HEAT AND MASS TRANSFER CORRELATIONS FOR SURFACES IN FLUIDISED BEDS

This paper presents a comparison of heat and mass transfer coefficient around a horizontal cylinder immersed in a 500 μm glass beads bubbling fluidised bed. In particular, experimental results on the local heat transfer coefficient, h, derived from Di Natale et al., (2008) and local mass transfer coefficient, k, given by Garim et. al., (1999) are used as the starting point for this analysis. The heat transfer coefficient measures pointed out the occurrence of different heat transfer zones which directly reflect the existence of different hydrodynamic conditions around the cylindrical surface (e.g. Buyevich et al. 1986; Rong et al., 1999) studies. The highest value of the heat transfer coefficient is observed at the boundary of the lateral mixing and the upper sliding zones, as the result of a counterbalance between particle surface renewal frequency and particle concentration. On the contrary, the mass transfer coefficient is the highest in the lower and lateral zones of the cylinder, where high mixing occurs. Indeed, mass and heat transfer phenomena in fluidised beds are substantially different but they are both linked to the void fraction profile near the surface, εw: mass transfer increases with εw while the heat transfer coefficient show the opposite behaviour. Surface void fraction profile can be obtained from the heat transfer coefficient measures by applying the Di Natale et al. (2008). The model results consistent with former investigations and the void fraction profile and provide for a good description of mass transfer coefficient profile near the surface