Finite rate simulations and analyses of wet/distributed flame structure in swirl-stabilized combustion

•Flame structure and oxidation process of wet combustion are investigated with CEMA.•Effect of steam and O2% on flame regime transition is investigated using ECMA.•Temperature not being a dominating variable to CEM marks flame regime transition.•Wet combustion promotes contribution of the reaction R148: 2CH3 + M = C2H6 + M to CEM.•Flame in CDC regime allows roughly a ∼ 3% change of O2 mole fraction in reactants. Towards developing humidified gas turbines (HGT) capable of running at high electrical efficiencies and low emissions, wet/steam-diluted combustion in a premixed swirl burner is investigated using large eddy simulation and a partially stirred reactor method. Chemical explosive mode and extended combustion mode analyses are performed to promote the understanding of wet flame structures. The former identifies the key features of the wet methane oxidation processes, and the latter extends the flame regime classification method to describing the combustion status of fluid parcels using local properties. Three combustion regimes are extensively discussed: the swirl stabilized (SS), colorless distributed (CDC) and non-combustible. Using the combined analyses of the two approaches, it is found that compared to dry flames, wet flames present more fluid parcels defined in the practical CDC regime where local heat release is low and Damköhler number is smaller than unity. The wet fluid parcels are capable of self-igniting via radical explosion, while dry fluid parcels self-ignite via thermal runaway. The species CH2O and temperature are the first and second highest contributors towards the explosivity of dry flames, while temperature is insignificant to that of wet flames. The species C2H6 is found an important source to the self-ignitability of wet fluid parcels in the practical CDC regime due to the activation of the three-body ethane formation reaction R148: 2CH3 + M = C2H6 + M in the low O2% wet combustion environment. Proper use of proposed methods to quantify wet flame behavior guides stable and low emission operation of practical HGT.