Effects of radiation heat loss on the flammable region of counterflow premixed flames at a large Lewis number

The extinction limits and flammable regions of a nonadiabatic counterflow premixed flame at a large Lewis number are investigated numerically. It is found that interaction between the radiative heat loss and the Lewis number effect results in four kinds of flame bifurcations. Furthermore, the results show that there exist four different kinds of flame regimes: normal flames, radiation-stretch-induced weak flames, radiation Lewis number effect-induced weak flames and unified weak flames at typical equivalence ratios. An increase in the radiation intensity reduces the extinction limit of a normal flame but extends the flammable region of weak flames to large stretch rates. A general graph showing the extinction limits and the flammable regions is obtained. It is found that the G-shaped curve obtained at a low Lewis number is reduced to a K-shaped curve at a large Lewis number. This G-K transition presents a good explanation for the experimental results obtained in microgravity experiments.