baffles for staggered tube arrangement. Local mass transfer coefficients were transformed into heat transfer coefficients by using the analogy between heat and mass transfer. The local, per-tube and integral heat transfer coefficient distributions are presented.     

A separated flow model for predicting two-phase pressure drop and evaporative heat transfer for vertical annular flow

A separated flow model has been developed that is applicable to vertical annular two-phase flow in the purely convective heat transfer regime. Conservation of mass, momentum, and energy are used to solve for the liquid film thickness, pressure drop, and heat transfer coefficient. Closure relationships are specified for the interfacial friction factor, liquid film eddy-viscosity, turbulent Prandtl number, and entrainment rate. Although separated flow models have been reported previously, their use has been limited, because they were tested over a limited range of flow and thermal conditions. The unique feature of this model is that it has been tested and calibrated against a vast array of two-phase pressure drop and heat transfer data, which include upflow, downflow, and microgravity flow conditions. The agreements between the measured and predicted pressure drops and heat transfer coefficients are, on average, better or comparable to the most reliable empirical correlations. This separated flow model is demonstrated to be a reliable and practical predictive tool for computing two-phase pressure drop and heat transfer rates. All of the datasets have been obtained from the open literature.     

Heat transfer to an unconfined ceiling from an impinging buoyant diffusion flame

Impinging flames are used in fire safety research, industrial heating and melting, and aerospace applications. Multiple modes of heat transfer, such as natural convection, forced convection and thermal radiation, etc. are commonly important in those processes. However, the detailed heat transfer mechanisms are not well understood. In this paper, a model is developed to calculate the thermal response of an unconfined nonburning ceiling from an impinging buoyant diffusion flame. This model uses an algorithm for conduction into the ceiling material. It takes account of heat transfer due to radiation from the fire source to the ceiling surface, and due to reradiation from the ceiling surface to other items. Using experimental data, the convective heat transfer coefficient at lower surface is deduced from this model. In addition, the predicted heat fluxes are compared with the existing experimental data, and the comparison results validate the presented model. It is indicated that this model can be used to predict radial-dependent surface temperature histories under a variety of different realistic levels of fire energy generation rates and fire-to-ceiling separation distance.     

Heat and mass transfer in unsaturated porous media at a hot boundary: I. One-dimensional analytical model

We present a model of heat and mass transfer in an unsaturated zone of sand and silty clay soils, taking into account the effects of temperature gradients on the advective flux, and of the enhancement of thermal conduction by the process of latent heat transfer through vapor flow. The motivation for this study is to supply information for the planned storage of thermal energy in unsaturated soils and for hot waste storage. Information is required on the possibility of significant drying at a hot boundary, as this would reduce the thermal conductivity of a layer adjacent to the boundary and, thus, prevent effective heat transfer to the soil. This study indicates the possibility that the considered system may be unstable, with respect to the drying conditions, with the occurrence of drying depending on the initial and the boundary conditions. An analysis performed for certain boundary conditions of heat transfer and for given soil properties, disregarding the advective flux of energy, indicated that there are initial conditions of water content for which heating will not cause significant drying. Under these conditions, fine soils may be better suited for heat transfer at the hot boundary, due to their higher field capacity, although their heat conduction coefficients at saturation are lower than those of sandy soils. At present, these conclusions are limited to the range of 50–80°C. Potential effects of solute concentration at the hot boundary are indicated.     

Freezing around a finite heat sink immersed in an infinite phase change medium

Freezing around a spherical heat sink immersed in an infinite phase change medium — a free boundary problem involving growth and decay of the free boundary — is analysed here. A one-dimensional conduction model is formulated and the resulting partial differential equations are solved by finite difference methods. The energy discharged from the phase change medium during the heat transfer process is analysed for latent heat thermal energy storage applications. Results are presented for a wide range of parameters that are encountered in energy storage devices. The cases of slab/cylindrical heat sink are reexamined for a range of parameters not covered by the earlier investigators     

Prediction of cooling-coil performance under condensing conditions

The possibility of predicting chilled-water cooling-coil performance under condensing conditions using dry-surface heat transfer correlations is examined. Experimentally determined wet-surface Nusselt number data are presented and compared with dry-surface data obtained from the same cooling coils. The wet-surface Nusselt numbers show considerable scatter; some of the results are higher than the corresponding dry-surface correlations, while others are lower. A sensitivity analysis is presented to illustrate that the wet-surface Nusselt numbers are very sensitive to the uncertainties in the measured inlet dew-point temperature and the measured heat transfer rate. It is demonstrated that the use of dry-surface Nusselt number correlations in a coil model result in wet-surface heat transfer predictions that are generally within 5 percent of the experimentally determined value.     

A review on transient test techniques for obtaining heat transfer design data of compact heat exchanger surfaces

Accurate and reliable dimensionless heat transfer characteristic is very essential for the analysis of heat exchangers. It is also required for the rating and sizing problems of heat exchangers. One of the important experimental methods used to determine the heat transfer coefficient between the heat transfer surface of the heat exchanger and the flowing fluid is transient test techniques. The transient test techniques are usually employed to establish Colburn factor versus Reynolds number characteristics of a high NTU heat exchanger surfaces like compact or matrix heat exchangers. In those situations, a single-blow test, where only one fluid is used, is employed to conduct the transient test. The transient technique may have the fluid inlet temperature having a step change, periodic or an arbitrary rise/drop. In this paper, various transient test techniques that are used for the determination of heat transfer characteristics of high NTU heat exchanger surfaces are discussed.