近临界流体微通道内流动稳定性和换热特性研究

微尺度条件下的化工、医药、传热与能源利用等系统的研究已经成为极具潜力和挑战性的课题.相应条件下流体流动和换热的分析必须考虑尺度效应所带来的系列问题.该研究采用了数值模拟方法对近临界二氧化碳流体在微尺度通道内的流动稳定性和换热特性进行了探索.研究发现,在近临界区域内由于流体较强的膨胀特性和较低的热扩散特性,在微尺度几何条件下会产生瞬态不稳定的漩涡流动.该种条件下微尺度对流换热和混合效率都得到了大幅提高.进一步,研究针对微尺度局部稳定性演化进行了机理分析并应用了参数估计,总结获得了微通道内近临界流体瞬态换热和混合的基本特性.     

三维快速多极边界元法分析地埋管群传热问题北大核心CSCD

基于三节点三角形线性单元,为克服单元跨叶子积分难题,将三维位势问题快速多极边界元法与几乎奇异积分的半解析算法相结合,实现了三维边界元法中几乎奇异积分的准确计算,该方法适用于U型地埋管薄体结构的换热分析.在制冷、制热两种工况下研究了U型地埋管壁厚对换热量的影响,并进一步分析了管群间的热相互作用.计算结果显示,当管壁导热系数一定时,管壁越厚,对管内流体和土壤之间的换热影响越大.当钻孔间距一定时,管群中埋管数量越多,热干扰现象越强烈,提高管群换热量的主要措施是降低管群间热干扰.因准确计算了几乎奇异积分,三维快速多极边界元法可以有效计算薄体和厚体耦合的三维热传导问题.该文方法和分析结果可为地埋管换热器系统的工程应用提供参考.     

Numerical Study on Hypersonic Flow and Aerodynamic Heating北大核心CSCD

基于有限差分法开发了高超声速流动与换热问题气热耦合仿真求解器,运用该求解器对三种典型高超声速流动与换热问题开展了仿真研究,得到了相应的气动参数、热流密度分布。高超声速后台阶的存在使表面气动参数、热流分布不再连续;随着缝深的提高,缝隙局部流速迅速降低,对流换热效应减弱;高超声速无限长圆管绕流中,边界层外部区域气动参数随时间变化不大,边界层内存在较大的温度梯度,壁面温度随时间升高。三个算例的仿真结果均与试验测量值进行了对比,验证了所开发的求解器的计算能力。     

热-力耦合原子-连续关联模型的框架及其算法

对热-力耦合的原子-连续关联模型进行了系统研究,给出了计及热-力耦合行为的金属微-纳米构件内材料的瞬态弹性常数,应力、应变、比热容等物理量的具体计算公式及其算法.利用原子运动中的“结构形变”部分来研究微-纳米尺度下多晶原子团簇的非均匀结构变形.将原子团簇晶格结构的变形与连续体的变形关联起来,在准简谐近似假设下,推导出依赖于微观结构变形和热振动的自由能密度、熵密度、内能密度表达式,从而给出了微-纳米尺度下的瞬态热-力学参数.     

主动冷却点阵夹层防热结构温度响应计算模型北大核心CSCD

针对点阵夹层结构主动热防护问题,建立了夹层结构面板和芯体导热与冷却剂对流耦合的非稳态传热理论模型,利用有限体积法离散控制方程并在MATLAB中进行了迭代求解.模型首次考虑了面板与夹芯杆之间的收缩热阻,并利用分离变量法得到了收缩热阻的近似解析解.基于单胞模型和周期性边界条件,模拟得到了模型所需的表面对流传热系数h_(b)和h_(fin).最后,选取多单胞计算工况进行数值模拟和理论模型对比,并讨论了收缩热阻对模型预测精度的影响.结果表明:理论模型能够准确预测夹层结构及内部流体的温度变化,理论与仿真之间的最大误差不超过1%;随着外加热流密度不断增大,忽略收缩热阻使得计算结果造成的误差不断增大;与数值模拟相比,理论模型可显著地减少计算时间并节省计算资源,尤其适用于非均匀、非稳态复杂热载荷下点阵夹层结构的温度响应计算.     

换热系统性能在线分析数学模型

本文提出了表面式换热系统性能分析的一种新方法,建立了性能在线分析数学模型.所建模型基于质量、能量守恒定律和换热过程的一般理论,可在线用于发电、化工、纺织等工业中的换热系统.     

反演二维瞬态热传导问题随温度变化的导热系数

基于边界元法反演二维瞬态热传导问题随温度变化的导热系数.采用Kirchhoff变换将非线性的控制方程转变为线性方程.边界元法用于构建二维瞬态热传导问题的数值分析模型.将反演参数作为优化变量,测点温度计算值与测量值之间的残差平方和作为优化目标函数.引入复变量求导法求解目标函数的梯度矩阵,梯度正则化法用于优化目标函数获得反演结果.探讨时间步长、测点数量和随机偏差对反演结果的影响.减小步长、增加测点数量收敛速度加快.降低了随机偏差,计算结果更精确.算例证明了算法的有效性与稳定性.     

含温度载荷的管板变形的解析解

根据换热器结构形式建立了用于计算管板变形的力学模型,参考ASME Ⅷ 1计算管板的有效弹性常数,并将外筒和换热管束分别等效为会因温度和压力载荷产生轴向变形的弹簧和弹性基础.应用Ritz法建立了管板挠度的解析解,将该解析解与三个不同规模换热器的有限元分析结果进行对比.结果表明,建立的解析解与有限元分析结果吻合良好,验证了推导的管板变形解析解的正确性.研究结果对固定管板式换热器的设计有较大的指导意义.     

基于特征正交分解的材料微结构参数化表征模型及等效性能优化设计

随着计算能力的不断发展,近年来基于材料微结构图像的材料等效性能数值模拟越来越受到学者们的重视.在此背景下,提出了一种针对材料微结构图像的高效参数化表征模型.通过特征正交分解(proper orthogonal decomposition,POD)对已有材料微结构图像数据进行特征分析,得到近似描述该类材料微结构的特征缩减基.应用移动最小二乘(moving least squares,MLS)法建立特征缩减基映射系数的响应面模型,拟合得到任意给定参量相应的缩减基映射系数.利用拟合缩减基系数可获得任意给定参量对应的微结构图像矩阵.该参数化表征模型被用于表征含椭球夹杂的两相材料(2-phase composite)的二维情形, 并进一步应用于这类复合材料宏观等效力学性能的优化设计.     

弹性半空间热冲击问题的广义热弹性解

基于Laplace变换技术及其极限定理,推导了基于分数阶积分的不同广义热弹性理论模型下弹性半空间受热冲击作用的渐近解,该渐近解可以准确地揭示热量在弹性体内传播的波动特性,并可以捕捉到受热冲击作用在弹性波波前位置处产生的阶跃现象.通过对热冲击下弹性波的传播及热弹性响应的渐近求解及结果分析,比较了不同广义热弹性理论对于热冲击问题的预测能力,并揭示了热传输能力的不同对于热弹性行为的影响.     

A method and a computer program for determining the thermal diffusivity in a solid slab

The authors describe a method for computing the thermal diffusivity of a solid, based on a computer assisted evaluation of the solution of the transient inverse heat conduction problem.The program computes either the unknown diffusivity or simulates the one-dimensional unsteady heat transfer problem. The user may model the boundary conditions by a choice of different functions.The program provides instruction and information at all stages of input and provides tabular output of results. It may be used by anybody wishing to solve or simulate heat transfer processes.     

Lumped thermal model for switched reluctance motor applied to mechanical design optimization

Switched reluctance motors (SRM) are attracting much attention because of their special advantages. Generated heat, due to losses, can reduce the life time of SRMs. Therefore, taking into account thermal modeling helps to improve their performance and increase their life time. In this paper, a lumped thermal model of SRM based on the analogue circuit of conductive and convective thermal resistances is proposed. First the heat transfer equations were applied for modeling of each motor part as a thermal equivalent circuit. Then, the thermal modeling of whole SRM was done via assembling all these sub-circuits. For validity of the obtained model on a wide range of geometrical and structural properties of motor, apt heat transfer relations have been used. This comprehensive model is used to investigate the influence of dimensions on heat transfer in SRM. Finally, an optimal design of the stator yoke dimensions has been reached concerning the mechanical aspects without degrading electromagnetic characteristics of SRM.     

一种新的变域传热问题的边界条件

回顾古典热传导方程建立的假设条件的基础上,分析了热层材料由于表面烧蚀而引起的传热区域内部的热漏现象.基于能量守恒原理,利用有限元分析法推导出变域热传导方程,并得到了热漏函数的表达式,提出了变域传热问题边界条件的改进形式.为了检验这种边界条件的合理性,利用Crank-Nicholson法对此数学模型进行空间和时间离散化,并进行了数值仿真求解.仿真结果证实,基于边界条件改进形式的数学模型使计算更方便,结果更符合实际,从而为工程应用提供理论分析的依据.同时,该数学模型也为研究动边界发汗冷却控制问题奠定了理论研究的基础.     

Coupled fluid-solid thermal interaction modeling for efficient transient simulation of biphasic water-steam energy systems

This paper proposes a fluid-solid coupled finite element formulation for the transient simulation of water-steam energy systems with phase change due to boiling and condensation. As it is commonly assumed in the study of thermal systems, the transient effects considered are exclusively originated by heat transfer processes. A homogeneous mixture model is adopted for the analysis of biphasic flow, resulting in a nonlinear transient advection-diffusion-reaction energy equation and an integral form for mass conservation in the fluid, coupled to the linear transient heat conduction equation for the solid. The conservation equations are approximated applying a stabilized Petrov-Galerkin FEM formulation, providing a set of coupled nonlinear equations for mass and energy conservation. This numerical model, combined with experimental heat transfer coefficients, provides a comprehensive simulation tool for the coupled analysis of boiling and condensation processes. For the treatment of enthalpy discontinuities traveling with the flow, a novel explicit-implicit time integration method based on Crank-Nicolson scheme is proposed, analyzing its accuracy and stability properties. To reduce problem size and enhance numerical efficiency, a modal superposition method with balanced truncation is applied to the solid equations. Finally, different example problems are solved to demonstrate the capabilities, flexibility and accuracy of the proposed formulation.     

Application of the two-dimensional differential transform method to heat conduction problem for heat transfer in longitudinal rectangular and convex parabolic fins

In this article, approximate analytical (series) solutions for the temperature distribution in a longitudinal rectangular and convex parabolic fins with temperature dependent thermal conductivity and heat transfer coefficient are derived. The transient heat conduction problem is solved for the first time using the two-dimensional differential transform method (2D DTM). The effects of some physical parameters such as the thermo-geometric parameter, exponent and thermal conductivity gradient on temperature distribution are studied. Furthermore, we study the temperature profile at the fin tip.     

Topology Optimization of Fins for Rapid Heat Storage and Release in Triangular-Inside Tube Units北大核心CSCD

针对传统相变蓄热器传热速率低的问题,提出了一种内三角管式的蓄热器,并基于拓扑优化原理,以强化换热为目的,对其进行肋片设计,重构了拓扑结果,进而提取其拓扑特征重新设计肋片,分析了不同肋片设计对传热能力的影响。结果表明:内三角管式蓄热器相比传统圆管式蓄热器,蓄放热性能大大提高;安装拓扑重构肋片的蓄热器可以使蓄、放热时间缩短,传热效率提高;在蓄热过程中,分叉的拓扑特征可以提高自然对流作用;在放热过程中,安装拓扑重构肋片的蓄热器(火 积)耗散更小,可逆性更好,换热效率更高。     

Determination of heat transfer properties of media with a single-needle probe

Needle probes with a line heater inside are often used in studying the heat transfer properties of loose rocks. The key problem of contact methods of measuring thermal properties of various media consists in finding thermal contact resistance at the probe/medium interface which must be taken into account in determining the thermal diffusivity of the medium. We describe a mathematical model of heating of a long needle probe in the medium under study, taking into account dimensions and thermal properties of the needle source and assuming that thermal contact between the source and the medium is not ideal. Based on the proposed model, we formulate and solve the inverse problem of finding the thermal diffusivity coefficient of the medium and the heat exchange coefficient at the probe/medium interface. The purpose of the article is to create methodology for determining thermal properties of various media in the field.     

应力降低因子和陶瓷材料抗热冲击阻力参数的性质及适用条件

将对流条件下薄板的瞬态导热问题的解析解引入自由弹性薄板的热应力场模型中,给出了相应应力降低因子的具体表达形式．为了便于比较,进一步定义了一个新的应力降低因子．详细讨论了应力降低因子及分别对应于高Biot模数和低Biot模数的第1个和第2个抗热冲击阻力参数及与中间量级的Biot模数相对应的近似表达式的性质及适用条件．将传热学与热弹性力学或断裂力学相结合的方法及有限元方法是该文所推荐的抗热震性能计算方法．研究表明,采用断裂临界温差和断裂临界无量纲时间相结合的方式能够直观简洁地表征陶瓷材料的抗热震性能．     

Transient modelling of flow distribution in automotive catalytic converters

The transient catalytic converter performance is governed by complex interactions between exhaust gas flow and the monolithic structure of the catalytic converter. Therefore, during typical operating conditions of interest, one has to take into account the effect of the inlet diffuser on the flow field at the entrance. Computational fluid dynamics (CFD) is a powerful tool for calculating the flow field inside the catalytic converter. Radial velocity profiles, obtained by a commercial CFD code, present very good agreement with respective experimental results published in the literature. However the applicability of CFD for transient simulations is limited by the high CPU demands.The present study proposes an alternative computational method for the prediction of transient flow fields in axi-symmetric converters time-efficiently. The method is based on the use of equivalent flow resistances to simulate the flow paths in the inlet and outlet catalyst sections. The proposed flow resistance modelling (FRM) method is validated against the results of CFD predictions over a wide range of operating conditions. Apart from the apparent CPU advantages, the proposed methodology can be readily coupled with already available transient models for the chemical reactions in the catalyst. A transient model for heat transfer inside the monolith is presented. An example of coupling between FRM and transient heat transfer inside the converter is included. This example illustrates the effect of flow distribution in the thermal response of a catalytic converter, during the critical phase of catalytic converter warm-up.