Cavitation Characteristics And Transient Processes Of A Pump-Turbine Analyzed By Three-Dimensional Simulation

Pumped-storage plants are widely constructed and used as they have the advantages of peak and frequency regulation and absorbing the extra energy.However,it is those advantages that put the pump-storage unit under violent transient processes frequently.The flow patterns are in chaos in all flow area during the transient process.In the runner lots of reserve flow lead the uneven distribution of the pressure on the blades,and in the channels between the vanee the flow seperation occurs.Various types of vortex ropes appear in the draft tube and they block the flow,and in some extreme transient processes,such as the valve closes very quickly,the reserve water hammer may happen.Those mentioned adverse flow patterns in different zones will perform as signal source and these turbulence will spread all over the flow area and will be superimposed with each other.Meanwhile the adverse flow patterns will lead the unbalanced forces on the runner.The swinging radial force may lead the rotor sweeps the stator and the sharp axial force may lift the whole unit.Based on these two accidents and the predecessors' work,the author simulate and analyze the cavitation characteristics and two transient processes of the pump-turbine by adopting the 1-D-3-D coupling method.The main contributions are as follows,Part 1:the anlysis of the cavitation characteristics of a protype pump-turbineThis paper simulates 6 conditions of the protype pump-turbine under different cavitation cofficients.The results show that the peak pressure pulsations increase at first and then decrease with the cavitation coefficient increases.The cavitation chamber,originating from the cone of the runner bottom,has no specific and clear shape,volume as well as the period.Thus,the frequency bands of the cavitation pressure signal are very wide,which means the amplitude of each frequency is small.Besides,the cavitation pressure will spread all over the flow field and the time-frequency characteristics in all areas are similar except that the first mode fresuency in the vaneless zone remains 9 times of the frequency of the runner revolution.Part2:the anlysis of the time-frequency characteristics and the flow patterns in the all four quadrants during the pump powered-off transient process of a protype pump-turbineThis paper also simulates the pump powered-off transient process under a certain vane opening.During the whole process the frequency can be classified into 4 types:the high frequency with low amplitude,the high frequency with high amplitude,the middle frequency with middle amplitude and the low frequency with high amplitude.The high frequency(189.42Hz)originates from the RSI and it influences the vaneless zone mainly.The low frequency(0.47Hz and 1.01Hz)is caused by the condition conversion.The low middle frequency(1.62Hz)is caused by the vortex rope with high power in the draft tube while the high middle frequency(13.29Hz)is caused by the vortex rope with small power.Besides,the adverse flow including the uneven distribution of the pressure,the reverse flow in the runner and various kinds of vortex ropes appear in the draft tube always come together and the worst situation is in the pump hump area.Part3:the influences of the macro parameters and the pressure pulsations during the runaway process after the closure of a ball valveFinally this paper simulates the transient process of a pump-turbine under two conditions with the closure of the ball valve.It is found that both the contradiction of the maintenance and the oscillating forces on the runner during the runaway process can be weakened when the ball valve closed quickly first and slowly after.Besides the pressure pulsations can be weakened in the whole flow passageway when the valve closes.The reverse' flow patterns at the middle vertical section of the vaneless zone are analyzed and 9 zones of reverse area are found.During the whole S-shape zone the reverse flow exists all the time and the flow rate will determinate the shape of the reverse flow area.The above mentioned 3 works are the keys to the research about the accidents including the rotor sweeping the stator and the lift of the unit.Firstly,the characteristics of the cavitation lay the foundation of the reverse water hammer and the analysis of the transient processes can enrich the understanding of the pump-turbine.Then the classification of the frequency can be useful for the research of the unbalanced force.At last the analysis of runaway process after the closure of the valve can be useful for the understanding of the influences about the forces and the pressure pulsations,which can lay the foundation for the research of the closure law of the valve to avoid the above mentioned accidents.