The Influence of Blade Tip Gap Variation on the Flow Through an Aggressive S-Shaped Intermediate Turbine Duct Downstream a Transonic Turbine Stage: Part I — Time-Averaged Results

The demand of further increased bypass ratio of aero engines will lead to low pressure turbines with larger diameters which rotate at lower speed. Therefore, it is necessary to guide the flow leaving the high pressure turbine to the low pressure turbine at a larger diameter without any loss generating separation or flow disturbances. Due to costs and weight this intermediate turbine duct has to be as short as possible. This leads to an aggressive (high diffusion) s-shaped duct geometry. To investigate the influence of the blade tip gap size of such a nonseparating high diffusion duct flow a detailed test arrangement under engine representative conditions is necessary. Therefore, the continuously operating Transonic Test Turbine Facility (TTTF) at Graz University of Technology has been adapted: A high diffusion intermediate duct is arranged downstream a HP turbine stage providing an exit Mach number of about 0.6 and a swirl angle of 15 degrees (counter swirl). An LP vane row is located at the end of the duct and represents the counter rotating low pressure turbine at larger diameter. In order to determine the influence of the blade tip gap size on the flow through such an s-shaped turbine duct measurements were conducted with two different tip gap sizes, 1.5% span (0.8mm) and 2.4% span. (1.3mm). The aerodynamic design of the HP vane, the HP turbine, the duct and the LP vane was done by MTU Aero Engines. The investigation was conducted by means of five-hole-probes with thermocouples, boundary layer rakes and static pressure taps at the inner and outer wall along the duct at several circumferential positions. Five-hole-probe measurements were done in five planes within the duct and in two planes downstream of the LP vane. A rough estimation of the duct loss is given at the end of the paper. Part II of this work deals with two-component Laser-Doppler Velocimeter (LDV) measurements at duct inlet directly downstream the HP blade to obtain unsteady information about the inflow. Additionally, oil film visualisation was used to get information about the surface flow at the outer and inner wall of the duct.