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Gas turbine (GT) liners, transition ducts and exhaust diffusers are large thin-shell ducts bounded by two barrels, typically characterized by multiple natural frequencies inside the operating speed range of the engine rotor. In most applications, GT ducts are supported on one side only and they are free to expand inside the gas turbine so to avoid thermal distresses. The GT ducts are typically damped structures able to prevent high cycles fatigue failures. Damping is provided by sliding features as insulation or bolted joints. This paper describes the redesign of a transition duct (TD) after it was discovered that in some operating conditions, duct could crack for high cycle fatigue (HCF). The TD connects the flow path of the gas generator turbine with the flow path of the power turbine. The new TD has been made more robust, but it has also been equipped of dampers capable to operate at high temperature. Starting from the analyses of field data, a predictive FEA model has been developed and validated. After a deep investigation of the TD modes that could be excited by flow path and/or by rotor vibrations, it was decided to add two dampers, one for each barrel of the TD. Due to internal space limitations, a new type of damper has been designed for the external barrel. Both dampers have been sized using FEA. Harmonic analyses rather than forced response transient analyses have been performed so to verify the effectiveness of the new design. In the simulations, dampers have been replaced by harmonic forces able to reproduce the friction force of the dampers. Validation of the method and damper calibration has been done by performing lab tests and full-size TD tests. |
