Laser diagnostics of hydrodynamics and gas-mixing induced by bubble bursting at the surface of gas-fluidized beds

Gas mixing in the splash zone of gas-fluidized bed reactors plays an important role in fluidized bed processes where fast homogeneous gas-phase reactions take place. An example is represented by the fluidized bed combustion of high-volatile solid fuels, where the large contribution to overall heat release due to homogeneous combustion of volatile matter and the "stratified" combustion pattern emphasize the importance of mixing/segregation phenomena in the splash zone. Gas mixing and hydrodynamics in the splash zone are determined by the complex interaction between bursting bubbles, the mainstream gas and the bed solids. As a first step toward comprehensive characterization of hydrodynamics and gas mixing in the splash zone of a gas fluidized bed, the present paper addresses the flow structures and the gas mixing patterns associated with the eruption of isolated bubbles at the surface of an incipiently fluidized bed. These are investigated by a combination of non-intrusive optical diagnostic techniques, namely planar laser light scattering (PLLS) and planar laser induced fluorescence (PLIF). The first is based on the use of a non-diffusive tracer (fine solid particles). The second is based on the use of a diffusive gaseous tracer (acetone). The two techniques provide complementary tools to assess the detailed structure of the gas and particle flow fields and the macro- and micromixing patterns associated with bubble bursting. The application of the two techniques to the analysis of events associated with the eruption of isolated bubbles proved to be successful. Results highlighted that the basic flow structure generated by bubble bursting is a toroidal vortex ring. The uprise velocity of the vortex and the gas entrainment rate from the mainstream have been determined from results obtained with both the PLLS and PLIF measurements as a function of the size of the bursting bubble. Macromixing, determined by gas entrainment, and micromixing, related to molecular diffusion, have been quantitatively assessed by the combined analysis of tracer concentration maps obtained with the PLLS and PLIF techniques. The relevance of the observed hydrodynamics and gas mixing patterns to the kinetics of fast gas-phase chemical reactions and to the performance of the splash zone of a fluidized bed as a homogeneous reactor is discussed, with a focus on volatile matter burn-out in fluidized bed combustors.