From the experiments, it is observed that variation of void fraction with volume fraction of smaller particles is ‘V-shaped’ as shown in the Figure. That is, there exists a minimum void fraction and two different volume fractions of smaller pebbles can give same void fraction. Effective thermal conductivity of binary particulate bed is the maximum when its void fraction is the minimum. The binary bed with less volume fraction of small particles has higher conductivity than the binary bed of higher volume fraction of small particles and having same void fraction. This is due to the fact that with increase in volume fraction of small component, the number of small-to-small and large-to-small contact point increases resulting in higher resistance to heat transfers.