Unsteady Investigation of the Effect of Purge Air on the Flow Field Inside a Turbine Vane Frame Using Particle Image Velocimetry

The urgent need to reduce the emission of greenhouse gases, in combination with high fuel expenses, motivates manufacturers to design more efficient civil aircraft engines. In the case of directly driven jet engines, this is possible through the increased by-pass ratios for high propulsive efficiencies. This tendency implies a change in the operating condition of the low-pressure turbine (LPT) towards lower rotational speeds at larger diameters. Turbine vane frames (TVFs) constitute a new generation of inter-turbine ducts that guide the airflow from the high-pressure turbine (HPT) to the LPT in radial and circumferential direction. The TVF setup integrates turning vanes, and thus removes the need for a separate vane blade-row in the first LPT stage. Consequently, the TVF yields a benefit for overall engine weight and length, resulting in overall efficiency gains. This paper offers new insight into airflow through a TVF duct. Experimental measurements have been conducted at the two-spool test rig at the Graz University of Technology, consisting of a single-stage HPT, the TVF, and the first LPT rotor. Engine-relevant flow conditions are achieved at the TVF inlet, including HPT tip clearance and purge air effects. Particle Image Velocimetry (PIV) was used to capture the flow field in between two struts of the TVF. Inside each strutted segment, splitter vanes are located along the second half of the TVF axial length. This paper presents new results for a TVF based on measurements from a PIV test section located upstream of the splitter vanes in the first half of the TVF duct. Flow data in the area of strong transient interactions between the HPT and the TVF is recorded and discussed in terms of aerodynamic performance. To reveal the unsteady behavior of the fluid, the flow field has been recorded at six serial stator-rotor positions. In addition, two data sets of varying HPT purge flows were obtained and discussed in order to characterize the effect of purge air inside the measurement domain.