Local extinction in an unsteady methane-air jet diffusion flame

Vortex-flame interactions constitute important elements of a turbulent flame and are building blocks for the development of turbulent-flame models. When a vortex originating inside the flame has sufficiently high normal velocity, it can pass through the flame, creating a localized hole in which no chemical reactions take place. An experimental and numerical investigation was conducted in an attempt to understand the local quenching process associated with the vortex-flame interaction in a methane jet diffusion flame. Axisymmetric toroidal vortices were generated periodically in a low-speed, laminar jet diffusion flame by driving the fuel jet at a frequency of 30 Hz. A time-dependent, axisymmetric computational fluid dynamics with chemistry (CFDC) model that incorporates detailed finite-rate chemistry for CH4-O2 combustion was developed for the simulation of this unsteady jet diffusion flame. The code was validated by direct simulation of an axisymmetric counterflow diffusion flame for different exit velocities. These calculations were found to yield more reliable predictions than those from conventional one-dimensional analyses. An accurate prediction for the quenching limit was made by replacing the rate expression of Peters for the chain termination reaction involving methyl radicals with that recommended by Warnatz. Calculations were made for the unsteady, coflowing, jet diffusion flame, and the local and temporal quenching processes observed during the vortex-flame interaction were successfully simulated. The structures of the weakly strained and near-quenching-limit flames were studied for the counterflow and coflow configurations. An order-of-magnitude increase in the production and destruction rates of methyl and oxygen radicals, respectively, was found in the flame zone of the coflowing, jet diffusion flame as it is strained to near-quenching limit - which is in contrast with that observed in a counterflow diffusion flame.