An alternate yet general form of the classical effective thermal conductivity model （Maxwell model） for two-phase porous materials is presented, serving an explicit thermo-physicM basis. It is demonstrated that the reduced effective thermal conductivity of the porous media due to non-conducting pore inclusions is caused by the mechanism of thermal stretching, which is a combi- nation of reduced effective heat flow area and elongated heat transfer distance （thermal tortuosity）.
The natural convective heat transfer performance and thermo-fluidic characteristics of honeycombs with/without chimney extensions are numerically investigated.The present numerical simulations are validated by the purposely-designed experimental measurements on honeycombs with/without chimney.Good agreement between numerical simulation and experimental measurement is obtained.The influences of inclination angle and geometric parameters such as cell shape,streamwise and spanwise length are also numerically quantified.With the increment in inclination angle,the overall heat transfer rate decreases for the honeycombs with/without chimney.For honeycombs with the same void volume fraction but different cell shapes,there is little difference on the overall heat transfer rate.To enhance the natural convective heat transfer of honeycombs,these techniques including increasing the length of honeycomb in the streamwise/spanwise direction,increasing the thermal conductivity of hon-eycomb structure or adding a chimney extension may be helpful.
YANG XiaoHuYAN HongBinWANG WenBinJIN LiWenLU TianJianICHIMIYA Koichi
The natural convective heat transfer performance of an aluminum hexagonal honeycomb acting as a novel heat sink for LED cooling is experi- mentally investigated. The concept of adding an adiabatic square chimney ex- tension for heat transfer enhancement is proposed, and the effects of chimney shape, height, and diameter are quantified. The average Nuav of a heated hon- eycomb with straight chimney is significantly higher than that without chimney, and the enhancement increases with increasing chimney height. At a given chim- ney height, honeycombs with divergent chimneys perform better than those with convergent ones. For a fixed divergent angle, the Nuav number increases mono- tonically with increasing chimney height. In contrast, with the convergent angle fixed, there exists an optimal chimney height to achieve maximum heat transfer.
