Emilio Rapposelli

Thermal Cavitation Experiments on a NACA 0015 Hydrofoil

The present paper illustrates the main results of an experimental campaign conducted in the Thermal Cavitation Tunnel of the Cavitating Pump Rotordynamic Test Facility (CPRTF) at Centrospazio/Alta S.p.A. Experiments were carried out on a NACA 0015 hydrofoil at various incidence angles, cavitation numbers, and freestream temperatures. in order to investigate the characteristics of cavitation instabilities and the impact of thermal cavitation effects. Measured cavity length, surface pressure coefficients, and unsteady pressure spectra are in good agreement with the data available in the open literature and suggest the existence of a strong correlation between the onset of the various forms of cavitation and instabilities, the thermal cavitation effects, and the effects induced by the presence of the walls of the tunnel. Further analytical investigations are planned in order to provide a better interpretation of the above results.

Experimental Characterization of the Cavitation Instabilities in the Avio “FAST2” Inducer

The present paper illustrates the main results of an experimental campaign conducted using the CPRTF (Cavitating Pump Rotordynamic Test Facility) at Alta S.p.A. The tests were carried out on the FAST2 inducer, a two-bladed axial pump designed and manufactured by Avio S.p.A. 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 a given 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 having a frequency of about 4.4Ω and a rotating stall at about 0.31Ω Ω Ω Ω. 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.

Two-Phase Flow and Inertial Effects on the Rotordynamic Forces in Whirling Journal Bearings

This paper presents the application of the isenthalpic two-phase flow approximation (Brennen, 1995) to the study of cavitation and ventilation effects in plane journal bearings with whirling eccentricity. A quasi-homogeneous bubbly liquid/vapor model, suitably modified to account for thermal effects (Rapposelli and d’Agostino, 2001), is used to describe the occurrence of flow cavitation. An homogeneous liquid/gas/vapor model without thermal effects is used to describe the simultaneous occurrence of cavitation and ventilation. The proposed model treats the fully-wetted and two-phase portions of the fluid in a unified manner in order to avoid the use of “ad hoc” matching conditions, whose applicability and accuracy is questionable in the presence of significant inertial and/or unsteady effects. A non-linear analysis that accounts for the inertia of the lubricant is used to determine the reaction forces caused by the shaft’s eccentric motion both in the viscosity-dominated regime and at intermediate values of the Reynolds number, where the inertia of the lubricant is no longer negligible. The classical iteration method for the Reynolds lubrication equation (Muster and Sternlicht, 1965; Mori and Mori, 1991; Reinhardt and Lund, 1975) has been extended to the unsteady two-phase flow case in order to account for flow acceleration effects in the presence of cavitation and/or ventilation. Significant deviations from the steady-state case are obtained at moderately high Reynolds numbers (Re = ωRc/νL ≅ 10). Comparisons with the scant experimental data support the validity of the proposed model. Results are shown in a number of representative cases in order to illustrate the influence of the relevant parameters.Copyright

CAVITATION EXPERIMENTS ON TURBOPUMP INDUCERS AND HYDROFOILS AT ALTA/CENTROSPAZIO: OVERVIEW AND FUTURE ACTIVITIES

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

Cavitation and Rotordynamic Activities at Centrospazio

Centrospazio has long been carrying out an articulated program of experimental, theoretical and numerical research on cavitation and two-phase flow dynamics in connection with liquid propellant rocket fuel feed systems. The areas of involvement dealt with in recent years range from the modeling and simulation of cavitation to the study of rotordynamic fluid forces in whirling and cavitating axial inducers and journal bearings. Specifically, this paper illustrates the development of a new cavitation model accounting in an approximate but physical way for the occurrence of thermal cavitation and liquid quality effects, and the realization of the CPRTF (Cavitating Pump Rotordynamic Test Facility), a water loop for the measurement of rotordynamic fluid forces on whirling and cavitating turbopump impellers, and the TCT (Thermal Cavitation Tunnel), a modified version of the CPRTF for cavitation experiments in fluid dynamic and thermal cavitation similarity. Representative results of the application of cavitation model to journal bearings, hydrofoils and helical inducers and the typical performance of the CPRTF and TCT are presented.