Suppressing internal non-equilibrium condensation in steam turbines by adjusting the expansion process

Against the background of deep peak shaving, steam turbines face serious risks of wetness loss and water erosion, which seriously affect the safe operation of thermal power plants. This study establishes a mathematical model for the non-equilibrium condensation phenomenon inside a Laval nozzle and validates the model with experimental data. The influence of the nozzle's expansion rate and expansion process on the non-equilibrium condensation characteristics is systematically investigated. Results show that increasing the back pressure of the main steam (M-S) nozzle reduces the expansion rate and suppresses the occurrence of non-equilibrium condensation. Changing the inlet pressure does not affect the nozzle's expansion process (p0/pout≈6.35). When the nozzle's inlet pressure increases from 33.023 to 53.023 kPa, its outlet liquid mass fraction only increases by 6.2%. For operating conditions with high expansion rates, pre-expansion can effectively reduce the intensity of non-equilibrium condensation. Pre-expansion of steam in the nozzle can reduce the outlet liquid mass fraction by 2.14%, while in the cascade, it can reduce the outlet liquid mass fraction by 9.52%. For operating conditions with low expansion rates, pre-expansion has a promoting effect on condensation, increasing the outlet liquid mass fraction by 10.7% in the nozzle and 13.22% in the cascade. In summary, regulating the expansion rate and expansion process of steam can be employed to reduce the condensation intensity, which provides an important theoretical basis for mitigating the risk of water erosion in the last stage of steam turbines.