多孔介质内往复流动下超绝热燃烧系统温度场的理论分析

对于多孔介质内往复流动下超绝热燃烧（简称RSCP）系统，如果多孔介质的孔隙分布均匀、物性参数保持不变、化学反应为不可逆反应而且反应完全，那么，稳定燃烧过程中，多孔介质内最大温度和温度分布几乎不受往复半周期长短的影响。依据这一特点，用过滤燃烧模型，讨论了往复半周期无限长和无限短两种极端情况下最大温度值和温度分布，得出两种极端情况适用于通常工况的简单模型理论解。     

接箍导热系数测试方法研究

本文基于单宗量热物性参数导热反问题理论,对接箍的导热系数进行反演.建立正、反问题模型,利用黄金分割法反演了接箍导热系数,然后用CFD软件模拟了温度场分布,最后分析了测量位置对导热系数反演的影响.研究结果表明,黄金分割法能够适用于单宗量热物性参数的反演,通过测量外表面温度,能够准确地反演隔热油管接箍导热系数.     

多孔介质的热物性对往复流动下超绝热火焰的影响

多孔介质燃烧室的传热性能主要取决于多孔介质材料的热物性,本文在气固两相局部非热平衡假设基础上,建立往复式流动下多孔介质超绝热燃烧的二维数学模型,研究了多孔介质的比热、导热系数、衰减系数和体积换热系数等对温度分布和燃烧速率的影响,以期为多孔介质选材和往复流动下多孔介质超绝热燃烧器的优化设计提供理论依据。     

多孔介质层内同轴管式井下换热器的相似解

对多孔介质层内同轴管式井下换热器的自然对流传热过程进行了数值模拟。通过对井下换热器分段并依次在局部采用对流相似解法对其周围遵循Ｄａｒｃｙ定律的多孔介质层内自然对流过程进行了近似求解，得到了有关多孔介质层物性参数对井下换热器热输出影响的规律。     

多孔介质燃烧器的辐射输出效率和污染物

使用双温度体积平均模型、详细化学反应机理GRI3．0,对甲烷,空气预混气在多孔介质燃烧器内的预混火焰进行模拟．分析不同当量比和质量流量下的预热效率、辐射输出效率以及污染物排放情况,并对辐射输出效率随多孔介质热物性参数的变化进行敏感性分析．结果表明,增大相间对流换热系数或减小当量比、质量流量及固相消光系数都可以提高辐射输出效率,减小当量比或质量流量可以减少污染物排放．在所有的影响因素中,当量比的影响最大,发展超贫燃燃烧技术是获得高效低污染多孔介质燃烧器的关键．     

建筑材料多孔介质热物性参数测试实验研究

建立一种操作简单、易推广的实验方法,测试建筑材料多孔介质的导热系数、孔隙率、定压比热以及密度等基础热物性参数。通过实验测试建筑材料混凝土的干燥密度、干燥定压比热容、干燥导热系数及孔隙率等参数值,验证此实验测试方法的可行性,为推动建筑材料多孔介质热湿迁移物性基础数据的进一步完善提供收集依据。     

骨架发热多孔介质通道内单相流阻力与换热特性数值模拟

以Fluent 6.3为平台,采用局部非热平衡模型,对紊流及紊流过渡区范围内骨架发热多孔介质竖直通道内的非达西强制对流换热进行了数值模拟。采用三维N-S方程及标准k-ε湍流模型描述多孔介质内的流动,详细研究了孔隙有效雷诺数Re(400Re2000),表面热流密度q(q=5、30和90 kW/m2)和冷却剂入口温度Tin(Tin=20、50和80℃)的变化对多孔介质流道内流动阻力及换热特性的影响。结果表明:低热流密度下,表面热流密度的变化对流动阻力和换热系数的影响很小;小球直径对换热系数的影响显著,且随着雷诺数的增加而增加;换热系数随冷却剂入口温度的增加而减小。     

伞喷油雾与热多孔介质相互作用的数值模拟

为深入认识多孔介质发动机中均匀混合气的形成,用改进的KIVA-3V详细模拟了伞喷油雾与热多孔介质之间的相互作用．在KIVA-3V中增加了油滴碰撞热多孔介质壁面的碰撞模型、传热模型．为检测数值模型的合理性,在Senda等人的实验条件下进行了数值计算．油束碰壁后油滴和油蒸气分布的数值计算结果与实验结果吻合较好．在简化多孔介质结构的基础上和不同的环境压力及喷雾锥角时,模拟了伞喷油雾与热多孔介质的碰撞过程．计算结果表明,伞喷油雾的喷雾锥角及空间压力对油滴在多孔介质中的分布有着很大的影响,在多孔介质厚度一定时,通过调节这些参数,能够形成均匀混合气．     

多孔介质发动机的双区燃烧模型

多孔介质(PM)发动机是基于多孔介质燃烧技术的新型发动机,能够实现均质和稳定燃烧。在考虑了区间质量分布、壁面传热、区间质量交换等因素的基础上,结合多孔介质换热模型,建立了多孔介质发动机的一种双区模型,对其燃烧过程进行模拟。着重讨论了进气温度和压强、压缩比、过量空气系数等参数对多孔介质发动机性能的影响。计算结果表明多孔介质对混合气的预热作用,促进了液体燃料汽化和燃烧反应发生,多孔介质初始温度对发动机的压燃着火起决定性作用。     

生物质热燃气多孔介质直燃机理与脱焦特性研究

为研究含焦油的生物质热燃气在多孔介质中的燃烧机理与焦油燃烧脱除特性,采用固相实体颗粒堆积法模拟多孔介质,通过分析燃烧过程中反应器内温度、热流密度以及反应动力学速率等参数场的分布特征,揭示了当量比对生物质热燃气多孔介质燃烧过程的显著影响作用.研究表明,焦油燃烧脱除过程中直接氧化反应速率高是决定焦油出口浓度小、转化率高的关...     

Simulation of laminar flow and convective heat transfer in conduits filled with porous media using Lattice Boltzmann Method

In the present work, laminar flow and convective heat transfer between two parallel plates of a conduit were simulated using Lattice Boltzmann Method (LBM). The conduit core was filled with a porous medium fully and partially. The effect of porous medium was considered by introducing the porosity into the equilibrium distribution. Viscous and inertia flow resistance effects of porous medium were incorporated in the form of force terms in the Boltzmann's equation. To simulate the temperature field, a simplified thermal lattice BGK model with doubled population method was employed. Comparing the results of the present study to the analytical solutions, a reasonable agreement was observed. The effects of various parameters like Darcy number, porous medium thickness, etc. on the conduit thermal performance were investigated. It was found that all these parameters had significant influence on thermal performance of the channel in certain conditions.     

Effect of thermal radiation on unsteady mixed convection flow and heat transfer over a porous stretching surface in porous medium

An analysis is performed to investigate the effects of thermal radiation on unsteady boundary layer mixed convection heat transfer problem from a vertical porous stretching surface embedded in porous medium. The fluid is assumed to be viscous and incompressible. Numerical computations are carried out for different values of the parameters involved in this study and the analysis of the results obtained shows that the flow field is influenced appreciably by the unsteadiness parameter, mixed convection parameter, parameter of the porous medium and thermal radiation and suction at wall surface. With increasing values of the unsteadiness parameter, fluid velocity and temperature are found to decrease in both cases of porous and non-porous media. Fluid velocity decreases due to increasing values of the parameter of the porous medium resulting an increase in the temperature field in steady as well as unsteady case.     

Transient and non-Darcian effects on natural convection flow in a vertical channel partially filled with porous medium: Analysis with Forchheimer–Brinkman extended Darcy model

The present study addresses the transient as well as non-Darcian effects on laminar natural convection flow in a vertical channel partially filled with porous medium. Forchheimer–Brinkman extended Darcy model is assumed to simulate momentum transfer within the porous medium. Two regions are coupled by equating the velocity and shear stress in the case of momentum equation while matching of the temperature and heat flux is taken for thermal energy equation. Approximate solutions are obtained using perturbation technique. Variations in velocity field with Darcy number, Grashof number, kinematic viscosity ratio, distance of interface and variations in temperature distribution with thermal conductivity ratio, distance of interface are obtained and depicted graphically. The skin-friction and rate of heat transfer at the channel walls are also derived and the numerical values for various physical parameters are tabulated.     

Combined Mode Conduction and Radiation Heat Transfer in a Porous Medium and Estimation of the Optical Properties of the Porous Matrix

This article deals with the analysis of combined mode conduction and radiation heat transfer in a porous medium, and simultaneous estimation of the optical properties of the porous matrix. Simultaneous solution of the gas- and solid-phase energy equations encompasses local thermal nonequilibrium, while the convective heat exchange term couples the gas- and the solid-phase energy equations. A localized uniform volumetric heat generation zone is the source of heat transfer in the porous matrix. With volumetric radiative information needed in the solid-phase energy equation computed using the discrete transfer method, the solid- and gas-phase energy equations are simultaneously solved using the finite difference method. For a given set of boundary conditions and operating parameters, the computed temperature distribution serves as the exact temperature profile necessary in the estimation of parameters. In the estimation of parameters using inverse analysis, the objective function is minimized using the genetic algorithm. Effects of measurement error, number of generations, population size, crossover probability, and mutation probability are studied in regard to the accuracy of results and the computational time required. Reasonably accurate estimations of extinction coefficient, scattering albedo, and emissivity of the porous matrix are obtained.     

Analysis of a fully wetted moving fin with temperature‐dependent internal heat generation using the finite element method

The temperature field of a moving longitudinal porous fin with varying internal heat generation with respect to temperature has been studied under natural radiation and convection effects. The Darcy model was implemented for the analysis and the parameters, whose effect on the thermal process were grouped and nondimensionalized. By using the finite element method, the obtained highly nonlinear second order ordinary differential equations were numerically solved. The relevant parameters were studied by means of graphs and subsequently their importance in the rate of heat transfer was interpreted.     

Numerical analysis of thermal response tests with a groundwater flow and heat transfer model

The Kelvin line-source equation, used to analyze thermal response tests, describes conductive heat transfer in a homogeneous medium with a constant temperature at infinite boundaries. The equation is based on assumptions that are valid for most ground-coupled heat pump environments with the exception of geological settings where there is significant groundwater flow, heterogeneous distribution of subsurface properties, a high geothermal gradient or significant atmospheric temperature variations. To address these specific cases, an alternative method to analyze thermal response tests was developed. The method consists in estimating parameters by reproducing the output temperature signal recorded during a test with a numerical groundwater flow and heat transfer model. The input temperature signal is specified at the entrance of the ground heat exchanger, where flow and heat transfer are computed in 2D planes representing piping and whose contributions are added to the 3D porous medium. Results obtained with this method are compared to those of the line-source model for a test performed under standard conditions. A second test conducted in waste rock at the South Dump of the Doyon Mine, where conditions deviate from the line-source assumptions, is analyzed with the numerical model. The numerical model improves the representation of the physical processes involved during a thermal response test compared to the line-source equation, without a significant increase in computational time.     

Experimental investigation on simultaneous measurement of temperature distributions and radiative properties in an oil-fired tunnel furnace by radiation analysis

The paper reports experimental investigations on simultaneous measurement of temperature distribution and radiative properties in an oil-fired tunnel furnace by radiation analysis. Two color CCD cameras were used to obtain visible thermal radiation in the furnace. A radiation imaging model was established by the calculation of radiative transfer equation in the furnace. The temperature distribution and radiative properties can be obtained from the inversion of the radiative imaging model. The validity of radiative imaging model was verified by the numerical analysis of cavity radiation and isothermal system radiation, and the accuracy of reconstruction method was validated by simulation reconstruction. The experimental analysis was divided into two parts. Firstly, the temperatures of wall surface were calculated from the radiative image of refractory wall and compared with the measured temperature of a thermocouple. The difference between the two methods was only about 20 K. Secondly, the temperature distributions in the furnace, absorption coefficients of combustion medium, and emissivities of refractory wall were reconstructed. Because of a single burner in the tunnel furnace, the temperature distributions in the XY vertical sections in the furnace were with temperature higher in the center and lower near the refractory wall surface, and the temperatures decreased along the length of the tunnel furnace. The measured emissivity of refractory wall showed that the refractory material of RPA-MC30 is with high reflectivity in visible spectrum.     

Analysis of temperature gradient bifurcation in porous media – An exact solution

The phenomenon of temperature gradient bifurcation in a porous medium is analyzed by studying the convective heat transfer process within a channel filled with a porous medium, with internal heat generation. A local thermal non-equilibrium (LTNE) model is used to represent the energy transport within the porous medium. Exact solutions are derived for both the fluid and solid temperature distributions for two primary approaches (Models A and B) for the constant wall heat flux boundary condition. The Nusselt number for the fluid at the channel wall is also obtained. The effects of the pertinent parameters such as fluid and solid internal heat generations, Biot number and fluid to solid thermal conductivity ratio are discussed. It is shown that the internal heat generation in the solid phase is significant for the heat transfer characteristics. The validity of the one equation model is investigated by comparing the Nusselt number obtained from the LTNE model with that from the local thermal equilibrium (LTE) model. The results demonstrate the importance of utilizing the LTNE model in the present study. The phenomenon of temperature gradient bifurcation for the fluid and solid phases at the wall for Model A is established and demonstrated. In addition, the temperature distributions for Models A and B are compared. A numerical study for the constant temperature boundary condition was also carried out. It was established that the phenomenon of temperature gradient bifurcation for the fluid and solid phases for the constant temperature boundary condition can occur over a given axial region.     

The lattice Boltzmann modeling on the nanofluid natural convective transport in a cavity filled with a porous foam

In this work, the natural convective transport was numerically investigated for nanofluids in a metal-foam cavity. A lattice Boltzmann (LB) model for the nanofluid natural convection in a porous medium was established by using the volume-averaging method. The velocity and temperature fields were obtained, and flow and thermal characteristics of the nanofluid convection in a porous medium were presented. The effects of the Rayleigh Number, the Darcy Number, the porosity, the solid thermal conductivity of porous medium, the nanoparticle thermal conductivity and the nanoparticle concentration on natural convection were examined. The average velocity was put forward to evaluate the convection effect and the natural convection onset was also discussed. It is shown that the Nusselt number of the natural convection increases with an increase in the Darcy number, the Rayleigh number, the porosity and the effective thermal conductivity. The change from the heat conduction regime to the convection regime is clearly shown from the numerical result, which verifies the onset point of the nanofluid natural convection in a porous medium. The highly conductive porous foam and the nanofluid can promote the thermal performance of the natural convection, which own great potential in practical thermal applications.     

The effect of porous insert position on the enhanced heat transfer in partially filled channels

In the present work, the effect of porous insert position on enhanced heat transfer in a parallel-plate channel partially filled with a fluid-saturated porous medium was studied. The fully-developed laminar flow and convective heat transfer in the channel were simulated using Lattice Boltzmann Method (LBM). The walls of the channel were subject to a uniform constant temperature. The flow field and thermal performance of the channel were investigated and compared for two configurations: first the porous insert was attached to the channel walls, and second the same amount of the porous material was positioned in the channel core. Comparing the results of the present study to the analytical solutions, a reasonable agreement was observed. The effects of various parameters like Darcy number, porous medium thickness, etc. on the conduit thermal performance were investigated in both channel configurations. It was found that the position of the porous insert has significant influence on the thermal performance of the channel.