A Lattice Boltzmann Model for Fluid-Solid Coupling Heat Transfer in Fractal Porous Media

We report a lattice Boltzmann model that can be used to simulate fluid-solid coupling heat transfer in fractal porous media. A numerical simulation is conducted to investigate the temperature evolution under different ratios of thermal conductivity of solid matrix of porous media to that of fluid. The accordance of our simulation results with the solutions from the conventional CFD method indicates the feasibility and the reliability for the developed lattice Boltzmann model to reveal the phenomena and rules of fluid-solid coupling heat transfer in complex porous structures.     

温室内多孔蓄热墙传热与流动特性

本文采用一维稳态饱和多孔介质能量双方程模型,对温室内多孔蓄热墙的传热与流动特性进行了研究.结果表明:多孔蓄热墙的固体骨架与空气之间的换热取决于空气的入口流速、多孔材料的孔隙率、渗透率和固体骨架的导热系数,由于多孔蓄热墙接受的太阳辐照是不均匀的,因此推荐采用具有不同孔隙率的新型复合多孔蓄热墙,以减少这种不利的影响.     

微多孔介质非牛顿流体格子Boltzmann模拟

根据修正Blake-Kozeny方程对Herschel-Bulkley模型的非牛顿流体在微多孔介质中流动的定量描述,通过增加外力项建立了非牛顿流体在宏观多孔介质中的格子Boltzmann模型,并将该模型应用到平板多孔介质中非牛顿流体的电渗驱动流模拟。对多孔介质渗透性参数和非牛顿流体流变性参数的影响进行模拟和分析,获得了各种参数对多孔介质中非牛顿流体流动速度的影响规律。     

Kinetic theory based lattice Boltzmann equation with viscous dissipation and pressure work for axisymmetric thermal flows

A lattice Boltzmann equation (LBE) for axisymmetric thermal flows is proposed. The model is derived from the kinetic theory which exhibits several features that distinguish it from other previous LBE models. First, the present thermal LBE model is derived from the continuous Boltzmann equation, which has a solid foundation and clear physical significance; Second, the model can recover the energy equation with the viscous dissipation term and work of pressure which are usually ignored by traditional methods and the existing thermal LBE models; Finally, unlike the existing thermal LBE models, no velocity and temperature gradients appear in the force terms which are easy to realize in the present model. The model is validated by thermal flow in a pipe, thermal buoyancy-driven flow, and swirling flow in vertical cylinder by rotating the top and bottom walls. It is found that the numerical results agreed excellently with analytical solution or other numerical results.     

聚光太阳能PV/T空气集热器能量和(火用)效率分析

文中建立了带有散热翅片的复合抛物面聚光太阳能PV/T空气集热器内部传热过程的一维稳态数学模型,对传热过程进行了数值模拟,对集热器热、电、(火用)和净电效率进行了计算.分析了空气质量流量、入射光强度、风速对集热器的空气温度及系统各效率的影响.结果表明:随着入射光强度的增加,空气进出口温差、热和(火用)效率是增大的,而电效率则有所降低.随着空气流量的增加,系统的净电效率和进出口温度差是降低的.通过计算可知集热器的(火用)效率在18%～11%,热效率可达65%,净电效率低于2%,并明显受空气质量流速的影响.     

Investigation of Stability and Hydrodynamics of Different Lattice Boltzmann Models

Stability and hydrodynamic behaviors of different lattice Boltzmann models including the lattice Boltzmann equation (LBE), the differential lattice Boltzmann equation (DLBE), the interpolation-supplemented lattice Boltzmann method (ISLBM) and the Taylor series expansion- and least square-based lattice Boltzmann method (TLLBM) are studied in detail. Our work is based on the von Neumann linearized stability analysis under a uniform flow condition. The local stability and hydrodynamic (dissipation) behaviors are studied by solving the evolution operator of the linearized lattice Boltzmann equations numerically. Our investigation shows that the LBE schemes with interpolations, such as DLBE, ISLBM and TLLBM, improve the numerical stability by increasing hyper-viscosities at large wave numbers (small scales). It was found that these interpolated LBE schemes with the upwind interpolations are more stable than those with central interpolations because of much larger hyper-viscosities.     

Étude théorique du comportement thermique d'un capteur solaire plan à air à couverture combinée plastique-vitre

The air heater studied in this paper is made of an external transparent plastic cover, such as plexiglas, the role of which is to protect the collector from accidentally thrown stones and an internal transparent glass cover which produces the necessary greenhouse effect for heating the absorber. An analysis of the unsteady state heat exchanges in such a collector is presented. It is shown that at quasi-steady state the energy balance equations of the components of the heater cascade into a single first order differential equation, which is able to predict the thermal behaviour of the collector. The solution of this differential equation is written down as an explicit expression of the local temperature of the fluid flowing in the collector in terms of the time-dependent incident solar intensity. The effect of various parameters such as the inlet fluid temperature, the mass flow rate and the depth of the air channel on the thermal performances of the combined plastic-glass air collector is also studied.     

30m~2槽式太阳能集热器性能模拟研究

本文针对30 m~2槽式太阳能集热试验装置,对太阳能集热器的性能进行了模拟。分析了太阳能集热器光热转化过程,建立了太阳能能量转换与传递模型,并开发了模拟计算程序,研究了集热器关键结构参数和运行参数对集热性能的影响规律。具体考察了太阳辐照强度、工质流量、环境风速、吸热管管径等对集热效率的影响。结果表明:太阳能集热器的集热效率随太阳辐照强度的增加而增大,随工质流量的增加而增大,存在最佳的吸热管管径使得集热效率最大。研究结果将为太阳能集热器的设计提供参考依据。     

使用条件生成对抗网络生成预定导热率多孔介质

多孔介质在工程领域中的应用非常广泛,其中有效导热率和孔隙率为多孔介质材料非常重要的性质,得到一个符合需要的有效导热率和孔隙率的多孔介质材料具有重要意义.本文使用四参数随机生成方法制作了训练数据集,搭建了一个条件生成对抗网络(CGAN),使用预定的有效导热率和孔隙率作为输入,生成一个满足输入条件的多孔介质结构.特别地,由于多孔介质的孔隙结构分布对材料的有效导热率影响巨大,提出局部结构损失函数参与网络训练,使得网络更好地学习到孔隙分布与导热率之前的关系.通过使用格子Boltzmann方法验证神经网络生成的多孔介质结构的有效导热率,结果表明该方法能够快速且准确地生成预定参数的多孔介质结构.     

沉降分布孔隙率多孔介质模型及数值研究

研究沉降分布孔隙率多孔介质流动和传热,根据"O"形圈理论和现场测定确定孔隙率系数,建立坐标方向孔隙率分布函数;考虑流体密度变化,并引入Brinkman-Forchheimer的扩展Darcy模型,能量方程采用界面连续条件,建立沉降分布孔隙率多孔介质流动和传热求解模型.采用差分法对模型进行离散化,应用高斯-赛德尔方法迭代求解.数值分析表明:沉降分布孔隙率条件下多孔介质内流体流动速度在壁面附近较大,中心部位较小,壁面附近孔隙率的增大使得低流速区域减小,较高流速区域增大;当孔隙率小值时,温度按线性减小;当孔隙率大值时,温度在高低温壁面附近迅速减小,在中部减小较缓,热量按导热和对流共同传递;孔隙率增大能使平均怒谢尔数增大,对流换热作用增强.     

多孔介质高温蓄热的热性能分析

高温蓄热是太阳能热发电、高温热利用中的重要组成部分。本文对基于多孔介质和熔融盐流体的高温蓄热过程进行了计算分析,对蓄热时间、流体进口温度、进口速度对斜温层温度分布的影响进行了分析。结果表明进口温度对斜温层厚度的影响较小,进口速度的增加会导致斜温层厚度的增加。同时对流体和多孔介质的温度差进行了分析,得到了应用于局部热平衡和非热平衡的雷诺数判据。     

分形多孔介质中流体流动的LBM模拟

基于分形理论构造了规则Sierpinski地毯结构来模拟多孔介质孔隙结构,采用Lattice Boltzmann Method(LBM)分别对不同孔隙率与进出口压差下的分形多孔介质中的流体流动进行了数值模拟.结果表明,分形多孔介质的某些流场结构也具有分形特征;体积流量和流场进出口压差呈线性关系,符合Darcy定律;流体穿过分形多孔介质的体积流量与孔隙率成指数函数关系.     

A New Numerical Solution of Fluid Flow in Stratigraphic Porous Media

A new numerical technique based on a lattice-Boltzmann method is presented for analyzing the fluid flow in stratigraphic porous media near the earth's surface. The results obtained for the relations between porosity, pressure, and velocity satisfy well the requirements of stratigraphic statistics and hence are helpful for a further study of the evolution of fluid flow in stratigraphic media.     

Effects of variable properties on MHD heat and mass transfer flow near a stagnation point towards a stretching sheet in a porous medium with thermal radiation

The effect of variable viscosity and thermal conductivity on steady magnetohydrodynamic(MHD) heat and mass transfer flow of viscous and incompressible fluid near a stagnation point towards a permeable stretching sheet embedded in a porous medium are presented,taking into account thermal radiation and internal heat genberation/absorbtion.The stretching velocity and the ambient fluid velocity are assumed to vary linearly with the distance from the stagnation point.The Rosseland approximation is used to describe the radiative heat flux in the energy equation.The governing fundamental equations are first transformed into a system of ordinary differential equations using a scaling group of transformations and are solved numerically by using the fourth-order Rung-Kutta method with the shooting technique.A comparison with previously published work has been carried out and the results are found to be in good agreement.The results are analyzed for the effect of different physical parameters,such as the variable viscosity and thermal conductivity,the ratio of free stream velocity to stretching velocity,the magnetic field,the porosity,the radiation and suction/injection on the flow,and the heat and mass transfer characteristics.The results indicate that the inclusion of variable viscosity and thermal conductivity into the fluids of light and medium molecular weight is able to change the boundary-layer behavior for all values of the velocity ratio parameter λ except for λ = 1.In addition,the imposition of fluid suction increases both the rate of heat and mass transfer,whereas fluid injection shows the opposite effect.     

Lattice Boltzmann simulation on thermal performance of composite phase change material based on Voronoi models

Phase change materials(PCMs) are important for sustaining energy development. For the thermal performance enhancement, the composite PCM with metal foam reconstructed by the Voronoi method is investigated in this work. The lattice Boltzmann method(LBM) is used to analyze the melting process on a pore scale. The melting interface evolution and temperature contour of the composite PCM are explored and compared with those of pure PCM. Moreover, structure parameters including the pore density, porosity and irregularity are investigated comprehensively, indicating that the additive of metal foam strengthens the melting performance of PCM obviously. Compared with pure PCM, the composite PCM has quick rates of the melting front evolution and heat transfer. The heat conduction plays a great role in the whole melting process since the convection is weakened for the composite PCM. To improve the melting efficiency, a larger pore density and smaller irregularity are recommended in general. More significantly, a suitable porosity is determined based on the requirement for the balance between the melting rate and heat storage capacity in practical engineering.     

Lattice Boltzmann modeling of microchannel flow in slip flow regime

We present the lattice Boltzmann equation (LBE) with multiple relaxation times (MRT) to simulate pressure-driven gaseous flow in a long microchannel. We obtain analytic solutions of the MRT-LBE with various boundary conditions for the incompressible Poiseuille flow with its walls aligned with a lattice axis. The analytical solutions are used to realize the Dirichlet boundary conditions in the LBE. We use the first-order slip boundary conditions at the walls and consistent pressure boundary conditions at both ends of the long microchannel. We validate the LBE results using the compressible Navier–Stokes (NS) equations with a first-order slip velocity, the information-preservation direct simulation Monte Carlo (IP-DSMC) and DSMC methods. As expected, the LBE results agree very well with IP-DSMC and DSMC results in the slip velocity regime, but deviate significantly from IP-DSMC and DSMC results in the transition-flow regime in part due to the inadequacy of the slip velocity model, while still agreeing very well with the slip NS results. Possible extensions of the LBE for transition flows are discussed.     

Experimental Investigation on Pressure Drop and Heat Transfer Characteristics of Copper Metal Foam Heat Sink

Abstract

The effect of the cooling performance of a copper metal foam heat sink under buoyancy-induced convection is investigated in this work. Experiments are conducted on copper metal foam of 61.3% porosity with 20 pores per inch. The pressure drop experiment is carried out to find the permeability and foam coefficient of the porous media. It is found that the property of porous media changes by changing the angle of inclination of the porous media from a horizontal to a vertical position while keeping the orientation and porosity the same. The Hazen-Dupuit Darcy model is used to curve-fit the longitudinal global pressure drop versus the average fluid speed data from an isothermal steady-flow experiment across the test section of the porous medium. The study concludes that the permeability and foam coefficient for copper foam is found to be 1.11 × 10^?7 m² and 79.9 m^?1, respectively. The heat transfer study shows that the thermal performance of copper metal foam is 35–40% higher than the conventional aluminum metal heat sink under an actual conventional mode.     

生物过滤器中非均匀性流动的数值研究

生物过滤技术因其具有有效性、低成本和环境友好等优点引起了人们的广泛关注.该技术主要通过生物过滤器去除含有H2S等废气的有毒有害气体.运用格子Boltzmann方法对三种生物过滤器模型中多孔介质的非均匀性流动进行了数值模拟.数值模拟结果表明,多孔介质的性质和进口流动条件对临界Rayleigh数有显著影响,临界Rayleigh数随着多孔介质的孔隙度和Darcy数的增大而逐渐变小,并随着进口Reynolds数的增大而逐渐变大.所得结果可望为生物过滤器的优化设计提供一个合理的理论依据.     

Effect of melting heat transfer and thermal radiation on Casson fluid flow in porous medium over moving surface with magnetohydrodynamics

In this study, the effects of variable fluid properties on heat transfer in MHD Casson fluid melts over a moving surface in a porous medium in the presence of the radiation are examined. The relevant similarity transformations are used to reduce the governing equations into a system of highly nonlinear ordinary differential equations and those are then solved numerically using the Runge–Kutta–Fehlbergmethod. The effects of different controlling parameters, namely, the Casson parameter,melting and radiation parameters, Prandtl number,magnetic field, porosity, viscosity and the thermal conductivity parameters on flow and heat transfer are investigated. The numerical results for the dimensionless velocity and temperature as well as friction factor and reducedNusselt number are presented graphically and discussed. It is found that the rate of heat transfer increases as the Casson parameter increases.     

Lattice Boltzmann simulation of double diffusive natural convection in heterogeneously porous media of a fluid with temperature-dependent viscosity

Double diffusive natural convection inside a porous cavity with non-uniform porosity has been numerically studied. The cavity was filled with two parallel porous layers with different porosity. Considering the effects of temperature-dependent viscosity, simulations have been done via the Lattice Boltzmann method (LBM) at representative elementary volume (REV) scale. In this study, the effect of porosity, buoyancy ratio, the viscosity-variation number and thermal Rayleigh number on heat and mass transfer rates was investigated. The streamlines, isotherms, isoconcentrations, average Nusselt number and average Sherwood number curves of different parameters were discussed in detail. It was observed that the governing parameters has significant impact on the fluid flow, temperature and concentration distributions. In addition, the average Nusselt and Sherwood numbers are increase with an reduce in the viscosity-variation number. Further, as the absolute value of buoyancy ratio and thermal Rayleigh number increases, the effect of porosity and viscosity changes on the heat and mass transfer enhancement was augmented.