Effect of Methane on the Ignition Process of Spray Flame Using Large Eddy Simulation

The ignition process of spray flame in a methane environment was studied using large eddy simulation. By comparing the combustion process of n-dodecane spray (single fuel, SF) and n-dodecane/methane dual fuels (DF), the effect of methane addition on the low- and high-temperature ignition was investigated. Results showed that although the ignition delay times (IDTs) for the SF and DF cases are very similar, methane in the ambient gas mainly prolongs the time interval between the low- and high-temperature combustion for the DF case. A high gas temperature of 900 K can increase the reactivity and promote the early oxidization of n-dodecane. Thus, the formation of formaldehyde appears very early at 900 K. Then, the interactions between the early oxidation process of methane and the multistage ignition process of spray are investigated. By delaying the n-dodecane injection timing, different active environments including intermediate radicals are created, which can delay or even shorten the ignition process of n-dodecane spray depending on the gas temperature. However, the formation of formaldehyde is less dependent on the injection timing at low temperatures, indicating that methane has a negligible influence on the onset of low-temperature reactions. But at high temperatures, the early oxidation process in methane increases the gas temperature, which plays the dominant role in shortening the IDT.