Investigations on Sudden Unbalance Effects in Overhung Bowed Rotor System

Abstract

This work presents an analytical model for studying the effect of sudden unbalance acting on a rotor dynamic system in presence initial shaft bow of the rotor system. The effect of hydrodynamic bearing supporting the rotor system is also incorporated in the study. Based on the measured unbalance responses from the system, it is possible to identify the severity of the unbalance force using an inverse model. The time-domain responses are used to obtain both FFT plots and phase diagrams along with rotor whirl orbits. By observing the Campbell diagram of the system, it is possible to identify the differences in the rotor response in comparison with a uniform unbalance system using subcritical frequencies. Initial shaft bow is introduced into the system and the effect of this shaft bow on the rotor vibration is analysed. Simulation is carried out by changing different parameters of the system. It is observed that even a small amount of shaft bow greatly influences vibration signature of the system. Different plot such as time response, Orbit Plot and Phase Plot are obtained and compared with the plots of ideal system. This comparison of the vibration signature helps us to identify the fault in real system. Further the effect of hydrodynamic bearing is also studied by introducing the parameters of bearing affecting the system such as hydrodynamic forces and damping coefficient. The presence of hydrodynamic bearing influences the vibration signature of the system considerably. Modelling of large scale rotor system is done using Finite Element Method, which facilitate in formulation of complicated rotor system and complex problem involved with it. Central Difference Method of time-integration is used to obtain the response of the system in all cases. The methodology is illustrated with a turbofan rotor system supported on three oil-film bearings. An experimental work is also conducted to obtain the natural frequencies of the rotor without bow and sudden unbalance using vibration exciter test.