Influence Of Pore Distribution On The Effective Thermal Conductivity Of Porous Materials And Optimization

Porous heat—resistant material has become an advanced research hotspot due to the great potential for energy saving and consumption reduction with the global economy and industry rapidly development.The effective thermal conductivity is an important parameter of porous materials for heat insulation performance.In this paper,models of porous material were built,the2D geometric models were built based on the microstructure picture,heat transfer theories were used for calculating and analyzing the thermal conductivity by numerical simulation.The contribution of thermal radiation to the effective thermal conductivity was analyzed using the commercial software combined with user defined programs.The influence of porosity,pore space and size distribution and the pore morphology on heat-insulating property was investigated systematically.The distribution of porosity was optimized using MATLAB program based on the optimal control theory.The following conclusions could be drawn:?1?The effective thermal conductivities calculated by FEM considering both conduction and radiation heat transfer,was consist with the experiment results.Thermal radiation makes greater contribution to the effective thermal conductivity of porous material with higher porosity and large pore size under higher temperature.For mullite porous material with porosity of 67%,radiant heat flux accounts for more than 50%of total heat when the temperature is higher than1873K,and now,most the heat is trnsfered by radiation.?2?Departure factor of pore position c,acentric factor of pore diameter b and standard deviation of pore diameter?were introduced to describe the non-uniform distribution of pores for corundum porous material.For the material which porosity was less than 50%,non-uniform distribution of pores results in the decrease of effective thermal conductivity.While for the material which porosity was more than the 50%,the influence of pore distribution on thermal conductivity is complicated at high temperature.The non-uniform distribution of pore were good for decreasing effective thermal conductivity when the pore diameter is less than 0.1mm;the non-uniform distribution of pore will increase when the pore diameter is more than 0.1mm.?3?In order to research the influence of pore morphology on effective thermal conductivity,the porous material models with octahedral pore,icosahedral pore and spherical pore were built and heat transfer was modeled.It can be find that with the decrease of pore sphericity,the effective thermal conductivity decreased for the material with fixed porosity,compared to the model with spherical pore,the effective thermal conductivity of the model with octahedral pore and the model with icosahedral pore were reduced by 28.7%and 21.2%respectively.?4?The pore distribution in porous materials was optimized using the quasi Newton method.it can be concluded that the porous material when the thermal conductivity of solid phase decreases with increasing temperature,the high temperature side with high porosity,the low temperature side with low porosity after optimization calculation;the porous material that thermal conductivity of solid phase increased with increasing temperature represented an opposing trend;the larger the amplitude of the porosity of the adjacent discrete points,the better the insulation performance;while the average porosity is (P_min+P_max)/2,improvement rate of insulation property was for the best.