In this research we have addressed the study of the qualitative behavior of nonlinear variable speed axial compressors model exhibiting surge and stall instabilities. Such a study can shed some light on the development of an effective control approach being capable of simultaneously controlling the speed and instabilities. The controller can stabilize the system around an effective operating point and improve performance and reliability of variable speed axial compressors widely used in aeronautic industries. Although previous studies [1, 2] have focused on developing a model for non-constant speed axial compressors, qualitative characteristic of such a model is still unclear and its active control including both rotating stall and compressor speed is still a challenging problem. In this study we are particularly interested in investigating effects of the acceleration of the compressor rotor on qualitative properties of the model. To this end, bifurcation analysis of an axial compressor model with spool dynamics was performed and the simulation of the model was developed along the way. Preliminary surprising results revealed that the type of instability, surge or rotating stall, not only depends on the final speed as thought before but is also deeply affected by the rate of the rotor acceleration. Impacts of the initial speed on the qualitative properties of the model were demonstrated as well. Furthermore previous work [1, 2] showed that amplitudes of stall harmonics grow during the speed transition and cause a temporary pressure drop at the compressor output. Our simulation results supporting the bifurcation analysis of the model revealed that during speed transitions both amplitudes of high order stall harmonics and the number of dominant harmonics also depend on the rate of the acceleration and the initial speed.

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