Investigation on flow instability in the hump region of the large vertical centrifugal pump under cavitation conditions based on proper orthogonal decomposition

Complex flow patterns in a large vertical centrifugal pump (LVCP) when operating in the hump region under cavitation conditions should deserve more attention. This research investigates the interaction between cavitation and vortices in LVCP from the perspective of rigid vortex transport characteristics and modal decomposition. The hump characteristics of LVCP are more pronounced under cavitation. The backflow vortex cavitation at the impeller inlet and the leading edge cavitation of the diffuser vanes can be found under part-load conditions. Rigid vortex analysis reveals the low pressure backflow vortices are generated between the impeller inlet and the inlet pipe. The fusion of leading edge separation vortices with pressure surface separation vortices and shedding high pressure trailing edge separation vortices in the diffuser generates the unique stall vortices with a high pressure gradient. Under part-load conditions, the combination of rigid vortex stretching, rigid vortex dilation (RVD), coriolis force, and baroclinic torque (BT) cause further increase in the strength of cavitation tail vortices. The RVD and BT caused by diffuser vanes leading edge cavitation can accelerate the development and fusion of leading edge separation vortices, pressure surface separation vortices, and trailing edge separation vortices in the diffuser, then generate unique stall vortices with high pressure gradient in advance. Proper orthogonal decomposition analysis indicates that the unique stall vortices in the diffuser occupy the main energy of the flow pattern. The flow pattern in the diffuser is superimposed by a variety of vortices with different frequency characteristics, and these vortices show a particular low frequency signal well below fn. The cavitation at the diffuser vanes leading edge interacts with vortices to induce leading edge separation vortices and unique stall vortices with low frequency characteristics more easily, and these vortices always dominate the flow pattern evolution.