CFD analysis of two-phase flow in siphon outlet passage with retaining weir in the low-head pumping station

Abstract The siphon outlet passage is a typical type of discharge passage used in large-scale low-head pumping stations and is widely employed in agricultural irrigation pumping stations. However, its application is limited by the sump water level, preventing some pumping stations from utilizing this design. In this paper, the siphon outlet passage with a weir in a large low-head pumping station is taken as the research object. Through three-dimensional numerical simulation, the hydraulic characteristics of the siphon outlet passage with a weir were explored, and the influence of different weir crest elevations on the hydraulic performance and air exhaust process of the outlet passage was systematically analyzed. The research shows that for a weir crest elevation of 79.07 m, compared to a weir crest elevation of 77.57 m, the head loss of overall descending section of the outlet passage is reduced by 17%, and the flow pattern is significantly improved. The air exhaust completion time is shortened by 66.5 s, the hump mean pressure increases by 57%, and the velocity distribution uniformity at the hump section, which directly influences the air exhaust efficiency and hydraulic performance, improves by 4.5%. Furthermore, the retaining weir structures were characterized by crest elevations ranging from 77.57 m to 79.07 m, slopes varying from 0.4 to 1.0, and four distinct geometric types. A weir with a gently sloping upstream face reduces head loss by 24% compared to a weir with a steep slope. The ogee-type weir with a crest elevation of 79.07 m and gentle transition yields better performance. This study can furnish theoretical references for the design of retaining weirs in outlet sumps of low-head pumping stations.