25TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES COMPRESSOR STABILITY MANAGEMENT IN AIRCRAFT ENGINES Aspi R. Wadia*, David Christensen* and J.V.R. Prasad** *GE Aviation, Cincinnati, Ohio, **Georgia Institute of Technology, Atlanta, Georgia Keywords: Compressors, Stability, Auto-correlation, Gas Turbines Abstract The safety for modern gas turbine engines NOMENCLATURE can be greatly improved with the development of accurate on-line measurement to gauge the ASC = Active Stall Control aerodynamic stability level for fans and C (t) = Auto-correlation measure compressors. This paper describes the DV = Discharge Valve (used to throttle the development and application of a robust real compressor) time algorithm for gauging fan/compressor i = Index aerodynamic stability level using over-the-rotor P = Pressure Signal dynamic pressure sensors. This real time scheme Pr = Pressure ratio computes a correlation measure through signal t = Current sample time multiplication and integration. The algorithm RPM = Compressor shaft speed (revolutions per uses the existing speed signal from the engine minute) control for cycle synchronization. The algorithm shaft = Shaft time period is simple and is implemented on a portable wnd = Correlation window size computer to facilitate rapid real-time implementation on different experimental platforms as demonstrated both on a full-scale 1 Introduction high-speed compressor rig and on an advanced aircraft engine. In the multi-stage advanced Rig and engine test processes and in flight compressor rig test, the compressor was moved operation and safety for modern gas turbine toward stall at constant speed by closing a engines can be greatly improved with the discharge valve. The stability management development of accurate on-line measurement system was able to detect an impending stall to gauge the aerodynamic stability level for fans and trigger opening of the valve so as to avoid and compressors. Having a stability compressor surge. In the full-scale engine test, management capability during engine the engine was configured with a one-per-rev development testing would ensure stall free distortion screen and transients were run with a operation resulting in enhanced engine significant amount of fuel enrichment to acceleration / thrust capabilities and significant facilitate stall. Test data from a series of reductions in development cycle time by continuous rapid transients run in the engine avoiding attendant hardware failures due to test showed that in all cases the stability unexpected surge events in the compression management system was able to detect an components. impending stall and manipulated the enrichment Since reliable early stall warning part of the fuel schedule to provide stall free measurement systems and algorithms are in the transients. experimental phase, modern high stage loading Copyright @ 2006 by General Electric Company. 1 Published by the International Council of the Aeronautical Sciences, with permission. A.R. WADIA, D. CHRISTENSEN, J.V.R. PRASAD compressors are currently being designed with stall, the amplitude of the blade passing conservative steady state stall margin frequency was reduced but new frequency peaks requirements that have to include effects due to occurred and grew until stall. By blanking out deterioration, engine control tolerance, engine the blade passing frequency and its harmonics transient/thermal effects and inlet distortion. from the spectra and by means of numerical Conservative stall margin requirements result in differentiation, taking absolute values and efficiency penalties that impact the overall finally integration, the stability parameter was engine fuel burn and temperature margins. A calculated from the remaining signal. robust and reliable stability management system In addition to the study of pre-stall could alleviate the conservative stall margin behavior, a variety of active control devices requirements by active regulation of routine have been designed and tested in the last few control devices available, such as variable stator years. One popular way of compressor operating and/or fuel schedules and bleed and/or exhaust range extension has been obtained by using air nozzle actuation. injection for stabilizing compressor flow in the Active compressor stall control by vicinity of the tip gap. The stabilizing effect of suppressing the stall inception phenomenon was air injection can be achieved in two different originally proposed by Epstein et al. [1] in 1986. ways. Starting a constant air injection at several The stall inception process was investigated circumferential positions when an on-line further by Camp et al. [2], Hoying et al. [3], monitoring stability parameter derived from Breuer et al. [4] and Garnier et al. [5]. Camp high frequency pressure data exceeds a and Day proposed two typical stall inceptions: a threshold can eliminate ‘spikes’ and delay the long-wavelength pattern referred to as ‘modal onset of stall to lower mass flows as oscillation’ and a short-wavelength pattern demonstrated by Freeman et al. [9] in a single typically of the order of one or two blade spool turbojet engine. In addition to constant passages referred to as ’spike’. Using numerical blowing, modulated air injection to damp out experiments, Hoying [3] showed that the short modal disturbances that start growing at certain length-scale inception process was linked to the operating conditions near the stall line to further behavior of the blade passage flow field extend the operating range has been reported by structure, specifically the tip clearance vortex Spakovszky et al. [10], Weigl et al. [11] and and claim that a criterion for stall inception for Suder, et al. [12]. the short length-scale phenomena requires that Leinhos et al. [13] used a combined the tip vortex trajectory be perpendicular to the approach of both constant and modulated air axial direction. Tryfonidis, et al. [6] investigated injection into the low-pressure compressor pre-stall behavior of several high-speed (booster) in a twin spool turbofan engine. The compressors and proposed a new technique turbofan engine studied had a tip critical first based on spectral analysis of the spatial Fourier booster rotor that exhibited different types of harmonics of measured case wall pressures. Day stall inception at different speeds. At low speeds et al. [7] reported test results from four high- spikes initiated the instability, while there were speed compressors and suggested that an some weak indications for a modal type stall alternative approach to active stall control is to inception in the mid-speed regime and strong disregard the physical details of the flow evidence that a disturbance rotating at LPC rotor generating the incoming signals and to simply frequency caused stall in the high speed region pay attention to deriving a stall risk index from [14]. In their test, the injected air was either the sum of all the inputs. Hoenen and Arnold [8] taken from an external source or from bleed proposed a monitoring algorithm for the ports at the high-pressure compressor exit. prediction of unstable compressor operation that Tahara et al. [15] proposed a unique stall- involves analyzing the signal patterns of warning index based on pressure signals by high dynamic pressure transducers mounted above response transducers on the casing wall at the the first rotor. As the compressor approached rotor leading edge. Tests conducted on a 2 COMPRESSOR STABILITY MANAGEMENT IN AIRCRAFT ENGINES research compressor with both uniform inlet and both in a full-scale high-speed compressor rig with inlet distortion revealed that their stall test and in an advanced aircraft engine test. correlation degradation first appeared at the mid In the multi-stage advanced compressor rig chord location and advanced toward the leading test, the stability management system was edge with decrease in flow coefficient. The successfully demonstrated by attempting to stall correlation degradation increased monotonically the compressor by increasing the compressor with decrease in the flow coefficient and had the pressure ratio at constant corrected speed by potential to generate a stall-warning signal closing the rig discharge valve (throttling to sufficiently in advance of spike inception for the stall). Test data will be presented that shows that stall avoidance actuation to respond in a timely the auto-correlation was able to sense the onset manner. of stall and trigger corrective actions in This paper describes the development of a sufficient time to avoid compressor surges. robust real time fan/compressor aerodynamic Rapid transients, often referred to as bodes, stability management system using over-the- were run in a full-scale engine test. The engine rotor-tip dynamic pressure sensors. The real was configured with a one-per-rev distortion time algorithm used for gauging the screen and transients were run with a significant aerodynamic stability level is based on the amount of fuel enrichment to intentionally stall method first reported by Dhingra et al. [16] in the engine. Test data from a series of eight 2003. This real time scheme computes an auto- continuous rapid transients (bodes) run in the correlation measure through signal engine test will be presented that show that the multiplication and integration. The auto- correlation method performed consistently in all correlation, reported herein, has a value of unity the testing. In all cases, the impending stall was for a purely periodic signal (stable operation)