SURFACE-TO-SUSPENSION HEAT-TRANSFER MODEL IN LEAN GAS-SOLID FREEBOARDFLOW

Heat transfer in dense fluidized beds have been extensively studied. H owever, there is not much detailed information about the mechanism of surface-to-suspension heat transfer in the freeboard region. In the pr esent work, a newly designed heating plate was used to measure the pla te-surface-to-particle-suspension heat transfer coefficients in the fr eeboard. The experimental unit consisted of a 30 cm i.d. fluidized bed reactor packed with fluidized catalytic particles of mean particle si ze 90 mum. Three types of plate orientations were used to test directi onal effects of surface on heat transfer rate. Height of the freeboard was 171 cm, and the superficial gas velocity was varied from 0.28 to 0.64 m/s. Local solids concentrations in the freeboard were also obtai ned by a nozzle-type sampling probe. Data on axial distribution of sol ids concentration were used to find out the solids kinematics in the f reeboard region. Finally, a surface-to-suspension heat transfer model was developed to elucidate the surface to particle heat transfer mecha nism in this lean phase system. The model is based on the transient ga s-convective heating of single particles when sliding over the heating plate and the assumption of instantaneous attachment-detachment equil ibrium between particles and the plate surface.