Radiative Extinction of Counterflow Premixed Flames

This report presents experiments on counterflow premixed flames at small stretch rates and relevant numerical calculation involving radiation loss from the flame zone. Extinction experiments on counterflow methane/air and propane/air premixed flames at small stretch rates are conducted under microgravity to observe extinction characteristics of near-limit premixed twin flames. The results show two kinds of flame response, that is, C-shaped extinction curve due to radiative loss for methane/air mixture and ordinal monotonous extinction curve for propane/air mixture.

To analyze these experiments, numerical investigation addressing extinction and flammable regions of radiative counterflow premixed flames for Lewis numbers from unity to 1.82 are conducted to clarify the mechanism of these two kinds of flame response. The computation exhibited flame bifurcation resulting in a G-shaped extinction curve showing the extended limit of flammability for Lewis number near unity and a K-shaped extinction curve showing the composition flammability limit being equal to that of one-dimensional flame for Lewis number of 1.82. G- and K-shaped extinction curves respectively explain the experimental C curve for methane/air and monotonous curve for propane/air mixtures. Furthermore, the relationship between the composition flammability limit of one dimensional propagating flame and that of stretched flames is clarified.