Low-Speed Annular Cascade Tests of an Ultra-Highly Loaded Turbine With Tip Clearance: Part 1 — Near Design Incidence Free

Annular cascade tests were carried out to study the performance of an ultra-highly loaded turbine cascade (UHLTC) with a design turning angle of 160 deg. The UHLTC is for applications to future high-temperature gas turbine engines. This paper describes details of the secondary flows and the associated total pressure losses of the UHLTC obtained at a test incidence of −2.7 deg. The cascade flows were measured with a small five-hole Pitot probe located at 21 traverse measurement planes upstream, inside and downstream of the UHLTC. From the measurements, detailed flow structures and the loss evolution process were analyzed. Flow visualization tests were also carried out to see more details of the flows on the blade surfaces, on the endwalls and in the blade tip gap. Various flow separations and various small vortices associated with the passage and leakage vortices, such as corner vortices and edge vortices, separation bubbles, and the associated reverse flows, were seen. These were clarified from various flow lines showing separation, attachment/reattachment and division of each flow. The results obtained from the flow visualization were compared with those from the traverse measurements. Large total pressure losses occur inside the cascade passage as well as downstream of the cascade. Various strong passage vortices, strong leakage vortex, strong swirling flows upstream and downstream of the cascade and their associated various flow separations, are the main causes of the loss generation. The coefficient of total pressure loss generated inside the cascade was 0.28 at the test near-design incidence. The actual turning angle of the flow from the cascade inlet and the cascade outlet was 146 deg. Some schematic drawings of the flow structures in the present UHLTC were also given. The basic flow structures did not differ significantly from those seen in the conventional cascades with much smaller turning angles, except for stronger passage vortices, larger internal loss and larger downstream mixing loss due to the very high turning angle of the UHLTC.