HEAT TRANSFER TO AN AIR-SOLID SUSPENSION AT HIGH CONCENTRATION AND LOW VELOCITY
This study is concerned with heat transfer between the wall of a tube 38 rom diameter and a concentrated gas-solid suspension flowing upwards. The volumetric fraction of the solid has been varied from 0.5 to 25% and the Reynolds number Re from 3000 to 30000. The particles studied were glass beads (180 μm) and commercial cracking catalyst (20 μm). The heat transfer coefficient obtained is high, -up to 326 W/m2 deg. Cand the relative increase in heat transfer coefficient (Δh/h0) could reach a value of 15. This relative increase was found to be proportional to R0.8 × Re−1.1, where R is the ratio of the solid to gas flow rate. The downward flow of particles along the wall of the tube, at low gas velocities, seems to be responsible for the large values of Δh/h0 and this downward motion was in fact observed in a glass tube by means of a high speed camera.