The Influence of Shroud and Cavity Geometry on Turbine Performance: An Experimental and Computational Study—Part I: Shroud Geometry

Abstract

Imperfections in the turbine annulus geometry, caused by the presence of the shroud and associated cavity, have a significant influence on the aerodynamics of the main passage flow path. In this paper, the datum shroud geometry, representative of steam turbine industrial practice, was systematically varied and numerically tested. The study was carried out using a three-dimensional multiblock solver, which modeled the flow in a 1.5 stage turbine. The following geometry parameters were varied: inlet and exit cavity length, shroud overhang upstream of the rotor leading edge and downstream of the trailing edge, shroud thickness for fixed casing geometry and shroud cavity depth, and shroud cavity depth for the fixed shroud thickness. The aim of this study was to investigate the influence of the above geometric modifications on mainstream aerodynamics and to obtain a map of the possible turbine efficiency changes caused by different shroud geometries. The paper then focuses on the influence of different leakage flow fractions on the mainstream aerodynamics. This work highlighted the main mechanisms through which leakage flow affects the mainstream flow and how the two interact for different geometrical variations and leakage flow mass fractions.

Comparative investigation of unsteady flow interactions in endwall regions of shrouded and unshrouded turbines

“…10 and 11, the passage vortex in the shrouded rotor passage is very weak and hard to be identified [30]. Inside the unshrouded rotor passage, as the development of the passage vortex is suppressed by the strong tip leakage vortex, the passage vortex cannot be observed due to stronger tip clearance effects (described above).…”

“…In comparison to real applications, this value is relatively large, as gap of 0.7-1% blade span are commonly employed. However, the small inlet and exit cavities as well as the large gap were chosen in order to reduce the pure cavity-to-main flow interaction by increasing the leakage jet momentum and reducing main passage flow ingress into the shroud [30], thus facilitating the investigation of upstream wake-secondary flow interactions in shrouded tip endwall region. Besides, due to the large clearance gap, the effect of tip passage vortex inside the unshrouded rotor passage is greatly reduced, which also facilitates the investigation of the unsteady flow interaction in unshrouded tip endwall region.…”

Uncertainty Quantification of Leakages in a Multistage Simulation and Comparison With Experiments

“…11, right side), forming a region of higher relative yaw angle in the near casing of the main annulus (Fig. 12, right side), because of mechanism explained in [14].…”

“…Furthermore, during long-term turbine operation the sealing effectiveness deteriorates due to rubbing and worn sealing elements. Because of this, leakage fractions for shrouded blades can vary from 1% to 4 % within HP turbines [14]. Therefore it is important to investigate the influence of the different shroud radial gap on the main blade path.…”

“…However, the loss-generating mechanisms are highly dependent on the particular shroud and stage geometry, therefore it is difficult to develop universal design rules and to correlate different geometrical and operational turbine parameters to generic shroud design. A map of the possible turbine efficiency changes caused by different shroud modifications was obtained by Rosic et al [14][15] who investigated the influence of each geometric parameter on the mainstream aerodynamics. The intention was to summarize these effects and to highlight efficiency trends that can be used by turbine designers as guidelines for their particular shroud geometry rather than to create universal design rules.…”