Effects of particle size and bubbling behavior on heat transfer in gas fluidized beds

The fluidization and heat transfer behaviors of a bubbling fluidized bed were studied using Computational Fluid Dynamics (CFD). The simulations were conducted with varying particle sizes and inlet gas superficial velocities. Solid volume fraction, solid temperature, air temperature, solid velocity vectors, and air velocity vectors distributions were analyzed and compared for the various operating conditions. Solid volume fraction profiles showed a symmetrical and non-uniform distribution during the initial phase of fluidization. The size of bubbles generated during the initial phase of fluidization increased with increasing inlet air superficial velocity and decreasing particle size. The coupled analysis of solid volume fraction and temperature profiles indicated that both conductive and convective heat transfer between the two phases were significant. The simulation results showed that rate of heat transfer from air to particles was dependent on the amount of interfacial surface area, which increased with increasing voidage within the bed. Voidage increased with decreasing particle size or increasing inlet air superficial velocity. However, based on the range of operating conditions studied, it could be deduced that there seemed to be an optimal set of operating conditions where heat transfer was most efficient. Over-fluidization led to poor heat transfer due to channeling which resulted in heat being unevenly distributed. Under-fluidization resulted in poorer convective heat transfer between the two phases but this was compensated partially by an increase in conductive particle-to-particle heat transfer due to high solid concentrations. (C) 2016 Elsevier Ltd. All rights reserved.