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.