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.

While studies on compressor flow instabilities under the presence of inlet distortions have been carried out with steady distortions in the past, the investigation presented here focuses on the influence of transient inlet distortions as generated by variable geometry engine intakes of super- and hypersonic aircrafts on the characteristic and the nature of the instability inception of a LPC. The flow patterns (total pressure distortion with a superimposed co- or counter-rotating swirl) of the distortions are adopted from a hypersonic concept aircraft. A LARZAC 04 twin-spool turbofan was operated with transient inlet distortions, generated by a moving delta-wing, and steady total pressure distortions starting close to the LPC’s stability limit until it stalled. High frequency pressure signals are recorded at different engine power settings. Instabilities are investigated with regard to the inception process and the early detection of stall precursors for providing data for a future stability control device. It turned out that the transient distortion does not have an influence on the surge margin of the LPC compared to the steady distortion, but that it changes the nature of stall inception. The pressure traces are analysed in the time and frequency domain and also with tools like Spatial FFT, Power Spectral Density and Travelling Wave Energy. A Wavelet Transformation algorithm is applied as well. While in the case of clean inlet flow the compressor exhibits different types of stall inception depending on the engine speed, stall is always initiated by spike-type disturbances under the presence of steady or transient distortions though modal disturbances are present in the mid speed range that do not grow into stall, but rather interact with the inlet flow and produce short lengthscale disturbances. The obtained early warning times prior to stall are adversely affected by transient distortions in some cases. The problem of appropriate threshholding becomes evident. The best warning times have been acquired using a statistical evaluation of the Wavelet coefficients which might be promising to apply in a staged active control system which could include different phases of detection and actuation depending on the current precursor.

While studies on compressor flow instabilities under the presence of inlet distortions have been carried out with steady distortions in the past, the investigation presented here focuses on the influence of transient inlet distortions as generated by variable geometry engine intakes of super- and hypersonic aircraft on the characteristic and the nature of the instability inception of a LPC. The flow patterns (total pressure distortion with a superimposed co- or counterrotating swirl) of the distortions are adopted from a hypersonic concept aircraft. A LARZAC 04 twin-spool turbofan was operated with transient inlet distortions, generated by a moving delta wing, and steady total pressure distortions starting close to the LPC’s stability limit until it stalled. High-frequency pressure signals are recorded at different engine power settings. Instabilities are investigated with regard to the inception process and the early detection of stall precursors for providing data for a future stability control device. It turned out that the transient distortion does not have an influence on the surge margin of the LPC compared to the steady distortion, but that it changes the nature of stall inception. The pressure traces are analyzed in the time and frequency domain and also with tools like Spatial FFT, Power Spectral Density, and Traveling Wave Energy. A Wavelet Transformation algorithm is applied as well. While in the case of clean inlet flow, the compressor exhibits different types of stall inception depending on the engine speed, stall is always initiated by spike-type disturbances under the presence of steady or transient distortions. Modal disturbances are present in the mid-speed range that do not grow into stall, but rather interact with the inlet flow and produce short length scale disturbances. The obtained early warning times prior to stall are adversely affected by transient distortions in some cases. The problem of appropriate thresholding becomes evident. The best warning times have been acquired using a statistical evaluation of the Wavelet coefficients, which might be promising to apply in a staged active control system. This system could include different phases of detection and actuation depending on the current precursor.

The aerodynamic stability of aero engine compressors must be assured by active control systems in all operating conditions when the design surge margin is reduced in order to improve efficiency. While this has been investigated only on compressor rigs and single-spool engines in the past, this study focuses on the active control of the LARZAC 04 twin-spool turbofan. The objective is to demonstrate potential benefits, problems and solutions and also to provide a data base for numerical modeling and simulation of the capabilities of active control. Three different control strategies have been employed each of which refers to a specific operating condition and instability inception of the engine: The attenuation of disturbances travelling at rotor speed by modulated air injection into the LPC in the high speed range, the recovery of fully developed LPC stall at low speeds with a minimized amount of air and finally a constant air recirculation (HPC exit to LPC inlet) for stabilizing the compression system at different speeds based on the monitoring of a stability parameter. The injector is mounted upstream of the LPC and has ten circumferentially distributed nozzles for air injection into the tip region of the first rotor. The injected air which is either taken from an external source or from bleed air ports at the HPC exit is controlled by high-bandwidth direct-drive-valves. Disturbances travelling at rotor speed can be detected and attenuated with modulated air injection leading to a delay of stall onset. Fully developed rotating stall in the LPC was eliminated by asymmetric injection based on modal control strategies with less air than needed with constant air injection. By using online-stability-monitoring it is possible to initiate constant air recirculation when approaching the surge line, though the current design of the injector does not allow for large extension of the operating range for all spool speeds.

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 analyzing 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 analyzing 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 midspeed 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 independently of the type of precursor. Setting up a reliable stall avoidance control based on this analysis scheme seems to be promising. [S0889-504X(00)00401-3]