Modeling and control of compressor flow instabilities

Stable operation of both axial and radial compressors is constrained by the occurence of aerodynamic flow instabilities: rotating stall and surge. Developments in the understanding of the physics behind these instabilities and ideas of how t o stabilize the compressor system, have opened the door t o a new era in the field of compressor control. This study gives an overview of the current state in modeling and control of surge and rotating stall in axial and radial compressors. I t summarizes different types of aerodynamic flow instabilities tha t can occur in these machines. Compressor performance is discussed on the basis of compressor maps and important features of these instabilities are treated. But, this work focuses on active control systems applied in experimental systems. Compressor models that are used frequently in controller designs are described and the effect of assumptions on the prediction capability of these models is discussed. Furthermore, this report deals with control concepts t o guarantee stable operation and an outline is given of sensors, actuators and controllers applied in laboratory set-ups. The main contribution of this study is the comparison of aerodynamical flow instabilities found in axial and radial compressors, of several models, and of controllers implemented on experimental set-ups. It appeared that the behavior of a compressor system subsequent to instability onset is reasonably grasped but the considered models can not describe all types of instabilities encountered. Nevertheless, the Moore-Greitzer model seems an useful starting point for further research since it describes the compressor behavior subsequent t o the onset of rotating stall and surge. A better understanding of the mechanism(s) behind the onset of these instabilities may allow refinements to existing models and may give insights into new control methods. Currently, apart from a few exceptions, only complex-valued proportional feedback controllers are used. More advanced, nonlinear controllers are expected to improve the performance of the compressor system.