Modelling of Air Pocket Entrapment during Pipe Filling in Intermittent Water Supply Systems

Abstract

Intermittent water supply (IWS) operations frequently involve water filling and emptying cycles that are strongly influenced by air-water interaction. Storm Water Management Model (SWMM) software has been recently proposed to simulate pipe filling events in IWS systems. As the tool is conceived to simulate both free surface and pressurized flows, it also has the potential to analyse the air entrapment. However, there is no numerical model capable of accurately and efficiently simulating the air behaviour during these events, nor of predicting the locations of air pockets created during the pipe filling process.An experimental pipe rig was assembled to better understand the pipe filling process in terms of pressure variation, the propagation of the filling wave and the air entrapment locations under different initial conditions. The pipe rig has a classic reservoir-pipe-valve configuration. Different behaviours are observed in this experimental setup during the pipe filling tests. An entrapped air pocket is created at the high point for lower flow rates, which is not dragged when the pipe is full. This air pocket can go from a similarly free surface flow inside the pipe to a complete water filled flow, depending on the flow rate. For low flow rates, a high head loss is introduced due to a hydraulic jump inside the pipe. For higher flow rates, the air is dragged, no air is entrapped and only the local head losses from the change of direction at the high point are observable.Following the collected experimental data, SWMM is used to assess to which extent it can predict entrapped air pockets location and their volume. Different filling processes can occur and an air model should be included to simulate the tests carried out in the pipe rig. The results obtained show that SWMM is capable of predicting air pocket locations but not the air pocket volumes. Further research is necessary to improve SWMM in this context.