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International audience; The paper presents an experimental campaign aimed at the characterization of the relationship between cavitation-induced instabilities and forces acting on the shaft relevant to space application turbopumps. The experiments have been carried-out on a 6-bladed, unshrouded centrifugal turbopump. The described apparatus allows for contemporary measurements of the flow instabilities and the force exchanged between the impeller and the shaft. Pressure fluctuations are frequency analyzed allowing for understanding the instability nature (axial, rotating) and their main characteristics (e.g. amplitude, rotating direction). The frequency content of the force components highlights a strong relationship of the z-component (along the rotating axis) with axial instabilities. On the other hand, rotating cavitation may involve force oscillations along all the three components leading to unwanted and dangerous fluctuating unbalances perpendicular to the rotating axis.

The experimental characterization of the rotordynamic fluid forces acting on a whirling centrifugal impeller has been investigated at different flow rates and cavitating conditions. The recently developed method for continuously measuring the rotordynamic forces at variable whirl ratios has been readapted and successfully applied for measuring the same forces at constant whirl ratio but variable cavitation number. The flowrate has a major influence on the stability of the rotordynamic forces at positive whirl ratios where a threshold flowrate separates the stable zone from the unstable. At negative whirl ratios, the normal force is typically unstable independently on the flow rate. Cavitation has always a destabilizing effect at positive whirl ratio while it can stabilize the rotordynamic forces at negative whirl ratio.

The paper illustrates the results of an experimental campaign conducted in the CPRTF (Cavitating Pump Rotordynamic Test Facility) at ALTA S.p.A., aimed at characterizing the rotordynamic forces acting on a whirling four-bladed, tapered-hub, variable-pitch inducer, designated as DAPAMITO4. The roles of the imposed whirl motion of the rotor, flow coefficient, cavitation number and liquid temperature have been investigated. A novel experimental technique, consisting in measuring the continuous spectra of the forces as functions of the whirl ratio, has been developed and validated. This technique gives the possibility of extracting valuable information from the experiments by clearly identifying the qualitative and quantitative behavior of the forces, and is therefore useful to catch the unlikely foreseeable complexity of the rotordynamic forces and their consequences on the stability of axial inducers.Copyright © 2011 by ASME

The present paper illustrates the results of an experimental campaign conducted in the CPRTF (Cavitating Pump Rotordynamic Test Facility) at ALTA S.p.A. aimed at characterizing the rotordynamic forces acting on a whirling four-bladed, tapered-hub, variable-pitch inducer, designated as DAPAMITO4. The forces acting on the impeller have been measured by means of a rotating dynamometer mounted just behind the inducer. The roles of the imposed whirl motion of the rotor, flow coefficient, cavitation number and liquid temperature have been investigated. The destabilizing role of cavitation has been confirmed. The experimental results are consistent with previous findings obtained by the authors, as well as with former data published by Caltech researchers. The observed dependence of the tangential and normal components of the rotordynamic force on the whirl-to-rotational speed ratio does not follow the quadratic functional behavior often assumed in the open literature. Rotordynamic forces of large amplitude and destabilizing nature especially occur in the presence of cavitation, potentially compromising the stability of the pump operation.