Aerodynamic Stability of Steam Turbine Final-Stage Blades Under Low-Flow Operating Conditions

Operating steam turbines under low-flow conditions poses severe threats to structural safety due to flow-separation-induced aeroelastic instability. However, the exact sources of localized unsteady aerodynamic excitations in vortex-dominated regions and their explicit contribution to blade flutter under deep low-flow regimes remain unclear. To address this problem, this paper proposes a quantitative aeroelastic stability assessment method that combines unsteady pressure monitoring within key vortex zones with a one-way fluid–structure interaction (FSI) energy-based framework. Taking the 661 mm last-stage blade of a 350 MW LP steam turbine under 20% THA operating conditions as a representative study, numerical simulations were carried out to capture three-dimensional separated flow structures, isolate dominant unsteady excitation sources via fast Fourier transform (FFT) pressure spectrums, and evaluate traveling-wave flutter across various inter-blade phase angles (IBPAs) for the first three structural modes. The results indicate that flow separation initiates at the rotor root and forms a large-scale recirculation vortex blocking nearly 50% of the spanwise passage. Unsteady pressure monitoring demonstrates that this root recirculation vortex exhibits the maximum pulsation amplitude, serving as the primary source of unsteady aerodynamic excitation. Under fluid–structure coupling, the first three modes all reach their minimum aerodynamic damping coefficients at an IBPA of 45°. When accounting for mechanical damping, the total system damping for the first and second modes becomes negative at 45° IBPA, with the first mode dominating the flutter response. The main novelty of this work lies in revealing the spatial–temporal mechanism by which localized separation vortices govern blade flutter under deep low-flow conditions, providing a precise mapping between localized vortex dynamics and global aeroelastic stability to guide the safe operation of turbine blades.