Numerical investigation on the mechanism of impeller hub corner separation flow and induced energy loss in the bulb tubular pump

Impeller hub corner separation flow (IHCS) has a significant influence on energy conversion of the bulb tubular pump, and its unsteady characteristics are investigated with CFD-based method. The generation mechanism and power loss characteristics of IHCS are investigated by the entropy production method and pressure fluctuation analysis. The main cause can be attributed to the large transverse pressure gradient near the hub at the trailing edge of the impeller, which is aggravated by the circumferential movement trend and the diffuser reverse flow, while the IHCS is significantly weakened with increased flow rate. The undesirable flow behavior is more likely to cause a significant increase in energy loss near the hub region compared to that in the rim region. The relative vortex stretching induced by the velocity gradient is the main cause of the horn-like vortex (HLV), and its intensity and resulting energy loss tend to decrease along the vortex trajectory. The HLV changes the dominant frequency of the pressure fluctuations in the nearby flow field, the value of which increases from 1f _r (the blade passing frequency) to 2f _r with increased amplitude, mainly due to the lower pressure regions on the impeller suction surface (SS) and HLV vortex core. Due to the effect of rotor-stator interaction (RSI), the HLVs generated between two adjacent impeller blades are cut into several sections by the diffuser vanes and propagate and dissipate along the mainstream direction.