Laminar premixed flame extinction limits. I. Combined effects of stretch and upstream heat loss in the twin-flame unburnt-to-unburnt opposed flow configuration

Abstract

Numerical methods have been used to examine the combined effects of stretch and localized upstream heat loss on the properties of a stoichiometric laminar premixed methane-air flame. A symmetric unburnt-to-unburnt opposed flow configuration was investigated, in which fresh combustible premixture was discharged towards the stagnation plane from identical coaxial plug flow nozzles maintained at constant temperature, and separated by a fixed distance. A continuation method was used in tandem with Newton solutions for individual flame structures, in order to study the flame behaviour as the stretch rate (or nozzle inlet velocity) was varied. High- and low-stretch extinction limits were observed as positions of vertical tangency on the steady-state solution curves of flame property versus stretch rate. The complete series of solutions between these positions formed a set of isolae with both stable and unstable branches. The chemical behaviour at the limits and along both branches is discussed, with particular emphasis on the behaviour near the low-temperature low-stretch-rate limit. The importance of changes not only in the size of the radical pool, but also in the ratio of hydrocarbon (CH 3 ) to non-hydrocarbon (H, O and OH) species therein, is demonstrated both near the limits and along the unstable solution branch. Attention is drawn to the significance of the conclusions in the context of flame extinguishment. The presentation and discussion of the flame behaviour is preceded by a full description of the computational approach.