Structures of CO Diffusion Flames Near Extinction

Abstract

Computational results are reported for structures of laminar counterflow diffusion flames between carbon monoxide and air, initially at room temperature and pressure from 1 to 100 atm, with total hydrogen-atom mole fractions in the system ranging from zero to about 0.02. All strain rates considered are within a factor of ten of the critical extinction strain rate. This critical strain rate is calculated as a function of pressure and of hydrogen content and is shown to lie below measured values under most conditions. For hydrogen-free flames, activation-energy asymptotics is employed and supports the computational results. It is reasoned that trace hydrogen amounts in air and preferential hydrogen diffusion through nonplanar diffusive-thermal instability contribute to enhanced flame robustness in the experiments, while increasing buoyant convective heat loss with increasing pressure promotes extinction at the higher pressures.