| Local two-phase flow measurements were obtained in a pilot-scale, external loop airlift bioreactor. The gas-liquid system was air and water; the gas phase was not recirculated. Sparging rates ranged from 1.0-8.0 cm/s, using a multiorifice, ring sparger. The axial dependence of the cross sectionally averaged gas volume fraction was determined using a track-mounted gamma densitometer. Hot-film anemometry was used to measure the radial and azimuthal dependence of the liquid velocity and turbulence for two axial riser locations. The same spatial dependence was ascertained for the gas velocity and volume fraction using resistivity probe techniques. The axial distribution of liquid phase pressure was measured with precision inclined manometers.; The axial dependence of the void fraction was found to be modest. The radial dependence of the gas and liquid velocities and the void fraction was substantial, especially near the sparger. Local property changes with azimuthal position were small, and the flow may be characterized as being azimuthally symmetric. The developing flow effects were pronounced, as evidenced by the distinct changes in the radial profiles of fluid flow properties with axial position. Under certain operating conditions, liquid acceleration effects near the sparger resulted in greatly reduced slip velocities.; For the two-phase riser flow, a model was developed directly from the point equations of continuity and motion, resulting in a differential, two-fluid form. Macroscopic mechanical energy balance equations were used to describe the flow in the gas-liquid separator and the downcomer. The model incorporates empirical parameters for the frictional effects only. The developing two-phase flow characteristic of airlift risers was observed to create large frictional effects at the wall.; The model results were compared to the experimental results of this study and those of Merchuk and Stein (1981). Agreement between the predicted and measured values was typically within 10% for both cases. The model was also solved for a hypothetical reactor having a liquid volume which was an order of magnitude greater than the vessel used in this study. The results had the expected qualitative trends. |
