Stability Analysis Of Francis Turbine Under Partial Load Condition

For purpose of reducing the impact of random intermittent energy on the instability of the power grid,hydropower will be transformed from the role of base load to highly flexible and adjustable energy,which will make more Francis turbine units operate normally under off-load conditions to balance the grid parameters.Under partial load conditions,the turbine unit will face the problem of dynamic load imbalance,which is threatened by high amplitude pressure pulsation,strong hydraulic vibration,high decibel induced noise and so on.With the improvement of turbine design and manufacturing level and material progress,the application of Francis turbine moves forward from low specific speed to high specific speed or even ultra-high specific speed.High specific speed turbine operation efficiency zone is relatively narrow,unit output is easy to produce instability.In order to respond to the large-scale development of external power energy and technical progress,the high specific speed hydraulic turbine has put forward substantial progress requirements,not only to have higher efficiency,but also to have better stability and reliability.In line with the security in the new situation to high specific speed Francis hydro-turbine governing system safe and efficient operation purpose,based on the combination of theoretical analysis and numerical simulation,the structure of the turbine,perspective,such as energy and flow field are analyzed in detail under partial load operation condition high specific speed Francis turbine flow field structure,energy and instability of regularity and mechanism,to explore and pave the way for the follow-up fluid machinery related research,in order to promote the development of hydropower energy industry.This paper mainly includes the following aspects:(1)Based on the fluid-structure interaction theory,based on load variation parameter,the structure of Francis turbine impeller structural characteristics under partial load operation condition,analysis of the wheel structure stress and deformation under different load conditions such as structural strength assessment of key issues,to explore the different load condition wheel structure prestressed modal and wet modal,analysis of the wheel structure natural frequency and resonant modes.The results can provide theoretical reference for the healthy operation of high specific speed turbine.(2)Based on the theory of entropy production of Francis turbine under partial load operation condition internal energy loss of system research,quantitative analysis of Francis turbine under partial load operation condition within the different regional characteristics of energy loss,so accurately the location of the Francis turbine to produce hydraulic loss,implementation of Francis turbine under partial load condition energy stability of accurate prediction.The conclusions can provide theoretical support for optimizing the design of the turbine and expanding the high efficiency zone of the turbine with high specific speed.(3)Based on fast Fourier transform,the movement process of the Francis turbine flexible load changes,the different load condition,impeller and draft tube of the spiral case key parts such as pressure fluctuation law,analysis of pressure fluctuation and switch frequency,pressure impulses,and coupling between frequency doubling mechanism,from the perspective of static pressure,turbulence kinetic energy and vorticity,to explore the influence law of partial load condition of Francis turbine flow instability.The conclusions can provide some theoretical guidance for reducing hydraulic excitation during flexible operation of the mixed turbine under partial load condition.(4)Based on the theory of vortex dynamics,the evolution mechanism of tailwater vortex belt under different load conditions was analyzed according to the load variation during the flexible operation of the Francis turbine.Based on the theory of intrinsic orthogonal decomposition,the modal decomposition of the wake vortex zone was carried out to explore the coherent structure of the wake vortex zone under partial load conditions and capture the evolution law of vortexes of different scales.The results can provide some suggestions for reducing tailwater oscillation and improving the instability of flow pattern in the whole passage of the mixed turbine during the flexible operation.