The Effect of Rotor Tip Clearance Size onto the Separated Flow Through a Super-Aggressive S-Shaped Intermediate Turbine Duct Downstream of a Transonic Turbine Stage

The demand for a further increased bypass ratio of aero engines will lead to low pressure turbines with larger diameters, which rotate at a 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 (ITD) has to be as short as possible. This leads to an aggressive (high diffusion) and, furthermore, to a super-aggressive s-shaped duct geometry. In order to investigate the influence of the blade tip gap size on such a 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: An super-aggressive intermediate duct is arranged downstream of a transonic high pressure (HP)-turbine stage providing an exit Mach number of about 0.6 and a swirl angle of –15 deg. A second low pressure (LP)-vane row is located at the end of the duct and represents the counter-rotating low pressure turbine at a larger diameter. A following deswirler and a diffuser are the connection to the exhaust casing of the facility. In order to determine the influence of the blade tip gap size on the flow through such a super-aggressive s-shaped turbine, duct measurements were conducted with two different tip gap sizes, a 1.5% span (0.8 mm) and a 2.4% span (1.3 mm). The aerodynamic design of the HP-turbine stage, ITD, LP-vane, and the de-swirler was done by MTU Aero engines. In 2007 at the ASME Turbo Expo, the influence of the rotor clearance size onto the flow through an aggressive ITD was presented. For the present investigation, this aggressive duct has been further shortened by 20% (super-aggressive ITD) so that the flow at the outer duct wall is fully separated. This paper shows the influence of the rotor tip clearance size on to this separation. The flow through this intermediate turbine duct was investigated by means of five-hole-probes, static pressure taps, boundary layer rakes, and oil flow visualization. The oil flow visualization showed the existence of vortical structures within the separation where they seem to be imposed by the upstream HP-vanes.