Shape of Characteristics of a Transonic Compressor

Abstract

In this paper results from a transonic single-stage axial compressor test facility are presented and discussed concerning the shape of characteristics. For this purpose data of two similar configurations with and without casing treatments are tested. Observing the various characteristics one significant aspect appears: For nearly the whole working range (despite choke) the curve is unexpectedly flat and in several cases the curve even starts rolling (turning) over. This effect is more relevant in the transonic cases but also present for subsonic rotational speeds. However, the compressor is operating in stable mode until at stalling mass flow transient pressure traces reveal instabilities. From the same measurements clear spike-type stall indications are detectable, being distinctive for tip-critical rotors, where flow features at the tip are responsible for stall. For further investigation the data of the stage performance measurements are analyzed for different radial blade height positions in more detail. Apparently, the roll-over of the characteristics results from the upper mid-span region and not, as expected, directly from tip or hub. The reason for this is seen in an exaggerated increase of blockage spreading out from tip towards the upper mid-span while throttling the compressor. On one side this causes a region of comparatively high total temperature ratios but more significantly this region is characterized by high losses mainly due to mixing effects. By this, the efficiency values in the upper mid-span formerly intended by design are decreased over proportionally. By probe measurements and numerical results it could be affirmed that it is not primarily about stator flow phenomena but the effect is already apparent at the rotor exit plane and directly linked to the tip leakage vortex. Finally, the results are discussed in terms of a stall type model detecting the type depending on the curve gradient at stall onset.