An analysis of stage performance in automotive turbocharger centrifugal compressors

As the concept of engine downsizing becomes ever more integrated into automotive powertrain development strategies, so too does the pressure on turbocharger manufacturers to deliver improvements in map width and a reduction in the mass flow rate at which compressor surge occurs. A consequence of this development is the increasing importance of recirculating flows, both in the impeller inlet and outlet domains, on stage performance. The current study seeks to evaluate the impact of the inclusion of impeller inlet recirculation on a meanline centrifugal compressor design tool. Using a combination of extensive test data, single passage CFD predictions, and 1D analysis it is demonstrated how the addition of inlet recirculation modelling impacts upon stage performance close to the surge line. It is also demonstrated that, in its current configuration, the accuracy of the 1-D model prediction diminishes significantly with increasing blade tip speed. Having ascertained that the existing model requires further work, an evaluation of the vaneless diffuser modelling method currently employed within the existing 1-D model is undertaken. The comparison of the predicted static pressure recovery coefficient with test data demonstrated the inherent inadequacies in the resulting prediction, in terms of both magnitude and variation with flow rate. A simplified alternative method based on an equivalent friction coefficient is also presented that, with further development, could provide a significantly improved stage performance prediction. Nomenclature A Flow area (m 2 ) AR Area ratio (-) b Passage height (m) B Blockage (-) Cf Equivalent skin friction coefficient (-) CP Static pressure recovery coefficient (-) Cp Specific heat at constant pressure (J/kgK) D Diameter (m) ṁ Mass flow rate (kg/s) p Static pressure (Pa) p0 Total pressure (Pa) R Radius (m) U Blade speed (m/s) V Absolute velocity (m/s) W Relative velocity (m/s) π Total-total pressure ratio (-) γ Ratio of specific heats (-) α2 Impeller tip flow angle relative to radial (°) φ01 Stage flow coefficient (-) η Isentropic total-total efficiency (-) ρ Density (kg/m 3 ) β Flow angle relative to meridional (deg) βbl Blade angle (deg) CORA Compressor Off-Design Radial Analysis 1-D One-dimensional SFM Swirl flow meter VLD Vaneless diffuser Subscripts: b Blade crit Critical ini Initial r Radial direction u Tangential direction 1 Stage inlet 2 Impeller exit 3 Vaneless diffuser exit 4 Stage exit