Advanced Piv Measurements in High-Speed Turbomachinery Flows

Recent interests in the development of advanced turbines include both the efficient use of cooling flow and the reduction of energy loss attributed to the injection of secondary air streams into the gas path. This study focuses on the investigation of optimized endwall contouring and purge flow emerging the stator-rotor rim seal gap. The high speed short-duration turbine test facility of the von Karman Institute will host new designed components and Particle Image Velocimetry (PIV) will be performed to acquire an extensive experimental database for optimized rotor endwall designs. The advantage of PIV is to allow instantaneous spatial data acquisition with low-intrusiveness. This capability is powerful for the analysis of complex three-dimensional flow structures which are normally present in turbine stages. Multiple test configurations are studied in order to acquire both radial and meridional planes at turbine rotor inlet and blade passage. At the moment, the open literature is very scarce about PIV measurements of unsteady flow structures related to endwall contouring, rim seal design and purge rates in high speed turbine wind tunnels.

Installation of a PIV equipment into the high speed rig is challenging due to the packed hardware geometry with multiple design features and the transient operation of the rig itself. In order to get optical access through the turbine casing, endoscopes will be used for both camera sight and laser sheet introduction. The equipment must be designed and aligned precisely in order to acquire high-quality images and to avoid any disruption in the test facility. A laser endoscope will be introduced into a partially transparent stator vane which is designed taking into account the laser refraction on its transparent surface in the attempt to eliminate flow disturbance and flow-induced vibrations on the scope. 

Triggering the PIV system is also one of the biggest challenges especially for short-duration facilities, for which the blow down test lasts only 0.5 seconds. All data acquisition must be accurately synchronized. In order to analyze the fluid motion with respect to the rotor blades, clocking positions of blades will be identified with respect to the image acquisition timing by introducing a PIV synchronization system combined with a blade position sensor. The particle seeding will be carefully designed in order to reduce noise caused by strong laser light reflections on the metallic components. Advanced PIV techniques, such as time-resolved and stereo PIV will be performed to maximize the velocity field database by means of a carefully designed optical installation. The outcome of this research is not only the generation of a database of flow structures in turbine stages, but also to provide further practical guidelines for the implementation of PIV into high speed turbomachinery rigs.