Vibration analysis of compressor blade tip-rubbing

There has been a significant increase in air traffic volume, particularly over the past twenty years. In order to cope with this increase in demand, it has been necessary to increase the efficiency of aircraft engines. Over the years, this has been achieved by reducing the clearance between blade tips and the engine casing. As a consequence of the reduced clearance, tip-rubbing frequently occurs in the engine during operation. The primary aim of this project is to address the vibrations involved in a tip-rubbing phenomenon when a blade of the Intermediate Pressure (IP) compressor in a Trent 900 engine interacts with the casing. Current trends towards blade optimisation tend to make the blade thinner and thus more flexible; therefore, it is very important to be able to successfully predict and prevent nonlinear response of a blade when tip-rubbing occurs. Current literature on the study of nonlinear vibration of a blade in a tip-rub event is limited; this project is understood to be the first to attempt an understanding of the issue. In this thesis, analytical models are presented that predict the nonlinear responses of rotor-stator interactions. These are helpful in understanding nonlinear parameters that can have an effect on the system response. Simulations were started by determining the stresses in the blade due to centrifugal rotation. Resonant frequencies of the blade were determined by modal analysis. Finally, the tip-rubbing event was simulated. The results were used to output frequency response curves which were used to identify if the blades were behaving nonlinearly as a result of tip-rubbing. The primary conclusion from this project is that tip-rubbing can excite nonlinear vibration in the compressor blades. However, the simulation results were affected adversely by hourglassing of the casing segments and should not be considered completely accurate.