Computational studies of diffusion cool flame structures of n-heptane with/without ozone sensitization with a reduced chemistry

The diffusion cool flame structures of n-heptane are studied numerically with a comprehensively reduced chemistry. For this study, n-heptane/oxygen diffusion flame is self-sustained in a counterflow flame configuration with ozone sensitization. N-heptane is diluted by nitrogen to lower flame temperature and a small amount of ozone is added to oxygen to enhance oxygen atom production. N-heptane diffusion flame shows both hot-flame and cool-flame behaviors in high and low temperature regimes, respectively. Its S-shaped curve has two upper branches of hot and cool flame branches. The upper branch of cool flame is observed at low strain rates with the order of magnitude from O(1) to O(10²) s⁻¹ and its zone is much narrower than that of hot flame. N-heptane cool flame survives irrespective of ozone addition, but ozone extends the viable or stable flame zone to higher strain rate. Cool-flame temperatures are below 800 K and radical-induced ignition comes into play. N-heptane is highly diluted by nitrogen and extinction strain rate is more sensitive to mole fraction of n-heptane rather than mole fraction of ozone. Heat release rates of elementary reaction steps and concentrations of major radicals are far lower in cool flames than in hot flames. Main characteristics of cool flame structures at low strain rates and low temperatures are provided although the present low-temperature kinetics is not complete.