EXTINCTION OF TURBULENT COUNTERFLOW FLAMES WITH REACTANTS DILUTED BY HOT PRODUCTS

The effects of simultaneous dilution and preheat of reactants by mixin g with hot combustion products are examined in terms of the stability of turbulent counterflow flames. Premixed flames were stabilized in th e opposed flow of premixed natural gas/air mixtures within the flammab ility limits and an opposing jet composed of hot products at temperatu res up to 1750 K and oxygen mole fractions down to 0.02. The gain in s tability 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 b y straining, in agreement with expectations from laminar counterflow p remixed 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 sa me extinction strain rate and that there was no extinction at air temp eratures higher than about 1700 K. Laminar counterflow flame predictio ns of extinction are shown to be in agreement with the measurements an d also show that stability is improved in the special case of adiabati c mixing of the air with hot combustion products, so that the temperat ure rise and the oxygen content are related, and this explains why fla mes stabilized by recirculation zones, where hot products are recircul ated to mix with the incoming reactants, can be stable with their high stretch rates.