Cavitation Instabilities in Turbopump Inducers

A set of 4-bladed inducers with various amounts of cutback was tested with the aim of suppressing the rotating cavitation by applying alternate leading edge cutback. Unsteady cavitation patterns were observed by means of inlet pressure measurements and high-speed video pictures. It was found that the region with the alternate blade cavitation and asymmetric cavitation were enlarged with the increase of the amount of the cutback. As a result, the region with the rotating cavitation was diminished. At low flow rate, two types of alternate blade cavitation were found as predicted theoretically on 4-bladed inducer with smaller uneven blade length. One of them is with longer cavities on longer blades, and the other is with longer cavities on shorter blades. Switch was observed in these alternate blade cavitation patterns depending whether the cavitation number was increased or decreased. For an inducer with larger amount of cutback, the rotating cavitation and cavitation surge were almost suppressed as expected for a wide range of flow rate and cavitation number, although the cavitation performance was deteriorated. However, we should note that an asymmetric cavitation pattern occurs more easily in inducers with alternate leading edge cutback, and that the unevenness due to the cutback causes uneven blade stress.

Conventional signal decomposition of raw flow visualizations with a limited field of view fails to accurately identify cavitation instabilities in turbopump inducers. Therefore, a new deep learning-based framework is proposed to directly analyze visualization data and identify such instabilities. The framework employs a hybrid convolutional-recurrent neural network to learn the spatio-temporal features of instability modes. The trained model successfully captures alternate blade cavitation in a two-bladed inducer and super-synchronous rotating cavitation in a three-bladed inducer. Notably, the approach requires neither pre-processing of the visualization data nor extensive measurement durations, as only a few shaft revolutions are sufficient for accurate identification. By eliminating the dependence on unsteady pressure transducers, the proposed framework provides a robust tool for identifying turbopump inducer cavitation instabilities and can be extended to other systems where low-order modal patterns are dominant.

The thermodynamic effect on a cavitating inducer is investigated from joint experiments in cold water and Refrigerant 114. The analysis is focused on leading edge cavitation and cavitation instabilities, especially on alternate blade cavitation and supersynchronous rotating cavitation. The cavity length along cylindrical cuts at different radii between the hub and casing is analyzed with respect to the local cavitation number and angle of attack. The similarity in shape of the cavity closure line between water and R114 is examined and deviation caused by thermodynamic effect is clarified. The influence of rotation speed on cavity length is investigated in both fluids and analyzed on the basis of a comparison of characteristic times, namely, the transit time and a thermal time. Thermodynamic delay in the development of leading edge cavities is determined and temperature depressions within the cavities are estimated. Thresholds for the onset of cavitation instabilities are determined for both fluids. The occurrence of cavitation instabilities is discussed with respect to the extent of leading edge cavitation. The thermodynamic delay affecting the occurrence of cavitation instabilities is estimated and compared with the delay on cavity development.

Cavitation instabilities in hydraulic machines, hydro turbines and turbopump inducers, are reviewed focusing on the cause of instabilities. One-dimensional model of hydro turbine system shows that the overload surge is caused by the diffuser effect of the draft tube. Experiments show that this effect also causes the surge mode oscillations at part load. One dimensional model of a cavitating turbopump inducer shows that the mass flow gain factor, representing the cavity volume increase caused by the incidence angle increase is the cause of cavitation surge and rotating cavitation. Two dimensional model of a cavitating turbopump inducer shows that various modes of cavitation instabilities start to occur when the cavity length becomes about 65% of the blade spacing. This is caused by the interaction of the local flow near the cavity trailing edge with the leading edge of the next blade. It was shown by a 3D CFD that this is true also for real cases with tip cavitation. In all cases, it was shown that cavitation instabilities are caused by the fundamental characteristics of cavities that the cavity volume increases with the decrease of ambient pressure or the increase of the incidence angle.

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.

Temperature effects on the critical cavitation number and rotating cavitation in a turbopump inducer have been experimentally investigated in water. Static pressures upstream and downstream of the inducer have been measured to determine the cavitation performance, and cavitation instabilities have been detected using unsteady pressure sensors and a high-speed camera. Two kinds of cavitation instabilities have been identified—rotating cavitation and asymmetric attached cavitation. To quantify temperature effects, nondimensional thermal parameter has been adopted. Increasing water temperature, or increasing nondimensional thermal parameter, lowers the critical cavitation number. Increasing nondimensional thermal parameter also shifts the onset of rotating cavitation to a lower cavitation number and reduces the intensity of rotating cavitation. However, for values larger than 0.540 (340 K, 5000 rpm), the critical cavitation number and the rotating cavitation onset cavitation number become independent of the nondimensional thermal parameter. The onset of the head coefficient degradation correlates with the onset of rotating cavitation regardless of temperature.

An analysis of steady cavitation on flat plate cascade is performed by a singularity method based on closed cavity model. As a result, it is found that two kinds of the steady cavitation patterns exist. One is equal length cavitation in which the cavity lengths of all blades are the same. The other is alternate blade cavitation in which the cavity length changes alternately from blade to blade. Although the present model fails to predict the range of cavitation number where alternate blade cavitation occurs, it predicts alternate blade cavitation in terms of cavity length fairly well. The static pressure losses in both the equal length cavitation and the alternate blade cavitation are estimated. A parameter study is carried out to educe the general characteristics of alternate blade cavitation.

Alternate blade cavitation, rotating cavitation and cavitation surge in rocket turbopump inducers were simulated by a three dimensional commercial CFD code. In order to clarify the cause of cavitation instabilities, the velocity disturbance caused by cavitation was obtained by subtracting the velocity vector under non-cavitating condition from that under cavitating condition. It was found that there exists a disturbance flow towards the trailing edge of the tip cavity. This flow has an axial flow component towards downstream which reduces the incidence angle to the next blade. It was found that all of the cavitation instabilities start to occur when this flow starts to interact with the leading edge of the next blade. The existence of the disturbance flow was validated by experiments.

Numerical analysis of unsteady cavitating flow in an axial inducer

The seady flow and stability analyses of cavitation in fiat plate cascades corresponding to 2, 3, 4 and 5 bladed impeller were carried out to find out all of the steady cavity patterns and destabilizing modes. For this purpose each blade is treated independently. In steady flow analysis the alternate blade cavitation was found only for impellers with even number of blades. It was shown that the alternate blade cavitation occurs since the local flow near the cavity closure reduces the angle of attack to the opposing blade when the cavity closure point approaches the leading edge of the opposing blade. For 2 or 4 bladed impeller, it was confirmed that there exists no additional destabilizing mode to those found in an analysis assuming inter-blade phase difference. It was shown that the modes with total cavity volume fluctuation depend on the inlet duct length while the modes without total cavity volume fluctuation are independent on the system.

Cavitation dynamics, such as cavitation surge and rotating cavitation, are natural performance limits for turbopump inducers and pose a significant risk to the development and operation of liquid rockets. In the last 63 years, progress has been made in understanding cavitation instabilities and their relationship to cavitation number, inlet flow coefficient, rotating speed, inducer size, and thermal effects. However, we are still unable to predict the cavitation behavior of a real inducer installed in rocket engines or contribute much to the design of a more cavitation-resistant machine. This paper comprehensively reviews findings from computational fluid dynamics simulations, reduced-order modeling, and experiments in cryogenic fluids and water, focusing on the effects of compressibility on multiphase flow calculations, the assessment of cavitation compliance and mass flow gain factor, and their influences on cavitation stability analysis, as well as the transfer function for cavitating inducers. This review will provide insights for future research to achieve more accurate performance predictions, improve flow modeling, and enhance the interpretation of experimental results.

Identification of inducer cavitation instabilities using high-speed visualization

An experimental study is conducted at Mach number 1.71 on rectangular open cavity of length-to-depth ratio () 2 with subcavity at the front wall. Subcavity at the front wall has been established as a passive control device. In the current study it is found that subcavity at the front wall can also act as a passive resonator. The dominance of fluid-dynamic or fluid-resonant oscillations accordingly causes the subcavity at the front wall to act as a passive control device or as a passive resonator in supersonic cavity flow. In the current study the presence of the fluid-dynamic and fluid-resonant cavity oscillations is clearly brought out and the inherent differences between the two types of oscillations are also highlighted. Fluid-resonant cavity oscillations are not often observed experimentally in most of the experimental works related to supersonic cavity flow. A total of six cavity models with different subcavity lengths are investigated and are also designated by the ratio of subcavity length to main cavity length (, 0.10, 0.20, 0.25, 0.30, and 0.40). High-speed schlieren flow visualization and unsteady pressure measurements are employed to gain further insight into the flow physics. High-speed schlieren flow visualization clearly indicates the presence of six different types of waves and five different flow features that are associated with the cavity flow field. Statistical analysis techniques, namely, fast Fourier transform, spectrogram, correlation, and coherence, are employed for analyzing the unsteady pressure data. Fluid-dynamic-type cavity oscillations are dominant in , 0.10 and 0.20 cavities. Pressure oscillations inside the cavity are suppressed as is increased from 0.00 to 0.20. As is increased from 0.20 to 0.25, the type of cavity oscillations changes from fluid-dynamic to fluid-resonant. This change in the type of cavity oscillations results in increased pressure oscillations inside the cavity for , 0.30, and 0.40. Distinct cavity tones at certain frequencies in power spectrum plots are classified into fluid-dynamic mode and fluid-resonant mode. Modal frequencies of fluid-resonant modes differ widely from those predicted by modified Rossiter formula. Correlation analysis suggests the presence of fluid-resonant behavior in , 0.30, and 0.40 cavities. With reference to cavity, there is maximum reduction of 30.2 dB in sound pressure level for the second fluid-dynamic mode, and there is also 4.4 dB reduction in overall sound pressure level for the cavity.

The influence of inducer blades’ geometry on the propagation of cavitation instabilities was examined using three and four-bladed inducer, to develop the design criteria that can operate stably over a wide operation range. In industrial centrifugal pumps used for transporting liquefied natural gas at low flow rates and low NPSH, the self-excited oscillations caused by cavitation have been observed. In previous studies, both the effect of the number of inducer blades on pump performance and the effect of the blades’ outlet angle on cavitation instability phenomena were investigated. The main objective of this study was to investigate the effect of the configuration of blade number and outlet angle on the suppression of cavitation instability phenomena at low flow rates.

Oscillation types, multistability, and basins of attractors in symmetrically coupled period-doubling systems