Endwall Flows in Transonic Turbine Cascades

The results of an investigation of the midspan and endwall flows in transonic linear turbine cascades are documented here. A family of five turbine cascades with different levels of flow turning and aerodynamic loading were used for this study to quantify the effects of these parameters on loss generation in compressible flows. Experimental data on these flows are scarce in the open literature. In addition, the application of two different passive flow control techniques for reducing the endwall losses is examined here: these are referred to as endwall contouring, and airfoil pressure-side modification. The experimental investigations were conducted in the Pratt and Whitney Canada (PWC) High-Speed Wind Tunnel Laboratory at Carleton University. The measurements were made using a seven-hole pressure probe downstream of the cascades at both design and off-design Mach numbers. In addition to the measurements, surface flow visualization was conducted to assist in the interpretation of the flow physics. The results from complementary numerical investigations are also presented, and compared with the experimental data. Overall, the examined secondary flow structures are in agreement with previous low-speed findings. However, in contrast to low-speed results, the downstream mixing losses are mainly attributed to the dissipation of primary kinetic energy in the present study. Raising the exit Mach number results in weaker secondary flow structures and smaller secondary losses. The experimental results demonstrate that endwall contouring is a viable option for mitigating the secondary flows, particularly for the more highly-loaded cascade. The modification of the airfoil pressure surface is also found to provide a significant benefit in terms of endwall loss reduction. The differences between the measurements and the computational results highlight the need for detailed experimental investigations.