Abstract
The aerodynamic penalties associated with the tip gap flow in axial turbines remain a challenging problem for turbine manufacturers. As modern gas turbines with small-core architectures are brought online, the influence of the tip gap continues to grow. While technologies to reduce the losses associated with the tip gap flow have been implemented into the blades themselves, little attention has been paid to the stationary tip seal, or casing, around the rotor wheel. In this study, an introduction to the use of axisymmetric groove enhancements for the casing of the rotor tip is examined computationally. These studies use the National Experimental Turbine (NExT) geometry, an engine-representative high-pressure turbine blade.
Steady, Reynolds-averaged Navier–Stokes simulations are used to assess the basic characteristics of axisymmetric grooves, such as depth, location, and arrangement. The objective of this introductory study was to determine the feasibility of impacting the tip leakage vortex formation and the associated losses in rotor efficiency. Furthermore, analyses were done with different tip gap heights along with both flat- and squealer-tipped blades. Tip seals with a single groove are demonstrated to improve rotor aerodynamic efficiency relative to ungrooved seals by up to 0.4 points when applied to flat-tipped rotor blades and up to 0.15 points with squealer tips. Alternating arrays of grooves show improvements for flat-tipped blade performance by up to 0.76 points while having a little additional aerodynamic effect on the squealer tip compared to the best single-groove designs. Finally, grooved tip seals appear to exert greater influence on the aerodynamic performance of the turbine rotor when at larger tip gaps, indicating that grooved tip seals alter the sensitivity of rotor performance to the tip gap.
