Abstract: Based on the fractal theory,the micro-scale geometric structure of nano-granule porous medium was illustrated and the equivalent fractal unit model was proposed.The effective thermal conductivity of the nano-granule porous medium under the vacuum condition has some relation with the thermal conductivity of solid substrate,vacancy porosity,fractal dimension,fractal diameter,pressure and thermal conductivity of rarefied air,and environmental condition.The calculation models for thermal conductivities of air phase and solid phase,the equivalent thermal radiation and the total effective thermal conduction were constructed.The results show that the effective thermal conductivity of nano-granule porous medium increases with the increase of fractal diameter and rarefied air pressure within the material,but decreases with the increase of vacancy porosity.The optional diameter of the nano-granule is recommended.The calculated results are in good agreement with the experimental data.The study is useful for enhancing the adiabatic performance of nano-granule porous medium and developing new vacuum adiabatic materials.

Key words: fractal theory, nano-granule porous medium, vacuum insulation, thermal conductivity

摘要： 采用分形理论,描述了纳米颗粒多孔介质材料的微尺度空间结构,建立了分形等效单元体模型,分析了影响其真空下有效热导率关键因素为固体基质热导率、填充率、分形维数、分形直径、残余气体压力及热导率等,并导出了气相、固相热传导计算公式和热辐射等效热导率计算公式及多孔介质材料总有效热导率计算公式。研究表明,纳米颗粒有效热导率随着分形直径、残余气体压力的增大而增大,并给出了纳米颗粒多孔介质材料作为真空材料的最佳直径。同时,模型计算值与实验测量值比较,具有较好的一致性。该理论分析方法对新型真空绝热材料的研制和绝热性能的提高具有实用价值。

关键词: 分形理论, 纳米颗粒多孔介质, 真空绝热, 热导率