Experimental characterization of instability onsets in a high speed axial compressor

The last years have seen a considerable growth of the aviation sector together with more severe environmental constraints. In order to reduce the environmental footprint of aircrafts, a higher efficiency is demanded to engines, leading to more loaded compressor stages, being though more prone to stall. Rotating stall and surge are accurately avoided as they reduce the performance of the machine and affect its structural integrity. Since these phenomena appear at the limit of the stable operating range, they have to be taken into account in the design of a new stage. CFD predictions still differ from the actual stall occurrence, deriving the necessity for the experimental characterization of any new compressor stage. This PhD project aims at providing an extensive experimental characterization of the stable and unstable operating range of a high speed low pressure axial compressor stage. The outcomes of the present research will shed light on the mechanisms leading such machine to stall and they will therefore contribute to the improvement of its design methodology with respect to its stability limit. Since a detailed characterization of the unstable range is only possible through the knowledge of the flow structures appearing in stable operating conditions, the stable operating range is also deeply investigated. The global performance of the stage is defined at first with time-averaged measurements. The detailed description of the flow field in stable regime is then provided with the time-resolved measurements: a typical blade passage is investigated by applying the phase locked average technique to the data acquired on the rotor casing and at the rotor outlet. What emerges is that the rotor is hub loaded and that the tip leakage flow remains in the blade passage also in near stall condition. The hub corner separation is likely to develop into hub corner stall, whereas the tip region is the most affected by losses, which appear to be due to both the tip leakage flows and rotating instabilities. The flow field at the stator outlet is also represented and shows a lower impact on the stage pressure ratio. The time-resolved data related to the unstable conditions have been acquired while continuously closing the throttling valve from the near stall operating point and quickly reopening it. Five speed lines have been investigated and various stall inception patterns appear at each rotational speed. The compressor stage tested exhibits an abrupt stall inception behavior at higher speeds, while modal waves dominate the lower rpm range. Modal waves are affected by an acoustic mode of the facility, resulting therefore to be modulated by a low frequency component at first, and then to become continuous. One of the speed lines investigated presents a sadle type characteristic with a second stability zone preceding the flow breakdown. The post-stall behavior is characterized: only one stall cell occupying approximately 50% of the annulus and travelling at around 20% of the rotational speed appears at all the rotational speeds investigated. The stall cell shape is shown in its radial, circumferential and axial extension, remarking the presence of defined flow patterns at the rotor inlet. The analysis of the experimental data results to be useful to improve not only the blade design, but also the analysis tools to employ for the conception of new compressors. Ameliorations are proposed for the experimental set-up, and for the application of numerical tools utilized for the definition of the aerodynamic and mechanical design.(FSA - Sciences de l'ingénieur) -- UCL, 201