| Pumped storage power station uses the surplus power at the trough to pump water to the upstream reservoir for storage,and then uses it to supplement power at the peak load.It can also cooperate with other grid systems to carry out peak load regulation and valley filling.It is recognized as a reliable peak load regulation power source in the world,so the stability of unit operation is crucial.The external characteristic curve of conventional pump turbine usually produces hump phenomenon when it is running under pump condition.When the unit runs in the hump area,it is extremely unstable,which will lead to serious safety and benefit problems such as strong damage and vibration of the unit.It is very important to improve the hump characteristics.At present,the splitter blade structure has good application in the fields of water pump,fan,etc.It has the advantages of improving the flow pattern,stabilizing the flow field and improving the performance.It is still in the research stage in the field of water pump and turbine,and there are few reports on practical engineering application.In this paper,the pump turbine with splitter blades of a pumped storage unit in China was taken as the research object,aiming at the hump problem of its pump condition,simulation was taken as the main method,the experiment as the auxiliary method,three guide vane openings were selected,and the internal flow mechanism and flow induced noise characteristics under the hump condition with external characteristics were deeply explored.The main content of this paper are as follows:1)CFX was used to carry out steady numerical simulation on the opening of the three guide vanes of the model pump-turbine pump to obtain the external characteristic curve.The simulation results were compared with the model test results to discuss the reliability and feasibility of the numerical simulation.The internal flow characteristics under the three kinds of guide vane openings were analyzed.The results show that under the three kinds of guide vane openings,the variation law of internal flow characteristics with flow rate has certain similarity.Under the working condition of hump area,the draft tube outlet(in front of runner inlet)appears spiral backflow and bypass phenomenon.There are backflow and secondary flow at the runner inlet.The backflow changes from the rear cover plate to the front cover plate with the decrease of the flow rate.The uniformity of the water flowing out of the runner becomes worse and the runner's working ability decreases.This kind of unstable structure extends into the double row cascade,making asymmetric flow gradually appear in the guide vane,the mainstream cannot pass smoothly,and it will consume energy itself.The increase of loss in runner and guide vane basins is the main cause of hump phenomenon.2)Unsteady numerical simulation was carried out for typical hump operating points with guide vane opening ?=17.5 o.Based on the steady calculation results,the internal flow characteristics of runner and double row cascade were analyzed in detail,and combined with experiment,the pressure fluctuation characteristics of monitoring points in various watersheds were analyzed.The results show that under the working condition of hump region,the flow in the runner and guide vane basin deteriorates,and the runner inlet forms an obvious low pressure region.The flow in the runner channel is affected by the development of the inlet vortex,which increases the loss and greatly reduces the working capacity.Vortex of opposite directions appears in the guide vane and stay vane,and the position of vortex structure is distributed in periodicity.The main frequency of pressure pulsation obtained by numerical simulation is in good agreement with the experimental results.Under rated conditions,the internal pressure pulsation of the pump turbine unit is mainly affected by the static and dynamic interference between the runner and the guide vane.The main frequency of pulsation in the runner and between the runner and the guide vane is about 1.6BPF,the main frequency of pulsation in double row cascade basin is mainly 0.5BPF,while the pressure pulsation in the draft tube and volute is mainly affected by 2BPF.In the hump region,especially at the lowest point of hump,pressure fluctuation is affected by the weakening of static and dynamic interference.The low frequency component of pressure pulsation increases and the main frequency changes to the low frequency direction.The main frequency of the runner basin is changed to 1.5 times of the axial frequency,in double row cascade,the low frequency component is even more serious,the main frequency is about 0.5 times of axial frequency,which is mainly caused by complex flow patterns such as internal vortex and secondary flow.The main frequency of the monitoring points near the volute diaphragm is about 0.3 times of the axial frequency,and the main frequency of the monitoring points at other sections of the volute is about 0.6 times of the blade frequency.The main frequency of pressure pulsation of draft tube at the highest and lowest points of hump is still mainly affected by the dynamic and static interference between runner and draft tube,which is around 1.4BPF and BPF.The peak value of pressure pulsation in the runner basin is the largest under all working conditions,and shows a decreasing trend towards the upstream and downstream,with the peak value in the draft tube being the smallest.3)Based on CFD+Lighthill acoustic analogy theory,the sound source information obtained from unsteady calculation is extracted,and the internal sound field of model pump turbine is calculated by LMS Virtual.Lab software.First of all,through the analysis of the sound pressure level spectrum at its inlet and outlet,it is found that the noise characteristics are similar to the pressure pulsation characteristics,and the main frequency of the sound pressure spectrum is basically same as the pressure pulsation.With the increase of frequency,the sound pressure level of the outlet field point is larger than that of the inlet field point as a whole.According to the analysis of the sound pressure nephogram of the whole basin under the three guide vane openings,the peak value of the sound pressure level appears at 2BPF under the rated working condition.Further analysis of sound pressure nephogram under different operating conditions with each opening at 2BPF shows that the intensity of noise source caused by internal flow of the unit is relatively high under hump operating conditions,and the operation of the unit in hump area should be avoided as much as possible to reduce the noise of pump turbine.According to the research on the characteristics of sound field,it is found that the volute diaphragm is the main gathering area of noise source,and the unsteady pulse force generated by the dynamic and static interference between runner and guide vane is the main reason for flow induced noise.The sound source mainly propagates to the volute outlet in the direction deviating from the tongue along the XY plane,the sound pressure in the direction of draft tube outlet is much smaller than that in the direction of volute outlet.It has obvious characteristics of dipole sound source,and the sttenuated speed of sound pressure under rated working conditions is greater than that of hump area. |
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