孔隙率对Al_2O_3高孔隙率多孔介质EHC的影响

Effects of Porosity on Effective Heat Conductivity of Highly Porous Media of Al_2O_3

新型多孔介质燃烧技术主要采用泡沫陶瓷 ( Ceramic Foams)、波纹状陶瓷 ( Fabric LamellaStructure)等高孔隙率多孔介质。基于气、固相局部热平衡假设的有效导热系数 EHC( EffectiveHeat Conductivity)描述了气流与固体骨架中导热、弥散和热辐射多种传热方式的综合效果 ,是一个重要的传热特性 ,对其研究非常不足。通过实验与数值模拟确定不同孔隙率 Al2 O3 陶瓷的 EHC:基于测定的温度分布 ,给定 EHC的初值 ,采用有限体积法进行流场数值模拟 ;比较测量与计算的温度均方根差 ,通过对 EHC搜索寻优 ,间接确定泡沫陶瓷的有效导热系数。结果表明 Al2 O3 陶瓷的EHC随温度增加而增加 ,对速度不敏感 ;孔隙率高的陶瓷其 EHC较大。给出试样 EHC与温度和空管速度的拟合式 ;将颗粒床 EHC的 Z&B模型外推到 Al2 O3 高孔隙率陶瓷 ,并给出

Effective heat conductivity (EHC) is an important property in simplifying simulations of flow, heat transfer and combustion in the application of porous media in heat exchanger and newly developed porous media burner technology. It is representative of cumulative effects of conduction, radiation and dispersion in porous medium under the local thermal equilibrium assumption. However the knowledge of EHC in highly porous media (porosity>83%) is very limited compared to that of packed beds. In this paper the EHC of Al 2O 3 ceramics was determined by an inverse technique of the combination of experimental and numerical simulation. The temperature profiles in porous media were measured with K thermocouples under steady state conditions for inlet air having various temperature and mass flow rate. Based on the measured temperature, the two dimensionally simulated temperature profile was obtained using a finite volume code with a couple of guessed radial and axial effective heat conductivities. The best combination of radial and axial EHC was found when the minimal error based on the least squares method between the measured and simulated temperature fields was reached. The specimens tested were foams having different porosities of 4, 8 and 12 pore/cm, and fabric lamella structure of higher porosity. As the results show in Figs.2 through 7, the EHC increases with the increase of temperature. A slight velocity dependence of EHC was observed for foams as shown in Fig.3. Foams having higher porosity showed higher EHC. The EHC of Al 2O 3 ceramics was correlated to temperature and artificial velocity as listed in Table 2. Based on the EHC models of Z&B for packed beds, the reversed model for Al 2O 3 ceramics was developed and predictions were presented. The results are useful for the application of highly porous media in combustion and heat exchanger system.