## 1 Introduction  
The South-to-North Water Transfer Project is one of the most important water conservancy projects in China in the 21 st century. It is the largest inter-basin water transfer project in the world. Its total investment is estimated to exceed US 576 billion, involving438 million people and over 40,000,000,000 m\({}^{3}\) of water. It mainly solves the problems of water shortage and distribution in Northern China [1; 2]. A total of 35 pumping stations have been constructed in Phase I of the east route of the South-to-North Water Transfer Project. Among the 21 new pumping stations, the number of large vertical axial flow pumping units accounts for more than 66%, which is the main structural type of pumping unit used in the project. The siphon outlet conduit is one of the common outlet structures for large-scale low-head pump units. It is suitable for situations where the water level of the outlet pool varies little. Its cut-off mode has the advantages of simplicity, reliability and economy. However, the siphon outlet conduit has the disadvantages of a complex shape and inconvenient construction. The advantages and disadvantages of the siphon outlet conduit are comprehensively analyzed. In the first stage of the east route of the South-to-North Water Transfer Project of China, the siphon outlet conduit had advantages and disadvantages. Seven of the 14 new vertical pumping stations in the project employ siphon outlet conduits accounted for 50%, such as Liu Laojian No.2 station, Siyang pump station, Suining No.2 station and Denglou pump station in Jiangsu Province. The internal flow characteristics of the outlet conduit will not only affect the safe and stable operation of the pump unit but also affect the hydraulic efficiency of the pump unit. The hydraulic loss of the outlet conduit accounts for a large proportion of the factors affecting the hydraulic efficiency. The internal flow of the siphon outlet conduit is complicated due to the influence of the axial flow pump. Domestic and international scholars have carried out relevant research work on the siphon outlet conduit. [PERSON] [3] used the SST k-\(\omega\) turbulence model and the Zwart cavitation model closed multiphase flow control equations to numerically analyze the cavitation characteristics inside the siphon outflow conduit. [PERSON] [4] analyzed the influencing factors of hydraulic loss in the condition of a gas-liquid two-phase flow in the siphon flow conduit by the physical model test method and measured the bubble velocity, pressure drop and gas volume fraction in the siphon flow conduit under different operating conditions. [PERSON] [5] used numerical simulations to analyze the transient flow characteristics inside the siphon outlet conduit of a vertical axial flow pumping device during shutdown conditions. [PERSON] [6] carried out full flow conduit numerical calculations for a siphon axial flow pump unit by the CFD method and quantitatively analyzed the influence of the residual circulation at the guide vane outlet and flow rate on the hydraulic loss of the siphon outlet conduit. [PERSON] [7] used the Euler multiphase flow model and RNG k-\(\epsilon\) turbulence model to simulate a gas-liquid two-phase flow in the siphon process of the siphon outlet conduit of a pump station. [PERSON] [8; 9] modified the geometrical dimensions of the siphon outlet conduits of the pumping stations one by one, observed the changes of the flow patterns inside the conduits by the CFD method and gradually optimized the flow path profile and carried out the comparison of the siphon and straight pipe outlet conduits used in vertical pumps. [PERSON] [10] applied aerodynamic principles to develop a mathematical model for the calculation of the maximum start-up head of a siphon outlet conduit at a pumping station and compared it with the experiments. [PERSON] [11] investigated the effect of the siphon section length and cross-sectional area on the efficiency of the pumping unit through physical model tests and studied the siphon outlet conduit vacuum breaker.