Evolution of Turbulence and Its Modification by Axial Casing Grooves in a Multi-Stage Axial Compressor

Abstract

Abstract This paper examines the distribution of turbulent kinetic energy (TKE), normal Reynolds stresses, and production rates of stress components in the JHU refractive index-matched axial compressor at pre-stall and near the best efficiency (BEP) flow rates. Modifications to the turbulence caused by installing semicircular axial casing grooves (ACGs) are also investigated. The stereo-PIV measurements are performed in axial planes covering entire passages, from the hub to the tip, both between and within blade rows. The TKE is highly anisotropic and spatially inhomogeneous, with each of the normal stress components evolving differently. Most of the trends can be explained by examining the dominant production rate terms. Upstream of the rotor, at pre-stall flowrate, the TKE is high in the inlet guide vane (IGV) wakes, and in the tip region. The turbulence increases significantly upon entering the rotor with peak RMS values of axial velocity fluctuations reaching as high as 71% of the spatially averaged mean axial velocity. Near the rotor leading edge (LE), the TKE is elevated over the entire outer 30% of the span, expanding to 50% near the rotor trailing edge. Upon entering the stator, there is a rapid reduction in turbulence in the outer half of the passage aided by negative turbulence production due to axial extension. However, further downstream near the stator trailing edge, the turbulence increases along the lower suction side of the stator blade as the boundary layer separates intermittently. By stabilizing and homogenizing the flow, the ACGs reduce the turbulence production, hence the TKE, in the rotor and the stator. The only exception is an increase in turbulence at the interface between the outflow from the grooves at their upstream end and the passage flow. Near BEP of the untreated endwall, the TKE is much lower in all the investigated planes except for the region influenced by the groove outflow. The TKE downstream of the rotor and the stator for both smooth endwall and with the ACGs appears to be quite similar. Yet, the ACGs cause a reduction in the TKE peak but expand the area of elevated turbulence in the tip region. Distributions of measured eddy viscosity highlight the extreme non-equilibrium conditions over the entire machine, questioning the fundamental assumptions of local equilibrium in eddy viscosity-based Reynolds stress models.