In order to reduce the compressor cost and weight, a 15% radial reduction was applied to the diffuser passage length of a high pressure ratio centrifugal compressor. The diffuser endwall was diverged to achieve an appropriate area ratio from diffuser inlet to outlet. Two different vane shapes were considered with the radially reduced diffuser passage. The first diffuser vane was a scaled version of the baseline airfoil vane shape and this was used to develop an understanding of the impact of the passage length reduction and diverging endwall. The second vane geometry was parameterized using an unconventional method derived from published literature and optimized to improve performance at DP. The radially reduced designs matched the performance of the baseline configuration. Furthermore, despite not being one of the goals, an extension of the stable operating range of 36% and 20% at the highest speed condition was achieved for the scaled and optimized designs, respectively. The current study focuses on understanding the physical reasons that generated this significant enhancement to the compressor operating range. In order to acquire the desired understanding, experimental data is presented to validate the Computational Fluid Dynamics (CFD) model and a detailed numerical investigation is presented to explain the changes in the flow field within the radially reduced designs. Detailed scrutiny of the flow field revealed a secondary flow feature in the diffuser passage which resulted in higher diffuser passage losses at DP but stabilised the flow as it developed for lower mass flow rate operating points.

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