Characterization of MILD Combustion of a Premixed CH4/Air Jet Flame versus Its Conventional Counterpart

This study is performed to characterize moderate or intense low-oxygen dilution (MILD) combustion (MC) versus conventional combustion (CC) of a premixed CH4/air jet flame in a hot coflow under identical inlet and ambient conditions. The present CC and MC correspond to the cases using a bluff-body (BB) and a no bluff-body (NBB), respectively. It is demonstrated that the NBB combustion develops by entraining ambient hot low-oxygen gas so as to dilute the reactant mixture and simultaneously heat up beyond the minimum autoignition temperature (T-ai), leading to the MC. By contrast, in the BB case, the conventional flame is established and stabilized by a steady heat source of the recirculation zone (RZ) behind the BB with highly intense mixing and rapid ignition. A large reaction zone with uniform temperature distribution (i.e., low temperature rise) is found in the MC mode, while the CC has a much smaller size of the intense reaction zone with the concentrated high temperature and species distributions. Significantly, it has been first revealed that, in the BB case, there is a secondary combustion in the MC mode formed far downstream from the BB flame under the environmental condition of a high-temperature hot coflow.