MILD combustion versus conventional bluff-body flame of a premixed CH4/air jet in hot coflow

Abstract

The present work is to investigate the MILD (moderate and intense low-oxygen dilution) combustion of a premixed methane jet in hot coflow against its conventional counterpart, i.e., a typical bluff-body flame, under identical inlet and boundary conditions. This paper demonstrates that the MILD combustion develops as a stable ‘flame’ lifting far downstream from the nozzle while the conventional flame evolves immediately behind the bluff body. More specifically, all chemical reactions are found to occur far more slowly over a greatly larger reaction zone for the MILD combustion than for the conventional one. Also, for the MILD combustion, the laminar flame speed (SL) is very small, far below the local flow speed (Ux), whereas chemical and mixing times are compatible so that the Damköhler number Da ∼1.0. In contrast, the conventional combustion takes place with SL ≥ Ux and Da = 10–1000. Moreover, the MILD combustion eventually emits little NOx, only less than 3% of the emission from the conventional counterpart. Fundamentally, the MILD combustion produces the NOx emission mainly through the N2O-intermediate and NNH routes while the thermal NOx mechanism dominates the conventional flame. In addition, this paper provides a comprehensive explanation to each of the above differences.