  Reversible pump turbines are praised for their operational flexibility
leading to their recent wide adoption within pumped storage hydropower plants.
However, frequently imposed off-design operating conditions in these plants
give rise to large flow instability within RPT flow zones, where the vaneless
space (VS) between the runner and guide vanes is claimed to be the base. Recent
studies have pointed out the possibility of these instabilities stretching to
other flow zones causing more losses and subsequent machine operational
performance degradation. This study therefore intends to investigate the VS
flow instability, its propagation characteristics, and the effect of machine
influx and runner blade number on the same. CFD-backed simulations are
conducted on ten flow conditions spanning from turbine zone through runaway
vicinities to turbine brake (OC1 to OC15), using three runner models with
different blades (7BL, 8BL, and 9BL). While VS pressure pulsation amplitudes
increased with runner blades number decrease, the continuously decreasing flow
led to gradual VS pressure pulsation level drop within the Turbine zone before
increasing to Runaway and dropping back to deep turbine brake zone. The effect
of the same parameters on the transmission mode to VS upstream flow zones is
more remarkable than the downstream flow zones.
