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[EN] In typical liquid rocket engines, turbopumps used to feed the propellant are crucial components: the presence of flow instabilities is the most stringent limitation in the suction performance, power density and operational life of these machines. This work represents the extension of the Bayesian estimation method applied to effectively detect and characterize backflow vortex instability successfully designed and tested by Prof. Luca d’Agostino and his collaborators. The method was used for the quantitative comparison of the backflow vortex instabilities detected in several 3- and 4-bladed inducers varying load, tip clearance and different cavitating regimes. The results are consistent with the reported data in common literature, that is particularly scant, confirming these models can be helpful in efficiently addressing the preliminary parametric optimization of rocket engine inducer design., [ES] En los motores líquidos de cohetes típicos, las turbobombas utilizadas para alimentar el propulsor son componentes cruciales: la presencia de inestabilidades de flujo es la limitación más estricta en el rendimiento de succión, la densidad de potencia y la vida operativa de estas máquinas. Este trabajo representa la extensión del método de estimación bayesiano aplicado para detectar y caracterizar eficazmente la inestabilidad del vórtice de reflujo, diseñado y probado con éxito por el Prof. Luca d'Agostino y sus colaboradores. El método se usó para la comparación cuantitativa de las inestabilidades de vórtice de reflujo detectadas en varios inductores de 3 y 4 palas que varían la carga, la holgura de la punta y diferentes regímenes de cavitación. Los resultados son consistentes con los datos de la literatura común que confirma que estos modelos pueden ser útiles para abordar de manera eficiente la optimización paramétrica preliminar del diseño del inductor del motor de cohete., [CA] En els motors de coets líquids típics, les turbobombes que s'utilitzen per alimentar el propulsor són components crucials: la presència d'inestabilitats de flux és la limitació més estricta en el rendiment d'aspiració, la densitat de potència i la vida operativa d'aquestes màquines. Aquest treball representa l'extensió del mètode d'estimació bayesià aplicat per detectar i caracteritzar eficaçment la inestabilitat del vòrtex de retorn dissenyat i provat amb èxit pel Prof. Luca d'Agostino i els seus col·laboradors. El mètode es va utilitzar per a la comparació quantitativa de les inestabilitats del vòrtex de retrocés detectades en diversos inductors de 3 i 4 fulles que varien la càrrega, la separació de la punta i els diferents règims de cavitat. Els resultats són coherents amb les dades informades a la literatura comuna que confirma que aquests models poden ser útils per abordar de manera eficient l'optimització paramètrica preliminar del disseny de l’inductor del motor de coets.

This paper reports the experimental characterization of the cavitation-induced flow instabilities of a high head three-bladed inducer at design condition detected simultaneously by means of piezoelectric pressure transducers located at different axial and azimuthal stations on the casing of the pump and strain gages mounted on the pressure side of each blade. The simultaneous analysis performed in the stationary and rotating frames, supported by high-speed movies, suggests that the mechanism of formation of the detected subsynchronous rotating cavitation resembles that of modal stall in compressors, being the cavitation region the source of compliance. In fact, at decreasing cavitation number, a strong attached cavitation, developed preferentially on one blade and capable of completely surrounding and unloading the following blade, starts to be destabilized as consequence of the progressive intensification of a modal oscillation. The developed complex instability, consisting in a strong oscillation of the attached cavitation, is detected in both the frames as the simultaneous presence of a subsynchronous rotating cavitation and a cavitation surge.

International audience; The paper describes results of recent experiments carried out under similarity conditions in the Cavitating Pump Rotordynamic Test Facility for the dynamic characterization of cavitation-induced flow instabilities as simultaneously observed in the stationary and rotating frames of a high-head, three-bladed axial inducer with tapered hub and variable pitch. The flow instabilities occurring in the eye and inside the blading of the inducer have been detected, identified and monitored by means of the spectral analysis of the pressure measurements simultaneously generated in the absolute and relative frames by multiple transducers mounted on the casing near the impeller eye and on the inducer hub along the blade channels. The comparison of the results obtained in the two reference frames over a wide range of flow coefficients and cavitation numbers contributed to shed some light on the complex dynamic interactions between the cavitation-induced instabilities in the eye and in the blade channels of typical high-head inducers.

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.

A high-head three-bladed inducer has been equipped with pressure taps on the hub along the blade channels with the aim of more closely investigating the dynamics of cavitation-induced instabilities developing in the impeller flow. Spectral analysis of the pressure signals obtained from two sets of transducers mounted both in the stationary and rotating frames has allowed to characterize the nature, intensity, and interactions of the main flow instabilities detected in the experiments: subsynchronous rotating cavitation (RC), cavitation surge (CS), and a high-order axial surge oscillation. A dynamic model of the unsteady flow in the blade channels has been developed based on experimental data and on suitable descriptions of the mean flow and the oscillations of the cavitating volume. The model has been used for estimating at the inducer operating conditions of interest the intensity of the flow oscillations associated with the occurrence of the CS mode generated by RC in the inducer inlet.

The present paper illustrates the main results of an experimental campaign conducted in the Cavitating Pump Rotordynamic Test Facility. The tests were carried out on the FAST2 inducer, a two-bladed axial pump designed and manufactured using the criteria followed for VINCI180 inducer. The transparent inlet section of the facility was instrumented by several piezoelectric pressure transducers located at three axial stations: inducer inlet, outlet and at the middle of the axial chord of the blades. For each axial station at least two transducers were mounted at selected angular spacing, in order to cross correlate their signals for coherence and phase analysis. The most interesting detected instabilities were: a cavitation auto-oscillation at about 5-12 Hz, a high-order cavitation surge at a frequency of about 4.4Ω and a rotating stall at about 0.31Q. Some experiments were carried out under forced vibration conditions: it was observed that a whirl frequency of about 1 Hz can provide excitation for violent surge-mode oscillations. A "cavitation surge" instability was also observed at higher whirl frequencies.

The present paper illustrates and discusses the first results of the characterization of the dynamic transfer matrix of cavitating turbopumps by means of a series of forced-vibration tests carried out on a high-head three-bladed inducer in the Cavitating Pump Rotordynamic Test Facility (CPRTF) at Alta, Pisa, Italy. The flow has been excited by vertically vibrating with adjustable frequency and intensity the free-surface tank used to close the loop of the suction and discharge lines of the inducer. The pressure and flow rate fluctuations for the determination of the dynamic transfer matrix have been obtained from unsteady pressure measurements at suitable locations on the inlet/outlet lines of the inducer. Two loop configurations with different inlet/outlet impedances to/from the test inducer have been used in order to obtain at each excitation frequency a set of four linearly independent measurements for the detetermination of the four elements of the dynamic transfer matrix. The preliminary results are in qualitative agreement with the scant data available in the literature for comparable inducers.

The paper describes a reduced-order analytical model for the characterization of the dynamic transfer matrix of complex test setups including cavitating pumps. The model, even if based on several simplifying assumptions (quasi 1-dimensional flow, small oscillations, incompressible working fluid, quasi-static response of all the components of the system), is able of providing good indications about the order of magnitude of the expected pressure and flow rate oscillations in the system under given flow conditions and, more in general, about the experiment design. The model has been applied to Alta’s Cavitating Pump Rotordynamic Test Facility with the custom-designed DAPAMITO3 axial inducer, in order to start the design process of an experiment for the characterization of the inducer dynamic matrix. It has been found that a good mechanism for providing an external excitation to the facility can be represented by a device able of mechanically vibrating the water tank in a vertical direction, while the most suitable way for obtaining the second linearly independent test configuration, needed for the experimental characterization of the cavitating pump dynamic matrix, is represented by a variation of the suction line inertance. Finally, it has been shown that it is possible to measure the flow rate oscillations in the suction and discharge lines by means of the difference between the measurements taken by two pressure transducers, placed at two different sections of the relevant pipe line.

The paper introduces and briefly outlines the research activities on cavitation and turbopump hydrodynamics conducted in the last 20 years at the Department of Aerospace Engineering of Pisa University, at Centrospazio, a nonprofit space propulsion laboratory established in 1989, and at Alta S.p.A., a private spin-off company founded in 2000 and active in the aerospace sector. Both the experimental activities conducted in a dedicated facility called Cavitating Pump Rotordynamic Test Facility (CPRTF) and the analytical/numerical work is described. A valuable experience has been gained by the research group throughout the years, and important results have been obtained in the definition of experimental techniques for the detection of flow instabilities, the understanding of thermal cavitation effects on pump performance and instabilities, and the development of reduced order analytical tools for the preliminary design and performance prediction of axial inducers.

The present paper illustrates the setup and the preliminary results of an experimental investigation of cavitation flow instabilities carried out by means of a high-speed camera on a three-bladed inducer in the cavitating pump rotordynamic test facility (CPRTF) at Alta S.p.A. The brightness thresholding technique adopted for cavitation recognition is described and implemented in a semi-automatic algorithm. In order to test the capabilities of the algorithm, the mean frontal cavitating area has been computed under different operating conditions. The tip cavity length has also been evaluated as a function of time. Inlet pressure signal and video acquisitions have been synchronized in order to analyze possible cavitation fluid-dynamic instabilities both optically and by means of pressure fluctuation analysis. Fourier analysis showed the occurrence of a cavity length oscillation at a frequency of 14.7Hz, which corresponds to the frequency of the rotating stall instability detected by means of pressure oscillation analysis.

The aim of the present paper is to provide some highlights about the most interesting experimental activities carried out during the years 2000–2004 through the CPRTF (Cavitating Pump Rotordynamic Test Facility) at Centrospazio/Alta S.p.A. After a brief description of the facility, the experimental activities carried out on a NACA 0015 hydrofoil for the characterization of the pressure coefficient on the suction side and evaluation the cavity length and oscillations are presented. Then, the results obtained to characterize the performance and the cavitation instabilities on three different axial inducers are showed: in particular, a commercial three-bladed inducer, the four-bladed inducer installed in the LOX turbopump of the Ariane Vulcain MK1 rocket engine and the “FAST2”, a two-bladed one manufactured by Avio S.p.A. using the criteria followed for the VINCI180 LOX inducer. The most interesting results are related to the effects of the temperature on the cavitation instabilities on hydrofoils and inducers. Experiments showed that some instabilities, like the cloud cavitation on hydrofoils and the surge on inducers, are strongly affected by the temperature, while others seem not to be influenced by the thermal effects. In the final part of this paper, some indications of the main experimental activities scheduled for the next future are provided.Copyright © 2005 by ASME