Research On Modeling And Vibration Features Of The Unbalanced Coupled Bending-torsional Rotors

In recent years,the variable frequency drive(VFD)technology has been greatly developed and widely deployed in high energy-consuming industries with the carbon peak and neutrality goal proposed by governments around the world.However,the use of VFD technology also introduces strong coupling effects,such as electromechanical coupling and bending-torsional coupling,to the rotor-bearing system.Due to the bending-torsional coupling mechanism,the lateral vibrations and torsional vibration would affect each other,and the lateral excitation may lead to severe torsional vibration problems,which threatens the safety production of manufacturers.In this thesis,the VFD rotating machinery is studied: firstly,the problem of modeling a coupled bending-torsional(CBT)rotor caused by mass unbalance was investigated;secondly,an efficient numerical method to solve the high-dimensional finite element(FE)rotor model containing nonlinear nodes efficiently was proposed;finally,based on previous research,a novel torsional vibration monitoring method was developed.The specific contents are as follows:1.An independent nonlinear unbalanced mass element was established and incorporated into the rotor model to build the FE model of the CBT rotor system with six degrees of freedom(DOF)for each node.The established rotor model can simulate dynamic behaviors of the CBT problems caused by the unbalanced mass.In addition,an FE model of the whole rotor-bearing-coupling-motor system of a real rotor test-rig was established,and the experiment results indicate that the CBT rotor model is reasonable and accurate.2.To improve the efficiency of transient simulation to the CBT rotor model,a general algorithm named with linear and nonlinear nodes separation method is proposed.This method has high computational efficiency in solving the transient vibration response of high-dimensional motion equations with local nonlinear DOF since it adopts the explicit-implicit separation calculating strategy and combines the Newmark-β integration method and the NewtonRaphson iterative method.The proposed method can not only for solving the CBT rotordynamic problems but also for studying vibration features of nonlinear rotors.3.Numerical studies and experimental validations were carried out,revealing the features of the CBT vibration of a single shaft rotor caused by mass unbalance.Based on the established CBT rotor model and the proposed algorithm,two cases,constant rotating speed and cast-down,were numerically simulated.The results showed that the torsional excitation would generate sideband frequency components in the spectra of lateral vibrations,and lateral excitation would also affect the torsional vibration spectrum.The effect of gravity is considerable and non-negligible in CBT modeling.Additionally,the results of experiments with two different scales of setups,a laboratory test-rig and an industrial centrifugal compressor platform,are well agreed with the numerical ones.4.A method for extracting weak torsional vibration characteristics from measured lateral vibration signals was proposed,which paved a new way for torsional vibration monitoring.The Hilbert-based technique can reverse the torsional vibration signal demodulated in lateral vibration signals.The forward whirl signal was constructed to enhance the torsional features.Furthermore,it was found that the forward whirl signal has better stability and stronger antinoise ability compared to the horizontal,vertical,and backward signals applied to the proposed method to extract the torsional vibration features.Numerical simulation and experimental results indicate that the method can detect torsional vibration signals accurately,with a high resolution in frequency estimation.This method can use the existing lateral sensors to achieve supplementary torsional vibration monitoring for units without a dedicated torsional vibration tester installed.