Transient Internal Flow and Energy Conversion Characteristics of a Siphon Vertical Axial-Flow Pump Hydraulic System During Startup

Siphon vertical axial-flow pump systems are widely used in large-scale low-head pumping stations, irrigation and drainage projects, urban flood control, and water diversion engineering, where safe and stable operation is essential for hydraulic system reliability. Compared with steady operating conditions, the startup process involves rapid variations in impeller speed, flow rate, pressure distribution and hydraulic energy transfer, resulting in highly transient hydraulic behavior and obvious instability in the internal flow field. To clarify the transient hydraulic response of a siphon vertical axial-flow pump system during startup, an unsteady numerical model was established by coupling the impeller speed control equation with a dynamic boundary condition implemented through a user-defined function. The time-dependent evolution of flow rate, head, blade loading, pressure distribution, internal flow pattern and energy conversion characteristics was then investigated during the whole startup process. The results show that the pump system undergoes a distinct transition from an initially unstable flow state to a quasi-steady operating condition. At the early startup stage, the flow passage is not fully established, and the pump operates under a low-flow transient condition, leading to strong flow disorder, uneven blade loading and a sharp fluctuation in head. With the increase in rotational speed and flow rate, the internal flow gradually becomes organized, the pressure distribution on the blade surface tends to be more uniform, and the hydraulic energy transfer process becomes progressively stable. The pump head first exhibits an abnormal transient peak, then decreases rapidly, and finally recovers to a stable value as the flow field is fully established. The study reveals the transient evolution mechanism of hydraulic performance and energy conversion during startup, providing a theoretical basis for improving startup control strategy and operational stability of siphon vertical axial-flow pump systems.