High pressure ratio centrifugal compressors with vaned diffusers are required in modern gas turbine engines. A potential method to further improve the performance of such compressors is to apply splitters to the vaned diffuser. This paper numerically investigates design choices of splitters in a vaned diffuser of a high pressure ratio centrifugal compressor. The splitters are derived from the main vane but with different chord length. The total vane number has been adjusted to maintain the same diffuser flow capability relative to the impeller. When the splitter is designed in the vaned region of the datum diffuser, the flow separation has been successfully suppressed with increasing relative splitter chord length from 0.10 to 0.70. At high mass flow rate conditions, the stage efficiency and the diffuser pressure recovery coefficient increases by 1.0% and 0.04 respectively due to the suppression of the flow separation. However, at design mass flow rate, the stage efficiency and the diffuser pressure recovery coefficient drops slightly by 0.3% and 0.01 respectively because the enhanced friction loss of vane surfaces is now dominant. When the splitter vane reaches the semi-vaneless region by further increasing its chord length from 0.8 to 0.95, the shock structure changes from two parallel shocks to two sequential shocks at first, but turns to the datum shock structure at last. The stage efficiency and the diffuser pressure recovery coefficient of the sequential shock structural case significantly drops by 0.7% and 0.03 respectively due to the increased shock strength and thus the higher shock loss. It is indicated that the application of splitter vanes in the vaned region of the diffuser is beneficial when the induced friction loss from splitter surfaces is less significant or the reduced loss from the suppressed flow separation is more dominant, which is the case that the flow separation happens relatively downstream or the case operating at large incidence conditions.

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