A Pre-Design Model to Estimate the Effect of Variable Inlet Guide Vanes on the Performance Map of a Centrifugal Compressor for Automotive Applications

The onset of aerodynamic instabilities in proximity of the left margin of the operating curve represents one of the main limitations for centrifugal compressors in turbocharging applications. An anticipated stall/surge onset is indeed particularly detrimental at those high boost pressures that are typical of engine downsizing applications using a turbocharger. Several stabilization techniques have been investigated so far to increase the rangeability of the compressor without excessively reducing the efficiency. One of the most exploited solutions is represented by the use of upstream axial variable inlet guide vanes (VIGV) to impart a pre-whirl angle to the inlet flow. In the pre-design phase of a new stage or when selecting, for example, an existing unit from an industrial catalogue, it is however not easy to get a prompt estimation of the attended modifications induced by the VIGV on the performance map of the compressor. A simplified model to this end is presented in the study. Figuring out a typical industrial pre-design phase, the model assumes the availability of the original performance data of the compressor without pre-whirl and only very few geometrical parameters. Based on fluid dynamic considerations and some additional models and correlations, a procedure is defined to correct the attended stage pressure ratio and efficiency as a function of the pre-whirl angle imposed by the VIGV. The model has been successfully validated using an experimental literature case study and is thought to represent a new useful preliminary tool for turbocharger designers.