DETERMINATION OF PARTICLE AND GAS CONVECTIVE HEAT-TRANSFER COMPONENTS IN A CIRCULATING FLUIDIZED-BED

In order to study the role of particles in augmenting heat transfer from the wall of a circulating fluidized bed (CFB), simultaneous heat and mass transfer experiments were carried out. For heat transfer, particles are important hydrodynamically, augmenting gas convection, and are a source of internal energy, i.e. particle convection. For mass transfer. only the former occurs. Simultaneous heat and mass transfer experiments using naphthalene as a sublimation material were performed in a 20 cm diameter circulating bed operating at atmospheric conditions. The presence of particles in the circulating bed causes an order of magnitude increase in the bed to wall heat transfer in comparison to single-phase turbulent gas flow. In contrast, the mass transfer is increased by 50% over single-phase gas flow. The gas convection component of the total heat transfer, found from the mass transfer experiments. varied from 10 to 20% of the total heat transfer. In this range of solids concentration between 12 and 80 kg/m3, particle convection dominates. The superficial gas velocity has little influence on the particle convection or on the gas convection component. The particle convection varies with the density of particles in the core, probably due to variations in the wall fraction covered by particle clusters. Gas convection is insensitive to the density of particles in the core.