Lewis number effect on extinction characteristics of radiative counterflow CH₄-O₂-N₂-He flames

This work concentrates on general extinction characteristics of low-stretched premixed flames, that is, a change in extinction curve of radiative counterflow premixed flames with the Lewis numbers. To substantiate the occurrence of the change in extinction curves based on the Lewis numbers, flammable boundaries and flame bifurcation were studied by experiment and computation with detailed chemistry. Mixtures of methane and artificial air (O₂-N₂-He, the O₂ mole fraction being fixed at 0.21, and those of N2 and He being varied) having four different Lewis numbers, that is, 0.97, 1.2, 1.4, and 1.8, were used. Experiments were carried out under microgravity of 10 s, and computation employing C1 chemistry was conducted taking radiative heat loss into account. The experimental results and those of detailed computation qualitatively agreed with each other and showed a change of extinction characteristics with the increase of Lewis numbers. We call this change "G-K transition," because the configuration of the extinction curve gradually changed from G-shaped to K-shaped with the increase of Lewis numbers. This transition clearly showed, the following: (1) the relation between counterflow flame and propagating plane flame, (2) the existence of a critical Lewis number where the lower limit of counterflow flame and the conventional 1-D limit coincide, and (3) the existence of a multiple solution region resulting from flame bifurcation. Furthermore, two pieces of supporting evidence for flame bifurcation were obtained by microgravity experiments. One is a discontinuous extinction curve due to separated extinction curves of the K-curve for Lewis number 1.8. The other is flame pulsation due to transient flame motion in the multiple solution region initiated by the diffusive-thermal instability of high Lewis number.