Strong transient characteristics in axial flow waterjet pump during rapid starting period with special emphasis on saddle zone

The rapid starting condition of axial flow waterjet pumps is relatively common in high-speed marine vessels, often leading to cavitation damage and vibration noise. This paper aims to reveal the strong transient mechanism of the rapid start-up process in axial flow waterjet pumps. The relationship between the strong transient effect and the saddle zone during the rapid starting period is analyzed numerically and experimentally. The flow dynamics are analyzed with different valve openings by velocity triangle method. The results showed that the saddle zone of the waterjet pump is the region from 0.45 to 0.65 times the rated flow rate. During the rapid starting period, the external characteristic parameters on the H-Q plane gradually approach the steady-state performance curve, and the pump head decreases when the pump runs in the saddle zone regardless of the valve opening. The distribution law of velocity circulation is broken when the pump starts rapidly to the saddle zone, leading to the tip blockage vortex and outlet backflow vortex. The channel blockage and energy dissipation caused by these vortices are the main reason for the head drop. The conclusion is of certain value for application of unsteady operation for axial flow waterjet pumps.