Leakage Loss Reduction by Shroud Cavity Shape Improvement in Reaction Turbine

Abstract In a steam turbine blade row, leakage flows through the radial clearance between the shroud at the tip of the blade and the sealing fins resulting in a reduction in efficiency, known as leakage loss. During turbine operation, radial clearances are affected by various factors such as thermal deformation resulting in elongation difference between the rotor and casing, deflection due to the rotor’s weight, and changes in the state of the bearing oil film. The design strategy commonly adopted to reduce the amount of leakage flow includes: reducing seal fin tip clearance, increasing the number of seal fins, etc... All these countermeasures are implemented while satisfying the clearance under operating conditions. Besides the leakage flow, another source of losses to be reduced is the difference in circumferential velocity component between the main flow leaving the moving blade passage, and the flow coming out of the shroud cavity of these same blades. The focus in recent years has been on reducing this flow discrepancy, also called mixing loss. It should however be noted that the situation is different for reaction and impulse turbines. Generally, as the shroud cavity circumferential velocity after a stationary blade passage is slower than that after a nozzle passage, reducing the problematic velocity component coming out of the shroud cavity in a reaction turbine will not be as effective as it would be for an impulse turbine. A final point to consider is manufacturability. Structures installed to correct the shroud cavity outflow in the axial direction often require non-axis-symmetrical geometries, hardly machined by just the use of a vertical lathe. Considering the background above, the research presented in this paper focused on investigating the shroud cavity shape suitable for leakage flow reduction. An improved shroud cavity shape was developed and the mechanism used by this new design to reduce leakage flow by using the generated counter vortex is discussed.