Active Stability Control of the Compression System in a Twin-spool Turbofan Engine by Air Injection

This investigation presents an active stability control system on a twin-spool turbofan engine to recover from unstable operating conditions or to avoid them. Therefor air injection in the tip region ahead of the first low pressure compressor (LPC) rotor is used to damp any instabilities and their precursors. A high frequency instrumentation is applied to detect the flow disturbances and to feed the control system with the measured values. An injection casing, consisting of 10 injection ducts and pivoted nozzles at their ends is equipped with fast acting direct drive valves to provide the required air flow. This air is delivered by an external source. In this paper it is focused on the phenomena in the low and medium speed range of the LPC where fully developed rotating stall appears prior to surge. In the control section it is shown that the stabilizing effect of an additionally injected constant air flow can be further increased by using active control methods, leading to a dynamically modulated injection. In this case less air is required to obtain a comparable improvement or for the same amount of air the stable operating range can be further enlarged. A numerical model was built which is able to reproduce the experimentally gained results in good manner. NOMENCLATURE Symbols Fs [kN] thrust f, frot [Hz] frequency, rotor frequency n [RPM] mechanical spool speed ps, pt [Pa] static pressure, total pressure Π, PI [-] pressure ratio nθLPC [%] relative corrected spool speed, (nLPC/√Tt,LPC,inlet)rel. W [kg/s] mass flow WRED[kg/s•√K/bar] corrected mass flow Abbreviations AIC active instability control BPR bypass ratio DDV direct drive valve DSP digital signal processor FFT Fast Fourier Transformation ISA international standard atmosphere HP, HPC high pressure, high pressure compressor LP, LPC low pressure, low pressure compressor PSD power spectral density RMS root mean square TET turbine entry temperature