Abstract
The study is focused on the analysis of the flow structure within the vaned diffuser of a transonic high-pressure centrifugal compressor stage. The analyzed time-dependent flow field comes from unsteady computations of the stage using a 3D Navier-Stokes code with a phase-lagged technique, at an operating point close to the design point. A good comparison with available experimental data allowed the use of CFD for investigating the details of the flow in order to assess the effect of the unsteadiness in the diffuser flow development. Applying various data processing techniques, it is shown that the unsteadiness is due to the jet and wake flow structure emerging from the radial impeller and to the pressure waves brought about by the interaction between the vane bow shock wave and the impeller blade. The interaction between the pressure waves and the vane pressure side boundary layer leads to a pulsating behavior of separated bubbles within the diffuser. The pressure waves are similar in shape and strength whatever the blade height. The observed change in the flow field from hub to tip is due to migration of the low momentum fluid contained in the wake toward the pressure side/hub corner.
Highlights
Impeller-diffuser interaction in centrifugal compressors with vaned diffuser plays an important role in the compression process
The unsteady flow that develops in the vaned diffuser of a transonic high-pressure-ratio centrifugal compressor has been analyzed in detail from CFD results
The unsteadiness comes from the combined effects of the jet/wake structure and the α+ pressure waves brought about by the interaction between the vane bow shock wave and the impeller blade
Summary
Impeller-diffuser interaction in centrifugal compressors with vaned diffuser plays an important role in the compression process. The vaned diffuser has to tolerate the distorted upstream flow due to jet-wake structure coming from the impeller, whereas the impeller is submitted to the potential effect of the vaned diffuser. Krain [1], Inoue and Cumpsty [2], and Ziegler et al [3, 4] tested different diffuser configurations to show the effect of different parameters on the pressure recovery through the diffuser. The main result is the importance of the inlet axisymetric flow angle which is governed by the radial gap between impeller blade trailing edge and diffuser vane leading edge. All the tested compressors had subsonic conditions at diffuser inlet
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