Investigations of Turbulent Flow in a Centrifugal Compressor Vaned Diffuser by 3-Component Laser Velocimetry

Abstract

The unsteady 3D impeller exit and vaned diffuser flow of a high-subsonic centrifugal compressor has been investigated with an LDV system. Time-resolved 3D velocity measurements were taken along a streampath at 8 positions from impeller exit downwards through the vaned diffuser and at 18 positions from hub to casing at each station. The compressor was operated at its best point at a rim Mach number of Mu = 0.75. Time-resolved (phase averaged) angle and velocity profiles are presented for 2 positions along the streampath. The time-averaged velocity, deterministic fluctuation intensity, turbulence intensity, and in-plane Reynolds sheer stress profiles, presented for all stations, show the evolution of flow and permit comparisons to in-house CFD calculations to be made. The flow leaving the impeller enters the diffuser with an asymmetric and distorted velocity profile. It is shown that the deterministic fluctuations caused by the jet/wake are quickly damped along the streampath. The results illustrate the deceleration of the flow arriving near the hub in the diffuser channel. The deceleration is accompanied by a sharp increase of turbulence. Near the casing, where the approach velocity is low, no deceleration occurs and the Reynolds stresses are high. Turbulence in the in-plane flow can be regarded as isotropic whereas the axial fluctuations clearly show a high amount of anisotropicity. The narrow diffuser passage required special optical measures for permitting close-to-wall LDV measurements. The experiences are described.