The effects of Reynolds number and roughness on compressor performance

Authors

F. Dietmann - University of Stuttgart, Germany
M.V. Casey - PCA Engineers, Lincoln, United Kingdom

Abstract

An empirical correlation (due to Casey and Robinson (2011)) for the effect of Reynolds number and roughness on the performance of compressors is examined and refined. The original correlation quantifies the inefficiency due to friction losses with an empirical coefficient Bref which differs for different machine types. The variation of this coefficient was originally given as a function of the specific speed and determined from extensive test data for many compressors, in which the Reynolds number, roughness and size were varied. In this paper the value of the empirical coefficient is determined from a theoretical analysis and the experimental data is reanalysed and extended to provide an improved correlation. Two theoretical approaches are given. The first determines the inefficiency using a method due to Traupel to give the frictional dissipation losses as a function of flow coefficient for a range of radial compressor stages. The second uses a well-known correlation for global efficiency, which is corrected for the non-frictional losses to derive the inefficiency due to the friction losses alone. The original test data and some additional data has also been analysed with a slight modification of the equation for the friction factor as a function of Reynolds number and roughness, and this removes some of the scatter in the experimental data. On this basis three independent methods for determining the relevant coefficient are now available. A suggestion is made for an improved correlation based on these three approaches.