Nitric oxide formation during flame/vortex interaction

The formation of nitric oxide was investigated in methane diffusion flames under the perturbation of laminar vortices. Laser-induced fluorescence and Raman spectroscopy were used to measure nitric oxide, major species, and temperature, and to obtain the time evolution of the scalar dissipation rate at the stoichiometric surface during the flame/vortex interaction. Vortices with two characteristic times were thrust into the flame and their effects were compared with steady flames at the same scalar dissipation rate. The experimental measurements showed that the vortex did not affect significantly the peak temperature, CO, and CO₂ mass fraction during the interaction. NO, instead, was strongly affected. The peak mass fraction of NO in flames under vortex perturbation differed by almost a factor of 2 from steady flames with similar scalar dissipation rates. One-dimensional numerical simulations, used under the well-justified assumption that the vortices induced negligible curvature, yielded results in good agreement with the experimental measurements. The numerical computations were also used to investigate the effect of the characteristic time of the vortex on the different NO formation paths, that is, thermal, and prompt. The peak mass fraction of thermal NO was shown to be strongly dependent on the timescale of the unsteady perturbation, while prompt NO was almost independent. The emission index, that is, the production of NO normalized by the consumption of the fuel, behaved quasi-steadily for the two formation paths. The results were rationalized by considering the flame structure and the activation energy of the different formation paths.