Investigation On Flow Instabilities And Cavitation Characteristics Of The Turbopump And Inducer

This work is supported by the State Key Program of National Natural Science Funds of China "Cavitation Characteristics and its Countermeasures of Hydraulic Machinery"（Grant No.51239005）, National Natural Science Funds of China "Low Frequency Characteristics Induced by Inlet Deformation Within Centrifugal Pumps"（Grant No.51349004）, and Jiangsu Provincial Project for Innovative Postgraduates "The Mechanism and Characteristics of Low-Frequency Cavitation in the Centrifugal Pump"（Grant No. CXLX11₀577）.As one of the critical components of the liquid propellant rocket engines, the turbopump should be capable of meeting the requirement of operating under high pressure and temperature condition. As the development of aerospace technology, the turbopump with high efficiency and cavitation performance becomes one of the key impacts of the space rocket transportation system. Current rocket turbopump often employs an inducer upstream to improve the suction performances. The main role of an inducer is to pressurize the flow sufficiently to increase the head as well as the inlet pressure of the impeller of the turbopump to avoid unacceptable cavitation and flow instabilities. Therefore, the hydraulic performances, cavitation characteristics associated with the flow instabilities in the inducer and the turbopump were studied based on CFD and experimental methods in this dissertation. The main work and creative achievements are:1. The present research work both at home and abroad on the rocket engine inducer and turbopump has been systematically reviewed, including the type of inducer, the internal flow theory as well as the onset and the characteristics of flow instabilities, for example, rotating stall, rotating chock, surge, rotating cavitation, asymmetric blade cavitation, backflow vortex cavitation and high-order rotating instabilities.2. The main test facilities, test items in the CPRTF （Cavitating Pump Rotordynamic Test Facility） at Alta, Pisa, Italy, as well as the corresponding experimental procedures such as hydraulic performances, cavitation performances and pressure fluctuation were systematically overviewed. Meanwhile, the basic theory and approaches for numerically simulating the unsteady flow and cavitating flow in the turbopump and inducer has been also briefly introduced. 3. The numerical approach of predicting the internal flow and the hydraulic performances of the VAMPIRE turbopump and DAPROT3inducer under different working fluid temperatures were systematically investigated by considering the following factors:the mesh elements, the boundary conditions, turbulence models, the lengths of inlet and outlet pipes, and the locations of the inlet and outlet pressure taps. The results showed that:these factors such as turbulence models, the lengths of the inlet and outlet pipes, the positions of the inlet and outlet pressure taps for measuring the pressure as well as the working fluid temperatures, had a significant influence on the hydraulic performances of the DAPROT3inducer, especially at low flow rates; the H-Q curves of the DAPR0T3inducer based on the short inlet and outlet pipes as well as the small tip clearance （0.8mm） is positioned above those based on the long inlet and outlet pipes. However, these above factors had less effect on predicting the hydraulic performances of the VAMPIRE Turbopump. Finally, the hydraulic performances of the VAMPIRE Turbopump and the DAPROT3inducer predicted by CFD are in good agreement with their corresponding experimental data.4. The cavitating flow in both the VAMPIRE turbopump and the DAPROT3inducer with two different tip clearances under different water temperatures were investigated for a wide range of flow rates based on numerical and experimental methods. It can be found that at low flow rates, the temperature increase has less effect on the suction performances of the DAPROT3inducer with the tip clearance of0.8mm; as the tip clearance increased, the cavitation induced flow instabilities occurs, causing an oscillating portion to the head-drop curve of the DAPROT3inducer. In addition, in comparison with the cavitating flow patterns on each blade leading edge based on the flow visualization test on DAPROT3inducer, it can be found that the vapor volume distribution obtained from the numerical simulations were in good agreement. The cavitation characteristics of the VAMPIRE turbopump at low flow rates are different from those at high flow rates, which could be easily affected by the working fluid temperature.5. The pumping and the suction performances of the VAMPIRE turbopump with or without the DAPROT3inducer under low and high water temperatures have been investigated based on CFD and experimental methods, and it could be found that the hydraulic performances of the VAMPIRE turbopump with the DAPROT3inducer has obviously changed, especially at low flow rates. The cavitation performances of the VAMPIRE turbopump have been significantly improved.6. The homogeneous cavitation model based on Rayleigh-Plesset equations could provide a more accurate prediction of the cavitating flow in the VAMPIRE turbopump as the head-drop was less than3%at low temperature, while for a high temperature that amount of the head-drop was less than5%. However, this cavitation model has the limitation in predicting the serious cavitation occurring in the turbopump when its head-drop amount was in the range between5%and10%.7. The numerical simulations for the unsteady flow in the DAPROT3inducer were carried out for different flow rates. The characteristics of flow instabilities in the DAPROT3inducer were first clarified in present study. Meanwhile, a series of pressure fluctuation tests were further carried out to investigate the cavitation induced flow instabilities. It can be found that the main frequencies in the DAPROT3inducer were or even lower than the rotational frequency in low flow rates, indicating the complex flow phenomena like flow instability occurring.8. The characteristics of flow instabilities as well as the cavitation phenomenon in a centrifugal pump typed IS65-50-160operating at low flow rates were studied by experimental and numerical methods, respectively. Cavitation proved to occur over a wide range of low flow rates, producing a characteristic creeping shape of the head-drop curve and depend on the parameter σ/2α, which is the relationship between the flow incidence angle and cavitation number. The experimental results of the pressure fluctuation showed that the unsteady behavior of the internal flow in the centrifugal pump operating at low flow rates had the characteristics of a peculiar low-frequency oscillation with the frequencies ranged from48.2Hz to51.2Hz. The pressure fluctuations were closely correlated to the flow instabilities induced by the occurrence of cavitation phenomena at low flow rates such as rotating cavitation.9. Based on numerical and experimental results of the cavitating IS65-50-160centrifugal pump and the cavitating VAMPIRE turopump, it could be found that the cavitation instabilities such as rotating cavitation or the asymmetric blade cavitation had occurred in both of them.