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, rotating cavitation, and cavitation surge, as well as the characteristics of those instabilities, based on one [1] and two [13][14] dimensional stability analyses. 1.0 IMPELLER PERFORMANCE AND CAVITATION CHARACTERISTICS To be used for a one dimensional stability analysis of the instabilities, the impeller performance and the cavitation characteristics are modelized in this section. 1.1 Impeller Performance We consider a cascade as shown in Fig.1, rotating with the velocity of T U in a uniform axial velocity of U . The blade spacing h is assumed to be sufficiently small as compared with the blade length l and the flow in the cascade is perfectly guided by the blades. The axial and tangential velocity disturbances are represented by 1 u δ and 1 v δ , respectively, at the inlet, and the axial velocity disturbance 2 u δ at the outlet. The cavity of volume c V per blade appears at the inlet. The velocity triangle at the inlet is shown in Fig.1. Tsujimoto, Y. (2006) Flow Instabilities in Cavitating and Non-Cavitating Pumps. In Design and Analysis of High Speed Pumps (pp. 7-1 – 7-24). Educational Notes RTO-EN-AVT-143, Paper 7. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 01 NOV 2006 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Flow Instabilities in Cavitating and Non-Cavitating Pumps 5a. CONTRACT NUMBER