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

Abstract

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±0.04mm and their sphericity, ϕ, to be 0.77±0.09. Values of Umf were 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 Umf were 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 Umf were over-predicted by Wen and Yu’s equation, when using the optically measured mean size of the particles. The best predictions of Umf were made by using the optically measured mean values of both dp and ϕ, together with Ergun’s equation and the above equation coupling εmf and ϕ. Such a procedure is proposed for estimating Umf in general. No evidence was found for εmf varying with temperature.