Secondary Flow Patterns Through a Turbine Vane Frame

The Turbine Vane Frame (TVF) is an integrated component interposed between the High-Pressure Turbine (HPT) and the Low-Pressure Turbine (LPT). It carries structural loads, provides shrouded paths for oil and service lines, guides the flow in the transition from small HPT radius to large LPT radius, and imparts to the flow the level of swirl required by the first LPT stage. Integrating these requirements in a single component offers significant size and weight benefits; it poses however a significant challenge to the aerodynamic design. The TVF takes the form of an S-shaped annular duct with low aspect ratio turning vanes. The strong development of secondary flow within the TVF is therefore a matter of particular interest; the scope of this paper is to expand the understanding of secondary flow patterns with respect to the widely accepted models based on linear blade cascades. Numerical and experimental results are leveraged to obtain insights into the flow structures; a method to evaluate vortical structures from experimental data based on a modified version of the Q Criterion is introduced. Experiments are performed in the Sector Cascade rig at the Institute for Thermal Turbomachinery and Machine Dynamics at the Graz Unversity of Technology. Two TVF geometries with differing strut counts are furthermore compared.