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Tip-clearance effects in axial compressors 10.17863/CAM.57067 Young, Anna The flow in the tip-clearance gap of a compressor has a profound effect on its performance, but understanding of the actual flow phenomena involved is limited. The current work aims to improve the situation through a comprehensive set of tests on a compressor with different clearance sizes and levels of eccentricity. The size of the tip-clearance gap is shown to affect both the flow coefficient at which the compressor stalls and the mechanism by which stall inception occurs (spikes or modes). Furthermore, the industry ’rule of thumb’ that the stalling flow coefficient of an eccentric compressor is determined by the maximum tip-clearance is shown to be overly cautious. In this thesis it is shown that the more stable, small tip-clearance part of the annulus has a stabilising influence on the large clearance sector. The second part of the thesis focuses on pre-stall disturbances in the rotor tip region and their link to stall inception. Past work has found that the signature generated by passing rotor blades becomes more irregular as the compressor approaches stall. Various attempts have been made to develop stall warning systems based on this irregularity, but without understanding its underlying cause. It has been revealed that the increase in irregularity depends on both the tip-clearance size and the eccentricity level in the compressor. When the clearance is small and uniform, reducing the flow rate leads to a modest increase in irregularity. If the tip-clearance is large, there will be a sharp rise in irregularity all around the annulus as the compressor moves towards stall. In a compressor with eccentric tip-clearance, the irregularity will only increase in the large tip-clearance sector. This presents a major obstacle to reliable stall warning as tip-clearance size and eccentricity change during each flight and over the life of an engine. Having examined the conditions under which pre-stall irregularity is most pronounced, the cause of the irregularity itself is investigated. The tip region is shown to be occupied by discrete disturbances that have been named ’blue holes’. The techniques developed as part of the work described in this thesis enable these disturbances to be investigated in detail. The experimental data gives strong evidence that the blue holes are caused by radial vortices, and this is confirmed by computational results. The connection between blue holes and stall inception is also investigated. In a compressor with small tip-clearance, the formation of blue holes is found to lead directly to spike formation. A compressor with larger tip-clearance supports blue hole activity while continuing to operate stably. These blue holes are initially within the blade passage and do not appear to cause stall until they reach the rotor leading edge plane. University of Cambridge 2012-04 Thesis doctoral PhD en https://www.repository.cam.ac.uk/handle/1810/309969 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/aace6349-4711-4bd5-9c49-40753130a1ae/download 7a67473b07759467d032795cacef697c https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/2ddc89c1-beea-4497-acaa-3afb8fec5dbb/download 87eda9de84448d1f82354d60eee3eb5f https://www.rioxx.net/licenses/all-rights-reserved/ compressors blue holes