Analysis of the dynamics of heat transfer between a hot wire probe and gasfluidized beds

Hot wire anemometry used inside air fluidized beds of glass (175 mu m), FCC (75 mu m) and silica (85 mu m) powders (Archimedes numbers of 510, 29 and 16, respectively) allowed the measurement of the time-resolved local heat t ransfer coefficient. Time averages of this coefficient reproduce the same b ehaviour found by other authors with different experimental techniques. A s tochastic model for the heat transfer rate has been developed on the basic hypothesis that heat transfer fluctuations are due to the continuous renewa l of packets of solid particles along the wire. The most relevant simplifyi ng hypothesis is that the contact time between the wire and the packet is m uch shorter than the characteristic heating time of the packets. With this model, probability density distributions of the heat transfer coefficient a re evaluated. Comparison between experimental and theoretical results is fa irly good in all experimental conditions relative to fully developed aggreg ative fluidization. The model is less reliable in conditions of incipient a nd homogeneous fluidization, where the simplifying hypotheses may not apply . Calculated values of packet to particle size ratios, lambda/d(p), are aro und 8 for glass, between 14 and 36 for FCC and between 17 and 32 for silica . The increasing number of particles inside a packet seems, therefore, to b e correlated, on one hand, to the decreasing Archimedes number, and on the other, to an apparently reduced particle mobility of powders belonging to t he Group A of the Geldart [D. Geldart, Types of gas fluidization, Powder Te chnol., 7 (1973) 285-292] classification. (C) 1999 Elsevier Science S.A. Al l rights reserved.