Comparative studies on the propagation of rotating stall in a liquefied natural gas cryogenic submerged pump-turbine in both pump and turbine mode

With the tightening of liquefied natural gas (LNG) supply, unsteady stall flow under partial flow rate will likely cause significant difficulties with respect to the application of a cryogenic submerged pump-turbine (PT) at LNG-receiving terminals. In this study, the unsteady propagation characteristics of stall cells in pump mode (PM) and turbine mode (TM) of the PT are investigated numerically using the timescale-based hybrid turbulence model. The predicted performance curves show good consistency with on-site experimental data. As the rotating speed and fluid undercooling increase, the triggering of stall flow tends to a lower flow rate in PM, but this is reversed in TM. Under the superimposed action of separation and backflow vortex, stall flow in the impeller suffers from dynamic emergence to dissolution in PM, but quasi-static stall vortex is dominant in TM, benefiting from the rectification of the guide vane. By comparison, running in PM is prone to severe deep stall, and the stall cells have a higher propagation frequency and strength, which also induces greater local energy loss than in TM.