Characterization of the flow field response to vaneless space reduction in centrifugal compressors

Abstract

The unsteady three-dimensional flow field for two centrifugal compressors of nearly identical design (one with a marginally smaller impeller-diffuser gap) is interrogated to assess the difference in the time averaged performance and unsteady loading characteristics of the impeller blades. Computational calculations are conducted for three different operating points. Results show that the difference in the time-average performance of the two compressors is due to the change in the level of loss and blockage generated from a difference in the strength of the impeller-diffuser interaction between the two compressors. The unsteady component of loading on the impeller blade originates from pressure waves propagating with a phase difference on the suction and pressure surfaces of the blades. The frequency of the waves is set by the diffuser vanes passing frequency and the wavelength by the product of the diffuser vane passing period and the speed of the wave. It is demonstrated that the difference in static pressure from the pressure surface to the suction surface at the leading edge of the diffuser vanes sets the amplitude of unsteady loading on the blades of the impeller.

(cont.) The level of pressure on both surfaces of the diffuser vane at the leading edge is determined by the value of the local angle of incidence. It is concluded that for a given diffuser design, changes in the impeller configuration or impeller-diffuser gap that result in a variation of the diffuser incidence will lead to a different unsteady loading distribution on the blades of the impeller.