Endoskopic PIV-Measurements in an Enclosed Rotor-Stator System with Pre-Swirled Cooling Air


Advanced cooling schemes are one important reason for the performance of modern gas turbine engines. Experimental investigations were performed at the ITS to determine the cooling efficiency of a so called pre-swirl cooling air system where cooling air is expanded through nozzles and directed across the upstream wheel-space of the turbine to feed air holes that are located at the periphery of the turbine disk. The PIV technique was adapted to allow for velocity measurements in this crucial part of the engine.
Measurements were performed at several distances from one nozzle exit in order to examine the influence of nozzle to nozzle spacing. Flow velocities were also acquired relative to the rotor disk in order to investigate the influence of receiver hole position.
Using PIV in such a complex rotor-stator system implied many problems that were new compared with those found in standard applications. The extremely narrow gaps and the total enclosure of the upstream wheel-space made the design of an optical access to the system very difficult. A laser light sheet had to be introduced and placed accurately only few millimetres from the rotor wall and stator surfaces. Thereby, contact with the engine parts had to be avoided to minimize reflections. With respect to the installation of the required camera, any modifications of the experimental apparatus affecting the flow inside the pre-swirl system were not tolerable. Last but not least, the short distance between the detector and the measuring volume, the acquisition of data in front of a moving, i.e. rotating surface and the expected high flow velocities normal to the measuring volume further aggravated the boundary conditions of this particular application.
Nevertheless, this study has shown that PIV is a suitable and efficient method to investigate the flow in enclosed rotor-stator systems. Radial distributions of the tangential velocity show that pre-swirl efficiency decreases dramatically with increasing distance from the nozzle exit, supporting the conclusion that the nozzle to nozzle spacing is one parameter that influences the system performance. The study has further shown that the geometric shape of a pre-swirl system must be taken into account in the design of such an air transfer system.