Characteristic Residence Times of Laminar Nonpremixed Flames at Extinction

Abstract

The relationships between the inverse of the characteristic residence time a _q which is a measure of the strain rate at extinction, the “scalar dissipation rate” at extinction χ the adiabatic flame temperature T _st, and the stoichiometric mixture fraction Z _st in diffusion flames are examined. Numerical calculations are performed to determine the structure and critical conditions of extinction of diffusion flames stabilized between counterflowing streams, one containing methane and the other containing oxygen. These reactant streams are diluted with nitrogen or with nitrogen and argon. In the flame the value of the scalar dissipation rate is found to vary over a wide range and its value evaluated at Z _st is used to characterize the system. At constant values of T _st the values of a _q and χ_q are found to decrease with decreasing values of Z _st. The value of χ_q decreases at a faster rate than that of a _q. At constant values of Z _st the values of a _q, and χ_q are found to decrease at the same rate with decreasing values of T _st. At constant values of both T _st and Z _st the values of a _q and χ_q are found to be constant for various concentrations of reactants and inert species in the counterflowing streams.

Key words:

• Flames (diffusion
• laminar)
• extinction