Influence of Rotor Axial Thermal Growth on Rotordynamic Forces of High-Low Labyrinth Seals in Steam Turbines

Rotors in steam turbines experience significant axial shifting during start-up and shut-down process due to thermal expansion. This axial-shifting could significantly alter the flow pattern and the flow-induced rotordynamic forces in labyrinth seals, which in turn, can considerably affect the rotor-seal system’s performance. This paper investigates the influence of the rotor-axial-shifting on leakage rate and rotordynamic forces for high-low labyrinth seals under different geometrical and operational conditions. A well-established CFD-perturbation model was employed to predict the rotordynamic coefficients. A surprisingly large effect was found for rotordynamic characteristics due to changes in seal configurations caused by rotor axial shifting. It was also found that less destabilizing effect arose from rotor-axial-shifting in the leakage flow direction whereas a more destabilizing effect arose from shifting against the leakage flow direction. A tentative explanation was proposed for the large sensitivities of dynamic forces to the off-design operations with rotor-axial-shifting.