Oxidation-Induced Damage Modeling in Micro Gas-Turbine Combustion Chambers

Combustion chambers of micro gas-turbines are generally operated under very lean global air-fuel ratios for two main reasons: to constrain the production of pollutants (NOx), and to cool-down the metal walls with cooling air. The abundancy of oxygen contained in the air flowing over metal surfaces, together with high temperatures due to combustion, constitute an oxidizing environment. The thin oxide layer can crack due to alternating system operative conditions, potentially initiating a crack detrimental to the combustion chamber's life. An oxidation damage term should be computed in order to provide information of the component's life. In this paper, an oxidation-induced damage model for superalloy INCONEL 718 is applied to a combustion chamber fitting the MTT Enertwin micro gas-turbine. Oxidation growth and material parameters are obtained from experimental data. The phasing of thermal loads with mechanical strains is also considered. The results show a distribution of the oxidation-induced damage on the combustion chamber walls which depend on temperature and mechanical strain rate, suggesting that this damage mechanism is relevant under the given operation conditions. Therefore, the approach should be incorporated in a broader life-assessment methodology, which includes oxidation, creep and potentially mechanical fatigue.