Flow Instabilities in Cavitating and Non-Cavitating Pumps

It is well known that flow instabilities called rotating stall and surge may occur in non-cavitating turbomachines at flow rates smaller than design. Rotating stall is a local instability at the turbomachinery which is basically not dependent on the hydraulic system in which the turbomachine is installed. The stalled region rotates faster than impeller. Surge is a system instability in a hydraulic system which includes a turbomachinery and a capacitance (tank) which stores the working fluid depending on the pressure at the capacitance. For pumps, if a certain quantity of air is trapped in the pipeline it serves as a capacitance and a surge may occur even if the pipeline does not include exernalt capacitance. Both rotating stall and surge occur at smaller flow rates where the performance curve has a positive slope. On the other hand, cavitation instabilities called rotating cavitation and cavitation surge may occur even at the design flow rate. Rotating cavitation is a local instability in which the cavitated region rotates, for the most cases, faster than impeller. Cavitation surge is a system instability caused by cavitation. For cavitation surge, the cavitation at the inlet of turbomachinery serves as a capacitance and it can occur in a system without any external capacitance. The present lecture is intended to explain the mechanisms of the instabilities, rotating stall, surge, rotatin