Numerical study of ultra low solidity airfoil diffuser in an automotive turbocharger compressor

For the application of advanced clean combustion technologies, such as diesel HCCI/LTC, a compressor with high efficiency over a broad operation range is required to supply a high amount of EGR with minimum pumping loss. A compressor with high pitch of vaneless diffuser would substantially improve the flow range of the compressor, but it is at the cost of compressor efficiency, especially at low mass flow area where most of the city driving cycles resides. In present study, an ultra low solidity compressor vane diffuser was numerically investigated. It is well known that the flow leaving the impeller is highly distorted, unsteady and turbulent, especially at relative low mass flow rate and near the shroud side of the compressor. A conventional vaned diffuser with high stagger angle could help to improve the performance of the compressor at low end. However, adding diffuser vane to a compressor typically restricts the flow range at high end. Due to the limitation of flow range, vaned diffuser is mostly used on turbocharger compressor for large diesels where the work conditions don't vary too much; e.g., generation sets. Due to the wide operation range requirement in a light duty automotive diesel application, a centrifugal compressor with ultra low solidity airfoil diffuser was designed. Calibrated CFD simulation was carried out to predict its performance. It was also compared with the performance of the compressor with the same impeller but vaneless diffuser. The preliminary results showed that the ultra low solidity airfoil diffuser could improve the stage efficiency while maintaining a wide flow range of the compressor.