VISUALIZATION OF FLOW CHARACTERISTICS IN A 2-D BUBBLE COLUMN AND 3-PHASE FLUIDIZED-BED

Macroscopic flow structures of gas-liquid and gas-liquid-solid fluidiz ation systems are studied through flow visualization using a two-dimen sional column under various operating conditions. The gas distributor in the column comprises multiple injectors which are individually regu lated to generate desired gas flow rates, bubble injection frequencies , and bubble sizes. Colored bed particles and neutrally buoyant partic les as solid and liquid tracers, respectively, are used for flow visua lization through video photography. In a gas-liquid system operated un der liquid-batch conditions, bubble streams injected near both sidewal ls are observed to migrate toward bed vertical axis, and vortices appe ar along the sidewalls when gas velocity exceeds 4-6 mm/s. A considera ble amount of liquid descends along the sidewalls in a vortical flow p attern. The gross circulation pattern occurring at high gas velocities is associated closely with induced liquid or liquid-solid flows resul ting from rising bubbles and bubble wakes. When the gross circulation occurs, four flow regions can be distinguished. The formation of and m echanism for gross circulations can be interpreted in part based on tw o simplified flow conditions involving single bubbles rising in a stat ionary liquid and single chains of bubbles injected in a batch liquid. The effects of particle size, inlet liquid velocity and gas flow dist ribution on the macroscopic flow structure are also examined.