平板热管多孔毛细芯等效导热系数预测

Prediction of effective thermal conductivity of a porous capillary wick in a vapor chamber

本文以平板热管多孔结构毛细芯为研究对象,深入分析了毛细芯等效导热系数的影响因素及变化规律.针对多孔毛细芯微观结构随机分布且固相骨架相连的特点,采用扩散受限聚集模型对多孔毛细芯进行三维重构,对受限空间气体热导率进行计算,并采用有限容积法对模型在稳态导热条件下的传热性能进行了数值计算,研究了在孔隙率和颗粒直径对多孔毛细芯等效导热系数的影响规律.研究表明,受限空间内气体导热系数远低于自由空间内的导热系数,标准大气压下仅为0.00986 W/m K;由于尺寸效应的影响,固体导热系数约为块材导热系数的0.9倍;经计算,毛细芯等效导热系数随孔隙率的增大呈减小趋势,且在颗粒直径越小的情况下变化越显著,如颗粒直径为40μm时,孔隙率0.4情况下的气固耦合等效导热系数为12.14 W/m K,约为孔隙率0.7时对应的导热系数(3.89 W/mK)的三倍;颗粒直径对多孔毛细芯导热系数影响显著,小孔隙率下等效导热系数随颗粒直径的增大而减小,大孔隙率情况下趋势相反.孔隙率为0.5的毛细芯等效导热系数与实验测试值偏差为12.1%,文章提出的模型适应性较好.

In this paper, a porous capillary wick in a vapor chamber is used as a research specimen and the influencing factors of effective thermal conductivity of the wick are theoretically analyzed. Based on the microstructure of the wick, which is randomly distributed with solid phase connection, three-dimensional diffusion limited aggregation models are reconstructed in this paper. The thermal conductivity of the gas in the confined space is calculated and the heat transfer characteristics under steady-state heat conduction are studied using numerical simulation. The results show that the thermal conductivity of the gas in the confined space is much lower than that in the free space, which is only 0.00986 W/(m K) under the atmospheric pressure. Besides, the thermal conductivity of the solid phase is 0.9 times that of the bulk material. The influence of different parameters on the effective thermal conductivity of the porous media is analyzed. The results show that the effective thermal conductivity of the capillary wick decreases with the increase in porosity, and the smaller the particle diameter is, the more significant is the trend. When the particle diameter is 40 μm and porosity is0.4, the effective thermal conductivity is 12.14 W/(m K), which is about three times the corresponding thermal conductivity(3.89 W/(m K)) when the porosity is 0.7. The particle diameter also has a significant effect on the thermal conductivity of the porous capillary wick. In the case of small porosity, the effective thermal conductivity decreases with the increase in particle diameter, while in the case of larger porosity, the trend is opposite. The effective thermal conductivity of the capillary wick with porosity of 0.5 has deviation of 12.1% from the experimental value, which shows good adaptability of the proposed model.