INFLUENCE OF SHROUD SEAL DIMENSIONS ON AERODYNAMIC PERFORMANCE OF STEAM TURBINE STAGES: PART II-HOLE-PATTERN SEAL

The hole-pattern seal is usually used as a replacement of honeycomb seal due to their similarities in geometry and performance (leakage and rotordynamic). Compared with the honeycomb seal, the hole-pattern seal is easier to be manufactured and installed thus it is welcomed by the manufactures. However, most current literatures about the hole pattern seal mainly addressed the rotordynamic characteristic of the shaft seal. Almost no research attempts to give an insight into the aerodynamic performance of the hole-pattern seal applied in turbine stages. Therefore, the main objective of the present paper is to investigate how the hole-pattern seal geometries, i.e. sealing clearance, hole-diameter and hole depth, affect the aerodynamic performance of the steam turbine stages. With the commercial CFD (Computational Fluid Dynamics) software ANSYS CFX11.0, the leakage rates and aerodynamic efficiencies for the two stages with hole-pattern shroud seals were obtained and compared with those configured with honeycomb shroud seals at a range of seal dimensions. The results show that the leakage rate from the hole-pattern shroud seal is a bit higher than that from the honeycomb shroud seal at the same geometrical parameters (i.e. sealing clearance, hole/cell-diameter and hole/cell-depth). However, for these two configurations, the aerodynamic efficiencies are very close at the small sealing clearance cases. Big differences are shown at the large sealing clearance cases due to the difference in hole-area ratio. For the turbine stages with various hole-diameters and hole-depths, the aerodynamic performance of the turbine stages with honeycomb/hole-pattern seals are mainly affected by the flow patterns at the seal outlet if the sealing clearance is fixed. The sealing clearance has little effect on the flow pattern in the cells/holes, but it has a significant effect on the flow fields in the seal outlet chamber, thus affects the secondary flow development in the downstream flow paths.