## 1 Introduction  
A pumping station is the most common technical means to solve the imbalance of water resources in the world, especially for agricultural irrigation and drainage, water transfer and urban and rural construction. To date, China has more than 500,000 large, medium and small pumping stations; the total installed power exceeds 11.4 million kW [1, 2], and quite a few of these pumping stations are large low head pumping stations. The axial-flow pump is the main pump type applied to low-lift pumping stations and is featured by a simple structure, flexible installation and various forms. Nearly 300 large axial-flow pumps are needed for the third phase of South-to-North Water Diversion Project in China alone, while the demand for small and medium-sized axial-flow pumps is larger, indicating that the demand for axial-flow pumps is very large [3,4]. The outlet conduit is the transition section between the outlet of the guide vane and the outlet pool, and its main role is to guide the orderly and smooth diffusion of water into the outlet pool and recover the kinetic energy of water as much as possible. According to previous studies, the hydraulic performance of the outlet conduit has an important influence on the efficiency of the axial-flow pump device, and its head loss accounts for a large proportion of the total loss of the pumping station; in particular, for the large low-lift axial-flow pump pumping stations, it accounts for about 15-25% of the total [5,6]. The width of the cross section of the outlet conduit of a large low head axial-flow pump station is more than 3 m [7]; thus, considering the structure, it is necessary to set the division pier in the outlet conduit to support it and reduce the span of the roof of the outlet conduit so as to reduce the investment of the civil construction of the pumping station. This division pier structure has been widely used in the outlet conduits of some large low-lift pumping stations in China, Japan and the United States. However, the location, length and other factors of the division pier arrangement often affect the hydraulic performance of the outlet conduit, such as affecting the flow pattern of the outlet conduit, increasing the hydraulic loss in the conduit and inducing the vortex to cause severe vibration; this poses hidden dangers to the safety of the outlet conduit [8]. However, at present, we still lack a deep understanding of the quantitative results of these impacts. Previous studies have shown [8,9] that there is an obvious phenomenon of flow deviation in the outlet conduit of the division pier, and the hydraulic performance of the conduit has a great influence, such as causing flow pattern disorder and serious vibration and generating a large amount of noise. [PERSON] et al. [10] used a model test to compare the multi-parameter analyses of different types of outlet conduits and found that, due to the improper setting of the division pier, the hydraulic loss in the straight outlet conduit was significantly higher than that in the siphon outlet conduit, which failed to reflect the superiority of some hydraulic properties of the straight outlet conduit. In the work by [PERSON] et al. [11,12], in order to obtain a suitable large low head vertical axial-flow pump outlet conduit and its design scheme, a model test was carried out on the outlet conduit of a pumping station; it was found that the water flowing out of the pump easily impacted the water surface of the division pier of the outlet conduit and easily formed vortex on the back of the division pier, resulting in increased hydraulic loss in the outlet conduit.