This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada

Resource Abstract

According to the rotor dynamics, the damage case "fan-blade-off" (FBO) causes high load peaks in the rotor system of a jet engine and is relevant for the aircraft certification. This leads to significant stresses on the engine components. For the safe operation of the aircraft immediately after the event of damage, the reliable shutdown of the engine is absolutely necessary. In order to regulate these peak loads caused by the imbalance, one of the three rotor bearings of the low-pressure rotor is forced to break. After the bearing fracture, the rotor is supercritical mounted with respect to the first bending natural frequency, so that the engine can be shut down in a controlled manner with lower load peaks. By representing the engine design as a simulation model, it should be possible to set system parameters such as stiffness and damping characteristics during the development phase of an engine. In this way, the desired dynamic behaviour of a specific load case can be approximated cost-effectively. The final objective is to design the engine components in a goal driven manner and fit for purpose. In accordance with the current state of the art, transient finite element analyses (FEA) are used to analyse engine dynamics. Following the example of the Rolls-Royce Trent 900 three-shaft turbofan jet engine, a simulation model is created using the finite element method (FEM) program "ANSYS Mechanical APDL". The objective is to represent the rotor dynamic characteristics of the engine rotors by generating Campbell diagrams. The simulation model serves as a purely academic example model of the reference engine for the verification of modelling methods. A convergence study was used to compare the modelling approaches with respect to the available finite element types. By assuming constructive simplifications, an engine casing was modelled and then combined with the engine rotors in a whole engine model using linearized bearing stiffnesses. The engine model is primarily used for transient FE simulation of an FBO load case. Using the Campbell diagrams obtained, critical operating points of the engine are deliberately passed through. Using simplifying assumptions, the FBO load case is simulated as an imbalance force. The resulting bearing fracture is pre-defined in the simulation as a time event and thus leads to a non-linear model behaviour. The dynamic behaviour of the engine simulation model is analysed by evaluating rotor orbits, bearing forces and casing deformations. The calculation results reflect the expected qualitative rotor dynamic behaviour with respect to the resonance excitations and the self-centering effect of an aircraft engine damaged by a fan-blade-off. Within the scope of these investigations, an efficient model order reduction was also achieved. The CRAIG-BAMPTON reduction method was successfully applied to the engine rotors and the casing, thus optimizing the required computing time. The simulation models can also serve as a basis for a comparison with multi-body system simulation models of the engine.