Friction-induced backward rub of rotors in non-annular clearances: Experimental observations and numerical analysis

Abstract

During backward rub in a turbomachine, rotor is compelled by frictional forces into a severe precessional sliding motion inside stationary parts such as seals or journal bearings in opposite direction of rotor's rotation. Backward rub occurs with high frequency and high amplitude of vibration and can deteriorates rubbing surfaces just for few moments. This paper presents and investigates the idea of clearance profile alteration from annular form to non-annular 3-lobed form in order to affect the reverse rub's characteristics. For this purpose, a test setup consisting of a lightly damped rotor-stator system is designed and built in SUT's condition-monitoring laboratory for testing the efficacy of the idea. Degree of deviation of the clearance from the annular state is defined by “preload factor”. Different test specimens were fabricated for realizing clearances with different preload factors. Test results showed that the backward frequency and the rotor's vibration amplitude are effectively reduced in sufficiently high preload values. For interpreting the observed results, a numerical model is constructed to precisely capture the physics underneath the attenuation of the rub. A contact model for the rotor and the lobed stator is developed based on unilateral contact constraint and friction condition. Lagrange multiplier technique is implemented along with direct linearization of the constrained equation of motion for conserving the frictional energy in the discrete time domain. The numerical solution is obtained by an implicit time marching algorithm for different friction coefficients and preload factors. Good agreement is observed between the numerical and experimental results. Both results indicate that the non-annular 3-lobed clearances can mitigate the backward rub by retarding the tangential friction force transmission between rotor and stator and thus by reducing backward frequency and amplitude of vibration.