Aero Design Challenges in Wide-Operability Turbocharger Centrifugal Compressors

The prevalence of turbocharged engines in passenger car applications has been increasing rapidly, particularly in the past decade or so. The tightening of emissions requirements and goals for engine downsizing has significantly increased the demand on turbocharger compressor stable operating ranges and efficiency levels, for both diesel and gasoline engines. The operability limits of single-stage turbocharger centrifugal compressors are being severely tested, and customization of some key aerodynamic technologies is required to meet the challenge. This paper addresses some critical design challenges in wide-operability, single-stage turbocharger compressors for advanced automotive diesel engine applications. Starting with a few technological ground rules, a brief exploration of the design space for two notional advanced engine operating lines is presented. This serves to highlight basic correspondence between fundamental design choices and operability trends at the skeletal sizing level, while also considering the primary focus for detailed design customization of the 3D blading and flow path. A critical assessment of two key technology areas for addressing the aero design challenges is provided by way of a direct comparative example. Impeller passage diffusion has long been identified as an area for improving stage performance. It is shown that through a departure from the classic ruled-element blading approach used for most turbocharger centrifugal compressors, blade bowing and other techniques for controlling the impeller diffusion can improve the wide-range operability, albeit at the expense of manufacturing complexity. Similarly, the tailoring of a ported-shroud style casing treatment is presented as a necessary and integral design feature for tackling the performance goals of tomorrow. RANS CFD results are used to gain insight into the underlying flow field structure and help establish a framework for future enhancements.