An investigation of unsteady impeller-diffuser interactions in a centrifugal compressor

William Barry Bryan, Purdue University

Abstract

An investigation of steady and unsteady flow phenomena in centrifugal compressors has been performed. In particular, the effect of vaned diffuser geometry on the compressor unsteady aerodynamics has been considered, with particular emphasis on the diffuser vane unsteady loading generated by the impeller circumferentially nonuniform flowfield. A series of experiments was performed in the Purdue Centrifugal Compressor Facility to quantify the compressor performance, impeller blade and diffuser vane steady surface static pressure, vaneless diffuser steady and unsteady velocity field, diffuser vane unsteady surface static pressure, as well as surge and rotating stall occurrence. These measurements were made for various flow rates, number of diffuser vanes, diffuser vane leading edge/impeller exit radius ratios, diffuser vane stagger angles, and nonuniform circumferential vane spacing. In conjunction with the above experiments, theoretical predictions of the unsteady vaneless diffuser wake velocities and the unsteady diffuser vane loading were developed. The linearized unsteady Euler equations were solved to predict the wake behavior, with a conformal transformation applied to existing axial flow cascade theory for unsteady diffuser vane loading predictions. It was found that the compressor performance could be improved through use of a vaned diffuser, with the greatest performance improvement with zero steady loading on the vanes. The unsteady wake velocity in the radial diffuser was seen to decrease rapidly with increasing radius. The decrease was greater than predicted by the theory, with the unsteady circumferential pressure gradient and the phase angle between the radial and tangential unsteady velocities determined to be important parameters. The diffuser vane unsteady loading was determined to be a strong function of flow rate and number of diffuser vanes, with the diffuser vane leading edge/impeller exit radius ratio important for smaller numbers of vanes. The correlation with theory was poor with a high diffuser vane row solidity, although fair correlation was seen with moderate solidity. Finally, the vaned diffuser row had a significant impact on surge and rotating stall margins and behavior. It was discovered that significant improvement could be made by proper adjustment of the diffuser vane row geometry.

Degree

Ph.D.

Advisors

Fleeter, Purdue University.

Subject Area

Mechanical engineering

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