Master thesis and internship[BR]- Master's thesis : Numerical investigation of a variable tandem outlet guide vane in a low pressure compressor cascade[BR]- Internship

Abstract

With a view to improving the performance of axial compressors, the present work investigates the possibilities of adapting the last compressor stage (OGV) to varying compressor operating conditions. In particular, when the inlet flow angle is modified to off-design values, this work proposes solutions that extend the range of incidence operations of the compressor stage. The strategy for boosting performance is to combine a technology already used for OGVs, tandem blades, with the principle of variable stator valves. Numerical simulations using a RANS model with SU2 software were first carried out on the unmodified tandem configuration around a reference flow angle of 50°, considered to be the design angle of the compressor, in order to show the influence of the flow angle on the overall performance of the tandem. It was shown that the nominal tandem configuration is highly dependent on the inlet angle, and that flow separation phenomena on either blade (sometimes both) introduce high losses that limit the OGV’s compression capacities. In addition, flow turning was not ideal with this configuration. Indeed, the flow left the stage with a tangential component, whereas it is crucial to have an almost exclusively axial flow velocity. Because of these factors, the incidence range of this nominal configuration was 14.7°. Following this, further simulations showed very good compressor stage performances when the tandem’s front blade was rotated to match the upstream flow. Three strategies for rotating the blade were investigated: rotation around the trailing edge, rotation around the middle point of the central chord and rotation around a point outside the blade. Rotation of the front blade significantly reduced compressor stage losses and, in most cases, prevented the separation phenomena that occurred with the initial configuration, while achieving better compression ratios at each inlet flow angle. The third strategy only investigated high flow angles (above 50°), as geometry changes are limited. Although the first two strategies showed similar results at low inlet flow angles, rotation around the trailing edge performed better than the other two tuning strategies, particularly at high flow angles. Using the latter, the incidence range can be extended to 34.2°, more than double the initial range. The present work highlighted the interest to use such a configuration, especially if operating operations lead to high inlet flow angles. Finally, simulations also showed that the optimum blade rotation angle was often close to that which minimizes the relative incidence of flow on the front blade leading edge for inlet flow angles around the reference angle (50°). However, for extreme angles (around -15 to +10° beyond this reference angle), the optimum angle of rotation is sometimes relatively different from this minimum angle of relative incidence, as more complex phenomena take on greater importance (gap dimensions, general shape of the tandem, etc.).