Journal of Engineering Sciences, Assiut University, Vol. 34, No. 6, pp. 1843-1864 , Nov. 2006 ACTIVE SUPPRESSION OF AERODYNAMIC INSTABILITIES IN THE AXIAL FLOW COMPRESSORS ___________________________________________________________________________________________________________________________ Ahmed Sayed Hassan Mechanical Engineering Department, Assiut, University, Assiut, Egypt. (Received August 2, 2006 Accepted October 9, 2006 ) Aerodynamic instabilities due to rotating stall initiation and surge trigger in the axial flow compressors were investigated using different mathematical models. Limit of stability of the rotating stall was detected using simple model that considers transport of energy by the rotor wakes passing through the stator. Active suppression of the aerodynamic instabilities in the axial flow compressors were demonstrated using mechanical compensated system with controlled plenum simultaneously with air injection through reed valve. A nonlinear controller based on a bifurcation theory was used to design feedback control system for eliminating surge and rotating stall as well hysteresis. The bifurcation analysis was performed to determine the effect of throttle gain on the compressor limit of stability and then modifying the throttle characteristic to eliminate hysteresis and suppresses surge. The Moore–Greitzer model was extended to include the influences of the air injection on the axial flow compressors performance characteristics and to enhance the limit of stabilities of both the stall and the surge. Comparisons of the present theoretical results with the available previous experimental and theoretical results were carried out. KEYWORDS: Axial flow compressors, Rotating stall, surge, Limit of stability. 1- INTRODUCTION The axial flow compressors are widely used in various engineering applications. Their functions are to increase the pressure of the fluid and they are designed to operate in steady axisymmetric flow. As one decreases the flow rate through the compressor, the pressure rise increases and this improves the performance. Below a critical value of the flow rate, phenomena of aerodynamic instabilities occur, which are classified as (deep) surge, rotating stall and classic surge. However, various efforts to either avoid or control of those phenomena have been studied [1-26]. Rotating stall is the limit cycle resulting from higher-order, rotating-wave disturbances, while surge is a large- amplitude, low frequency (compared to rotating stall) oscillation of the total, annulus- averaged mass flow rate. Aerodynamic instabilities in axial flow compressors and possibilities to suppress them with feedback control are being investigated in an increasing number of theoretical studies often accompanied by experiments. Some of 1843