Cyclic Lifetime Validation of Annular Combustor Liner Segments for Heavy Duty Gas Turbines

Abstract

The cyclic lifetime of combustor liner segments for heavy duty gas turbines has been validated by means of full scale high pressure testing. This testing is part of a systematic combustor component validation ensuring top quality designed parts and a proper integration into the advanced GT24 and GT26 gas turbines. The accuracy of lifetime predictions for such components is highly dependent on the quality of the predicted temperature profiles and induced stress-strain distributions. Three-dimensional computer simulations of both hot combustion gas flows and high velocity cooling air provide detailed knowledge of the flow and temperature fields within a combustor. When linked to finite element representations of the mechanical structure, the resulting models can be used to give predictions of interface contact behaviour, coating integrity, creep deformation and fatigue lifetime. Full size component testing under representative engine conditions provides a means to ensure that a component fulfils its design objective. It also provides a substantiation of the design rules and the analytical models used for combustor liner lifetime prediction. The physical size, and the long time period needed to accumulate a representative number of cycles limits the practicality of full cyclic lifetime component testing in heavy duty gas turbine engines. Rig testing of parts provides a means of lifetime testing at reasonable cost and provides additional advantages relating to monitoring, instrumentation, flexibility and speed. An annular combustor sector test rig operating at high pressure in a cyclic mode and cycling between low and high firing temperatures has been used to cyclically test a single-burner sector of the first GT26 combustor, the so called EnVironmental (EV) combustor. The automatic control and monitoring system allowed accurate and consistent cycling conditions to be maintained. Continuous data logging provided an evolving picture of the conditions being experienced by the components. Between test runs, visual examinations and measurements were carried out by boroscope to assess the structural behaviour. Detailed modelling of the temperature field over the liner allowed the local stress-strain response to be predicted using a Robinson unified material model. Fatigue crack development was simulated by finite element analysis incorporating the effects of accumulated residual stresses. Close correspondence has been demonstrated between the measured temperatures and the predicted temperature fields for the testing conditions used. Regular visual examination of the development of damage during the course of the test has confirmed the accuracy of the mechanical integrity analysis process. Knowledge of the relationship between rig testing conditions and normal engine operating conditions has confirmed the ability of the combustor parts to exceed the specified cyclic life even under the severe conditions used in the test.