• whirl
• cavitating turbopumps
• inducer
• whirl frequency ratio
• rotordynamic forces
• rotordynamic moments

The present work illustrates the main results of an experimental campaign conducted in the Cavitating Pump Rotordynamic Test Facility (CPRTF) at Alta S.p.A., Pisa, on a three bladed, tapered-hub, variable-pitch inducer, named DAPROT3, and on a mixed flow pump, named VAMPDAP. The latter consists of the combination between the DAPROT3 inducer and a centrifugal pump placed downstream, called VAMPIRE. As a consequence of a displacement of the rotation axis, fluid induced rotordynamic forces arise, leading to a secondary motion of the shaft axis here referred as whirl motion. This precession of the shaft can be both unstable or stable depending on the direction of the rotordynamic force and the whirl frequency ratio. The combined effects of rotordynamic fluid forces and cavitation represent the dominant fluid mechanical phenomena that adversely affects the dynamic stability and pumping performance of turbopumps. In the experimental campaign, the rotordynamic forces and the hydraulic efficiency have been measured both in noncavitating and cavitating conditions, at different flow rates and temperatures, by means of a rotating dynamometer placed just behind the inducer. Two different methods have been exploited for the evaluation of fluid-induced rotordynamic forces. Indeed, in addition to the classic approach of discrete tests, a series of continuous tests have been performed in order to obtain a continuous spectra, allowing the possibility of accurately and unambiguously identifying the spectral minima and maxima, which are of special interest in the identification of the most dangerous operational condition of the test item. It has been found that the DAPROT3 inducer presents a different behavior of rotordynamic forces with respect to the typical trend observed in centrifugal pump impellers in presence of a whirl motion concordant with impeller rotation, according to previous experimental campaigns. It has been observed also that cavitation does not affect the rotordynamic forces in a significant way. Nevertheless particular situations may arise in which a certain level of cavitation is capable to alter the stability regions and in which the presence of a cold or hot flow may play an important role. Conversely the flow rate has a great influence and its variation may destabilize a pump that is stable at design condition. An increasing deviation has been obtained for decreasing flow rate as a consequence of the interaction between rotating backflow and whirl motion. The effect of cavitation on hydraulic efficiency has been found to be negligible with variations within 1%. On the other hand, the presence of rotordynamic forces may affect the efficiency up to 2% depending on the whirl speed both for cold and hot flow conditions. For what concerns the VAMPDAP pump, a deviation from the typical trend of fluid-induced rotordynamic forces has been observed as a consequence of the presence of the inducer placed upstream with respect to a radial pump, whose experimental diagrams respect the expectations. The effect of cavitation on this pump is relevant only at high flow rates, where for discordant whirl motion a decrease in rotordynamic force appears, whereas in other operating conditions it can be considered almost negligible. The experimental campaign has included the evaluation of thermal cavitation effect by performing tests at two different temperatures also for VAMPDAP pump. From the results it has been observed that no significant thermal effects are found in terms of rotordynamic forces. Moreover, the hydraulic efficiency is not affected from cavitation since the maximum variations are in the order of 0.9 % only at design and even lower at off-design conditions. At the same time the rotordynamic forces influence the efficiency in the order of 1% depending on whirl speed.