OBSERVATIONS OF FLUIDIZED-BED COALESCENCE USING CAPACITANCE IMAGING

A unique method for imaging voidage within a fluidized bed has been developed at the Morgantown Energy Technology Center (METC). This system allows high speed three-dimensional imaging of the voidage distribution in a bed to be recorded. From this imaging data a wide variety of visualizations can be created and various kinds of quantitative information extracted. Three materials with differing particle sizes were fluidized over a range of superficial velocities in a 15.24 cm diameter bed. The bed was imaged in a zone 1.25 to 2 bed diameters above the grid. This is a region which shows appreciable bubble growth and a transition of the flow regime from bubbling to slugging. The imaging allowed bubble sizes and rise velocities to be measured, revealed detailed voidage distributions, provided images of several forms of coalescence, and provided measurements of the expansion of the emulsion phase under some conditions. The imaging data point out several shortcomings of the two-phase representation of fluidized beds and the need for alternate models which better represent the voidage structure, at least in the grid-influenced region of fluidized beds. The technique has the potential to substantially improve design and scale-up of fluidized beds and other gas-solid systems by providing a detailed understanding of the gas-solids dynamics.