Aeroelasticity Analysis of Turbine Blade with Asymmetric Outlet Pressure

Elements of high-power steam turbines are subjected to significant unsteady loads, in particular, rotor blades of the last stages. These loads can, in some cases, cause self-excited oscillations, which are extremely dangerous and have a negative impact on the efficiency and service life of the blade cascade. Therefore, when designing new or modernizing existing steam turbine stages, it is recommended to study the aeroelastic characteristics of the blades. The paper presents the results of numerical analysis of the aeroelastic characteristics of the rotor blade cascade of the last stage of a 380 MW steam turbine, taking into account the asymmetric pressure distribution behind the stage. The pressure distribution is caused by the influence of the exhaust hood and was obtained experimentally. The analysis used a proven numerical method for simultaneously solving the coupled problem of unsteady aerodynamics and elastic oscillations of blades, which allows predicting the amplitude-frequency spectrum of unsteady loads and blade oscillations in a viscous gas flow. The modeling results are presented in the form of displacements of the peripheral blade section and aerodynamic forces acting on this section, as well as their spectra. Also, the dependence of the aeroelasticity coefficient on the inter-blade phase angle of oscillation was obtained. The results of the calculations showed the absence of flutter and self-oscillations according to the first natural form of blade oscillations in the partial mode of operation of a steam turbine. The analysis of the results did not reveal a significant effect of the pressure asymmetry at the outlet of the cascade, due to the significant difference between the excitation frequencies caused by the non-uniformity and the natural frequencies of blade oscillations.