Analysis Of Cavitating Flow In HTHP Mixed-Flow Pump And Influence On Structure Design

Nuclear reactor coolant pump, usually called nuclear main pump, is the driving equipment in the nuclear coolant cycling system. As the only revolving part in the nuclear islands, nuclear main pump must operate in the long term safely and reliably under the circumstances of high temperature, high pressure and strong radiation. As a common causes of damage in hydraulic machinery, cavitation and erosion make the performance of hydraulic machinery drop steeply with noises and vibrations, seriously, it will cause the damage or fracture of the blades. Therefore the cavitation in nuclear main pump should be avoided at the normal or disaster operations.In this paper, the comparison of cavitation phenomenon is carried out between nuclear main pump and ordinary mixed-flow pump, and the results show that it is feasible to simulate cavitation in reactor coolant pump by utilizing the existing cavitation model. Steady and unsteady numerical simulation are implemented for a2d naca66(mod) hydrofoil based on barotropic relationship model, and the simulation results agree well with the experimental data, that indicates the accuracy of this cavitation model in simulation of cavitating flows.In order to have a further analysis on reactor coolant pump cavitation, the characters of5blades impellers designed by velocity coefficient method and model transformation method are calculated in the cavitating flow conditions. The results show that the two types of impeller efficiency and critical net positive suction head(NPSHcr), are90.866%,92.844%, and51.93m, and16.35m respectively. The deviations of efficiency and NPSHcr between two design methods are nearly2%and68.5%.An optimization process is put into effect to improve the cavitation performances of impeller designed by velocity coefficient method. The result shows that the5blades impeller designed by velocity coefficient method, is not reasonable due to higher NPSHcr under design condition. Consequently, the pump impeller blades inlet edge shape, thickness, and the variation of blades thickness along flowing direction are optimized with objective functions of efficiency and head after increasing the impeller blade number to seven,. Cavitation performances among three types of impeller with different blade structures are compared and analyzed. Some useful conclusions are conducted. The NPSHcr is greatly reduced by extending the blade inlet toward the pump inlet properly, attenuating the blade inlet edge and optimizing the blade thickness. The optimized impeller efficiency and NPSHcr are90.857% and28.64m respectively, and NPSHcr is relatively reduced by45%compared with original5blades impeller. The effective improvement of the cavitation performance at the design condition provides helpful directions for further design and optimization of the reactor coolant pump impeller. Finally, cavitation characters are obtained by numerical simulation within the limits of0.8～1.2times design flow rate, the results indicate that the NPSHcr significantly enhance along with the increase of flow rates, meanwhile, the cavitation performance of the impeller goes worse.