Axial Flow Compression Systems Research Articles

LINEAR STATE FEEDBACK DESIGN FOR SURGE CONTROL OF AXIAL FLOW COMPRESSOR DYNAMICS

Based on Moore and Greitzer's model [15], the surge behavior of axial flow compression system was found to be attributed to the appearance of Andronov-Hopf bifurcationin [12]. In this paper, we focus on the design of control law to prevent and/or delay the occurrence of the bifurcations, which will then suppress the surge behavior. Both types of linear dynamic state feedback and linear washout filter controllers are proposed. The proposed washout design does not require the explicit knowledge of the operating point and keep the original equilibrium characteristic while the dynamic feedback design will change the equilibrium operating points for pre-surge and post-surge regime. In addition, the washout type control scheme can relax the implementation of the control law in the practical applications. Numerical results demonstrate the effectiveness of the proposed designs and the superiority of the washout filter scheme.

Fixed-order dynamic compensation for axial flow compression systems

We develop linear fixed-order (i.e., full and reduced order) pressure rise feedback dynamic compensators for axial flow compressors with throttle valve actuation. Unlike the nonlinear static controllers proposed in the literature possessing gain at all frequencies, the proposed dynamic compensators explicitly account for compressor performance versus sensor accuracy, compressor performance versus processor throughput, and compressor performance versus disturbance rejection. Furthermore, the proposed controller is predicated on only pressure rise measurements, providing a considerable simplification in the sensing architecture over the bifurcation-based and backstepping controllers proposed in the literature.

Closed-Loop Stability Monitoring of Axial Flow Compression Systems

Monitoring systems are proposed for the detection of incipient instability in axial flow compression systems. The work employs generic features associated with the response to noise inputs of systems bordering on instability. Based on these generic features, a closed-loop monitoring system is proposed. Numerical simulation is used to illustrate the operation of the proposed closed-loop monitoring system.

An overview of rotating stall and surge control for axial flow compressors

Modeling and control for axial flow compression systems have received great attention in recent years. The objectives are to suppress rotating stall and surge, to extend the stable operating range of the compressor system, and to enlarge domains of attraction of stable equilibria using feedback control methods. The success of this research field will significantly improve compressor performance and thus future aeroengine performance. This paper surveys the research literature and summarizes the major developments in this active research field, focusing on the modeling and control perspectives to rotating stall and surge for axial flow compressors.

Integrated Control of Rotating Stall and Surge in High-Speed Multistage Compression Systems

Aeroengines operate in regimes for which both rotating stall and surge impose low-flow operability limits. Thus, active control strategies designed to enhance operability of aeroengines must address both rotating stall and surge as well as their interaction. In this paper, a previously developed nonlinear control strategy that achieves simultaneous active control of rotating stall and surge is applied to a high-speed three-stage axial flow compression system with operating parameters representative of modern aeroengines. The controller is experimentally validated for two compressor builds and its robustness to radial distortion assessed. For actuation, the control strategy utilizes an annulus-averaged bleed valve with bandwidth on the order of the rotor frequency. For sensing, measurements of the circumferential asymmetry and annulus-averaged unsteadiness of the flow through the compressor are used. Experimental validation of simultaneous control of rotating stall and surge in a high-speed environment with minimal sensing and actuation requirements is viewed as another important step toward applying active control to enhance operability of compression systems in modern aeroengines.

Active control of compressor stall inception: a bifurcation-theoretic approach

Active control of the onset of stall instabilities in axial flow compression systems is pursued using bifurcation analysis of a dynamical model proposed by Moore and Greitzer. The state variables of this model are the mass flow rate, pressure rise, and the amplitude of the first-harmonic mode of the asymmetric component of the flow. A subcritical pitchfork bifurcation is found to occur at the inception of stall, resulting in a large-amplitude instability and associated hysteresis behavior. Using the throttle opening of the compression system for actuation, it is found that the eigenvalue that becomes zero at the onset of stall is linearly uncontrollable. This, along with uncertainty of the post-stall linearized model, motivates a consideration of nonlinear feedback control laws for mitigation of the jump and hysteresis behavior occurring at stall onset. Following the bifurcation control calculations introduced by Abed and Fu [Abed, E. H. and J. H. Fu (1986). Local feedback stabilization and bifurcation control, I. Hopf bifurcation. Syst. Control Lett., 7, 11–17. and Abed, E. H. and J. H. Fu (1987). Local feedback stabilization and bifurcation control, II. Stationary bifurcation. Syst. Control Lett., 8, 467–473], it is found that feedback incorporating a term quadratic in the first-harmonic flow asymmetry variable renders the pitchfork bifurcation supercritical. This introduces a new stable equilibrium near the nominal equilibrium after the nominal equilibrium itself has lost stability, thus eliminating the undesirable jump and hysteresis behavior of the uncontrolled system.

Surge-Induced Structural Loads in Gas Turbines

The axial flow compression system of a modern gas turbine engine normally delivers a large quantity of airflow at relatively high velocity. The sudden stoppage (and reversal) of this flow when an engine surges can result in structural loads in excess of steady state levels. These loads can be quite complex due to inherent asymmetry in the surge event. The increasing requirements for lighter weight engine structures, coupled with the higher pressure ratio cycles required for minimizing fuel consumption, make the accurate prediction of these loads an important part of the engine design process. This paper is aimed toward explaining the fluid mechanics of the surge phenomenon and its impact on engine structures. It offers relatively simple models for estimating surge-induced loads on various engine components. The basis for these models is an empirical correlation of surge-induced inlet overpressure based on engine pressure ratio and bypass ratio. An approximate estimate of the post-surge axial pressure distribution can be derived from this correlation by assuming that surge initiation occurs in the rear of the compression system.