On the flammability limit and heat loss in flames with detailed chemistry

Abstract

This paper reports a computational study of one-dimensional, planar premixed, nonadiabatic flames near lean flammability limits considering detailed chemistry and variable properties. Calculations made for CH₄/air mixtures of varying leanness between 4.5-6.0% of CH₄ show that (1) a steady propagation is obtained for all lean mixtures when heat loss is not considered, and (2) with the inclusion of a distributed heat loss a steady propagation is predicted above a certain CH₄ fraction; below this critical CH₄ fraction the solution decays, indicating nonflammability. The limit so determined varies from 5.18-5.6% of CH₄ with the magnitude of heat loss raised by a factor of 3. This small change in the limit for a large variation in heat loss factor is argued to indicate the fundamental nature of limit. The limiting fuel composition and flame speed found are nonzero and compare well with the results of zero-gravity experiments. The critical heat loss factor and flame speed at the limit obtained from the present work confirm the results of asymptotic theory. The principal effect of detailed chemistry is argued to be the correct reproduction of heat release rate vs. mixture ratio.