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 transfer coefficient. Time averages of this coefficient reproduce the same behaviour found by other authors with different experimental techniques. A stochastic model for the heat transfer rate has been developed on the basic hypothesis that heat transfer fluctuations are due to the continuous renewal of packets of solid particles along the wire. The most relevant simplifying hypothesis is that the contact time between the wire and the packet is much shorter than the characteristic heating time of the packets. With this model, probability density distributions of the heat transfer coefficient are evaluated. Comparison between experimental and theoretical results is fairly good in all experimental conditions relative to fully developed aggregative fluidization. The model is less reliable in conditions of incipient and homogeneous fluidization, where the simplifying hypotheses may not apply. Calculated values of packet to particle size ratios, lambda/d(p), are around 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 be correlated, on one hand, to the decreasing Archimedes number, and on the other, to an apparently reduced particle mobility of powders belonging to the Group A of the Geldart [D. Geldart, Types of gas fluidization, Powder Technol., 7 (1973) 285-292] classification. (C) 1999 Elsevier Science S.A. All rights reserved.
Analysis of the dynamics of heat transfer between a hot wire probe and gas fluidized beds
POLETTO, Massimo;
1999-01-01
Abstract
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 transfer coefficient. Time averages of this coefficient reproduce the same behaviour found by other authors with different experimental techniques. A stochastic model for the heat transfer rate has been developed on the basic hypothesis that heat transfer fluctuations are due to the continuous renewal of packets of solid particles along the wire. The most relevant simplifying hypothesis is that the contact time between the wire and the packet is much shorter than the characteristic heating time of the packets. With this model, probability density distributions of the heat transfer coefficient are evaluated. Comparison between experimental and theoretical results is fairly good in all experimental conditions relative to fully developed aggregative fluidization. The model is less reliable in conditions of incipient and homogeneous fluidization, where the simplifying hypotheses may not apply. Calculated values of packet to particle size ratios, lambda/d(p), are around 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 be correlated, on one hand, to the decreasing Archimedes number, and on the other, to an apparently reduced particle mobility of powders belonging to the Group A of the Geldart [D. Geldart, Types of gas fluidization, Powder Technol., 7 (1973) 285-292] classification. (C) 1999 Elsevier Science S.A. All rights reserved.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
