Effect of casing treatment to switch the type of instability inception in a high-speed axial compressor

Flow instability phenomena, such as surge and rotating stall, limit the operating range of compressors. Casing treatment is a commonly used technique to extend the operating range, and some corresponding physical explanations have been proposed for the effects of casing treatment on the flow field in compressors. While there is also a situation that a casing treatment that is effective for one compressor may be useless for another, as a result, which limits the practical application of casing treatments. Based on the knowledge that the radial distribution of blade loading has a great influence on the type of instability inception, a hypothesis is put forward: casing treatment could change the type of instability inception by altering the loading distribution, and the resulting influence on the occurrence of instability may be one of the reasons to explain some unsuccessful tests of casing treatment. This hypothesis is then verified experimentally and numerically in a transonic axial flow compressor operating at a specific speed where the loading of both rotor tip and stator hub is close to each critical value. The results present that casing treatment can alter the type of instability inception by redistributing the blade loading, and thus leads to an earlier part-span stall in the hub region for this compressor. The main concluding remarks in this paper could be practically beneficial for the design of casing treatment in compressors.