Investigation of Axial Thrust Characteristics and Nozzle Structural Optimization of the Steam Turbine Regulating Stage Under Off-Design Conditions

As thermal power units in China shift toward serving as flexible regulation sources in new-type power systems, accurately assessing the axial thrust of steam turbine regulating stages under off-design conditions has become critical. This paper employs numerical methods to investigate the axial thrust characteristics and nozzle structural optimization of the regulating stage under off-design conditions (VWO, THA, 75% THA, 50% THA). Steady-state results reveal significant deviations in the interstage hub forces predicted by 3D simulations compared with those from the conventional 1D formula under partial admission, prompting a correction. Unsteady results show that reducing the partial admission degree intensifies flow unsteadiness, increasing rotor blade axial force fluctuation from 1175 N (VWO) to 2057 N (50% THA). In terms of structural optimization, compared with not increasing the nozzle angle, increasing the nozzle angle by 2° reduces the total axial force on the regulating stage by 7.3%; compared with not extending the inlet guide arc segment, extending its length by 40 mm increases the axial force on the rotor blade by 1.6%, but decreases the maximum amplitude from 323.9 to 249.9. Based on these findings, the optimization direction for the nozzle structure is proposed.