Stall Inception in the Compressor System of a Turbofan Engine

Abstract

Compressor flow instabilities have been the subject of a great number of investigations during the past decade. While most of this research work was done on isolated test-rig compressors this paper presents stall inception measurements in the compressor system of a two-spool turbofan engine at various power settings. Several analysing techniques such as temporal low-pass and band-pass filtering, temporal and spatial Fourier transforms including power-spectral-density calculations of the spatial coefficients and a wavelet analysing technique are applied. For the low pressure compressor three different types of stall inception processes were observed depending on the rotor speed. At low-speed stall originates from spike-type precursors, while long wavy pressure fluctuations corresponding to modal waves were observed prior to stall at mid-speed for undistorted inlet flow. At high-speed the rotor shaft unbalancing dominates the stall inception process as an external forcing function. In the case of distorted inlet flow spike-type stall inception behavior dominates throughout the speed range. While filtering and the Fourier spectra give a good insight into the physical background of the stall inception process (but with a very short warning time), the wavelet transform indicates the approach of the stalling process a few hundred rotor revolutions in advance independendy of the type of precursor. Setting up a reliable stall avoidance control based on this analysis scheme seems to be promising.

Pressure Oscillations in a Single Stage Axial-Flow Low-Speed Compressor during Pre-Stall and Stall

“…The phase delay between the pressure signals equals the angle between the sensors. 6. For oscillations with low-frequency rotating stall, in all cases where the radial distances of both sensors from compressor casing differ (first sensor is mounted near the compressor casing, r t ' = 2 mm, and second sensor is in the mid-span of the blades, r 2 ' = 45 mm), the signals of both sensors and their frequency spectra, differ: when the pressure value of the first sensor increases during stall oscillations, the pressure value of the second sensor decreases.…”

Experimental analysis about the magnitude of the shaft frequency growth near stall in the axial compressor

“…In the research of Tryfonidis et al [5], Day et al [6] and Hoss et al [7], shaft f tended to increase when the compressor was run near stall. The analysis was focused on the wave energy variation, and they mentioned that not every compressor showed this phenomenon; only a certain compressor showed this prestall characteristic near stall.…”

“…Because of these nonuniformities, a signal which has a period for one revolution is generated repeatedly in every revolution, and shaft f represents the magnitude of shaft f . In the research of Tryfonidis et al [5], Day et al [6] and Hoss et al [7], this phenomenon, in which shaft f increase near stall, occurred in a certain axial compressor, and it was analyzed with respect to the wave energy variation near stall. However, no practical reasons were mentioned about this phenomenon.…”

The Pre-Stall Behavior of a 4-Stage Transonic Compressor and Stall Monitoring Based on Artificial Neural Networks

“…Strain gauge measurements in the literature report of bending stress in vanes exceeding stable operation by a factor of two during surge and by five under conditions of rotating stall (Rippl, 1995). For this reason, numerous investigations are under way to find reliable means for monitoring an eventual approach to the stability limit while running the machine (Ludwig and Nenni, 1978;Day et al, 1997;Höss et al, 1998;Walbaum and Rieß, 1998). Ways for alerts have been identified for individual test beds but a reliable warning of general validity also for foreign compressor designs is still unattained (Ludwig and Nenni, 1980;Schulze et al, 1998;Regnery, 1998;Grauer, 1998a, b), therefore the quest for proof of reliability initiates additional research.…”