Flow field in the vaned diffuser was calculated with CFD code. In order to take the large flow angle difference of the impeller discharged flow between hub and shroud into account, measured total pressure and flow angle downstream of the impeller with vaneless diffuser were used as the inlet boundary condition. Calculated results were compared with the measured total pressure distribution at the exit of the vaned diffuser and the results of oil flow visualization at hub and shroud. According to the results of the calculation and measurements, the possibility of existence of the separated region near pressure surface at hub was shown from the compressor choke to peak efficiency. In order to reduce this separated region, the vaned diffuser whose shroud wall was pinched from just downstream of the diffuser throat to vaned diffuser exit was calculated and tested. The improvement of flow field and the pressure recovery in the vaned diffuser was confirmed with the measurement of the static pressure and the total pressure at the vaned diffuser exit. The efficiency of the compressor was also improved from the compressor choke to peak efficiency. This study shows that the reduction of separation near pressure side at hub is effective way to improve the vaned diffuser performance.

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