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

Abstract

The demand of further increased bypass ratio of aero engines will lead to low pressure turbines with larger diameters rotating at lower speed. Therefore it is necessary to guide the flow leaving the high pressure turbine to the low pressure turbine at larger diameter without any separation or flow disturbances. Due to costs and weight this intermediate turbine duct has to be as short as possible leading to aggressive (high diffusion) S-shaped duct geometries. To investigate the influence of the blade tip gap size on 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: An high diffusion intermediate duct is arranged downstream of a HP turbine stage providing an exit Mach number of about 0.6 and a swirl angle of −15 degrees. A 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 performed with two different tip gap sizes, 0.8 mm and 1.3 mm. The aerodynamic design was done by MTU Aero Engines. While Part I describes the investigation by means of five hole probes with thermo couples, boundary layer rakes and static pressure tappings Part II uses Laser-Doppler-Velocimetry (LDV) for measurements at duct inlet directly downstream the HP blades to obtain unsteady information about the inflow and to quantify the differences between the two tip gaps. Additionally oil-film visualization was used to discuss the surface flow at the outer and inner wall of the duct. A comparison with a numerical simulation is also given. This work is part of the EU-project AIDA (Aggressive Intermediate Duct Aerodynamics, Contract: AST3-CT-2003-502836).