CONVECTIVE WALL-TO-SUSPENSION HEAT-TRANSFER IN CIRCULATING FLUIDIZED-BED RISERS

A mechanistic model, which incorporates recent findings on the fluid d ynamics in the riser of the circulating fluidized bed (CFB), is develo ped for predicting the suspension-to-wall heat-transfer coefficient in the riser. It is assumed that heat transfer between the gas-particle suspension and the riser wall takes place by the contact of both parti cle packets and an emulsion phase on the wall. A characteristic length (L), that is, a sliding distance of the emulsion phase along the heat -transfer surface, is introduced in the model, enabling the effect of the length of heat-transfer surface to be evaluated. It is found that the heat-transfer coefficient decreases with increasing L, but becomes increasingly insensitive to L when L is larger than 1 m. Agreement be tween model prediction and measurement is encouraging over a range of operating conditions, heat-transfer surface length, and riser diameter s.