The results of two tests conducted with a state-of-the-art high-speed research compressor are compared, which features three variable stator rows and 4.5 stages total. One test campaign was carried out with a smooth casing and the other with advanced axial slot type casing treatments over two rotor blade rows. Full characteristics, including the surge line, were measured for a number of different rotational speeds and variable stator settings. The measurements were analysed with respect to both overall as well as stage performance. Special attention was paid to the impact that the variable stator setting has on the surge line of the compressor and how this impact changes in the presence of the casing treatments. Literature review has shown that there are both numerical investigations as well as experimental tests. However, the fraction of multi-stage, high-speed rig tests is rather small, which is understandable considering the cost and complexity related to the design, manufacture, test and analysis of such a vehicle [3] , [5] , [9] . The treatments have been designed specifically for the embedded aerodynamic environment of a multi-stage compressor, where wakes from the upstream variable stator row are present and interact with both the rotor blades and the casing treatment slots. The geometric features are scalable and effective for both transonic as well as subsonic flow conditions, which can be found at rotors of the compressor's first and third stage, respectively. One major goal of this particular casing treatment design was to improve compressor stability at part speed operating conditions. At high power operating points, the efficiency level of the baseline compressor without casing treatments was to be maintained. It can be shown that the advanced casing treatments effectively improve the stage operating range of the compressor at both design and part speed conditions while minimizing the impact on efficiency. The stage-wise analysis gives further insight into the impact of the casing treatment in terms of stage matching and working range. By comparing the results with and without casing treatment at the same aerodynamic speed, but different variable stator settings, it becomes clear that the casing treatments are effectively allowing the compressor to operate at higher working lines or different VSV schedule while maintaining the same level of stability. Thus, the efficiency increase that is associated with these changes can be attributed to the presence of the casing treatments. This exchange between stability, efficiency and variable stator setting is a significant benefit and has to be taken into account when evaluating the casing treatments with respect to compressor overall performance parameters.
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