HEAT-TRANSFER COEFFICIENTS BETWEEN GAS-FLUIDIZED BEDS AND IMMERSED SPHERES - DEPENDENCE ON THE SPHERE SIZE

Heat transfer coefficients between gas-fluidized beds and immersed spheres of different sizes have been experimentally determined for different fluidization velocities, bed particle properties and temperatures, in the lower range of the bubbling bed regime. Results elucidate the pattern of the dependence of heat transfer coefficients on the different variables and demonstrate that the size of the immersed sphere and the fluidization excess velocity are the main factors influencing the heat transport process, together with the bed particle size. Experimental results cannot be fully interpreted on the basis of the available models and correlations developed for heat transfer surfaces of larger size and different shapes. Discussion of the results suggests a possible physical interpretation, consistent with the regular flow of bed solids past the immersed object so that the particle residence time can be correlated in terms of the excess gas velocity.