Current Research at the Gas Turbine Laboratory

A Unified Approach for Vaned Diffuser Design in Advanced Centrifugal Compressors

Jonathan Everitt, Jürg Schiffmann
Advisor: Prof. Spakovszky
Modern internal combustion engines utilize high pressure ratio turbochargers combined with NOx control strategies to improve efficiency and reduce emissions. In such an application, the centrifugal compressor has to simultaneously achieve high efficiency, high pressure ratio and a broad operating range. To meet these requirements, the trend has been towards highly loaded compressors, utilizing high speed impellers with backward-leaning blades for extended operating range and vaned diffusers for enhanced pressure recovery with compact geometry.
The flow out of the impeller presents multiple challenges for the vaned diffuser: it is transonic, unsteady, and highly non-uniform in both axial and circumferential directions. The relative importance of each of these factors is not well understood: different researchers have reached different conclusions regarding, for example, the importance of the impeller outflow non-uniformity. Diffuser design therefore largely depends upon historical correlations, CFD simulation and careful experimentation.
The objective of the investigation is therefore to rigorously establish the links between diffuser geometry, performance, component matching and stability. The technical approach combines first principles based modeling with high-fidelity calculations and experiments using a unique swirling flow diffuser test rig at the Gas Turbine Laboratory. The goal is to develop design criteria and to define performance metrics expressed in terms of overall vane parameters and appropriately averaged inflow properties that can be applied in the preliminary design stage.