In the present investigation the influence of different diffuser geometry parameters on the stage efficiency and operating range of radial compressor stages has been studied. In addition to a parallel-walled vaneless diffuser, 20 vaned diffusers were experimentally tested with three different impellers having the same meridional contour. All diffusers were designed for industrial compressor stages with subsonic diffuser flow and had parallel sidewalls, diffuser vanes with simple circular are profiles of constant thickness and rounded leading edges.

In the first part of this investigation the length of the vaneless space, the number of diffuser vanes, the vane length, the inlet vane angle of a diffuser, and the solidity have been altered systematically.

The results show that the throat area of a diffuser, and consequently the number of diffuser vanes, are the most important parameters affecting the operating range of a diffuser. The vaneless space extent and the diffuser vane inlet angle have a significant effect on diffuser efficiency, while their effect on operating range is much smaller. Changing these two parameters induces primarily a shift of diffuser characteristics. The diffuser vane length is a very important parameter at overload, where high losses in a diffuser channel occur when using a long diffuser vane.

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