Off design Reynolds stress tensor measurements in a centrifugal compressor vaneless diffuser

Abstract

Detailed off-design (16% below and 11% above design) flow measurements have been taken within a backswept centrifugal impeller vaneless diffuser using hot wire anemometry. 3-d velocity and six Reynolds stress components have been determined on eight measurement planes. High levels of turbulence kinetic energy were observed in both the blade and passage wakes, but these were only associated with high Reynolds stresses in the blade wakes. For this reason the blade wakes mixed out rapidly, whereas the passage wake maintained its size, but was redistributed across the full length of the shroud wall. Peak levels of Reynolds stress occurred in regions of high velocity shear and streamline curvature which would tend to destabilize the shear gradient. Stronger secondary velocities at the impeller exit at the above design flowrate led to significant axial velocities which delayed the development of an axisymmetric flow at this flowrate. The thickening of the hub boundary layer was more pronounced at the lower flowrate.