Dynamic Behavior Analysis And Resonance Prediction Of Pump-turbine Runner

For the high-head pump-turbines,the runner is installed in a chamber with narrow space to the nearby structure.The rigid walls will change the added mass of surround fluid,especially for the condition that the gaps are very small but the runner displacement is large.Therefore,it is very important to predict accurately the natural frequencies and mode shapes and the flow-induced dynamic responses of runner during the design stage.In this paper,the influence of rigid walls and gaps on the dynamic response behavior of a reduced scale pump-turbine runner was evaluated based on numerical simulation techniques.Firstly,based on acoustic-structural coupling method,modal analysis and harmonic response analysis were used to study the dynamic response behavior of a simple rotating blade model.Besides,traditional one-way fluid-structural interaction(FSI)and two-way FSI were used in the research.In this paper,an improved one-way FSI method was proposed and verified.This method combines the advantages of acoustic-structural coupling and multi-field coupling,it saves much time and computer resources than two-way FSI.By introducing the acoustic-structural coupling method,the fluid added mass effect was considered which was not taken into account in traditional one-way FSI.The improved one-way FSI method can be applied to predict the dynamic response and resonance characteristics of hydraulic machinery with complex structures.Secondly,simulation domain including labyrinth seals and other gaps were constructed,and acoustic-structural coupling was applied to investigate the modal behavior of a reduced scale pump-turbine runner in real work condition.The mode shapes of runner were studied and compared with test results.The influence of rigid walls on frequencies and frequency reduction ratio of different mode shapes were discussed.In order to investigate the influence of gap fluid on the modal characteristics,several simulation domains with different axial gaps and radial gaps were constructed.The results show that,the largest added mass factor is observed in resonance mode.In the case of no gap fluid,the added mass factor is only 20～30%of that which taken into account the real gaps.The added mass factors increase with the gap size decrease.The different influence of axial and radial gaps on mode shapes was discussed in this paper.The results reveal that,axial gaps have greater influence on the added mass factors for the in-phase(IP)modes than counter-phase(CP)and crown-dominant(CD)modes,while the CP and CD modes are much sensitive to the radial gaps.Thirdly,based on the improved one-way FSI method,which fully considering the influence of fluid added mass,the dynamic stress characteristics of runner under real working conditions was analyzed.In order to obtain the hydraulic excitation forces more accurately,3-dimentional full flow passage including all gaps was constructed for unsteady CFD simulation.5 typical operating conditions during no-load start-up process of pump-turbine unit were selected for the simulation,which provide accurate pressure loads as boundary conditions for the runner dynamic stress analysis.For the purpose of comparing the influence of fluid added mass on the dynamic stress of runner,different simulation domains were selected for transient structural analysis.The results show that,dynamic stress oscillogram has obvious periodicity,which agrees very well with the test results.The dynamic stress amplitude of the measurement point at band is larger than that at crown.The maximum amplitude of half peak-to-peak value of the dynamic stress is 51.9 MPa.It is confirmed that the fluid added mass not only significantly affects the dynamic stress amplitude,but also changes the dominant frequency of structural dynamic response.Finally,resonance phenomenon of the pump-turbine during start-up process was predicted,and resonance curve was obtained which is confirmed well agree with the test result.The influence of added mass and damping on the runner resonance was evaluated,and the cause of resonance was studied in detail.The results show that,resonance occurs at 91.3%of rated speed as the critical mode(2+4)ND was excited.The numerical method presented in this paper is validated to be very reliable for the prediction of the resonance curve.Compared with the test results,the accuracy of the resonance speed and amplitude is within 3%errors.The added mass and damping have strong influence on the resonance point,while for the non-resonance condition the influence is quite small.The prediction of the resonance curve has more important significance than using modal analysis and harmonic response analysis methods to evaluate the resonance of the structure.This paper provides a systematic and accurate method to analyze the dynamic response,which can be used for safety assessment during runner design stage.The research results presented in this paper have great significance for the safe and stable operation of the pump-turbine units.