doi:10.3850/978-981-08-7724-8_07-05

ABSTRACT

Based on the application of a detailed H2/O2/N2 reaction mechanism to a planar counter-flow fame model, numerical methods have been used to examine the effects of both flame stretch and radiative loss on the properties and structure of fuel-lean hydrogen-air flames near the composition flammability limit at atmospheric pressure. Two axi-symmetric opposed flow configurations were examined: (i) a single flame unburnt-to-burnt (UTB) configuration in which fresh unburned mixture is opposed by a stream of its own combustion products, and (ii) a symmetric unburnt-to-unburnt (UTU) configuration in which two flames are supported back-to-back, one on each side of the stagnation plane. Dependent on circumstance, the flame/flames may be extinguished predominantly by stretch related effects or by radiative loss. The detailed extinction behaviour depends on the diffusion characteristics within the near-limit mixtures, and in particular the Lewis number Le of the deficient component. The effect of high stretch is always to attenuate the range of flammability. However, for Le<1 the range is initially extended by low to moderate stretch, particularly in the UTU situation where downstream radiative loss is not present at extinction.. For realistic emissivities in lean H2/air, the computed low stretch extinction equivalence ratios are approximately 0.28 and 0.14 at stretch rates 0.1 and 1 s-1 respectively. A less pronounced extension to an equivalence ratio 0.23 at a stretch rate of 1.2 s -1 is found in the UTB configuration.

Keywords: Counter-flow flames, Interaction of flames with surroundings, Stretch, Flammability limits.

© 2011 CPD Unit, University of Leeds