Methodology for evaluating hot gas path defects in an exhaust jet

The components of modern jet engines represent state-of-the-art technologies. Their technological complexity results in costly components and thus expensive spare parts. Therefore, one of the main goals during the regeneration of jet engines is to repair, rather than replace, the components. So far, defects are mostly detected after a complete disassembly of the engine and an individual inspection of each component. One approach to reduce the costs of the regeneration process, therefore, is to acquire the information necessary for planning at an early stage and particularly before the disassembly of the jet engine. This approach requires new measurement methods. The present paper shows the Background-Oriented-Schlieren (BOS) method to be a promising contribution towards achieving this goal. Every defect in the hot gas path of a jet engine has a direct influence due to changes in the local temperature on the density of the flow. The BOS method provides a three-dimensional measurement of the density distribution in the exhaust jet. The localization and identification of non-uniformities in this distribution can be used to identify defects within the hot gas path. A three-dimensional unsteady CFD-simulation of the full annulus of a five-stage low-pressure turbine as well as a steady calculation of the exhaust jet is used to identify the signature in the exhaust density distribution of typical defects. As prototypical hot gas path defects, temperature variations at the inlet of the turbine were used to demonstrate that temperature non-uniformities can be detected with BOS. Hence, BOS measurements can be used for the identification of defects modeled a priori by CFD within the hot gas path before the disassembly of a jet engine. A methodology for evaluating the limits of detectability of non-uniformities with BOS is also provided.