Chemical Species Produced in Fires Near the Limit of Flammability

Abstract

Sustaining a diffusion flame requires sufficient concentrations of fuel and oxidizer to maintain the combustion reaction. When the quantity of oxygen in the environment falls below a minimum value, combustion processes will be extinguished. An experimental technique is described which was used to determine the oxygen concentration in a vitiated environment of combustion products mixed with air which only just supports a large-scale diffusion flame. Measurements were taken of limiting oxygen concentrations and chemical species produced in fires near this flammability limit for methane (natural gas) and ethylene fuels. As limiting conditions were approached, radiation from soot in the reaction zone became imperceptible, leaving only a weakly luminous blue flame. Even with significant reductions in both the flame height and luminosity at these conditions, the fire completely consumed the fuel and generated no measurable amounts of products of incomplete combustion. A comparison of limiting oxygen concentrations and limiting flame temperatures for these experiments with the results of other investigations shows reasonably good agreement despite widely varying experimental techniques.