Investigation of Hydraulic Transients of Two Entrapped Air Pockets in a Water Pipeline

Transient pressure associated with a rapidly filling pipeline containing two entrapped air pockets is investigated experimentally and numerically. A multiple-air-pocket elastic-water model considering multiple moving boundaries of water columns is developed by neglecting inertia and head loss of a short water column near air–water interfaces. The proposed model is validated by experimental data. Results show that when two air pockets in length are much different, the maximum pressure always arises in the smaller air pocket regardless of the blocking column’s length. The case of the upstream air pocket with a similar length to the downstream is the most complicated and dangerous because (1) the maximum pressure may alternately arise in two air pockets as the blocking column increases, and (2) interaction of two air pockets could cause a huge pressure surge, which is likely much higher than with only one air pocket. The existing single-air-pocket model cannot effectively simulate pressure surge of the two air pockets.