The gas-perturbed liquid model of Zhang et al. (1995) is modified in an eff
ort to improve its prediction of the minimum liquid velocity of fluidizatio
n, U-lmf, of a bed of solid particles in the presence of a low or moderate
cocurrent flow of gas. Variants of the model are considered where the buoya
ncy term is based on the gas-liquid mixture, instead of the liquid alone, a
nd with the frictional pressure gradient given by several alternative equat
ions to the Ergun equation employed in the original gas-perturbed liquid mo
del. All versions of the model provide similar dependence on such factors a
s gas velocity, particle diameter, particle density and liquid viscosity as
those seen experimentally. The mixture buoyed equation with the drag based
on an equation suggested by Foscolo et al. (1983) gives improved predictio
ns over the original Zhang et al. (1995) model, but the best overall agreem
ent is with buoyancy based on the liquid alone and the first term in the dr
ag equation with the Carman (1937) constant of 180 instead of Ergun's 150.
The predictions are sensitive to the minimum fluidization voidage, which is
measured, assumed, or estimated. Further work is required to investigate m
inimum liquid fluidization velocities experimentally for particles of densi
ty closer to that of the liquid, and for high-viscosity liquids.