Numerical Research Of Cavitation Flow In Axial Flow Pump

It is a normal phenomenon that cavitation occurs in hydraulic machinery, which has an obvious impact on the safe and stable operation of the unit. Therefore, researches on how to improve the cavitation performance and monitor the real-time status of the unit in operation have become hot topics. With remarkable development of the simulation technology, the reliability of simulation has been widely recognized. In this paper, the internal cavitating flow in an axial flow pump is analyzed by numerical simulation, and cavitation characteristics of the unit are predicted effectively, which provides a reliable reference for cavitaton research.Different turbulence models and cavitation models are applied to conduct the numerical simulation in NACA0009 airfoil. Among all the models applied, the results of SST turbulence model and Rayleigh-Plesset cavitation model show better agreement with the experimental data. Meanwhile, the simulation results show that numerical simulation is reliable enough to predict the cavitaion characteristics.Based on Rayleigh-Plesset cavitation model, 3D steady simulation is carried out to predict the cavitation characteristics in an axial flow pump. As the results show, under design operation, cavitation generates at the inlet edge close to the shroud on blade suction surface and expands gradually to the outlet edge on blade suction surface along axial direction, meanwhile to the impeller hub along radial direction. Under different operations, location distributions where cavitation generates and expands are not the same, thus leading to the difference of cavitaion characteristics in the pump. The axial flow pump obtains better cavitation performance under design operation than others.On the basis of keeping other parameters unchanged, numerical simulations are conducted to analyze the influence of different blade number and guide vane number on cavitation performance respectively. As the results indicate, the pump head gets higher and the cavitation performance becomes better as blade number increases. While the available net positive suction head(NPSH) reaches the critical value(NPSH), however, the more blades the impeller has, the faster the pump head declines as NPSH continues decreasing. As for different guide vane numbers, it has little influence on the energy and cavitation performance of the axial flow pump.In the cavitation process, several unsteady simulations are performed under different cavitation conditions. The time-frequency analysis reveals that the pressure fluctuations at different cross-sections in the axial flow pump are different: the frequency component of pressure fluctuation at impeller inlet is only affected by the impeller, thus making the dominant frequency to be blade passing frequency. Meanwhile the frequency component at impeller outlet and guide vane outlet is affected by the combination of impeller and guide vane, and the impeller has the main impact, thus making the dominant frequency to be blade passing frequency or its second harmonic. Due to the effect of static and dynamic interference, amplitudes of pressure fluctuation at different monitoring points show different trends as cavitation develops. As for radial force the impeller bears, the frequency component is affected by impeller and guide vane at the same time, and the amplitude at dominant frequency shows totally the same trend in both X direction and Y direction: it increases first until reaching the maximum value at the critical value(NPSH) and then decreases slowly.It is of great significance to analyze the cavitating flow and improve the cavitation performance in the axial flow pump by numerical simulation. Meanwhile, it's a good method to establish quantitative relationship between the development degree of cavitation and the pressure fluctuation as well as the radial force the impeller bears in axial flow pumps.