THE EFFECT OF GAS-PHASE DENSITY ON BUBBLE FORMATION AT A SINGLE ORIFICE IN A 2-DIMENSIONAL GAS-FLUIDIZED BED

In this study the effect of the gas-phase density on the process of bu bble formation at a single orifice in a two-dimensional gas-fluidized bed has been studied experimentally and theoretically. Specifically, a detailed comparison between experimentally observed and theoretically calculated bubble growth curves has been made in the case where the d ensity of the gas injected through the orifice (He and SF6) differs si gnificantly from the density of the primary fluidizing agent (air). Th e calculations have been carried out using an earlier developed, first principles hydrodynamic model of gas-fluidized beds which has been ex tended with a species conservation equation to calculate the compositi on of the fluidizing gas in the vicinity of the evolving bubbles. Besi des, the present experimental and theoretical results were compared wi th predictions obtained from adapted versions of approximate bubble fo rmation models previously reported in the literature. The advanced hyd rodynamic model appears to predict the experimentally observed diamete rs satisfactorily. In addition, the model correctly predicts the effec t of the gas-phase density on the experimentally observed bubble growt h. This effect can be explained satisfactorily in terms of the depende nce of the interphase momentum transfer coefficient on gas-phase densi ty. Finally, calculations with a three-dimensional version of our hydr odynamic model have been carried out to account for the effect of the front and back wall of the pseudo two-dimensional gas-fluidized bed us ed in our experiments. Our preliminary computational results indicate that the magnitude of the wall effect strongly depends on the boundary condition enforced for the gas-solid dispersion at these walls. In th e case that the no-slip boundary condition was enforced in the calcula tions for the solid phase, the wall effect was significant and a consi derable deviation between computed and experimentally observed bubble growth curves was found. However, when a more realistic partial slip b oundary condition for the solid phase was implemented the agreement be tween theory and experiment could be improved by altering the slip par ameter in the partial slip boundary condition expression. Copyright (C ) 1996 Elsevier Science Ltd