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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.

After a brief review of the fundamental aspects of cavitation relevant to the operation of high-performance inducers and turbopumps, the article summarizes their application to the analysis of pumping systems, illustrates the scaling of cavitation phenomena from model tests to full-scale operation, describes the occurrence of flow-induced instabilities in turbomachinery, and introduces the concepts of static and dynamic instability of pumping systems and their generalization to cavitating turbopump systems.

A reduced-order model for the preliminary design and performance prediction of radial turbopumps is illustrated. The model expresses the 3D, incompressible, inviscid, irrotational flow through helical blades with slow axial variations of their pitch and backsweep angles by superposing a 2D axial vorticity correction to a fully guided forced vortex flow with axisymmetric stagnation velocity in the meridional plane. Application of the relevant governing equations allows for the closed-form definition of the impeller geometry and flowfield in terms of a reduced number of controlling parameters. Mass and momentum conservations are used for coupling the flow leaving the impeller with the 2D reduced-order models of the flow in the diffuser and/or the volute, as well as for the evaluation of the mixing losses in the transfer between successive components of the machine. This information completes the geometric definition of the turbopump and determines its ideal noncavitating performance in accordance with the resulting flowfield. As a consequence of the neglect of viscous effects, the slip factor predicted by the present model exceeds those obtained from theoretical/semi-empirical formulas reported in literature for centrifugal pumps, but correctly captures their trend.

The article reviews the main forms of flow-induced instabilities detected in the liquid propellant turbopumps systems of modern rocket engines, with special reference to rotating stall, rotating cavitation, cavitation surge and higher order surge modes, illustrating their characteristics, origin and damage potential.

In the present paper, the results from an extensive experimental characterization of the rotordynamic forces acting on a whirling three-bladed, tapered-hub, variable-pitch inducer are presented. The campaign has been conducted in the Cavitating Pump Rotordynamic Test Facility (CPRTF) at ALTA S.p.A. The forces acting on the impeller have been measured by means of a rotating dynamometer mounted just behind the inducer. The roles of the rotor whirl motion, flow rate, cavitating condition, and liquid temperature have been investigated. The analysis has been conducted by means of the classical rotordynamic approach together with a recent experimental technique, consisting in measuring the rotordynamic forces continuous behavior (spectra) as functions of the whirl excitation frequency. This technique allows for evaluating information from experiments more rapidly and accurately with regards to previous methods. Therefore, it is useful to better capture the complexity of the rotordynamic forces and assess their consequences on the stability of axial inducers.

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.