ln this study, the insulation performance of mixed powders was investigated by measuring the effective thermal conductivity. Silica (Si02) which was semi-transparent for thermal radiation was used as a main powder and opaque powders for the thermal radiation, namely, titanium oxide (Ti02), carbon black, fly ash, and iron oxide (Fe203) were mixed in the main powder. The effective thermal conductivity was measured by the dual cylindrical method with a steady heat flow as shown in Fig.1 in the range of room temperature to 600°C. Experimental results were analyzed by the theoretical equation of Wang and Tien(1) for heat transfer in a semi-transparent medium. The heat transfer in the mixed powders was divided by two contributions of pure conduction and thermal radiation and the ratios of those were calculated qualitatively for the mixed powders.
The effective thermal diffusivity of dispersed materials has been studied by numerical simulation on the random model that has macroscopic homogeneity and isotropy. The two kinds of thermal diffusivity are obtained by pulse heated method (direct value) and by eq. (1) used thermal conductivity calculated from steady state comparison method (indirect one). These values are compared each other and we estimate critical condition for macroscopic homogeneity. We propose also clear graphical expression that is shown by the nondimensional thermal diffusivity vs. the ratio of specific heat capacity of particle to matrix.