Research On Cavitation Characteristics Of Francis Turbine With Large Variable Amplitude Head

As the most important component in the hydroelectric power generation system,the stability and safety of hydraulic turbines have been widely concerned by the industry.In the power grid,the stability and safety of the large-amplitude unit with the main function of peak-shaving and frequency-modulation are more important.This paper takes the unit of Zipingpu Hydropower Station in Sichuan Province as the research object,analyzes its full-channel cavitation flow characteristics under large amplitude conditions,and at the same time addresses the abnormal high-frequency vibration of the unit under specific water head and output conditions.The variable load test is carried out together,and a large amount of variable load test data is sorted out to map out the abnormal high frequency vibration area.At the same time,a comparative analysis of the recent maintenance reports of the power station was carried out,electromagnetic vibration and mechanical vibration were eliminated,and the abnormal high-frequency vibration of the unit was determined to be hydraulic vibration.Based on this,this article mainly launched the following research:(1)Regarding that the runner of the unit is produced by the Russian LMZ company,we did not provide us with detailed hydraulic model data.For this reason,during the B repair period,the runner was scanned by on-site three-dimensional laser,and the entity and parts of the runner were obtained through reverse engineering technology.The water body model provides key hydraulic model data for the safe and reliable operation of hydraulic turbines.(2)On the basis of reading a large number of related literatures,based on the density-corrected RNG k-εturbulence model and ZGB cavitation model,three different opening conditions under 70m,100m,129m water heads and the vibration area under 98m water head conditions The cavitation numerical simulation is carried out on the three working conditions of the initial vibration,the strongest vibration and the vibration vanishing point.And the simulation results are analyzed for internal cavitation flow.(3)Through the analysis of the results of cavitation of the whole flow passage under different water heads and openings of the turbine,it is found that the increase of the water head and opening will increase the uniformity of the circumferential distribution of the outlet pressure of the fixed guide vane and the movable guide vane.The movable guide vane of this type of turbine The pressure gradient on the high pressure side is uniform,while the pressure distribution on the low pressure side shows the characteristics of first decreasing and then increasing,and there is a relatively low pressure area under the low pressure measurement;under each water head,the cavitation degree of the suction surface of the blade increases with the opening of the movable guide vane.Under the condition of high water head,a local cavitation zone appears at the inlet of the blade suction surface near the lower ring as the opening increases,and the location of the cavitation zone is consistent with the cavitation damage position found during maintenance.(4)Through the analysis of the cavitation flow inside the draft tube and the runner,it is found that under the three working conditions,cavitation occurs at the lower ring position of the suction surface in the runner and at the water outlet edge of the blade,and the blade is initially vibrated at high frequency.And the strongest vibration condition point,the pressure pulsation at the monitoring point at the exit of the runner blade and the monitoring point at the inlet of the draft tube has a high-frequency component of 24f_n,which is consistent with the abnormal vibration frequency of the floor,and the runner is subjected to high-frequency components during rotation.The same high-frequency component also appears in the radial force.Finally,the calculated Karman vortex train frequency and abnormal vibration frequency generated by the runner blades under the three working conditions are compared and analyzed,and it is found that the strongest point of the vibration is 61.38Hz(about 24.5fn)and abnormal vibration.The frequency is similar,and it is determined that the abnormal vibration is the resonance between the Karman vortex row at the water outlet of the runner blade and the floor structure.