Heat transfer to a sphere immersed in a fluidized bed of coarse particles with transition from bubbling to turbulent flow regime

The present work concerns an experimental study on heat transfer in gas-solid fluidized bed of coarse (Geldart D) particles to a larger immersed sphere at high superficial velocities from 2 to 5.5 U-mf. The heat transfer coefficient was determined by measuring the temperature of the test sphere during its heating in a fluidized bed in the temperature range of 65-175 degrees C. The test spheres of different sizes and different materials were utilized. For the given gas-particles system the flow regime changes from rapidly growing bubbles to turbulent fluidization at superficial velocity U-c approximate to 3U(mf). It has been found that in rapidly growing bubbles regime, the heat transfer coefficient is higher for smaller test spheres while it is almost independent of the superficial gas velocity. In turbulent regime, the heat transfer coefficient increases with increase of gas velocity while the size of the test sphere exhibits less influence. In the rapidly growing bubbles regime, experimental data for heat transfer coefficient can be predicted adequately with correlation of Scott et al.. For the turbulent flow regime a new correlation for prediction of the heat transfer coefficient has been proposed.