This present work is aimed at providing detailed understanding of the flow mechanisms in a highly loaded centrifugal compressor with different diffusers. Performance comparison between compressor stages with pipe diffuser and its original wedge diffuser was conducted by a validated state-of-the-art multi-block flow solver at different rotating speeds. Stage with pipe diffuser achieved a better performance above 80% rotating speed but a worse performance at lower rotating speeds near surge, than that of stage with wedge diffuser. Four operating points including the design point were analyzed in detail. The inherent diffuser leading edge of pipe diffuser could alleviate the flow distortion upstream diffuser throat and created a better operating condition for the downstream diffusion, which reduced the possibility of flow separation in discrete passages at design rotating speed. At 60% rotating speed operating point, there was a misalignment between the leading edge absolute flow angle and the metal angle of diffuser, resulted in an acceleration near diffuser leading edge due to the large negative incidence angle. The sharp leading edge of pipe diffuser could largely accommodate this negative incidence as comparison of the round leading edge of wedge diffuser. As a result, the flow separation was depressed and a better performance was achieved in the pipe diffuser. At 60% rotating speed near surge, performance of the pipe diffuser dropped below wedge diffuser. Total pressure loss of pipe diffuser exceeded that of the wedge diffuser due to the larger friction loss near wall at throat and cone, meanwhile ineffective static pressure recovery for pipe diffuser was triggered by the strong boundary layer blockage in the front of pipe diffuser cone.

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