Two stators of a multistage centrifugal compressor with progressively smaller outer radii have been designed, built and tested. The aim was to achieve a significant reduction in outer diameter of the compressor stage without compromising performance. The reduction of size was achieved by reducing the diffusion ratio (outer radius/inner radius) of the vaneless diffuser in two steps. In the first step the outer diameter of the entire stage was reduced by 8% compared to the baseline design. In the second stage the outer diameter was reduced by 14%. The outer radius of the smallest design was limited by the impeller exit diameter, which was kept constant, as was the axial length of the stage. The large radius baseline design has been tested on a rotating rig in a 1.5 stage setup. This setup aimed at simulating multistage behavior by applying a pseudo stage upstream of the main stage. The pseudo stage consisted of a set of non-rotating preswirl vanes, in order to mimic an upstream impeller, and was followed by a scaled version of the return channel of the main stage. The experimental database was then used to calibrate a 1D analysis code and 3D-CFD methods for the ensuing design and optimization part. By applying extensive Design-Of-Experiments (DOE), the endwalls as well as the vanes of the stator part were optimized for maximum efficiency and operation range. In order to preserve multistage performance, the optimization was constrained by keeping the circumferentially averaged spanwise flow profiles at the exit of the smaller radius stages within close limits to the original design. The reduced radius designs were then tested in the same 1.5 stage setup as the baseline design. The results indicate that the reduction in size was feasible without compromising efficiency and operation range of the stage.

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