Experiments on a Small Pump Suction Well, with Particular Reference to Vortex Formations

Abstract

To investigate the incidence of air-entraining vortices in suction wells, experiments have been made with a suction tube of 2 1/2 inches diameter, fitted with a bellmouth intake, mounted in a series of wells of different shapes. With the tube set on the axis of a cylindrical sump into which water flowed radially between baffles at the bottom, only slight and transient air entrainment was noted, and this is ascribed to temporary irregularities in the flow near the water surface. Air entrainment was completely prevented by imparting a swirl to the flow at entry to the cylinder, and this swirl was found to pass into the suction tube with only slight reduction except near the axis. With the tube drawing from a rectangular well, air-entraining vortices formed at the edge of the wake of the external surface of the tube in the general flow in the well. The critical submergence (submergence of the bellmouth required to prevent air entrainment) is greatly affected by non-uniformity of the flow along the well. Any measure which reduces or diffuses the vorticity shed from the outer surface of the tube into the well has a beneficial effect; reducing the clearance between the rear wall of the sump and the suction tube reduces the streaming flow past it and so reduces the critical submergence, while a streamlined fairing fixed round the tube, or a cylindrical shroud placed within the well and coaxial with the tube, have both been found virtually to eliminate air entrainment.