PIV Application for Investigation of the Rotor Blade Tip Interaction with a Casing Treatment in a Transonic Compressor Stage

Abstract

This contribution describes the experimental investigation of the blade tip interaction with a casing treatment implemented to a transonic compressor stage using particle image velocimetry (PIV). The results obtained allowed for direct comparison with numerical simulations of the same compressor stage including the CT geometry, carried out using the DLR TRACE code following a new approach to efficiently perform time-accurate casing-treatment simulations. The single-stage transonic axial compressor was equipped with a casing treatment (CT), consisting of 3.5 axial slots per rotor pitch in order to investigate the predicted extension of the stall margin characteristics. Contrary to most other studies, the CT was designed especially accounting for an optimized optical access in the immediate vicinity of the CT, rather than giving maximum benefit in terms of stall margin extension. The nearly rectangular geometry of the CT cavities allowed one dividing bridge between two slots to be made of quartz glass with curvatures matching the casing. Thus the flow phenomena could be observed with essentially no disturbance caused by the optical access. Two periscope light sheet probes were specifically designed for this application to allow for precise alignment of the laser light sheet at three different radial positions in the rotor tip region (at 87.5%, 95% and 99% blade height). For the outermost radial position the light sheet probe was placed behind the rotor and aligned to pass the light sheet through the blade tip clearance. It was demonstrated that the PIV technique is capable of providing velocity information of high quality even in the tip clearance region of the rotor blades. Phase-constant measurements were carried out with a resolution of 8 phase angles per blade pitch in relation to the CT slots visible in the camera’s field of view. The chosen type of smoke-based seeding with very small particles (about 0.5 µm in diameter) supported data evaluation with high spatial resolution, resulting in a final grid size of 0.5 x 0.5 mm. The PIV data base established in this project forms the basis for further detailed evaluations of the flow phenomena present in the transonic compressor stage with CT and allows validation of accompanying CFD calculations using the DLR TRACE code. Based on the combined results of PIV measurements and CFD calculations of the same compressor and CT geometry a better understanding of the complex flow characteristics can be achieved.