基于LTNE的饱和多孔介质THMC耦合模型数值研究

为探究多孔材料中流体流动和温度变化,课题组对溶质和溶剂在孔隙内发生的多物理场耦合问题进行研究。首先,基于局部热非平衡理论(LTNE),分析发生的THMC耦合反应,推导出其控制方程组;其次,将其物理模型简化为内含圆孔无限大平面,通过COMSOL Multiphysics软件,选用一维轴对称坐标系模拟仿真,研究多孔介质应力、孔隙流体压力、溶质摩尔分数和流固体温度分布规律;最后,分析各物理场参数影响。结果表明:随时间增加,各因变量峰值和谷值逐渐沿径向伸展,各物理场之间相互影响愈加深远;反应过程中需衡量参数取值。该研究既可为相关实验和解析方法提供参考,也可直接应用于实际工程中,有助于深入了解饱和多孔介质热流固化完全耦合行为。

A Multiple-Relaxation-Time Lattice Boltzmann Model for Natural Convection in a Hydrodynamically and Thermally Anisotropic Porous Medium under Local Thermal Non-Equilibrium Conditions

To investigate the natural convective process in a hydrodynamically and thermally anisotropic porous medium at the representative elementary volume(REV)scale,the present work presented a multiplerelaxation-time lattice Boltzmann method(MRT-LBM)based on the assumption of local thermal non-equilibrium conditions(LTNE).Three sets of distribution function were used to solve the coupled momentum and heat transfer equations.One set was used to compute the flow field based on the generalized non-Darcy model;the other two sets were used to solve the temperature fields of fluid and solid under the LTNE.To describe the anisotropy of flow field of the porous media,a permeability tensor and a Forchheimer coefficient tensor were introduced into the model.Additionally,a heat conductivity tensor and a special relaxation matrix with some off-diagonal elements were selected for the thermal anisotropy.Furthermore,by selecting an appropriate equilibrium moments and discrete source terms accounting for the local thermal non-equilibrium effect,as well as choosing an off-diagonal relaxation matrix with some specific elements,the presented model can recover the exact governing equations for natural convection under LTNE with anisotropic permeability and thermal conductivity with no deviation terms through the Chapman-Enskog procedure.Finally,the proposed model was adopted to simulate several benchmark problems.Good agreements with results in the available literatures can be achieved,which indicate the wide practicability and the good accuracy of the present model.

UnsteadyMHD Non-Darcian Flow Over a Vertical Stretching Plate Embedded in a Porous Medium with Thermal Non-EquilibriumModel

An analysis is performed to study the influence of local thermal nonequilibrium(LTNE)on unsteadyMHDlaminar boundary layer flowof viscous,incompressible fluid over a vertical stretching plate embedded in a sparsely packed porous medium in the presence of heat generation/absorption.The flow in the porous medium is governed by Brinkman-Forchheimer extended Darcy model.A uniform heat source or sink is presented in the solid phase.By applying similarity analysis,the governing partial differential equations are transformed into a set of time dependent non-linear coupled ordinary differential equations and they are solved numerically by Runge-Kutta Fehlberg method along with shooting technique.The obtained results are displayed graphically to illustrate the influence of different physical parameters on the velocity,temperature profile and heat transfer rate for both fluid and solid phases.Moreover,the numerical results obtained in this study are compared with the existing literature in the case of LTE and found that they are in good agreement.

Heat Transfer and Energy Utilization of Waste Heat Recovery Device with Different Internal Component

Steel industry is high energy-consuming industry, and its waste?heat recovery is critically?important for energy utilization. In this study, pipeline bundle is used to enhance heat transfer in?waste?heat recovery device,?and?associated gas-solid heat transfer and energy utilization performance with different pipeline arrangement, pipe diameter and shape of internal component are further analyzed. The temperatures of gas and particle in device with pipeline bundle periodically fluctuate in horizontal direction, and those in staggered system distribute more uniformly than those in paralleled system. Compared with paralleled device, exergy and waste heat utilization efficiency of staggered device have been improved, and they are both higher than?those without pipeline. As pipe diameter increases, exergy and waste heat utilization efficiency first increases and then decreases, and they reach the maxima with optimal pipe diameter.?As the width of internal component keeps constant, influence of its shape on heat transfer is very little.

局部热非平衡多孔介质圆管非达西强迫对流换热分析

对流体饱和多孔介质圆管内局部热非平衡情形下非达西强迫对流的换热性能进行数值模拟.首先利用Brinkman流动模型和局部热非平衡模型建立研究问题的数学模型,预测强迫对流换热.然后使用COMSOL Multiphysics仿真软件对模型求解,获得无量纲渗流速度场、固体骨架温度场、流体温度场和努塞尔数(Nu).此外,详细分析Nu对某些关键参数的依赖性.研究发现,随着达西数(Da)和毕渥数(Bi)的增加,Nu先增加后趋于渐近值;贝克来数(Pe)的增加会导致Nu单调增加;相反,流体有效热导率与固体骨架有效热导率之比(即导热比κ)和流体有效动力黏度与实际动力黏度比(即黏度比M)的增加将导致Nu先减小后趋于渐近值.所得模型和数值结果既可用于提高工程中多孔介质圆管换热能力,也可为相关实验和解析研究提供参考.

Experimental and numerical analysis of the hydraulic and thermal performances of the gradually-varied porous volumetric solar receiver

A gradually-varied porous structure is designed to increase the thermal performance of the porous volumetric solar receiver.Based on the replica method and multilayer recoating technique, the silicon carbide porous ceramic with linear-changed geometrical parameters is fabricated. The performances of the uniform and gradually-varied porous volumetric solar receivers are studied by both experiment and numerical simulation. An optimization method combining genetic algorithm and computational fluid dynamics analysis is applied to determine the optimum porosity distribution. The results present that porous volumetric solar receiver with linear-changed geometrical parameters exhibits better thermal performance than the uniform porous volumetric solar receivers, especially when the thickness of the receiver is small. Larger porosity in the front is beneficial for increasing the solar radiation penetration depth, which limits the reflectance and thermal radiative losses. Smaller porosity in the rear traps more solar radiation and increases the convective heat transfer. When the receiver’s thickness is larger, the performance of the gradually-varied volumetric solar receiver is nearly identical to that of the uniform receiver with largest porosity. The double-layer configuration is found to be the optimized structure of the gradually-varied porous volumetric solar receiver. The thermal efficiency could be further improved using genetic algorithm with an 11 K increase of the outlet temperature.

Effect of asymmetrical wall heat flux and wall temperature ratio on mixed convection in a vertical micro-porous-channel with internal heat generation

Mixed convective heat transfer in a vertical parallel plate micro-porous channel with internal heat generation and viscous dissipation,varying wall heat flux ratio and wall tem­perature ratio at the boundaries is investigated using the Darcy-Brinkman model under local thermal non-equilibrium assumption.Numerical solution for both fluid and solid temperature distributions are obtained by applying the finite element method.The effect of pertinent param­eters such as Brinkman number,Rayleigh number,Darcy number,inter-phase heat transfer co­efficient,porosity scaled thermal conductivity ratio and solid internal heat generation are discussed.The results indicate that the Nusselt number increases with the increase in the solid internal heat generation as well as Rayleigh number in both wall heat flux ratio and wall tem­perature ratio boundary conditions.It is observed that with the quantitative increase in viscous dissipation parameter Br,Nusselt number Nu increases in the presence of internal heat gener­ation and it decreases in the absence of internal heat generation,for a specific range of values of wall heat flux ratio and wall temperature ratio.Beyond this range Nu increases with the increase in Dr regardless of internal heat generation.For the cases,constant wall temperature and wall heat flux ratios,good correlation is observed in the results obtained with that of available in the literature.