FLAME BIFURCATIONS AND FLAMMABLE REGIONS OF RADIATIVE COUNTERFLOW PREMIXED FLAMES WITH GENERAL LEWIS NUMBERS

Flame bifurcations. extinction, and flammable regions of radiative cou nterflow premixed flames and the planar propagating flames are investi gated numerically with general Lewis numbers. New insights and phenome na are presented by examining the interaction of radiation heat loss, Lewis number effect, and stretch. At a low Lewis number, the results s how that for equivalence ratio lower than the standard flammability li mit, radiation extinction limit and stretch extinction limit exist, re spectively, at low and high stretch rates. For a higher equivalence ra tio, this flame isola opens up and yields two stable flame branches, a normal flame branch, and a weak flame branch. A G-shaped curve showin g the limits and flammable regions of these two flame regimes is obtai ned. The results show that the inferior limit of counterflow flame is lower than the standard limit. At unity Lewis number, the results show that, for equivalence ratio lower than the standard limit, there is a lso a flame isola with two distinct extinction limits at low and high stretch rates. For equivalence ratio larger than the standard limit, a new flame branch with flame standing far from the stagnation plane em erges on the low stretch rate side of the flame isola. This flame is r elated to the standard limit with the decrease of fuel concentration a nd merges with the flame isola when the fuel concentration is larger t han a critical value. The extinction curve is also a G-shaped curve an d the inferior limit is lower than the standard limit. At Lewis number larger than a critical value, for equivalence ratio near the standard limit, there is only the weakly stretched flame branch with a single stretch extinction limit. With an increase of fuel concentration, a we ak flame isola emerges. As the fuel concentration further increases, t his weak flame isola merges with the weakly stretched flame branch yie lding three kinds of flame regimes. The resulting extinction curve is a K-shaped curve and the inferior limit is equal to the standard limit . The present results show that flame bifurcations and the existence o f weak flames are physically intrinsic phenomena of radiative counterf low premixed flames. It is found that the normal flame and the weak fl ame can jump to each other at their limits. The obtained G and K-shape d curves show a clear relation between the counterflow flame and the p lanar propagating flame. The occurrence of G-K transition with the inc rease of Lewis number also gives a good explanation to the physics of the experimental results. (C) 1998 by The Combustion Institute.