Flow instability transferability characteristics within a reversible pump turbine (RPT) under large guide vane opening (GVO)

Abstract Reversible pump turbines (RPT), though praised for their operational flexibility within pumped storage hydropower plants, suffer from large flow instabilities that take source from frequently imposed off-design operating conditions, where the vaneless space (VS) between the runner and guide vanes is claimed to be the base. This study therefore intends to investigate the VS flow instability development mechanism, and the effect of both the machine influx and runner blade number on its transferability to other flow zones. CFD-backed simulations are conducted on ten flow conditions spanning from turbine zone through runaway vicinities to turbine brake, using three runner models with 7, 8, and 9 blades respectively. Results have shown that, while VS pressure pulsation amplitude increased with the decreasing runner blades number, a continuous decrease of machine influx led to a correspondingly dropping VS pressure pulsation level within the Turbine zone before gradually increasing to Runaway and dropping again back to deep turbine brake operating zone. The effect of machine flow conditions and runner blade number on VS flow instability propagation mode to upstream flow zones is more remarkable than the downstream flow zones. This study contributes to a thorough understanding of RPT flow dynamics, especially under off-design operating conditions.