【作者】 陈元元；

【导师】 李本文；

【作者基本信息】 东北大学 ， 热能工程， 2018， 博士

【摘要】 多孔介质内的流动与传热过程是多孔介质工程应用中必不可少的研究内容,探明流体在多孔介质内的传输规律是多孔介质结构设计和优化的基础。饱和多孔腔体内的自然对流传热是多孔介质传输过程研究领域的经典问题,基于此问题可以对各种数值方法的精确性进行验证,亦可以对复杂流体及多场耦合现象展开基础性研究。在当前,发展高效、高精度的求解多孔介质内流动与传热的数值方法,深入研究变化的物性参数以及辐射-对流耦合传热下多孔介质内的流动与传热规律,仍是饱和多孔腔体内自然对流研究领域的热点。本论文首先采用Chebyshev配置点谱方法求解了饱和多孔介质方腔内Darcy自然对流换热问题,将模拟结果与已有文献的数值以及实验结果进行对比,验证了数值方法的有效性;近一步构造了一个局部热平衡假设下的精确解,对本数值方法的计算精确性进行了验证,结果表明二者之间的最大绝对误差为2.79×10-11,最大E2误差为2.14×10-12,证明了本数值方法具有很高的计算精度。其次,考虑到变物性对多孔介质中对流换热的影响,本文建立了多孔介质热扩散率随温度呈线性变化时矩形多孔腔体内的动量与能量传递模型,并引入热函数的概念,推导了变热扩散率下的热函数的控制方程及定解条件。在此基础上,采用配置点谱方法对控制方程进行了求解,结合数值计算结果,绘制了矩形多孔腔体内的热线、流线和等温线图,详细分析了Ra、热扩散系数、形状因子等无量纲参数对多孔矩形腔体内流场、温度场、传热速率的影响规律。研究结果表明,当腔体内的热扩散率大小分布不同时,腔内的流动形态、传热分布显著不同,热扩散率的变化在多孔介质的流动与传热中的作用不可忽视。最后,着眼于多孔介质高温应用中辐射在传热中的重要作用,本文研究了局部非热平衡假设下多孔介质辐射参与时方腔内的流动与换热现象。多孔介质固体骨架的辐射能量通过求解腔体的辐射传递方程而获得。在此基础上,详细分析了多孔介质的辐射特性参数(如Pl、光学厚度、散射反照率和壁面黑度)及其它无量纲参数(Ra、相间传热系数、相间导热系数比)对腔内流场、温度场、壁面努塞尔数以及腔内两相热非平衡程度的影响。本文的主要创新点如下:(1)首次将Chebyshev配置点谱方法用于多孔介质腔体内的流动与传热求解;自主构建了一个局部热平衡假设下的精确解验证了本数值方法的精确性;(2)结合形状因子分析了随温度线性变化的热扩散率下多孔介质矩形腔内的流动与传热现象,构造了变热扩散率时的热函数控制方程及定解条件;(3)在耦合求解辐射传递方程的基础上,详细研究了局部非热平衡假设下多孔介质辐射特性参数、Ra数、相间传热系数和相间导热系数比对多孔介质方腔内的流动、传热以及腔内两相热非平衡程度的影响。更多还原

【Abstract】 The fluid flow and heat transfer in porous media is an indispensable research topic in engineering applications of porous media.It is the basis of structure design and optimization of the porous media to ascertain the fluid transmission in porous media.The natural convection heat transfer in saturated porous cavities is a classic problem in the field of porous media transmission.Based on this problem,the accuracy of different numerical methods can be tested,and the basic research on complex fluid and multi-field coupling problems can be investigated.At the present,it is still the hotspot in the research field of natural convection in the saturated porous media to develop the numerical method with high efficiency and high precision,as well as to study the influence of the variable physical properties and the radiation-convection coupled heat transfer on the fluid flow and heat transfer in the porous media.In this paper,the Chebyshev collocation spectral method(CCSM)is used to solve the Darcy natural flow in a saturated porous cavity.The simulation results are compared with the numerical and the experimental results of the literatures to verify the reliability of the numerical method.An exact solution using local thermal equilibrium assumption has been constructed to test the accuracy of the numerical method.The results show that the maximum absolute error between the numerical result and the exact solution is 2.79×10-11 and the maximum E2 error is 2.14×10-12,which proved that the numerical method has a high accuracy.Secondly,considering the effect of variable physical properties on the convective heat transfer in porous media,a fluid flow and heat transfer model in a rectangular porous cavity with a variable thermal diffusivity has been presented.The thermal diffusivity is assumed to vary as a linear function of temperature.The concept of heat function has been adopted,and the governing equation of the heat function and the corresponding boundary conditions with variable thermal diffusivity has been constructed.The governing equations are solved by CCSM.Based on the numerical results,the heatlines,streamlines and isotherms in the rectangular porous cavity are ploted.The influences of the Ra,the thermal diffusivity coefficient and the aspect ratio on the flow field,thermal field and heat transfer rate have been analyzed.The results show that the different distribution of the thermal diffusivity in the cavity will lead to a significant difference in fluid flow and heat transfer.The effect of thermal diffusivity on the fluid flow and heat transfer in porous media can not be ignored.Finally,considering the high temperature applications of porous media,the fluid flow and heat transfer in the porous cavity with thermal radiation of the solid skeletons under the local thermal non-equilibrium is studied.The radiative energy of the solid skeletons is obtained by solving the radiation transfer equation.Based on the numerical results,the effect of the radiative properties(such as the PI,the optical thickness,the scattering albedo and the wall emissivity)and dimensionless parameters(such as the Ra,the interphase heat transfer coefficient,and the interphase thermal conductivity ratio)on the fluid flow and heat transfer are analyzed in detail.The main innovations of this paper are as follows:(1)The CCSM is developed to investigate the problems of fluid flow and heat transfer in porous media for the first time.An exact solution with LTE assumption is constructed to validate the accuracy of the numerical method.(2)The effect of temperature-dependent thermal diffusivity together with the aspect ratio on the free convection in a rectangular porous enclosure is numerically studied.The governing equation and the corresponding boundary conditions for heat function with variable thermal diffusivity has been constructed.(3)Based on the solution of the radiation transfer equation,the influence of the radiative properties and the dimensionless parameters i.e.the Ra,the interphase heat transfer coefficient and the interphase thermal conductivity ratio on the fluid flow and heat transfer in a porous cavity with local thermal non-equilibrium assumption is analyzed.更多还原