Force-transferring Mechanism Of Spiral Case And Characteristics Of Flow-induced Vibration Of Powerhouse In Hydropower Station

With the growing of the installed capacity and the available water head,the safe and stable operation of hydropower plant is facing new challenges.For a steel spiral case embedded under a pressurized condition,an initial gap between the spiral case and surrounding concrete appears due to the shrinkage of the spiral case.The value of clearance changes with the variation in water head during operation,and directly effects the outward transmission of the internal water pressure,thus playing an essential role in the load-bearing mechanism and structural integrity of the spiral case structure.In addition,during the operation period of hydropower plant,the hydraulic turbine will inevitably work under off-design conditions owing to the fluctuation in the water head,flow rate and consequent guide vane opening,as a result,flow stabilities such as flow separation,cavitation and vortex rope tend to occur in the turbine passage,therefore pressure pulsations are produced and vibrations appear in the powerhouse and unit.In the context of restructuring of international energy structure,renewable resources such as wind,photovoltaics and nuclear energy are integrated to the grid,while as a regulating power source,hydropower should bear a large portion of the responsibility for peak and frequency modulation,hence the operating conditions of hydraulic turbine are getting more and more complex,and the issue of related vibration has become the focus in both engineering and academic communities.In view of aforementioned issues,in this thesis,relevant research is conducted in combination with engineering practices that covers several following aspects:(1)In order to investigate the evolution mechanism of gap occurring in the spiral case embedded under a pressurized condition,a novel simulation technology is adopted for the construction of pressurized spiral case,moreover,by comparison with model test results,thorough validation is conducted in terms of gap value and contact state,stresses of spiral case and rebar,the displacement of generator pedestal and stay ring,cracking and damage of surrounding concrete.On this basis,analysis of contact and load-transmission behaviors is performed in the respects of the impact of spatial and temporal distribution on surrounding concrete and unbalanced force of stay ring in horizontal direction.The results demonstrate that:the results of the proposed simulation technology are consistent with those of experiment,relevant values are basically matching,which varifies the validity and accuracy of this approach,furthermore,potential phenomenon that the inner surface of concrete penetrates the surface of steel spiral case is able to be avoided;the area with large gap formed after releasing the pressure is mainly distributed in the waist and top of steel spiral case,before the internal water pressure reaches the specified value,the outer part of inlet and upper part of nose are the first to close,while inner part of the inlet turning and outer area of the volute end are still open when the pressure attains the specified value;when the expansion joint is the form of spiral case boundary,the setting of thrust ring contributes to the delay of the clearance closing in outer part of inlet,outer part in the direction of 45°and inner part of nose,besides,the closing behavior of the clearance is significantly improved.Moreover,the direct linkage between steel spiral case and penstock could produce similar effect as the thrust ring.(2)For the sake of exploring the characteristics of hydraulic vibration source in hydropower plant,based on the theory of computational fluid dynamics,RNG k-?turbulence model is adopted to carry out three-dimensional unsteady turbulent numerical simulation for the analysis of whole runner passage including spiral case,stay vane,runner and draft tube under various conditions.Based on the calculation results of the three-dimensional unsteady turbulence in the turbine,the pulsating pressure on the runner components is integrated.Moreover,the calculation method of hydraulic axial thrust pulsation characteristics is proposed by combining the analytical solution and numerical solution.The results show that the flow in spiral case is relatively smooth,pulsating pressure in this area is usually brought about by the upstream propagation of the fluctuations in vaneless space,runner and even draft tube;rotor stator interactions between the rotating and stationary components in the turbine unavoidably lead to the occurrence of the blade frequency or its harmonics;areas with intense pressure fluctuations are primarily concentrated in conical diffusion and elbow sections,the dominant frequency is 0.83Hz and 1.02 Hz,i.e.,one-fifth and one-quarter of the rotating frequency,respectively.Low-frequency pulsating pressure also appears in the vaneless space and spiral case influenced by the upstream propagation of low-frequency draft tube vortex rope.The hydraulic axial thrust,which shows distinct pulsating characteristics,is an important component of the vertical dynamic load of the unit.The cavity pressures between the crown and the head cover of the runner,the band of the runner and the base ring are the main sources of the hydraulic axial thrust.(3)In the past,the design of turbine runner and flow passage was not combined with that of powerhouse structure,as both design were separate.In order to improve this situation and carry out the analysis of flow-induced vibration characteristics in hydropower house structure based on fluid-structure coupling,in addition,the C~2 compactly supported radial basis function interpolation coupling matrix is presented and examined.On this basis,the whole fluid-structure coupling analysis model is developed so as to predict the hydraulic vibration occurring in the powerhouse via numerical approach.The results reveal that the C~2 compactly supported radial basis function demonstrates high precision,in the data-transferring process from fluid to structure and vice versa.The data-transferring model of fluid-structure coupling interface based on C~2 compactly supported radial basis function interpolation method is applicable to large-scale complex fluid-structure coupling analysis.Furthermore,its low mesh-dependence in combination with flow calculation of turbine passage and structural analysis of powerhouse is able to play a significant role in the interation of fluid and structure.Under the minimum head condition,the guide vane opening is relatively large,and the relative velocity of the water flowing into the runner region will form a certain angle of attack with the chord line of runner blade.Compared with the maximum head condition and design head condition,the pulsation pressure and viabration response of powerhouse structure are relatively larger.(4)In order to investigate the hydraulic vibration transmission mechanism of powerhouse in hydropower station,the vibration transmission path is analyzed.The calculation and analysis of plant vibration are conducted considering two vibration transmission paths of volute/draft tube~powerpouse,runner~axis~generator pedestal~powerpouse.Finally,the metal fatigue of the steel volute under the hydraulic loading is explored.The results reveal that the hydraulic axial thrust mainly causes the vertical vibration,especially the vibration of the generator pier,and the vibration transmission path of the volute/tail pipe-plant mainly causes the overall vibration of the plant structure.The volute/draft tube~powerpouse vibration-transmission path is more direct and influential,which is the main cause of powerpouse vibration.Quantitatively speaking,the steel volute faces no risk of fatigue failure under the cyclic load of hydrostatic pressure or pulsating pressure throughout the life of hydropower station.