Stall Warning by Blade Pressure Signature Analysis

Abstract

At low mass flow rates, axial compressors suffer from flow instabilities leading to stall and surge. The inception process of these instabilities has been widely researched in the past---primarily with the aim of predicting or averting stall onset. In recent times, attention has shifted to conditions well before stall and has focused on the level of irregularity in the blade passing signature in the rotor tip region. In general, the irregularity increases in intensity as the flow rate through the compressor is reduced. Attempts have been made to develop stall warning/avoidance procedures based on the level of flow irregularity, but little effort has been made to characterize the irregularity itself, or to understand its underlying cause. Work on this project has revealed for the first time that the increase in irregularity in the blade passing signature is highly dependent on both tip-clearance size and eccentricity. In a compressor with small, uniform, tip-clearance, the increase in blade passing irregularity that accompanies a reduction in flow rate will be modest. If the tip-clearance is enlarged, however, there will be a sharp rise in irregularity at all circumferential locations. In a compressor with eccentric tip-clearance, the increase in irregularity will only occur in the part of the annulus where the tip-clearance is largest, regardless of the average clearance level. In this paper, some attention is also given to the question of whether the irregularity observed in the prestall flow field is due to random turbulence or to some form of coherent flow structure. Detailed flow measurements reveal that the latter is the case. From these findings, it is clear that a stall warning system based on blade passing signature irregularity would be difficult to implement in an aero-engine where tip-clearance size and eccentricity change during each flight cycle and over the life of the compressor.

Investigation of Performance and Rotor Tip Flow Field in a Low Speed Research Compressor with Circumferential Groove Casing Treatment at Varying Tip Clearance

“…Other authors, for example, Wisler et al [15], connect the occurrence of RI in Mailach et al to the fact that one stage of the four-stage compressor has been weakened by a tip clearance increase. Young et al [16], however, found similar disturbances as Mailach et al and März et al that rotated relative to the rotor under near stall conditions in their investigated single-stage compressor with large tip clearance. According to Camp and Day [17] compressor stall can be caused by a local (mostly in the rotor tip) flow breakdown, the so called spikes, in only one or two blade passages that grow into stall cells.…”

Physics of Prestall Propagating Disturbances in Axial Compressors and Their Potential as a Stall Warning Indicator

“…After repeating this process for 100 revolutions, the mean of all RMS was nondimensionalised by the dynamic pressure at the rotor inlet. A detailed description of the presented analysis procedure is given by Young et al (2011) [5]. It is now possible to determine the variance of pressure fluctuations caused by the disturbances themselves.…”

Stall inception and warning in a single-stage transonic axial compressor with axial skewed slot casing treatment

“…However, unsteadiness increased uniformly over the entire frequency domain at N = 70% and N = 80%. Young et al [29] suggested that non-uniform tip clearance was the main cause of the pressure irregularity on the basis of the experimental results in a low-speed compressor. However, the reason for the increase in the pressure signal irregularity at the rotor tip is not yet understood clearly.…”