Heat transfer coefficient between a submerged surface and a fluidised of fine powders

This work deals with the experimental analysis of heat transfer coefficient between a fluidised bed of fine particles and submerged surfaces of different shapes. Experiments have been carried out using different bed materials (polymers, ballotini, corundum, carborundum and quartz sand) characterized by a mean particle diameters ranging between 30 and 90 µm and an Archimedes number between 2 and 50. Dry air at 25°C and 100 kPa has been used as fluidising gas. Three different objects are used as heat transfer probes: a sphere and two cylinders with different base diameter and same height. Experimental results show that the heat transfer coefficient increases with particle Archimedes number and is almost independent from particle thermal conductivity for a Kp between 0.9 and 16 W/(m•K). Finally, the comparison of heat transfer coefficient for the different probes shows that the effect of the surface geometry may account for a 30% variation in the heat transfer coefficient, with higher differences occurring for coarser particles.