Over the past decade a large amount of information has been accumulated on both centrifugal and axial compressors as a result of extensive research and development for gas turbine engines for aircraft. With the accumulation of this information it is important that a review be made of the ASME Power Test Code 10 for Centrifugal, Mixed-Flow and Axial Flow Compressors and Exhausters. As a result a committee has been appointed to revise ASME PTC 10 and bring it up to date. It is desirable as a part of this revised test code to include information on the problem of surging in compressors. Surging normally is defined as backflow which expresses itself both audibly and in violent instrument fluctuation. It is probable that the term surge should be extended to include any destructive forces that result from aerodynamic instability within the machine. If this is done, the problem of detection becomes more acute. For example, instabilities in both axial flow and centrifugal compressors which were not audible have resulted in failures. Rotating stall which has been reported in a number of papers has been investigated in both rotating blade rows and in stationary cascades. It is not the purpose of this paper to rehash the problem of rotating stall. Rather, it is intended here to discuss what aspects of aerodynamic instability should be considered by the user and the manufacturer and what common ground in compressor specifications can be met. It is recognized that although the compressor is normally the source of aerodynamic instabilities, these can be magnified by system arrangement. Therefore a compressor that operates quite satisfactorily in one system may fail in another. One of the main problems that is encountered in aerodynamic instability occurs with shifts in Reynolds number. Frequently the effects of Reynolds number change can be detected by running complete compressor maps over a range of inlet pressures. This in itself may require more testing than generally is expected by the supplier or the user. Therefore it is a second purpose of this discussion to emphasize the importance of thorough compressor test programs.

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