Effects of upstream wake phasing on the performance of transonic compressors

Abstract

The effect of the upstream wake phase on the work input (i.e., rise in stagnation enthalpy across the blade row) of a transonic rotor is examined computationally and analytically. It is found that the compressor work depends on the path followed by the wake vortices as they travel through the rotor passage. There can be an important impact on time-mean performance when the time-dependent circulation of the shed vortices in the wake is phase-locked to the rotor position. For the configurations tested, the calculated change in time-mean work input was approximately three percent. The effect on work input is explained in terms of the influence of the time-mean relative stagnation pressure nonuniformity associated with the unsteady (but phase-locked) wake vortex flow field. Changes in vortex path mean that the position of this nonuniformity is altered relative to the rotor. There is lower pressure rise, and thus lower work, when the rotor blade is embedded in the region of low time-mean relative stagnation pressure than when it is immersed in a region of high relative stagnation pressure. In addition to this essentially two-dimensional effect, it is demonstrated that the locations of the wake vortex paths can have substantial effect on the tip clearance flow, implying potential impact on pressure rise capability and rotor stability limits. Model calculations are given to show the magnitude and nature of this phenomenon.