Strained Premixed Laminar Flames with Two Reaction Zones

Abstract

The method of activation energy asymptotics (AEA) is used to study the structure and extinction characteristics of premixed laminar flames in two counterflowing reactant streams such that a layer of products is bounded by two reaction zones. The analysis includes the influences of realistic density variations and of large rates of strain. Account is taken of Lewis numbers different from unity and of different temperatures in the two reactant streams. It is found that abrupt extinction occurs either upon merging of the two reaction zones or prior to merging depending on the Lewis number and on the temperature difference of the two streams. In the adiabatic case Lewis numbers suitably greater than unity, i.e., greater than a critical value, are found to be needed for extinction to occur prior to merging. The critical Lewis number decreases as the temperature difference increases such that at suitably large differences extinction prior to merging occurs for Lewis numbers less than unity. Density nonuniformities associated with heat release are shown to increase the critical Lewis number with increases in such release if the reactant temperature is held fixed but not if the adiabatic flame temperature is held fixed. Expressions are developed for the calculation of reaction zone locations and of their variation with rate of strain, Lewis number and temperature difference. Predictions obtained from the analysis are found to be in qualitative agreement with experimental results.