Parasitic Loss Due to Leading Edge Instrumentation on a Low-Pressure Turbine Blade

Abstract

During the testing of development engines and components, intrusive instrumentation such as Kiel-head pitot probes and shrouded thermocouples are used to evaluate gas properties and performance. The size of these instruments can be significant relative to the blades, and their impact on aerodynamic efficiency must be considered when analyzing the test data. This paper reports on such parasitic losses for instruments mounted on the leading edge of a stator in a low-pressure turbine, with particular emphasis on understanding the impact of probe geometry on the induced loss. The instrumentation and turbine blades have been modeled in a low Mach number cascade facility with an upstream turbulence grid. The cascade was designed so that the leading edge probes were interchangeable in situ, allowing for rapid testing of differing probe geometries. Reynolds-averaged Navier–Stokes (RANS) calculations were performed to complement the experiments and improve understanding of the flow behavior. A horseshoe vortex-like system forms at the join of the probe body and blade leading edge, generating pairs of streamwise vortices which convect over the blade pressure and suction surfaces. These vortices promote mixing between the freestream and boundary layer fluid and promote the transition of the boundary layer from laminar to turbulent flow. The size and shape of the leading edge probes relative to the blade vary significantly between applications. Tests with realistic probe geometries demonstrate that the detailed design of the shroud bleed system can impact the loss. A study of idealized cylinders is performed to isolate the impact of probe diameter, aspect ratio, and incidence. Beyond a probe aspect ratio of two, parasitic loss was found to scale approximately with probe frontal area.