Study On The Dynamic Characteristics Of Simulating Rotor Test Rig In Windmilling Imbalance

As the current fan size of turbofan engine increases,the number of fan blades decreases,the clearance between the fan and the casing decreases and other development trends,when engine fan blades fall off,it will easily lead to rub-impact in windmilling imbalanced state,thus which have a serious impact on the whole body structure.Since the previous research focuses on the problem of rotor-stator rubbing,it mainly focuses on the high speed region of the engine.However,there are relatively few studies on the rotor-stator rubbing impact under the condition of low speed and large unbalance.In this paper,a typical windmilling imbalanced simulating test rig for low pressured rotor of a civil large bypass turbofan engine is simulated and analyzed.Combining with the experiment,the vibration characteristics the rotor under windmilling imbalanced simulating state are obtained.The specific research contents are as follows:According to the exsiting windmilling imbalanced simulating rubbing test rig,based on equivalent principles to properly simplify it,simplified three-dimensional,two-dimensional and one-dimensional rotor test rig models are obtained and analyzed by Samcef finite element software.The critical speeds of the first and second stages are obtained by simulation.And then the first and second critical speeds measured by the experimental test are compared with it to verify the effectiveness of the simplified rotor model.The one-dimensional model has sufficient accuracy in the shafting analysis and the calculation time is shorter.The rubbing characteristics verification is verified by windmilling imbalanced simulating experiments.Firstly,the rubbing test under large-unbalanced mass is performed.In the course of the experiment,due to the large collision force,the levers were twisted and the slider flew off.Due to the safety considerations of the rotor test rig,small-unbalanced mass was eventually performed.The rub-impact test result under experiment were used to verify the corresponding simulation results.Then the modeling method for small-unbalanced mass rubbing-impact simulation was used to simulate the rotor-stator rub-impact under low-speed and large-unbalanced mass with elastic casing in reality.Using the modeling method of small-unbalanced mass rubbing simulation,a finite element model of the rotor-bearing-casing system considering the elasticity of the casing was established.By changing different parameters(such as friction coefficient,rubbing stiffness,etc.),it was studied the impact of rotor-stator rubbing under low rotational speed and large-unbalanced mass.The results show that:(1)As the degree of rubbing increases,there are different levels of clipping on the time domain map.The number of collisions between the casing and the fan in the single cycle on the axis trajectory is more and more,the intensity of collision is getting bigger and bigger,and the phenomenon of collision and return occurs.(2)On the frequency spectrum chart,with the increase of the friction coefficient,harmonic components such as 1X/2,5X/2,2X,4X appear in addition to the power frequency.The amplitude of 5X/2 increases more significantly,and the 1X/2 frequency division also increases to a certain extent;when the rub-impact stiffness is small,in addition to the power frequency,1X/2,5X/2 equal components are evoked.When the rub-impac stiffness is large,it also evokes the frequency multiplication of components such as 2X and 4X,in which the amplitudes of 1X/2 and 5X/2 increase more significantly.Therefore,when modeling the engine structure,it is necessary to give special consideration to factors such as friction coefficient and rubbing stiffness.The above studies can provide certain reference for the engine windmilling imbalanced airworthiness review process.