Use of Endwall Contouring to Improve the Efficiency of Gas Turbines

Abstract

The widespread use of gas turbines in energy generation and aircraft propulsion means that small gains in efficiency can translate to large reductions in emissions produced. Next generation turbines may even be used in a fossil fuel free society, and key to this is optimal aerodynamic performance.

Purge flows are prevalent in modern gas turbine design to allow for turbine entry temperatures (TET) far beyond material capability. The history of the gas turbine has progressed with increased efficiency from correspondingly increased TETs and compression ratios, \(r_c\). The purge flow passes through a rim seal and interacts with the mainstream flow, modifying the blade secondary flow structures and reducing stage efficiency. These structures may be controlled using End Wall Contouring (EWC), though experimental demonstration of their benefit is seldom reported in the literature. Within a review of the literature, a disconnect is identified between predictions of EWC efficacy and obtained performance. Further to this, the difficult to access turbine stage obfuscates the mechanics of the EWC interaction with the flow.

The optically accessible turbine at the University of Bath was designed to directly measure and visualise the flow field within the blade passage for a rotor with EWC. The single-stage turbine enables phase-locked flow field measurements with volumetric Particle Image Velocimetry (PIV). Purge flow was supplied to investigate a range of operating conditions in which the secondary flow structures were modified. The modular turbine rotor allowed for expedient change of bladed rings, or blings. These blings featured a range of endwall designs from a cylindrical axisymmetric baseline to a range of non-axisymmetric EWCs.

Three-dimensional vortex systems known as secondary flow structures were experimentally measured within the rotating turbine stage. The Pressure-Side leg of the Horse Shoe Vortex (PS-HSV) and Egress Vortex (EV) of concurrent rotational direction were identified and tracked within the blade passage baseline cylindrical rotor. An increase in purge flow rate monotonically shifted the EV toward the Suction-Side (SS) of the adjacent blade. The migration of the PS-HSV towards the SS resulted the two aforementioned vortices to merge and form the Passage Vortex (PV). The coalesence was delayed with the inclusion of superposed purge flow, as confirmed by measurements of vortex strength assessed with the line integral method which was aligned to the rotational vector of the vortices.

The EWC rotor designs included features to alter the PS-HSV, and guide the EV in the passage to maintain displacement from the adjacent suction side. The robust post-processing algorithm was able to identify the system of secondary flow vortices in the EWC rotor blade passages. At high purge-mainstream mass-fraction (\textit{MF}), the EWCs were found to be effective at modifying the formation and movement of the PS-HSV. Despite differing design philosophies, the EWCs reduced the strength of the secondary flow vortices with purge.

Date of Award	25 Jun 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Carl Sangan (Supervisor), Oliver Pountney (Supervisor) & David Cleaver (Supervisor)