The influence of transient inlet distortions on the instability inception of a low-pressure compressor in a turbofan engine

While studies on compressor flow instabilities under the presence of inlet distortions have been carried out I with steady distortions in the past, th e investigation presented here focuses on the influence of transient inlet distortions as generated by variable geometry engine intakes of super- and hypersonic aircraft ora the characteristic and the nature of the instabilit y, inception of a LPC. The flow patterns (total pressure distortion with a superimposed co- or counterrotating swirl) of the distortions are adopted f rom a hyper-sonic concept aircraft A LARZAC 04 twin-spool turbofan was oper ated with transient inlet distortions, generated by a moving delta wing, an d steady total pressure distortions starting close to the LPC's stability l imit until it stalled. High-frequency pressure signals are recorded at diff erent engine power settings. instabilities are investigated with regard to the inception process and the early detection of stall precursors for provi ding data for a future stability control device. It turned out that the tra nsient distortion does not have an influence on the surge mai gin of the LP C compared to the steady distortion, but that it changes the nature of stal l inception. The pressure traces ai-e analyzed in the time and frequency do main and also with tools like Spatial FFT, Power Spectral Density, and Trav eling Wave Energy. A Wavelet Transformation algorithm is applied as well. W hile in the case of clean inlet flow, the compressor exhibits different typ es of stall inception depending on rite engine speed, stall is always initi ated by spike-type disturbances under the presence of steady or transient d istortions. Modal disturbances are present in the mid-speed range that do n ot grow into stall, but rather interact with the inlet flow and produce sho rt length scale disturbances. The obtained early warning times prior to sta ll are adversely affected by transient distortions in some cases. The probl em of appropriate thresholding becomes evident. The best warning times have been acquired rising a statistical evaluation of the the Wavelet coefficie nts, which might be promising to apply in a staged active control system. T his system could include different phases of detection and actuation depend ing on the Current precursor.