(287m) Voidage Instabilities in Liquid Fluidized Beds

Liquid fluidized beds in narrow tubes are well known to be susceptible to instabilities under certain operating conditions. One such instability is the appearance of one-dimensional voidage waves consisting of alternating regions of high and low particle concentrations along the bed. Several experimental studies on such phenomenon have been reported in the literature. In the present study, the convective nature of these voidage instabilities was investigated computationally using the Discrete Element Method (DEM) coupled with Computational Fluid Dynamics (CFD). Under unstable operating conditions, voidage instabilities in the form of waves of high and low particle concentrations could be observed traveling up the expanded bed when the base of the bed was allowed to oscillate sinusoidally. These clearly showed the unstable nature of the system towards external perturbations and the convective characteristic of the resulting instability. The solid fraction profiles also showed quantitatively the presence of voidage instabilities. In a stable fluidized bed, solid fractions were uniform and largely constant over the height of the bed. On the other hand, those for the unstable bed showed sinusoidal variations corresponding to alternating regions of high and low solid concentrations. One other interesting feature of such instabilities was that particles in the high solid concentration regions were observed to be moving upwards in the direction of the wave while those in the low concentration regions were observed to be settling downwards. In other words, particles switched between upward and downward motions as different phases of the wave passed through them. A comparison of the granular temperature profiles of the liquid fluidized bed operating in both the stable and unstable modes provided useful insights to the mechanistic nature and origin of such voidage instabilities. The stable bed exhibited almost negligible granular temperatures at all positions along the height of the bed while its unstable counterpart exhibited noisy but generally sinusoidal granular temperature profiles. These corresponded to the presence of low and high Reynolds stresses in the system respectively and seemed to suggest possible correlations between such stresses and the onset of instability. Interestingly, this observation is consistent with the conclusion made by Ham et al. (1990) whose flow visualization experiments and detailed quantitative measurements showed Reynolds stresses due to particle velocity fluctuations to be the main contributor to the formation of voidage instabilities.

Reference:

Ham, J. M., S. Thomas, E. Guazzelli, G. M. Homsy, M.-C. Anselmet. An experimental study of the instability of liquid-fluidized beds. International Journal of Multiphase Flow, 16, 171?185. 1990.