Influence of Axial Spacing on Spanwise Efficiency of a Transonic Axial Compressor Rotor Stage

Abstract

This study numerically examined the span-wise efficiency of a transonic compressor stage representative of a modern jet engine. Two simulations of a compressor stage with different axial spacing between blade-rows were studied to determine the influence on the fluid flow. The two spacing are denoted as Close and Far. The time-accurate Reynolds-averaged Navier-Stokes computational algorithm MSU-TURBO with a CMOTT turbulence model was used for both cases. All solutions are three-dimensional and model WG-rotor relative motion through a sliding-mash interface and phase-lagged boundary conditions. At reduced spacings the compressor stage is less efficiency, with the largest difference occurring in the mid-span region. One reason for this trend is the wake from the upstream wake generator does not completely mix with the main passage flow prior to entering the rotor passage with decreased axial gaps. Anthoer major unsteady factor identified that influences the compressor efficiency is the tip clearance flow. In the Far case, the tip vortex did not reach spans below 80%. However, for the Close case, the tip vortex reached to around 40% span. Examining the flow one rotor chord downstream revealed the region between 40% and 60% span to be the main difference between the two axial spacings. Additionally, the rotor tip leakage at the reduced spacing occurs around 30% chord, while the Far casing leakage flow does not appear until 50% chord. The consequence of the early appearance of the tip vortex in the Close case is the greater axial distance that the vortex has to drift to lower spans. The greater strength of the Close spacing leakage flow over the Far case may be another reason the vortex is able to influence the mid-span region. The conclusions reached in this analysis are: the upstream wakes interact with the rotor boundary layer and the tip flow is a major factor at reduced spacings. The purpose of this study was to identify the reasons for efficiency losses, therefore further investigations into the correlation between axial spacing and efficiency loss are needed.