Study on film cooling for aero-engine turbine endwalls with various small-scale surface structures

Abstract

In this study, a novel endwall modification by adding small-scale ribs onto a turbine endwall surface is proposed to reduce thermal loads on the endwall. The small-scale ribs are positioned in a streamline-curved and an axially-straight array patterns and, square and triangular shapes are imposed to the cross section of the ribs. The feasibility of improving purge air cooling by the ribs is investigated by using well-validated computational fluid dynamics (CFD) methods. Comprehensive comparisons of cooling performance among the ribbed endwall cases are made by examining cooling effectiveness, heat transfer, and aerodynamic losses. Detailed flow topologies are presented to reveal flow mechanisms involved in the actions of the ribs within the turbine passage flow. Particularly, secondary-order effects of the ribs on the turbine airfoil surfaces are also documented, which is expected to provide three-dimensional boundary conditions for the turbine aerodynamic and thermal analyses. Comparisons between the ribbed endwall cases and a smooth endwall case show that the endwall modification by adding small-scale ribs is effective in reducing thermal loads on the endwall via enhancing cooling effectiveness and, in some cases, reducing heat transfer levels. The thermal load reduction can be further improved by increasing purge air flow ratio. However, as additional vorticities are introduced by the ribs, slight aerodynamic penalty is generated. Among the four rib cases, the straight triangular-shaped rib case has the lowest overall cooling performance for the endwall. Additionally, secondary-order cooling effects are visible on the airfoil suction surface for all rib cases but only the straight rib cases can generate additional purge air coverage on the airfoil pressure surface, without significantly changing pressures in that region. This work offers a new option of thermal management techniques for the turbine endwall and the results provide guidelines regarding endwall modifications of such sort.