The labyrinth seal is widely used in turbomachinery to control leakage flow in clearance gap involved with stationary and rotating parts due to its good sealing performance and low manufacture cost. To achieve higher aerodynamic performance, modern turbo machines are developing towards the direction of tight clearance operation. In this case, rub damage between labyrinth seal rotor and stator parts becomes more significant. To mitigate the wear damage in labyrinth fin, researchers proposed the concept of flexible seal, which ensures the labyrinth fin with sufficient flexibility in rubbing events. Since the flexible seal has low wear damage in contacting process, it can be installed with a relative smaller clearance than the conventional labyrinth seal, thus has an excellent leakage controlling performance in clearance gap. However, in published literatures, almost all studies are focusing on discharge or mechanical performance of flexible seal itself. Less attention has been paid on aerodynamic performance of turbine stage configured with flexible seals. As a result, the present paper utilizes a numerical method to investigate the effect of flexible shroud seals, including the forward bending flexible seals and backward bending flexible seals, on aerodynamic performance of HP steam turbine stages. At first, the wear performance of the flexible seal is analyzed with the Finite Element Analysis method. It shows the wear in flexible strip is so small that only the installation clearance needs to be considered. Then, by replacing the labyrinth shroud seals with flexible shroud seals, the aerodynamic efficiency, outlet flow angle distributions, and reaction degree distributions in two-stages are obtained. At three installation clearances, interactions between leakage flow and main flow, as well as the flow patterns in flexible shroud seals, are visualized and also compared with the original design case. The numerical results indicate that turbine stages configured with forward bending flexible shroud seals have a very close aerodynamic performance with those with conventional labyrinth shroud seals at the same clearance, whereas the turbine stages configured with backward bending flexible seals have lower total-total isentropic efficiency than those with conventional labyrinth shroud seals. By replacing the conventional labyrinth shroud seals with forward bending flexible shroud seals (at the same clearance), the aerodynamic efficiency, outlet flow angles, limiting streamlines, secondary flow patterns in shroud region and reaction degree distributions in stages are almost not affected. Since the forward bending flexible seal allows relatively smaller installation clearance than the conventional labyrinth seal, application of this kind of seal in rotor blade tip gap is much beneficial to achieve lower leakage rate and higher aerodynamic performance in large power steam turbine stages.


Investigations Into Aerodynamic Performance of Turbine Stages With Flexible Shroud Seals

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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