Influence of Rotor Axial Shifting and Clearance on Leakage in Stepped Seal in Steam Turbines

Thermal expansion during starting and stopping processes in steam turbines in high-performance results in an axial shifting of the rotor. It could significantly alter the flow field in the seal. The stepped seal belongs to the category of complex structure labyrinth seal, which is widely applied in engineering for its better leakage control due to its multiple throttle mechanism which guarantee more satisfactory energy dissipation. By using Fluent, the flow and the leakage in stepped seal were predicted by employing two-dimensional steady flow model, the ¿-¿ turbulence model and numerically solving the Navier-Stokes equations. Altering the flow pattern and affecting the leakage due to rotor shifting were investigated. The variation of the predicted leakage rate due to rotor axial shifting was compared between Fluent and Perturbation, and the largest difference was less than 5%. The flow and the leakage in stepped seal were compared over a range of seal clearances and relative axial rotor positions at the same pressure ratio. It was detected that the leakage rate increased linearly with seal clearance, and a decline in leakage rate arose from rotor axial shifting in the leakage flow direction, whereas an increase resulting from shifting against the leakage flow direction was also found. Extreme conditions like short teeth shifting off the rotor block resulting in a significant increase in leakage were also explained.