Numerical study on autoignited and non-autoignited lifted flames of methane in coflow jets at elevated temperatures

The characteristics of autoignited and non-autoignited laminar lifted flames of methane fuel in high-temperature coflow air have been studied numerically. By varying the initial temperature and fuel mole fraction, three regimes were investigated. At relatively low initial temperature (i.e. T ₀ = 800 K), non-autoignited nozzle-attached flame was simulated at relatively low jet velocity and the result successfully reproduced previous experimental data. At higher initial temperature, two regimes were investigated: autoignited lifted flame with tribrachial-edge structure and autoignited lifted flame with mild combustion. The mild combustion regime existed for excessively diluted methane fuel with nitrogen. The chemical kinetics for the autoignition process was discussed. The local equilibrium temperature concept was adopted in explaining the mild combustion regime. Transition behavior from mild combustion regime to nozzle-attached flame was elucidated. As the fuel mole fraction increased, the maximum flame temperature increased while the flame lift-off height decreased reasonably linearly which resulted from the reactivity enhancement.