Heat transfer from immersed horizontal tubes of different diameter in a gas-fluidized bed

Following our earlier finding that the heat-transfer coefficient, hw, for a horizontal smooth tube immersed in a gas-fluidized bed depends upon its diameter, DT, a systematic investigation was undertaken and the results are reported in this paper. In particular, hw was measured for tubes with diameters of 28.6, 50.8 and 76.2 mm immersed in fluidized beds of silica sands of average particle diameters 497 and 773 μm, and glass beads of average particle diameter 270 μm, as a function of fluidizing air velocity at ambient temperature and pressure. It was found that hw follows the normal and previously observed dependence on G and d̄p, and also shows an interesting variation with DT; hw first decreases as DT is increased but increases as DT is further increased. The observation that hw has a minimum with respect to DT is suggested by some earlier studies. The understanding of this dependence of hw on DT involves a detailed interpretation of the changing hydrodynamic condition of the bed, particularly in relation to solids motion and related bubble dynamics. This process is discussed at length in this work as the fluidizing velocity is increased while keeping the other parameters unchanged.