Air Pocket Removal during Siphon Priming

Abstract

Siphon frequently is employed in pump stations as a pump outlet with a convenient flow cutoff mode for pump shutdown, and it usually works with axial-flow pumps with a large discharge and low head. After the pump starts, the initial air pocket in the siphon should be removed quickly from the siphon to generate a primed siphon flow, a process called priming. In this study, experiments were conducted to investigate the characteristics of air pocket removal during priming, in which two stages can be defined: air-compression and air-entrainment stages. During the air-compression stage, the air is compressed by the pumped water, and the increased air pressure can break the downstream water seal and expel some air out of the siphon. During the air-entrainment stage, the air is removed mainly in the form of bubbly flow. The air-entrainment stage can be divided further into two states: at a small water discharge, the priming process stays in State 1, in which the air pocket always remained in the siphon hump; and at a large discharge, the residual air pocket can be entirely swept out of the siphon in State 2, finally generating the primed siphon flow. The air-compression stage accounts for less than 5% of the priming time but discharges 20%–60% of the initial air. For the bubbly flow during the air-entrainment stage, the bubble diameters follow a log-normal distribution with the median of about 2.3 mm. The critical flow rate for generating primed siphon flow was predicted based on the bubble clearing velocity and force and on the motion analysis of the bubble, which can be a rough value for the calculation of the critical flow rate. The prediction indicates that the critical flow rate should increase with the descending angle and cross-section area of siphon. Siphons are inverted U-shaped pipes that commonly are used as pump outlets. We experimentally studied the air–water interactions during siphon priming in a laboratory-scale siphon model to elucidate the hydraulic removal of the air in the siphon. Because air-entrainment is the typical feature of siphon priming, high-speed photography was used to capture the bubble characteristics during siphon priming. Based on the in-depth observations of siphon priming at the bubble-scale, the priming time and the critical conditions for generating the primed siphon flow are discussed. The study shows that as the flow rate decreases, the priming time increases significantly and even the primed siphon flow cannot be generated; the priming time also increases with the increasing cross-sectional area and descending angle of the siphon. The empirical equation of the priming time was established, and the corresponding coefficients should be decided based on the siphon geometries. The prediction of the critical condition for generating the primed siphon flow was analyzed from the perspective of bubble motion. This study improves the understanding of air–water flow patterns during siphon priming and their influence on the siphon characteristics, which can be a reference for structural design and operational optimization in practical siphons.