Extinction of turbulent counterflow flames with reactants diluted by hot products

Abstract

The effects of simultaneous dilution and preheat of reactants by mixing with hot combustion products are examined in terms of the stability of turbulent counterflow flames. Premixed flames were stabilized in the opposed flow of premixed natural gas/air mixtures within the flammability limits and an opposing jet composed of hot products at temperatures up to 1750 K and oxygen mole fractions down to 0.02. The gain in stability of the premixed flames was small for temperatures from 300 to 1400 K, but temperatures higher than 1550 K always ignited flames of equivalence ratio as lean as 0.2 and these could not be extinguished by straining, in agreement with expectations from laminar counterflow premixed flames. This critical temperature is close to that below which chemical reaction is not self-sustaining. Turbulent diffusion flames were stabilized in the same arrangement with the hot product stream as oxidizer and it was found that for every 0.02 of oxygen mole fraction lost to dilution, the temperature had to increase by 100 K for the same extinction strain rate and that there was no extinction at air temperatures higher than about 1700 K. Laminar counterflow flame predictions of extinction are shown to be in agreement with the measurements and also show that stability is improved in the special case of adiabatic mixing of the air with hot combustion products, so that the temperature rise and the oxygen content are related, and this explains why flames stabilized by recirculation zones, where hot products are recirculated to mix with the incoming reactants, can be stable with their high stretch rates.