The Influence of Simulated Purge Flow on the Secondary Flow of a Low-Speed Turbine Cascade

The present thesis documents experimental measurements of a low-speed linear turbine cascade. The overall objective of the thesis was to enhance the understanding of turbine near endwall flows, termed secondary flows. Of particular interest were the loss generation and energy dissipation mechanisms present within the turbine blade row. Point measurements of the downstream pressure fields for different cascade configurations were made using a seven-hole pneumatic pressure probe. The generation of total pressure losses were examined in relation to the inferred secondary flow structures. Also, the corresponding turbulence field was measured for one of the cascade configurations using a hot-wire probe thereby providing insights into the role of the turbulence field in the generation of total pressure losses. The interpretation of the flow fields were aided with experimental oil film flow visualization and corresponding CFD simulations. In a real turbine engine there is an axial gap between the stationary and rotating bladerows. The term rim-seal refers to the geometry of the axial gap and the cooling flow, termed purge flow, which is typically injected into the mainstream from within the gap to prevent ingress of the hot mainstream gases. The present work examines the effects of the rim-seal geometry on the secondary flow and associated losses for different levels of purge flow. It was found that the presence of the rim-seal geometry significantly alters the secondary flow compared to that of a traditional flat endwalled turbine cascade. The addition of purge flow further enhances the secondary flow and the bladerow losses increase with increasing purge flow. It was therefore recommended that modern turbine designs consider both the rim-seal geometry and the purge flow during the design process. Previous studies have shown that a secondary loss mitigation technology, termed non-axisymmetric endwall contouring, significantly reduces the endwall losses generated through a turbine bladerow. The present work investigated a computationally optimized endwall design for the present turbine cascade for varying levels of purge injection. The experimental measurements found that the endwall contouring, with the upstream rim-seal geometry, was effective at reducing the bladerow losses for the range of positive net purge flows investigated.