Quantitative Analysis of Windage Loss in Last Stage Blades of Steam Turbine

Abstract

China's power system is undergoing a profound transition driven by renewable energy. With deep peak-load regulation, 600 MW-class steam turbines operate frequently at low load. Under reduced mass flow, the last-stage long-blade flow deviates from the design condition, potentially shifting from power-producing operation to windage and inducing local overheating near the blade tip. This study reviews representative empirical correlations for windage loss and develops a three-dimensional periodic CFD model for the last three stages of a low-pressure cylinder in a utility turbine, covering 10%–100% load conditions. The results show that decreasing load strengthens separation and recirculating vortex systems, producing a tip hot spot on the last-stage rotor blade; Meanwhile, the mass-flow-averaged outlet temperature of the last stage exhibits an anomalous increase, and the mass-flow-averaged temperature rise across the stage changes sign from negative to positive below approximately 13% load, indicating a transition into windage operation. Assuming no pronounced windage above 30% load, a baseline outlet-temperature curve is constructed to define the temperature increment ΔT attributable to windage heating; the corresponding windage heating power is then obtained via an energy balance. Finally, empirical regressions for N–f and Tmax–f are established (f: non-dimensional load), with a maximum error within 3%. The proposed approach enables rapid risk assessment and operational early warning for low-load windage.