To avoid friction, a clearance between the rotor blade tip and the cylinder is needed in a steam turbine. As a result, the leakage flow is formed under the pressure difference which mixes with the main flow and causes the mixing loss. So, a numerical calculation was conducted based on a high-pressure 1.5-stage steam turbine with the tip labyrinth seal. The leakage vortex system and the mixing progress at different tip clearances were analyzed. The related leakage losses were calculated. The results show that the leakage flow will lead to the steam deflecting at the rotor exit and cause incidence loss. Furthermore, in the downstream stator, the leakage flow near the suction side with high radial velocity causes the deflection of the outflow angles. The main influence region of leakage flow is distributed above 75% of blade height, whereas the most intense region is distributed at approximately 95% of blade height. It is found that the mixing loss is related to the size of the backflow vortex. The related leakage losses increase with tip clearance, in which the mixing loss is the major part and the mass-averaged entropy mixing loss coefficient is 7–11%.

Under low mass flow conditions, the velocity field and temperature field are very complicated in the low-pressure cylinder (LPC) of steam turbine, which seriously affects the safe operation of blades. In order to study the safety of the last stage blade under low mass flow conditions, the non-equilibrium condensation model is used to numerically simulate the flow of low-pressure cylinder. The strength performance of the last stage rotor blade is analyzed by the fluid–solid coupling method. The results show that the vortex appears in the flow passage as the load decreases. The last two stages are most affected by the vortex. Under low mass flow conditions, the exhaust steam temperature of LPC rises. Under 5% low-pressure turbine heat-acceptance (LPTHA) condition, the temperature rise has affected the last three stage blades. The maximum temperature is located at the tip exit edge of suction surface of stator blade of the last stage. With the load decreasing, the maximum equivalent stress and maximum deformation show the law of first decreasing and then increasing. Under 10% low-pressure turbine heat-acceptance condition, the maximum equivalent stress is 827.96 MPa, which is over the design value. Under 5% LPTHA condition, the maximum deformation is 6.6301 mm, which is also far beyond the design value. The research results can provide a reference for the operation of steam turbine under low mass flow conditions.