Flow in contra-rotating pump-turbines at stationary, transient, and cavitating conditions

Doktorsavhandling, 2024

This thesis investigates contra-rotating pump-turbines (CRPT) through computational fluid dynamics (CFD) simulations. The research was carried out within the ALPHEUS EU research project, which examined low-head pumped hydro storage using CRPTs. The aim is to analyse and suggest operations for the CRPT at stationary, transient, and cavitating flow conditions. Stationary conditions are analysed using steady-state and unsteady CFD. It is found that the CRPT can produce a hydraulic efficiency of about 90% in both pump and turbine modes for a wide range of operating conditions. Transient startup and shutdown sequences are extensively analysed with the objective of finding load gradient limiting sequences. It is uncovered that the transient sequences in pump mode are more severe than those in turbine mode. This is partly because reversed flow is encountered when the CRPT is not able to overcome the elevation difference between the reservoirs. Therefore, it is suggested that a valve needs to be part of the sequences to avoid reversed flow and control the change in flow rate. For an optimal pump mode startup, the runners need to initially speed up so that the CRPT precisely balance the reservoirs' elevation difference. In the remaining part of the sequence, the valve should open during about three-quarters of the sequence. The runner facing the lower reservoir should use most of the sequence to speed up, while the runner facing the upper reservoir should speed up in the final third of the sequence. For the pump mode shutdown, the valve should close before speeding down the runners, or the runners can speed down as the valve is almost closed. Corresponding sequences in turbine mode are also examined. The suggested startup sequence in turbine mode consists of an initial valve opening, shortly followed by the simultaneous speedup of the runners. The turbine mode shutdown, on the other hand, utilises a multi-stage valve closure as the runners are brought to a standstill. Cavitating flow simulations are carried out at stationary operating conditions in both pump and turbine modes to determine how cavitation impacts the performance of the CRPT. It is found that the pump mode is more sensitive to cavitation than the turbine mode. Nonetheless, irrespective of the mode, the presence of cavitation invariably leads to a degradation in the CRPT performance. This is because the cavitating region causes flow separation on the runner blades, which disturbs the efficient flow guidance in the blade passages.