Problems of Rotordynamic Modeling for Built-Up Gas Turbine Rotors with Central Tie Rod Shaft

Tie rod built-up rotor structures are widely used in power machinery for different types of gas turbine engines. Typical tie rod rotor structure consists from several disks and intermediate parts that are tightened together by central tie rod shaft. This type of construction allows assembling together compressor or turbine disks made from high strength materials whose welding is impossible or hard. Another benefit over solid cast rotor of the same size is lighter weight and possibility to replace damaged parts/disks during repair or retrofit. However modeling of this type of rotors is more complicated, time consuming and different from modeling of solid cast rotors or rotors with shrink fit disks/parts, since multiple interfaces between the built-up rotor components can reduce the shaft stiffness significantly. Fine meshed solid models are known to get a very accurate and close value with natural frequencies of real structures, however significant amount of time usually is required to get solution for them and further application of these models for rotordynamic simulations is not convenient. Thus beam models are still widely used, but cautions must be taken when preparing them, since obtained beam rotor model might be much more rigid than the real structure. Current paper is focused on rotordynamic modeling of typical built-up gas turbine rotor with central tie rod shaft. Paper describes a method how to correct beam model in order to achieve a better matching with fine meshed solid model. Described method was further used for rotor modeling of real 2 MW gas turbine rotor. Obtained simulation results were compared with experimental results from modal testing and good agreement was achieved.