Structure of stretched hydrogen - air diffusion flames near extinction

摘要

Numerical study of hydrogen/air diffusion flames at low stretch and near extinction is presented. For each flamelet the governing equations for the conservation of energy and species mass fractions based on "an extended reaction mechanism" are solved numerically, by a Gauss-elimination iterative technique, in the mixture fraction space for a fixed value of the dissipation rate χ. The flamelet is then stretched, by increasing χ and the obtained flamelet structures are stored in a flamelet library. The results revealed that the flame approaches extinction at χ= 400 s{sup}(-1) where the maximum flame temperature was 1200 K, after which the flame completely extinct at χ= 450 s{sup}(-1). The mass fractions of the reactive species (H{sub}2, O{sub}2, H{sub}2O, O, H, OH, H{sub}2O{sub}2 and HO{sub}2) are affected appreciably by stretching the flame towards the extinction limit. The individual reaction rates, of the 24 elementary reactions, show broadening near extinction. The detailed reactions involving OH, 0 and H showed considerable changes when χ was increased from 1 to 400 s{sup}(-1) while their net production rates were hardly affected.