Transient characteristics of the hydraulic transition process of emergency water supply multi-stage pump with unexpected shutdown

Emergency water supply is one of the most important lifeline projects for post-disaster survival support. Multi-stage pump can be the core power source for fluid transportation. A stable and reliable operation is crucial to rescue and support at disaster sites. In actual operation, the unexpected shutdown of emergency water supply multi-stage pumps can cause drastic changes in the performance parameters, such as the impeller rotating speed, flow rate, and pressure. The internal flow field in multi-stage pumps can pose a serious threat to the water supply safety. This study aims to investigate the transient response to the internal flow field during the hydraulic transition of multi-stage pump shutdown. A speed prediction was established to numerically calculate the internal flow field of the stage pump, according to the rotation balance equation of the impeller. The transient effects were analyzed from the dynamic characteristics of the impeller rotating speed, outlet flow rate, torque and the flow structures inside each stage of the impeller during the unexpected shutdown. The results indicate that the multi-stage pump shared four conditions during unplanned shutdown, pumping, braking, reversing and runaway. The rotational speed of the impeller showed a trend of first decreasing in the positive direction, and then increasing in the reverse direction, while finally stabilizing around the runaway speed of -4 210 r/min. The flow rate of the pump showed a trend of first positive decrease followed by a reverse increase, then stable reverse decrease and finally stayed around the runaway flow rate of -14.32 kg/s. The torque showed a trend of positive decrease followed by a positive increase, then stable positive decrease and finally stayed near zero. The continuous changes in the magnitude and direction of flow and speed inside the pump were attributed to the flow separation and backflow inside the impeller channel, accompanied by spatiotemporal evolution, such as the formation, development and fragmentation of vortices. The entropy output value was closely related to the unstable flow structure inside the multi-stage pump, while the loss caused by turbulent dissipation played a dominant role. The turbulent dissipation entropy production accounted for about 65.2% after reaching the runaway condition. Energy loss mainly occurred inside the impeller flow channel. There was a significant loss of the internal flow field in the braking condition. There was the most variation in the pressure fluctuation at the monitoring points in the first stage flow channel of the multi-stage pump. The amplitude of the pressure fluctuation coefficient showed a decreasing trend with the increase of the stage number. The dominant frequency of the pressure pulsation amplitude was positively correlated to the rotating speed of the multi-stage pump, corresponding to the blade passage frequency. The amplitude of dominant pressure pulsation frequencies gradually increased along the direction of fluid flow. There was the most significant change in the first stage flow channel during the unexpected shutdown of the multi-stage pump, which was accompanied by the most complex flow structure. The frequency characteristics of pressure pulsation can reflect the differential changes in flow instability during unexpected shutdowns. The research results can provide theoretical guidance for the safe and stable operation of emergency water supply systems.