Flow structures and gas-mixing induced by bubble bursting at the surface of an incipiently gas-fluidized bed

Flow structures and gas-mixing induced in the splash zone of a fluidized bed reactor by bubble bursting at the bed surface were investigated. A planar laser light scattering technique based on a nondiffusive gas tracer and coupled with computer-assisted imaging was used to characterize the macroscopic and microscopic features of flow structures. Bursting of either isolated bubbles or of chains of closely time-delayed bubbles was considered. The basic flow structure associated with bursting of isolated bubbles is a toroidal vortex ring. Multiple vortex rings are generated by bubble chains, eventually interacting with each other to give rise to more complex hydrodynamic patterns in a way that depends on the time-delay between successive bubbles. Quantitative assessment of the flow structures included axial displacement, rise velocity, angular velocity, volume growth, and intermaterial specific surface area. The influence of bubble size, of bed material size, and of the time-delay between subsequent bubble eruptions was investigated. The properties of the flow structures provided the basis for an assessment of the extent of gas macro- and micromixing in the splash zone. The issues associated with volatile matter burnout in the splash zone of fluidized bed combustors of high-volatile solid fuels were analyzed in the light of the findings of the present study.