An examination of Wen and Yu's formula for predicting the onset of fluidisation

Abstract

© 2018 Classical methods (measuring the pressure drop across a bed for different flowrates of air through the bed) were used to determine the superficial velocity for minimum fluidisation, Umf, of sieved particles of alumina. The particles were characterised optically using an instrument (Morphologi G3, Malvern Instruments), which, from pixelated, enlarged photographs, measured the particles’ mean diameter, dp, to be 0.48 ± 0.04 mm and their sphericity, ϕ, to be 0.77 ± 0.09. Values of Umfwere measured for the temperature in the electrically heated bed varying from 14 to 920 °C. The results were analysed using Ergun's equation; one outcome was that the voidage, εmf, in an incipiently fluidised bed was found to be related to the particles’ sphericity by: (1−εmf)/ϕ2εmf3=12.2±0.4. The measured Umfwere significantly larger than the values predicted by Wen and Yu's equation, if the mean diameter of the particles was taken, as recommended, to be the geometric mean of the upper and lower sieve sizes, used when preparing the particles. Alternatively, the measured values of Umfwere over-predicted by Wen and Yu's equation, when using the optically measured mean size of the particles. The best predictions of Umfwere made by using the optically measured mean values of both dpand ϕ, together with Ergun's equation and the above equation coupling εmfand ϕ. Such a procedure is proposed for estimating Umfin general. No evidence was found for εmfvarying with temperature.