Gas-mixing phenomena may play a significant role in fluidized bed combustion of solid fuels. Issues associated with gas mixing include: a) effectiveness of mass transfer between the bubble and the emulsion phases in the bed; b) degree of mixing between segregated gaseous streams in plume-like or bubbly flow in the bottom bed; c) extent of mixing between segregated gaseous pockets/streams in the splashing zone or in the upper freeboard. Among the others, issues b) and c) turn out to be relevant to fluidized bed combustion of high-volatile solid fuels (biomass, RDF, etc.). In this case, the rate of gas mixing often overcomes intrinsic kinetics as the rate-controlling step in volatile matter burn-out, especially under "stratified" combustion conditions. Despite several and significant contributions to the subject, understanding of gas-mixing in bubbling fluidized beds is still poor and calls for additional investigation. The present work aims at investigating gas-mixing in bubbling fluidized bed with specific reference to the above issue c). A laser assisted imaging technique has been used to characterize the hydrodynamic patterns associated with the bursting of either isolated bubbles or couples of closely time-delayed bubbles at the surface of a fluidized bed. Quantitative image analysis procedures were exploited in order to assess parameters defining the general fluid-dynamic behaviour and macromixing of the bubble-generated gas pockets with the mainstream gas. The formation of toroidal flow structures upon bubble bursting is highlighted in the case of isolated bubbles. The toroidal pockets entrain mainstream gas and grow accordingly while rising along the splash zone. Toroidal flow structures are observed also in the case of couples of closely time-delayed bubbles, but interference between leading and trailing pockets leads to more complex structures characterized by multiple incoherent eddies. The relevance of macromixing to volatile matter burning under conditions of stratified fluidized bed combustion is assessed and discussed.
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