Thermal–fluid–structure interaction (Thermal-FSI) analysis of radial–axial steam-turbine stage design considering observed failures

Variable renewable generation forces coal-fired steam turbines to operate in frequent start-up/shut-down cycles. This operating mode exposes hot-section components to steep thermal gradients and transient mechanical loads. Reports from a European unit with a radial–axial stage described blade jamming in casing grooves. They also reported fatigue cracks at the blade root during repeated start-up and shut-down events. This study examines whether a similar mechanism could occur in a high-pressure turbine radial–axial stage retrofitted at the Bełchatów Power Plant. A thermal–fluid–structure interaction analysis was performed for a blade made of St12T steel. The structural model included frictional blade–casing contact and clearance cancellation. The results show that the highest risk of interference occurs during specific transient phases. They also show stress concentrations at the blade-root transition, which supports the jamming-induced bending hypothesis. The results indicate that this damage mechanism is mechanically plausible under flexible operation. Possible mitigation measures include reinforcement of the root and fillet, optimization of clearances and fit, and temperature-based monitoring during operation.