Influence of spiral flow on the hydraulic performance of a siphon outlet conduit in an axial flow pump system

In axial flow pump systems, not only are the flow rates typically large but the water leaving the guide vanes almost invariably possesses a residual rotation. Since such pumps often generate only low heads, the energy content of this residual rotation often has a notable influence on the hydraulic and economic performance of the outlet conduit. The hydraulic losses and flow fields associated with a specific siphon outlet conduit under different rotational speeds are herein studied both experimentally and using a 3D turbulent flow numerical simulation. In order to experimentally verify the simulation results, the hydraulic losses are measured and flow patterns are observed for a siphon outlet conduit with a series of different guide vanes. It is observed that as the rotational speed increases the hydraulic loss within this outlet conduit increases gradually until the rotation speed reaches 300 ; as the rotational speed surpasses this value, the hydraulic losses increase sharply. Yet surprisingly, residual swirl can have benefits, with the general trend being that larger rotational speeds are associated with a smaller vortex zones and a better flow field in the outlet conduit, an observation confirmed both numerically and experimentally. This research has considerable significance, both theoretically and practically, for the design of outlet conduits and guide vanes in axial flow pump system.