IN THEIR paper "Consecutive
film and surface renewal mechanisms for heat and mass transfer from a wall", Wasan and Ahluwalia [l] formulate a model to describe the consequences of fluid replacement taking place to within some limiting distance of the transfer surface. Thus, they combine the effects of a consecutive "surface film" which presents high resistance to transfer and a zone where there is constant renewal of material. They then test the implications of this model against published fluidized bed, bed/wall heat transfer results using assumptions which are quite unrealistic for a gas fluidized system.
Their third assumption, namely that the solid particles are considered as stationary particles arranged in equally spaced horizontal layers, allows them to make some estimate of the postulated mean film thickness following the earlier ideas of Levenspiel and Walton [2] which are themselves of questionable physical reality. However, much more important is the fact that this assumption eliminates any opportunity for the model to include the dominating contribution to heat transport by particle convection in gas fluidized systems. In effect, the model which they are testing against published fluidized bed data is that of wall to fluid heat transfer in an "expanded" packed bed system where the particles are assumed to have negligible heat capacity (for we take it that the Prandtl number which they use is that for the fluid and that C. refers to the "gas" heat capacity at constant pressure although results for liquid fluidized systems are also considered).
In gas fluidized systems, the solids act as a local source or sink of heat and it is their ability to transfer heat which dominates the transfer process [3]. Particle volumetric heat capacities are of the order of one thousandfold higher than the gas heat capacity. Mickley and Fairbanks[4] developed the consequences of this further in their renewal model. From our own work [5], it seems that the heat transfer to the particles is by conduction through the shortest gas transfer paths between the surface and particle if radiant heat transfer may be neglected. Convective transfer through the gas makes negligible contribution.
Because of the relatively high diffusivity and low heat capacity of the gas, the gas temperature is controlled by the temperatures of the transfer surface