THE EFFECTS OF RADIATION ON STRETCHED FLAMES

We examine the behaviour of twin premixed dames in a symmetric counter flow, subject to radiation losses, a problem motivated by recent drop- tower experiments. A simple asymptotic strategy is adopted which compl ements recent numerical simulations and provides new insights. Paramet ers are chosen to emulate, in a rough way, lean methane-air mixtures, and the dame response to varying strain-rates is calculated for differ ent mixture strengths. For strengths below the standard flammability l imit, but above an inferior limit, sub-limit solutions are obtained fo r which the flame-position versus strain-rate defines an isola with lo w-and high-strain-rate quenching points. For mixture strengths greater than a critical value above the standard limit the isola opens up, an d there are two solutions (rather than none) for a vanishing strain ra te. An intermediate response characterized by two disjoint solution cu rves exists for a minute range of mixture strengths. Five different ca tastrophe (jump) events are identified, one of which is related to the standard flammability limit. It is shown that the inferior flammabili ty limit monotonically decreases with decreasing Lewis number (Le), an d that sub-limit solutions are possible for Le < Le(c) where Le(c) is a critical value greater than 1. Sub-limit solutions exist primarily b ecause of geometric effects.