Air-Water Flow Structure in a Circular Dropshaft

Circular dropshafts, commonly used in urban storm water systems for energy dissipation and flow control, are characterized by significant flow aeration. In the laboratory, a series of experiments were performed in a circular dropshaft having two identical horizontal inflow and outflow circular channels having diameters of d=0.12 m, and a circular drop chamber having diameter of D=0.358 m for observing flow patterns and measuring air bubble entrainment flow parameters. Experimental observations showed distinct flow regimes associated with nappe impact in the shaft pool, in the outflow channel, or in the opposite shaft wall. New detailed air-water flow measurements were conducted in a circular dropshaft with using double-tip fiber optical probe. Measurements of void fractions, bubble frequencies, and bubble sizes were conducted for different flow conditions of the circular inflow channel. In the shaft pool, the results obtained from measurements demonstrated the complexity of different air entrainment mechanisms.