含湿非饱和多孔介质的完全分形传热研究

为了探究在含湿情况下多孔介质有效导热率的变化,基于分形理论,考虑多孔介质在含湿时加热过程中相变的影响,结合加热过程中的热量守恒方程和傅里叶导热定律推导出计算有效导热率的新公式。将该模型相关数据代入进行计算,分析了孔隙率、含湿率、面积分形维数和迂曲分形维数对有效导热率的影响。研究发现,孔隙率与有效导热率呈负相关,含湿率与有效导热率呈正相关,分形维数与有效导热率呈负相关。该研究能够反映多孔介质内的传热进程,对于探究微孔结构物质的传热具有一定的指导意义。     

质子交换膜燃料电池的气体扩散层渗透率分形模型

本文提出了质子交换膜燃料电池（PEMFCs）气体扩散层（GDL）分形渗透率模型。这个模型是根据扩散层真实微观结构中的两个分形维数建立的。其中一个与毛细管流通道大小有关,另一个与通道迁曲度的描述有关。此外,气体分子的影响可以通过Adzumi方程计算。渗透率分形模型是多孔介质迁曲度分形维数、孔隙面积分形维数、孔径以及有效孔隙度的函数,模型中没有任何经验常数,可以用压汞法测量扩散层的微观结构。根据扫描电子显微镜图象,可用盒式维数法确定两个分形维数。为了检验模型的正确性,把该模型渗透率的预测数据与Toray提供的实验数据进行对比,发现该模型的渗透率预测与实验数据一致,证实了气体扩散层的分形渗透率模型的正确性。     

惰性多孔介质内天然气预混燃烧的数学模拟评述

天然气在惰性多孔介质内的预混燃烧是一个包含燃烧、辐射、对流及导热的复杂过程，从数学模拟的角度，比较了几种不同的甲烷-空气化学反应模型，研究了多孔介质内辐射传递方程的不同求解方法，并且分析了多孔介质的导热系数、对流换热系数等对燃烧器性能的影响。     

多孔介质特性参数对燃烧温度及压力影响的数值模拟研究

考查了两段式多孔介质内预混气燃烧的温度与压力分布情况。建立了甲烷／空气预混气体在多孔介质内燃烧的二维数学模型,运用FLUENT软件求解瞬态控制方程的方法计算出燃烧稳定后多孔介质内的温度、与压力分布,并考查了不同当量比、多孔介质辐射衰减系数和导热系数对温度和压力分布的影响。结果表明,甲烷／空气预混气体在多孔介质中燃烧,当量比越大温度峰值越高,压力梯度越大;小孔介质辐射衰减系数的改变对温度分布和压力分布没有明显的影响,而大孔介质辐射衰减系数对温度分布和压力分布有较大的影响;增加多孔介质的导热系数,会使固相与气相温度均有所升高,燃烧区域压力降低。     

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。     

钙基脱硫剂孔隙分形特性的实验研究

分形维数是描述分形结构特性的一个重要参数，其反映了结构的规划程度。通过实验的方法，研究钙基脱硫剂在不同温度，气氛及烧结时间等煅烧条件下，对孔结构分形维数的影响，以及分形特性对脱硫剂的硫化能力的影响。结果表明煅烧温度对CaO孔结构的分形维数影响较小；CaO孔隙的分形维数随着煅烧气氛中的CO2浓度的增加而减小，随着烧结时间的延长而减小，脱硫剂硫化过程中形成的不可进行孔隙量随着分形维数的增大而增大。     

骨架导热特性对部分填充多孔介质复合腔体内传热影响的模拟研究

利用有限元数值方法对部分填充多孔介质复合腔体内的自然对流进行模拟研究,模拟利用两区域法来计算复合腔体内的自然对流,在多孔介质区域采用Forheimer-Brinkman-Darcy方程,在纯流体区域使用Navier-Stokes方程,重点研究了多孔介质骨架导热特性对自然对流的影响。研究发现,在一定范围内随着多孔介质导热性的增强,流体温度升高,部分填充多孔介质腔体内自然对流有一定程度的增强。     

聚氨酯泡沫塑料热分析

针对聚氨酯开展导热性能分析,对聚氨酯导热系数、密度、闭孔率进行了实验研究,考察了聚氨酯密度、闭孔率对其导热系数的影响,并根据发泡过程中出现的孔洞开展模拟计算,建立基于多孔介质分形结构的物理模型,分析了孔洞对聚氨酯导热性能的影响。研究结果表明:随着密度的增大,聚氨酯的导热系数随之升高,聚氨酯的导热系数主要受固体孔壁结构的影响;聚氨酯闭孔率越低,导热系数越高,且过低的闭孔率会降低其密度;建立分形结构物理模型,通过数值模拟分析,闭孔率相同时,具有较大内部孔洞的聚氨酯导热系数较低。     

钙基脱硫剂分形孔结构的实验研究

分形维数是描述分形结构特性的一个重要参数，其反映了结构的规则程度，文章通过实验的方法，研究了钙基脱硫剂孔结构分形维数的变化特性，以及分形特性对脱硫剂的硫化能力的影响，结果表明CaO孔隙的平均孔径越大，分形维数会越小；脱硫剂硫化过程中形成的不可进入孔隙量随着分形维数的增大而增大。     

多孔介质中预混火焰燃烧速率的预示

本文提出了一种预估多孔介质中预混火焰燃烧速率的方法。在构成气，固两相合一模型的基础上，用光学厚极限条件下的扩散近似法简化其中的热辐射项，从而由基本能量方程导出计算火焰传播速度的迭代关系式，其中包含综合多孔介质传导和辐射的等效导热系数。然后应用此数值迭代法，分别计算出在多孔泡沫陶瓷中层流预混火焰及无多孔介质存在的自由火焰的燃烧速率。     

Determination of Effective Thermal Conductivity ForReal Porous Media Using Fractal Theory

INTRODUCTIONPorousmediaisacompositemediathatincludessolidframeandfluidandexistedwidelyintheeajrthbiosphere.Heatandmasstransferinporousmediaisbothanaturalphenomenoninearthbiosphereandaphysicalchemistryprocessinindustries,agricultureandhumanlife.Thusthestudyonheatandmasstransferinporousmediahasbecomeanimportanttasktoscielltistsandengineers.Heatandmasstransferinporousmediaisaverycomplexobject.Therearestillmanydifficultiestodescribethecoupledheatandmasstransferphenomena.Amongthesedifficultie…     

Towards Thermoelasticity of Fractal Media

An extension and generalization of thermomechanics with internal variables and thermoelasticity to fractal porous media is outlined. First, a field form of the second law of thermodynamics is derived. In conradistinction to the conventional Clausius–Duhem inequality, it involves generalized rates of deformation and internal variables. Upon introducing a dissipation function and postulating the thermodynamic orthogonality on any length-scale, constitutive laws of elastic-dissipative fractal media naturally involving generalized derivatives of strain and stress can then be derived. With a focus on thermoelasticity, a new form of Duhamel's differential equation of heat conduction is derived.     

A reiterated two‐scale probabilistic modeling and numerical simulation for heat equation in a fractal porous media

In this paper, we shall discuss the steady heat equation in fractal structure of a composite porous media. A reiterated two‐scale asymptotic method for a deterministic problem with many different length scales is given. For a stationary random structure of a composite porous media, a reiterated two‐scale probabilistic modeling is presented. Finally, some numerical results will be reported. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(3): 188–196, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20053     

Analysis of effective thermal conductivity for circular tubes made with porous media based on fractal theory

Accurately evaluating the relation between heat transfer performance and the complex structure of porous media is still a difficult task. Most previous fractal models of effective thermal conductivity (ETC) are developed to describe the heat-conducting characteristics of a unit cell or a representative elementary volume in porous media, and few models have paid attentions to the ETC for practical circular tubes made with a porous structure based on fractal theory. This paper proposes a new ETC model for a circular tube made with porous media based on fractals, and the validity of the present model is proved by previous models and testing data in the literature, then the effects of intrinsic thermo-physical properties of each component and pore structures on the ETC are discussed. The analysis results indicate that a circular tube made with porous media can improve its heat-insulating performance by about 25% compared with a common parallel circular tube. This can supply an alternative scheme for pipe insulation design in cold/hot fluid supplying systems or air conditioning systems.     

Thermal analysis of a 2-D heat recovery system using porous media including lattice Boltzmann simulation of fluid flow

The present work deals with the fluid flow simulation and thermal analysis of a two-dimensional heat recovery system using porous media. A basic high-temperature flow system is considered in which a high-temperature non-radiating gas flows through a random porous matrix. The porous medium, in addition to its convective heat exchange with the gas, may absorb, emit and scatter thermal radiation. It is desirable to have large amount of radiative heat flux from the porous segment in the upstream direction (towards the thermal system). The lattice Boltzmann method (LBM) is used to simulate fluid flow in the porous medium. The gas and solid phases are considered in non-local thermal equilibrium, and separate energy equations are applied to these phases. Convection, conduction and radiation heat transfers take place simultaneously in solid phase, but in the gas flow, heat transfer occurs by conduction and convection. In order to analyze the thermal characteristics of the heat recovery system, volume-averaged velocities through the porous matrix obtained by LBM are used in the gas energy equation and then the coupled energy equations for gas and porous medium are numerically solved using finite difference method. For computing of radiative heat flux in the porous medium, discrete ordinates method is used to solve the radiative transfer equation. Finally the effect of various parameters on the performance of porous heat recovery system is studied.     

Lack of oscillations in Dual-Phase-Lagging heat conduction for a porous slab subject to imposed heat flux and temperature

This study shows that the physical conditions necessary for thermal waves to materialize in Dual-Phase-Lagging porous media conduction are not attainable in a porous slab subject to a combination of constant heat flux and temperature (Neumann and Dirichlet) boundary conditions. It is demonstrated that the approximate equivalence between Dual-Phase-Lagging (DuPhlag) heat conduction model and the Fourier heat conduction in porous media subject to Lack of Local Thermal Equilibrium (La Lotheq) that suggested the possibility of thermal oscillations and resonance reveals a condition that cannot be fulfilled because of physical constraints.     

Interfacial heat transfer coefficient for non-equilibrium convective transport in porous media

The literature has documented proposals for macroscopic energy equation modeling for porous media considering the local thermal equilibrium hypothesis and laminar flow. In addition, two-energy equation models have been proposed for conduction and laminar convection in packed beds. With the aim of contributing to new developments, this work treats turbulent heat transport modeling in porous media under the local thermal non-equilibrium assumption. Macroscopic time-average equations for continuity, momentum and energy are presented based on the recently established double decomposition concept (spatial deviations and temporal fluctuations of flow properties). Interfacial heat transfer coefficients are numerically determined for an infinite medium over which the fully developed flow condition prevails. The numerical technique employed for discretizing the governing equations is the control volume method. Preliminary laminar flow results for the macroscopic heat transfer coefficient, between the fluid and solid phase in a periodic cell, are presented.     

A fractal model for the coupled heat and mass transfer in porous fibrous media

This paper developed a mathematical model for the coupled heat and mass transfer in porous media based on the fractal characters of the pore size distribution. According to Darcy’s law and Hagen–Poiseuille’s law for liquid flows, the diffusion coefficient of the liquid water, a function of fractal dimension, is obtained theoretically. The liquid flow affected by the surface tension and the gravity, the water vapor sorption/desorption by fibers, the diffusion of the water vapor and the phase changes are all taken into account in this model. With specification of initial and boundary conditions, distributions of water vapor concentration in void spaces, volume fraction of liquid water, distribution of water molecular content in fibers and temperature changes in porous fibrous media are obtained numerically. Effects of porosity of porous fibrous media on heat and mass transfer are analyzed. The theoretical predictions are compared with experimental data and good agreement is observed between the two, indicating that the fractal model is satisfactory.     

Review on effective thermal conductivity of bubble type porous media

多孔介质内部结构中发生的质量,动量,能量的传递是众多自然现象和生产,生活领域中发生的基本过程.有关多孔介质中的传热问题涉及许多科学领域,早就引起人们的广泛关注,研究人员也对其进行了长期的研究.等效导热系数方法即将多孔介质视为一种连续介质,将实际多孔介质中固体骨架与各种流体的传热模式(导热,对流,辐射)折合成一个综合的传热问题.此方法已成为研究多孔介质内部传热问题最常用的方法.最近一二十年,泡沫型多孔介质(如泡沫金属,泡沫碳等)的出现引起人们广泛的关注.本文针对此种新型多孔介质等效导热系数的研究做了综述,介绍了3种常用的研究方法,分别是实验测试法,理论推导法及数值模拟法,探讨了各种研究方法存在的问题.实验测试法准确性高,但每种传热模式对等效导热系数的影响很难确定,且成本高;理论推导法虽然物理意义明确,适用性广,但与实测结果有较大的偏差,且有些方法还需与实验相结合;数值模拟法建模较复杂,但模拟结果与实验数据较接近.