Several Dynamic Characteristics Of Aero-engine Case Model

Case is the connecting bridge and link between the rotor system and stator system of aero engines, the engine thrust through case is passed on to the plane, the case is the important components of sustaining loads and the base of the whole engine, in addition, the case system occupies an important position in the research and development of the whole aircraft engine design. In the request of the low quality and high thrust-weight ratio, the case of the modern advanced aero engines gradually developed into thin-wall cylindrical shell structures. The vibration problems of thin-walled structures become more prominent under the complex loading. It is noteworthy that the study of vibration characteristic for case structures mainly focused on completely structures and homogenous materials, in addition, there is less study on the fault(crack) case structures and composite case structures, especially in theory. Therefore, there have important theoretical values and practical significances to conduct a sophisticated research on the dynamic characteristics of the cracked case structures and the composite case structures.In this dissertation, theory model and dynamic analysis for the case are investigated in detail by analytical and numerical approaches. First, the bifurcation properties of the fan exit case under complex air exciting force are studied by the method of nonlinear dynamics. Next, the problems of elastic buckling and motion stability for typical composite case, i.e. orthotropic material case and laminated composite case, are analyzed using double modal displacements. Finally, the traits of modes of the typical failure(crack) case with circumferential surface crack are investigated using displacement-coordinating method. The specific research contents and the main achievements are as follows.According to the geometrical characteristics of the thin-walled structure, the thin-walled cylindrical shell model is established to analyze the vibration problems of the fan exit case, and then the vibration characteristics and the bifurcation characteristics are studied by using the method of singular analysis. The results show that natural frequency of the case increase with the radius-length ratio(R/L) and thickness-radius ratio(h/R). Under the certain unfolding parameters, the phenomenon of hysteresis and jump are appeared with the singularity analysis method. It can be observed that the excitation frequency and the fan speed have multiple relationships, further, the speed range of fan blades which corresponding to the interval of jump is obtained using the multiple relationships. In addition, an important conclusion that the vibration frequency in sudden jump range will increase the possibility of case crack is also reached by study.Take double modal displacements as example, the applicable scope for single modal displacement and double modal displacement is discussed, in addition, the motion stability of the case structures under a radical harmonic force is also studied using double modal displacement. Under the conditions of a small excitation force, solving the vibration problems of case structures by using single modal displacement both can satisfy the requirement of accuracy but also can simplify the calculation relative to multiplex modal displacements. Furthermore, with the increase of excitation force, solving the vibration problems of the system by using double modal displacement is more reasonable when the double modal displacement or multiple modal displacements appear. The simulation results show that the stable periodic motion and the complex chaotic motion will be appeared in the first order modal displacement with the increase of the excitation force.Based on Donnell’s nonlinear thin shell theory, the dynamic model of laminated composite case is established, and then the vibration characteristics of the system are analyzed by using the double modal displacements. The results shows that the reason for mode conversion between single mode and double mode of the system is the excitation force increasing, in addition, the excitation force intervals which corresponding to the different mode displacements are influenced by the thickness of inner and outer materials. Under some certain thickness of the materials, the bifurcation phenomenon and jump phenomenon are appeared during the mode conversion between single mode and double mode.The analysis of elastic stability of the orthotropic materials case with radial internal pressure show that all of the radial internal pressure, the thickness of case, the axial elasticity modulus and the circular elasticity modulus can improve the ability of case axial buckling resistance under the condition of strength allowed. Further, the ‘stiffness’ of the materials of the case structures is changed with the existence of the radial internal pressure, that is, the resonance of the system will be appeared just when the excitation frequency is higher than the natural frequency of the case without internal pressure effect. In addition, it is also found a tiny deviation phenomenon between the value of the natural frequency and the value of the peak point of the resonance curve, which is caused by the coupling of vibration modes.According to the actual crack forms of case systems, a dynamic model with a circumferential surface crack is established, then a new segment thought for solving the problems which the interfacial displacements of the structures are no longer successive as a result of the existence of crack, in addition, the displacement coordination method is also introduced to analyzed the vibration characteristics of the cracked case. Results indicate that axial bending moment play an important role in the local flexibility of the structures, which is the main factor to cause the vibration problems of the cracked systems. In general, it is has the most important influence when the crack appears near the peak point of the axial vibration mode, and there almost has no effect when the crack appears near the nodal points. However, the existence of the crack contrary decreases the vibration amplitudes if the crack appears near the fixed end. Further, the vibration problems of the system will be further intensified as the increase of the crack depth. The influences of the crack in the modal the amplitudes are different with different axial vibration modes.The research results of this dissertation provide a theoretical basis for the structure design and the parameter optimization of the aero engine case systems, in addition, it also have a certain practical guiding significance for the research of dynamic behavior of the case system.