The challenge of stereo PIV measurements in the tip gap of a transonic compressor rotor with casing treatment

Abstract

This contribution is aimed at summarizing the effort taken to apply stereoscopic PIV (SPIV) measurements in the tip clearance of a transonic compressor rotor equipped with a casing treatment. A light sheet probe was placed downstream of the stator and aligned to pass the light sheet through a stator passage into the blade tip clearance of the rotor. A setup with three cameras has been used in order to record the entire 2C velocity field and the smaller area of 3C field of view at the same time instance for comparison with earlier 2C PIV results. A homogeneous seeding distribution was achieved by means of a smoke generator, working with evaporated and subsequently re-condensed mineral oil. The main emphasis of the SPIV measurement was to establish a data set with high spatial resolution close to the compressor casing, where the aerodynamic effects of a CT are known to be strong. Additionally the SPIV data was intended to be used for comparison with numerical results and related code validation, as detailed laser-based flow field investigations in the vicinity of a casing treatment in a transonic compressor environment are barely reported in the literature. The paper will discuss some major aspects of the utilized PIV data processing and point out a variety of frequently underestimated error sources that influence the overall quality of the recovered data in spite of the fact that the individual PIV recordings seemed to be of very good quality. Thus the authors will not focus on the PIV results and related interpretation of the flow field, but on the optimization and procedures applied during setup of the experiment and data processing respectively.

Imaging Flow Velocimetry with Laser Mie Scattering

“…Under optimal conditions, PTV measurements allow flow velocity measurements with a minimal distance that is the radius of the scattering particle. DGV and PIV measurements inside a rotating machine are described in [182][183][184]. The occurrence of unavoidable light reflections causes measurement errors for PIV and DGV [185].…”

Post-Processing Methods of PIV Instantaneous Flow Fields for Unsteady Flows in Turbomachines

“…It has proved its ability to provide useful experimental data for various research topics: rotor stator interaction in radial pumps or fans [Cavazzini et al, 2009;Meakhal & Park 2005;Wuibaut et al, 2002], tipleakage vortex in axial flow compressors [Voges et al, 2011, Yu & Liu, 2007, swirling flow in hydraulic turbines [Tridon et al, 2010]. Nevertheless, in most cases, PIV was efficiently applied to catch phenomena which were correlated with the impeller rotation: PIV images were taken phase locked with the rotor.…”

Aero-thermal flow characterization downstream of an NGV cascade by five-hole probe and filtered Rayleigh scattering measurements

“…Considering the above mentioned aspects in the context of using probes in turbomachinery environments, due to their non-invasive nature, laser-optical diagnostics appear to be the natural choice for these kinds of applications. Regarding planar methods, in particular particle image velocimetry (PIV) was successfully applied in numerous studies, ranging from the characterization of stator passages and wakes (Bryanston-Cross et al, 1992;Woisetschläger et al, 2003) to applications in rotating machinery (Wernet, 2000;Voges et al, 2012). As an alternative approach to PIV, Doppler global velocimetry with frequency modulation (FM-DGV) was applied on a transonic cascade (Fischer et al, 2012) as well as a rotating turbine stage (Fischer et al, 2013).…”

“…Despite their general ability to deliver time-resolved three-component velocity fields in turbomachinery environments, both PIV and DGV suffer from their susceptibility to laser scattering from surfaces, so that affected image areas have to be masked and thus cannot be included in the data evaluation (Woisetschläger et al, 2003;Voges et al, 2012;Fischer et al, 2013). In addition, seeding particles tend to deposit on windows, which, due to the degrading quality of optical access, in consequence leads to reduced testing periods.…”