A DEM investigation of transitional behaviour in gas-fluidised beds

Abstract

Using DEM simulations, the paper examines the different types of behaviour as the gas velocity is increased to cover the complete range from fixed bed to homogeneous expansion, bubbling, turbulent and fast fluidisation. The paper highlights the transitions between the various regimes. At minimum fluidisation velocity, Umf, the structure of the bed is isostatic. When the gas velocity U is increased the system immediately breaks up into large clusters of contacting particles which gradually disintegrate with further increases in gas velocity until, at minimum bubbling velocity, Umb, the first bubbles start to appear. Conventionally, the regime Umf<U<Umb is referred to as homogeneous expansion. However, it is shown that the expansion is not homogeneous. Above Umb, the amplitude of the pressure drop fluctuations increases to a maximum when U=Uc, which marks the transition from bubbling to turbulent behaviour. The simulations also show that in the turbulent regime the average pressure drop increases with increasing gas velocity. This aspect appears not to have been reported previously in the literature. Finally, when U>Uk, corresponding to “fast fluidisation”, the particle system behaves as a granular gas. A new criterion is suggested to define the transition from turbulent fluidisation to fast fluidisation, defined by Uk.