Influence of Shroud Seal Dimensions on Aerodynamic Performance of Steam Turbine Stages: Part I — Honeycomb Seal

Abstract

Effect of leakage flow in honeycomb shroud seals on the aerodynamic performance of steam turbine stages in the high-pressure cylinder was numerically investigated by using the commercial CFD (Computational Fluid Dynamics) software ANSYS CFX11.0. The geometrical parameters of the honeycomb shroud seal, including the sealing clearance, cell depth and cell diameter, were selected as the research objectives to compute the aerodynamic performance of turbine stages at a wide range of dimensions. The numerical results show that the leakage rate in the shroud honeycomb seal is almost linearly increased with increase of sealing clearance. Correspondingly, the total-total isentropic efficiency of turbine stages decreases as well. As the cell depth increases, the total-total isentropic efficiency of the turbine stages is firstly increased and then almost kept constant, and the leakage rate in the honeycomb shroud seal is firstly decreased and then almost kept constant as well. For different honeycomb cell diameters, the leakage rate and stage efficiency are mainly determined by the flow structures in the honeycomb cells and seal outlet region. The present studies also show that, as the cell diameter increases, the total-total isentropic efficiency increases whereas the leakage rate decreases. Among the studied geometrical parameters (i.e. sealing clearance, cell depth and cell diameter), the variation of sealing clearance has a pronounced influence on the mixing loss in the main flow paths, but the variation of cell diameter has less effect on the aerodynamic performance of the turbine stages than that of sealing clearance. If the cell depth is not very small, the variation of cell-depth has a minor effect on the aerodynamic performance in the turbine stages.