Cristina Bramanti

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.

Development of hydrogen peroxide monopropellant rockets

Alta S.p.A. (Italy) and DELTACAT Ltd. (United Kingdom) are conducting a study, funded by the European Space Agency, into the development of hydrogen peroxide monopropellant thrusters using advanced catalytic beds. The present paper focuses on the design of two different demonstration thrusters with nominal ratings of 5 N and 25 N. Design requirements and specifications are presented, followed by the main results of a concept study, which was conducted to define the approximate dimensions needed. Some details about the specific design of the two prototypes and the choice of the main components are provided, with particular regard to the sensors and transducers to be used during the experimentation. Different catalytic bed configurations, including pure silver gauzes and pellets coated with manganese oxide or platinum, are going to be tested in the prototype thrusters, in order to find the optimum one for further industrial development. A dedicated test bench, designed and realized by Alta S.p.A. for tests on the thruster prototypes, is also illustrated.

Thermal Effects on Cavitation Instabilities in Helical Inducers

The main results of an experimental campaign conducted in the Cavitating Pump Rotordynamic Test Facility at Alta S.p.A. are presented. The experiments have been carried out on two different axial inducers (a three-bladed aluminum-made inducer of simple helical geometry and a prototype of the axial inducer of the Vulcain MK1 engine liquid oxygen turbopump) to characterize the instabilities affecting the pumps in a wide range of flow conditions; some experiments have also been carried out at higher temperatures to investigate the possible influence of the thermal cavitation effects on the observed phenomena. The transparent inlet section of the facility has been instrumented with several piezoelectric pressure transducers located at three axial stations: inducer inlet, outlet, and middle of the axial chord of the blades. For each axial station, at least two transducers were mounted at a given angular spacing to cross correlate their signals for coherence and phase analysis. The most interesting instabilities have been detected on the three-bladed inducer, including a cavitation surge, a rotating stall, and an auto-oscillation leading to a violent surge-mode instability. Some of these instabilities have also been found to be slightly affected by temperature. On the other hand, very few oscillating phenomena have been detected on the MK1 inducer, with a practically flat frequency spectrum at flow coefficients near its nominal operating point.

Experimental Characterization of Advanced Materials for the Catalytic Decomposition of Hydrogen Peroxide

Alta S.p.A. (Italy) and DELTACAT Ltd. (United Kingdom) are conducting a study, funded by the European Space Agency, into the development of hydrogen peroxide monopropellant thrusters using advanced catalytic beds. The present paper focuses on the design of a dedicated test bench to assess the effectiveness of different catalyst samples when immersed in hydrogen peroxide. To date, the apparatus has been used to study powdered silver and three oxides of manganese. The evaluation of an analytical technique for determining the observed decomposition rates is also presented. In addition to devising a quantitative method for assessing the efficacy of the candidate catalysts, some qualitative studies of proposed metallic catalysts, in the form of wires immersed in hydrogen peroxide, are also described. Of the catalyst materials tested so far, preliminary results suggest that dimanganese trioxide offers slightly better performance than both manganese dioxide and silver.

Setup of a high-speed optical system for the characterization of flow instabilities generated by cavitation

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 14.7 Hz, which corresponds to the frequency of the rotating stall instability detected by means of pressure oscillation analysis.

Initial Experiments on a Dual-Stage 4-Grid Ion Thruster for Very High Specific Impulse and Power

A new and innovative type of gridded ion thruster, the “Dual-Stage 4-Grid” or DS4G concept, has been proven under laboratory conditions under a preliminary research, development and test programme. The DS4G concept is able to operate at very high specific impulse, power and thrust density values well in excess of conventional 3-grid ion thrusters at the expense of a higher power-to-thrust ratio. A small low-power experimental laboratory model was designed and built, and its performance was measured during an extensive test campaign. The principal goals of the initial laboratory experiment were to prove the practical feasibility of the overall concept, demonstrate the performance predicted by analytical and simulation models, and investigate critical design parameters and technological challenges. In the present paper, the basic concept of the DS4G ion thruster is presented, along with the design, operating parameters and measured performance obtained from the first and second phases of the experimental campaign. Finally, the implications of these findings for future gridded ion thruster developments and future mission applications are addressed.

A Simplified Analytical Model for Evaluating the Noncavitating Performance of Axial Inducers

‡The present paper describes an analytical model for the preliminary prediction of the noncavitating flow field and performance of helical inducers. The proposed model is based on the traditional troughflow theory approximations with empirical corrections for outlet flow deviation and hydraulic losses due to inlet incidence effects and friction in the blade channels. Unlike most classical models, it allows ‐ even if under still rather restrictive assumptions ‐ for the prediction of the radial and circumferential flow velocity fields at the inducer exit section and for the approximate evaluation of the head coefficient as a function of the flow coefficient in terms of the static pressure rise generated by the inducer. The results are presented of the model validation by comparison with the experimental data obtained for several inducers tested in different facilities worldwide.

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.

Characterization of Cavitation Instabilities in Axial Inducers by Means of High-Speed Movies

The present paper illustrates the set-up 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 CPRTF (Cavitating Pump Rotordynamic Test Facility) at Alta S.p.A. The brightness thresholding technique adopted for cavitation recognition is described. A semi-automatic binary algorithm has been used for processing high speed videos of the cavitating inducer. 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. Fourier analysis showed the occurrence of a cavity length oscillation at a frequency of 15.7 Hz, which corresponds to the frequency of the rotating stall instability previously detected on the same inducer by means of pressure oscillations analysis.

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