Numerical investigation of air expulsion and transient pressurisation during rapid filling of water distribution pipelines

Controlling pressure in urban water distribution systems is a key operational challenge, as excessive pipeline pressure can lead to leaks, pipe bursts, and significant economic losses for utilities. This issue is relevant in networks with intermittent water supply, where frequent cycles of rapid filling and emptying generate transient pressures that, if not properly managed, can lead to structural damage and reduced operational reliability. This study investigates the air expulsion process and transient pressure response in a rapidly filling pipeline, with emphasis on the effect of a downstream orifice for controlled air release. A two-dimensional CFD model was developed in ANSYS Fluent using the Volume of Fluid (VOF) multiphase approach and validated against experimental measurements. The effects of orifice size and water column length on transient pressures and the associated air–water interactions were systematically examined. Results show that transient pressure behavior is strongly governed by orifice size. Without an orifice, the air-cushioning effect is maintained, leading to lower pressure peaks and smoother transients. Larger orifices promote rapid air release, weakening the cushioning effect and producing water-hammer–dominated transients with significantly higher-pressure amplitudes. In contrast, smaller orifices partially release air while compressing the trapped air until water slams against the pipe end, creating a temporary water block. In this case, the air is not fully expelled, and both cushioning and water-hammer effects occur simultaneously. These results enable the identification of orifice size ranges that control the transition between air-cushioned filling and water-hammer-dominated response for practical air-release design. The simulations also capture temperature variations in the entrapped air, providing additional insight into the thermodynamic interactions during pipeline filling and air expulsion. Overall, the numerical framework captures the full range of transient behaviours associated with rapid filling and air expulsion and offers practical guidance for designing safer filling strategies and controlling pressure in urban water pipelines.