Control of shroud leakage flows to reduce mixing losses in a shrouded axial turbine

Abstract

The losses caused by the leakage flows through the rotor tip clearance, and the mixing losses by the re-entering leakage into the main flow are considerable parts of the total losses in turbines. The main reason for the mixing losses is the different velocity components of main and leakage flows. This leads to shear stresses which cause increased turbulence and losses. This article presents a numerical investigation on three different configurations to control the leakage flows: (a) turning vanes are fixed onto the casing between the fins to turn the shroud leakage flow into the main flow direction in order to reduce the circumferential mixing losses; (b) honeycomb bands are inserted into the casing to weaken the leakage flow in the circumferential direction and reduce the circumferential mixing losses due to the special hexagon structure; and (c) downstream edge of the cavity is chamfered to reduce the radial velocity component of the leakage jet and the separation at the downstream edge, and also to reduce the streamwise mixing losses. A 1.5-stage axial turbine with high-aspect ratio blading was used in this study to investigate the sealing designs as mentioned. The flow simulation results of the three configurations were analysed and compared in this article.

Effects of leakage vortex on aerodynamic performance and loss mechanism of steam turbine

“…Besides, considering there is a very complex flow occurs in the tip clearance, a validation was also performed using the numerical data of Lin et al 24 Figure 17 shows the comparison of the current predictions with the numerical data for S dis distributions taken at a staggered labyrinth seal. S dis is the entropy generation rate, which is expressed by equations (14) and (15). It can be seen that, the distribution of viscous loss agrees well, especially in throttle regions and stagnation points.…”

“…The above researches only corrected the leakage loss, without considering that the leakage flow not only leads to the mixing loss, but also changes the downstream stator flow field and causes additional secondary flow loss. Gao et al 15 studied the effect of tip clearance on the aerodynamic characteristics of a supersonic turbine stage. Similarly, experimental and numerical investigations were proposed by Peters et al 16 to study the tip leakage flow, the flow angle at the rotor, and downstream stator blade exit with the six different tip clearances.…”

“…Gao et al. 15 studied the effect of tip clearance on the aerodynamic characteristics of a supersonic turbine stage. Similarly, experimental and numerical investigations were proposed by Peters et al.…”

A New Multi-Stage Turbine Stator Design for Improved Performance Retention

“…However, they did not investigate the robustness of the design to increases in clearance. Numerical calculations by Gao et al [12] suggested that the turning vane design is not robust to increases in clearance and would become less effective over the operational life of the turbine.…”

“…Fig.12:A two-dimensional diagram to illustrate how the pressure side boundary layer radial vorticity filaments are turned towards the positive streamwise direction at the separation point 12. …”