An Explanation for Flow Features of Spike Type Stall Inception in an Axial Compressor Rotor

Abstract

The unsteady behavior and threedimensional flow structure of spiketype stall inception in an axial compressor rotor were investigated by experimental and numerical analyses. Previous studies revealed that the test compressor falls into a mild stall after emergence of a spike, in which multiple stall cells, each consisting of a tornadolike vortex, are rotating. However, the flow mechanism from the spike onset to the mild stall remains unexplained. The purpose of this study is to describe the flow mechanism of a spike stall inception in a compressor. In order to capture the transient phenomena of spiketype stall inception experimentally, an instantaneous casing pressure field measurement technique was developed, in which 30 pressure transducers measure an instantaneous casing pressure distribution inside the passage for one blade pitch at a rate of 25 samplings per blade passing period. This technique was applied to obtain the unsteady and transient pressure fields on the casing wall during the inception process of the spike stall. In addition, the details of the threedimensional flow structure at the spike stall inception were analyzed by a numerical approach using the detachededdy simulation (DES). The instantaneous casing pressure field measurement results at the stall inception show that a lowpressure region starts traveling near the leading edge in the circumferential direction just after the spiky wave was detected in the casing wall pressure trace measured near the rotor leading edge. The DES results reveal the vortical flow structure behind the lowpressure region on the casing wall at the stall inception, showing that the lowpressure region is caused by a tornadolike separation vortex resulting from a leadingedge separation near the rotor tip. A leadingedge separation occurs near the tip at the onset of the spike stall and grows to form the tornadolike vortex connecting the blade suction surface and the casing wall. The casingside leg of the tornadolike vortex generating the lowpressure region circumferentially moves around the leadingedge line. When the vortex grows large enough to interact with the leading edge of the next blade, the leadingedge separation begins to propagate, and then the compressor falls into a stall with decreasing performance.