A numerical study on structure of premixed methane–air microflames for micropower generation

Structure of premixed methane–air microflames at normal and elevated temperatures and atmospheric pressure generated on a microtube was computationally studied, in order to understand the fundamental behavior of the microflames for micropower generation. Based on an earlier experimental investigation of the stability limits of the premixed microflames, the distributions of temperature, fuel and radicals for single microflames near the stability limits and in the stable region were predicted using a two-dimensional CFD simulation with a reduced kinetic mechanism and a detailed transport modeling. The predicted structure of microflames along the stability limits due to heat losses showed substantial fuel leaks between the microflame base and the microtube rim. This observation provided the burning mechanism that a microflame can be generated and sustained only if the injected mixture contains a certain concentration of fuel beyond a certain distance from the tube exit that can avoid quenching. The experimentally observed extended stability limits due to elevated temperature were readily explained by the predicted structure showing intensified burning and reduced heat losses. Finally, the predicted microflame structure showed that the predicted microflame length based on the maximum mass fraction of OH radical well represents the observed visible microflame length.