Thermomechanical coupling mechanisms of a rotary variable nozzle for supercritical CO ₂ turbines under extreme operating conditions

Addressing the operational stability challenges of supercritical carbon dioxide turbines under off-design conditions, this paper presents a novel inlet regulation mechanism featuring a rotary variable nozzle. Using a fully coupled fluid-thermal-structural multiphysics analysis, the reliability of this structure is investigated under extreme conditions of 500°C and 14 MPa. The results reveal that the nozzle opening governs the thermo-mechanical state of the mechanism. As the opening increases from 0° to 60°, the internal flow transitions from a localized gap jet to a fully developed diffused flow, reducing the maximum von Mises stress by 45% (from 485 MPa to 328 MPa) and increasing turbine isentropic efficiency by 4.8 percentage points. At the 120° opening, however, the maximum stress reaches 612 MPa, exceeding the material allowable by 33%, and the radial deformation (0.07 mm) exceeds the design clearance (0.04 mm) by 75%, posing significant seizure and low-cycle fatigue risks. The 60° opening maintains a clearance utilization below 30% and achieves the optimal balance between thermal uniformity and structural integrity. This research elucidates the complex coupling phenomena within the novel SCO 2 variable inlet structure, providing a critical theoretical foundation for its future reliability design and optimization.