A method of centrifugal compressor performance prediction

Predictions of compressor performance are often wanted for tentative designs where the detailed geometry of blade shapes and flow passages is not completely specified. A method of performance prediction for centrifugal compressors has been developed which requires knowledge of only overall dimensions and blade angles. This method can be applied to any centrifugal compressor with radial outflow (i.e. with no net axial component of velocity at rotor outlet and with diffuser centered about a radial plane); treatment of vaneless and vaned diffusers is included. The method is based upon analysis of a simplified flow model for each component of the system, with, in the case of the impeller, inclusion of an empirical factor governing growth of the blockage at outlet from the blade passages. The calculations involve a considerable amount of iteration, and are performed by means of a computer program. A description of the treatment is given, and various examples are presented of comparison with test data in order to illustrate the degree of success in prediction. Choking flow is generally well predicted, and the mass-flow/pressure-ratio characteristics produced have substantially correct form, although no universal means have been found of predicting accurately the onset of surge. For the cases examined the error in predicted efficiency level is within plus or minus 1 to 2 per cent at design speed, sometimes more at low speed. The prediction method also serves as a tool to ensure good matching between designs of impeller and diffuser, and can assist the understanding and analysis of conventional (i.e. flow-average) test rig measurements.