Air Entrainment and Acoustic Wave Celerities Following a Rapidly Moving Pipe Filling Bore

Abstract

Water mains stormwater sewers and tunnels may undergo processes of rapid filling, in which flow regimes transit from open channel into pressurized flows. In the process, air bubbles may be entrained, which may influence the resulting volume and distribution of air in the system upon priming. The amount of air that is entrained can have an effect on the the magnitude of the pressure wave celerities, an important parameter in the numerical simulation of unsteady tunnel filling or pipeline priming. However, air entrainment has not yet been studied in the context of rapid filling pipe flows. The purpose of this research is to study the nature of air entrainment and the effects on the celerity of pressure waves following episodes of rapidly filling pipes. Studies were conducted on a 10-m long, 1% and 2% slope, clear PVC pipeline using three diameters ranging from 50 mm to 152 mm. Inflow rates were systematically varied, and various repetitions (at least 10) were performed in each case to ensure consistency of results. Air entrainment is initiated by creating a backward moving, sometimes pipe-filling bore generated by a knife-gate valve closure at the down stream end. Upon complete pipe pressurization and air entrainment, a second pressure pulse was generated by maneuvering a solenoid valve. After each run the amount of air entrained is measured, as well as the speed of the acoustic wave and the speed of the bore. Research results include analyzing the relationship between the volume of air entrainment and various normalized parameters. The acoustic wave speed in these tests were also compared to baseline cases in which no air was entrained. These experiments hope to improve current understanding of unsteady flows following the rapid filling of closed pipes.