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1.
In this paper, the geometric structure of porous media is described using fractal techniques, and a section particle area fractal dimension d of a porous medium with various porosities is considered with a simplified model. Also an expression of the effective thermal conductivity for soil is presented via a fractal dimension and a model of heat transfer in soil. The results obtained in this paper indicate the effectiveness of the method for determining the effective thermal conductivity by using the section area dimension. © 2000 Scripta Technica, Heat Trans Asian Res, 29(6): 491–497, 2000 相似文献
2.
Determination of effective thermal conductivity for real porous media using fractal theory 总被引：1，自引：0，他引：1
INTRODUCTIONPorousmediaisacompositemediathatincludessolidframeandfluidandexistedwidelyintheeajrthbiosphere.Heatandmasstransferinporousmediaisbothanaturalphenomenoninearthbiosphereandaphysicalchemistryprocessinindustries,agricultureandhumanlife.Thusthestudyonheatandmasstransferinporousmediahasbecomeanimportanttasktoscielltistsandengineers.Heatandmasstransferinporousmediaisaverycomplexobject.Therearestillmanydifficultiestodescribethecoupledheatandmasstransferphenomena.Amongthesedifficultie… 相似文献
3.
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. 相似文献
4.
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。 相似文献
5.
Wuriti Sridhar Garishe Vijaya Lakshmi Khaled Al-Farhany Ganugapati Raghavendra Ganesh 《亚洲传热研究》2021,50(8):8134-8154
The main resource of this paper is to establish over fluid flows sheet using mathematical modeling for constant and variable thickness by including magnetic fields, electric fields, porous medium, heat propagation/immersion, and radiative heat relocation. The Implicit Finite Difference Method (IFDM) is applied to simplify using similarity conversions to implicate partial differential equations to convert into ordinary differential equations. IFDM has been implemented in MATLAB to tabulate numerical observations of the local parameters. Nusselt and Sherwood numbers are analyzed and measured for different parameters in different constant and variable thickness conditions of fluid properties. The influence of various parameters is explained through temperature, velocity, concentration, and nanoparticle volume fraction graphical representations. The coefficient of the skin friction for irregular fluid properties is shown to have a greater influence than that compared for constant fluid properties. Nevertheless, there is a reverse case in the local Nusselt number that is lower for the fluctuating fluid properties than with constant fluid properties. The results showed high-exactness computational outcomes are attained from the IFDM. 相似文献
6.
为了探究两种不同二维纳米填料对复合相变材料导热系数的影响,分别制备了以石墨烯纳米片和六方氮化硼纳米片为填料的石蜡基复合相变材料.采用瞬态平面热源法在20 ℃时测量了不同添加量下复合相变材料的导热系数.结果显示,石蜡基复合相变材料的导热系数随纳米填料添加量近似线性增长;六方氮化硼纳米片对复合相变材料导热系数的提升远低于石墨烯纳米片.此外,利用基于有效介质模型的预测公式与试验值进行了比较,计算发现形状,大小和导热系数相近的两种纳米材料,六方氮化硼纳米片的界面热阻却高出石墨烯纳米片两个数量级,是后者具有更显著强化效果的原因之一. 相似文献
7.
Composite materials, which consist of organic and inorganic components, are widely used in various fields because of their excellent mechanical properties, resistance to corrosion, low-cost fabrication, etc. Thermal properties of organic/inorganic composites play a crucial role in some applications such as thermal interface materials for micro-electronic packaging, nano-porous materials for sensor development, thermal insulators for aerospace, and high-performance thermoelectric materials for power generation and refrigeration. In the past few years, many studies have been conducted to reveal the physical mechanism of thermal transport in organic/inorganic composite materials in order to stimulate their practical applications. In this paper, the theoretical and experimental progresses in this field are reviewed. Besides, main factors affecting the thermal conductivity of organic/inorganic composites are discussed, including the intrinsic properties of organic matrix and inorganic fillers, topological structure of composites, loading volume fraction, and the interfacial thermal resistance between fillers and organic matrix. 相似文献
8.
In the present study, a magnetized micropolar nanofluid and motile micro‐organism with variable thermal conductivity over a moving surface have been discussed. The mathematical modeling has been formulated using a second‐grade fluid model and a revised form of the micropolar fluid model. The governing fluid contains micro‐organisms and nanoparticles. The resulting nonlinear mathematical differential equations have been solved with the help of the homotopy analysis method. The graphical and physical features of buoyancy force, micro‐organisms, magnetic field, microrotation, and variable thermal conductivity have been discussed in detail. The numerical results for Nusselt number, motile density number, and Sherwood number are presented with the help of tables. According to the graphical effects, it is noted that the buoyancy ratio and the bioconvection parameter resist the fluid motion. An enhancement in the temperature profile is observed due to the increment in thermal conductivity. Peclet number tends to diminish the motile density profile; however, the viscoelastic parameter magnifies the motile density profile. 相似文献
9.
10.
Lichuan Wei Zhao Lu Feng Cao Liyu Zhang Xi Yang Xiaoling Yu Liwen Jin 《国际能源研究杂志》2020,44(12):9466-9478
The reliable thermal conductivity of lithium-ion battery is significant for the accurate prediction of battery thermal characteristics during the charging/discharging process. Both isotropic and anisotropic thermal conductivities are commonly employed while exploring battery thermal characteristics. However, the study on the difference between the use of two thermal conductivities is relatively scarce. In this study, the isotropic and anisotropic thermal conductivities of the four commercially available lithium-ion batteries, ie, LiCoO2, LiMn2O4, LiFePO4, and Li (NiCoMn)O2, were reviewed and evaluated numerically through the heat conduction characteristics inside the battery. The results showed that there are significant differences in the temperature distribution in the battery caused by the isotropic and anisotropic thermal conductivities, which could affect the layout and cooling effectiveness of battery thermal management system. Furthermore, the effective thermal conductivities of porous electrodes and separator were determined to establish thermal conductivity bounds of lithium-ion batteries combined with the thicknesses of battery components. The thermal conductivity bounds could be applied to evaluate the rationality of the thermal conductivity data used in battery thermal models. 相似文献
11.
With the latent heat, the phase change material (PCM) is widely used in battery thermal management (BTM) to control the temperature. In this paper, the porous medium is employed to enhance the heat transfer of PCM. The lattice Boltzmann model for PCM/porous medium in pore scale is considered, where the mesh system with porous medium (fixed point) is generated by quartet structure generation set (QSGS) method. The effects of the Rayleigh number and porosity on the heat transfer process in BTM are investigated. The results show that decreasing the porosity will accelerate the melting rate. When the porosities are 0.9, 0.8, 0.7, and 0.6, the total melting times are decreased by 23.7%, 43.3%, 58.0%, and 75.4%, compared with pure PCM. The heat is transferred through the high‐conductivity framework. The natural convection in the porous medium is weak, and the conduction is the dominated heat transfer. As a result, the area of solid–liquid interface will be increased, and the heat‐transferred rate is accelerated. However, when the Rayleigh number is raised to 105, applying the porous medium with porosity of 0.9 will increase the total melting time, resulted from the stronger natural convection of PCM. The present study is helpful for design of PCM/porous medium‐based BTM. 相似文献
12.
介绍了将传统晶体硅商用太阳电池加装水冷器,改造为光伏-光热集成组件后的结构;通过热阻分析的方法,分析了该光伏-光热组件的传热特性;并通过实验,实际测量了该光伏-光热组件的传热性能,得到了导热热阻及其有效热导率,可为组件的进一步开发提供参考。 相似文献
13.
Embedding nanoparticles (NPs) in a matrix can effectively enhance the phonon scattering by the interface, reduce the lattice thermal conductivity, and improve the thermoelectric properties of the material. However, the understanding of how the distribution of embedded NPs affects the thermal conductivity is still not clear. To explore the underlying mechanism, frequency‐dependent Monte Carlo simulation and the effective medium method are applied to study the lattice thermal conductivity of Si/Ge composite (Si NPs embedded in Ge matrix). The effect of the free path distribution (FPD) of Ge phonon induced by the heterogeneous distribution of Si NPs is introduced into the effective medium method, and then, this method is used to calculate the lattice thermal conductivity of Si/Ge composite when Si NPs are unevenly distributed. Results show that decreasing the separation distance of adjacent NPs can slightly decrease the lattice thermal conductivity. Assuming that the FPD of Ge phonon induced by Si‐Ge interface scattering obeying lognormal distribution and that the deviation σ indicates the degree of inhomogeneity of Si NPs distribution, lattice thermal conductivity of composites with different σ is obtained. It is found that lattice thermal conductivity significantly decrease by more than 40%, with the increase of σ, especially for high‐Si concentrations. The present study indicates that the particle distribution in a composite can markedly affect the lattice thermal conductivity. 相似文献
14.
Convection heat transfer and pressure drop measurements were performed with a rectangular duct, having a cooled upper and a heated lower surface, which was packed with spherical particles. Air was used as the test fluid and four kinds of spherical particles having different diameters and thermal conductivities were used as the packing materials. The ratio of the diameter of the spherical particle to the distance between the cooled and heated surfaces, d/H, was varied from 0.173 to 1. The thermal conductivity of the particle layer was also measured under the still air condition. The thermal conductivity of the particle layer was not affected by the value of d/H. In the case of the one-stage arrangement of spherical particles (d/H = 1), the flow resistance took on a remarkably small value compared with the flow resistance of a homogeneous spherical particle layer. Moreover, the flow resistance of the particle layer formed with some layers of particles could be predicted by combining the flow characteristics of the one-stage particle layer and that of the homogeneous spherical particle layer. The heat transfer coefficient of the particle layer was larger than that of turbulent air flow on a flat plate. At a constant superficial air velocity, there existed a value of d/H which gave a maximum value of the average heat transfer coefficient. Nondimensional heat transfer correlation equations were derived in terms of parameters expressing the average characteristics of the spherical particle layers. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(3): 176–192, 1997 相似文献
15.
多孔介质导热的分形模型 总被引：12，自引：0，他引：12
多孔介质中热量传递与多孔介质内部的几何结构有密切的关系,讨论了多孔介质的分形结构和相关的分形维数,利用能量方程,导出了分形维数为D的有限尺度多孔介质中的广义热传导方程,在此基础上,假定热量在多孔介质中的传导路线也是一种分形结构,提出了一个筒化的多孔介质并联通道分形导热模型,求出了基于分形理论的多孔介质有效导热系数表达式。 相似文献
16.
The thermal conductivity measurement accuracy of sand was experimentally studied with a hot disk thermal constant analyzer and water morphologies, distribution, and evolution at the pore scale were observed with a charge coupled device (CCD) combined with a microscope. It was found that thermal conductivities of samples with low moisture content (<25%) could not be accurately measured. For samples with low moisture content, the analysis showed that the water in the region adjacent to the analyzer sensor mainly existed as isolated liquid bridges between/among sand particles and would evaporate and diffuse to relatively far regions because of being heated by the sensor during measurement. Water evaporation and diffusion caused the sample constitution in the region adjacent to the sensor to vary throughout the whole measurement process, and accordingly induced low accuracy of the obtained thermal conductivities. Due to high water connectivity in pores, the rate of water evaporation and diffusion in porous media of high moisture content was relatively slow when compared with that of low moisture content. Meanwhile, water in the relatively far regions flowed back to the region adjacent to the sensor by capillary force. Therefore, samples consisting of the region adjacent to the sensor maintained the constant and thermal conductivities of porous media with relatively high moisture content and could be measured with high accuracy. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20272 相似文献
17.
辐射屏法用于高温输热管网隔热保温的实验与工程应用 总被引：1，自引：1，他引：0
利用一定的条件,矩形密闭空间内的空气不产生自然对流产原理,经过理论计算,模拟实验,中间实验,最后将辐射屏法隔热保温进行了工程应用,并取得了很好的节能效果和经济效益。 相似文献
18.
In this paper, a novel thermal energy storage (TES) system based on a thermo‐sensitive magnetic fluid (MF) in a porous medium is proposed to store low‐temperature thermal energy. In order to have a better understanding about the fluid flow and heat‐transfer mechanism in the TES system, four different configurations, using ferrofluid as the basic fluid and either copper foam or porous carbon with different porosity (90 and 100 PPI, respectively) as the packed bed, are investigated experimentally. Furthermore, two thermal performance parameters are evaluated during the heat charging cycle, which are thermal storage velocity and thermal storage capacity of the materials under a range of magnetic field strength. It is shown that heat conduction is the primary heat‐transfer mechanism in copper foam TES system, while magnetic thermal convection of the magnetic fluid is the dominating heat‐transfer mechanism in the porous carbon TES. In practical applications in small‐scale systems, the 90‐PPI copper foam should be selected among the four porous materials because of its cost efficiency, while porous carbon should be used in industrial scale systems because of its sensitivity to magnetic field and cost efficiency. 相似文献
19.
Comprehensive understanding of evaporation in porous channels is important for the design of modern heat exchangers and thermal systems such as heat pipes. This paper presents a numerical study of evaporation in a vertical porous channel. The volume of the fluid method was applied to capture the interface between phases. The multi‐dimensional limiter for explicit solution method, in which there are additional constraints based on the flux‐corrected transfer method, was used to solve the interface transfer equation. Pressure drop, Nusselt number, and the drying rate were considered as performance parameters. Comparing simulation results with experimental data showed that this new solver can precisely solve the thermal phase change in a porous medium. The influence of parameters effective on the drying process, such as porosity, permeability, the dynamic contact angle was investigated. Based on the results of the pressure profile in the final state, four different regions are visible. Because of the close fit of the wall temperature to the average cross‐sectional temperature, the Nusselt number increases sharply at transition points from the liquid to the vapor phase. By increasing the dynamic contact angle, the average drying rate in the channel is reduced. 相似文献
20.
In this study, a numerical study is performed to determine the significance of local thermal nonequilibrium on mixed convection heat transfer of a copper water-based nanofluid in an inclined porous cavity. By employing the nonequilibrium hypothesis, the governing equations for nanofluid flow in a porous medium are solved by the Semi-Implicit Method for Pressure Linked Equation (SIMPLE) algorithm. From the obtained results, the nanofluid flow and thermal characteristics are analyzed through streamlines and isothermal plots whereas the heat-transfer rate of the system is scrutinized via the average Nusselt number. 相似文献