This paper presents a design process for optimizing the aerodynamic performance of the gas outlet casing on single-stage turbines with high specific flow capacity. A full-annulus flange-to-flange (inlet-to-outlet) steady-state CFD model of the turbine stage which takes into account the interaction between the rotor and the gas outlet casing and ensures an accurate inflow condition for the latter, is used to predict the turbine stage performance. A meta-model based optimization for the gas outlet casing is then performed and a simplified CFD model is used for sampling and optimization. The geometry of the gas outlet casing is fully parameterized to enable the simultaneous variation of diffuser and collector geometry and an ordinal regression optimization algorithm is adopted for the objectives of maximizing the static pressure recovery of the gas outlet casing and ensuring the design robustness. Extensive test measurement of the turbine stage with its baseline and optimized gas outlet casing geometries on a full scale turbocharger test bench validates the CFD results and confirms the significant improvement of the exhaust casing pressure recovery, which leads to an improvement of turbine efficiency between 1.3 and 2.4% points over the relevant considered operating range. Traverse measurement using five-hole probes and the flow field predicted by CFD are in good agreement. Evaluation of the CFD results highlights a significant loss in the collector despite a high pressure recovery at the end of the diffuser for the baseline gas outlet casing. For the optimized geometry, the more uniform flow at the diffuser outlet results in greatly reduced loss in the 90° turn in the casing, and thus higher pressure recovery and turbine efficiency.
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