The Influence of Transient Inlet Distortions on the Instability Inception of a Low Pressure Compressor in a Turbofan Engine

Abstract

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.

The role of variable pitch in motion blades and variable rotational speed in an industrial fan stall

“…During the stall recovery transient these rotating instabilities first emerged, and then collapsed. Other researchers have reported similar findings in a compressor with dynamic inlet flow distortion combined with tip injection 32 and in low speed axial fans. 33 As blade angle is reduced to 50 , the energy contained within the rotating instabilities reduces by an order of magnitude again (Figure 4(d)).…”

Flow and Noise Characteristics of Centrifugal Fan under Different Stall Conditions

“…As well accepted, two types of stall inception proposed by Camp and Day modal wave inception and spike inception were investigated experimentally [7]. Leinhos et al studied development process of stall inception under instantaneous inflow distortion in an axial compressor [8].…”

Aerodynamic instability prediction using fast wavelet subjected to transient circumferential distortion in an axial flow compressor

“…Schalkwyk et al [21] validated the Hynes-Greitzer distorted flow model as prediction to actively stabilize the low-speed three-stage axial compressor by 3.7% and 40% with different inlet circumferential distortion. Additionally, the transient inlet distortion of inlet rotating and swirl distortion has been focused on in last decades [22][23][24]. However, it is still necessary to develop a method to predict the aerodynamic instability more precisely, taking actual transient distortion generation process into consideration.…”