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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.

Even though the scalar dissipation rate at the stoichiometric surface, χ stoich , is recognized to be the most fundamental fluid time scale in laminar diffusion flames, their structure and extinction behavior are often characterized simply in terms of strain rate, a much more easily measurable observable. Yet, the two variables are different, especially in unsteady flamelets. An experimental technique based on line Raman imaging of major species is presented for the quantitative measurement of χ stoich in vortex-perturbed counterflow diffusion flames. Three formulations are evaluated, and it is shown that a formulation based on N 2 -mass fraction is the most appropriate, provided that N 2 is experimentally accessible and that there is no significant preferential diffusion. The technique is used to compare vortex-perturbed and quasi-steady extinction. The thesis that for a given composition of the counterflowing streams, extinction occurs at a given value of χ stoich , irrespective of the mode of perturbation, steady or unsteady, is verified experimentally and is contrasted with the observation that vortex-perturbed flames can sustain an almost double strain rate at extinction compared to steadily strained ones. The effect of two-dimensional phenomena on the results is discussed. Finally, a promising approximation of χ stoich using estimates of the thickness of mixing layer from temperature profiles, with significant simplifications in the required measurements, is investigated.

The interaction of laminar vortices with a methanol spray counterflow diffusion flame was studied experimentally with vortices generated from either the fuel side or the oxidizer side. The overall stoichiometry was such that the flame resided on the fuel side of the gas-stagnation plane. Local extinction and subsequent reignition were investigated as the circulation of the vortex was varied. It was found that extinction required vortices of larger circulation if generated from the oxidizer side. The effect was attributed to stretching and possibly, partial dissipation of the vortex, as it approached the stagnation plane before interacting with the flame. The robustness of the spray flame to vortex-induced extinction was compared with that of a similar gaseous flame. The spray flame was found to be comparatively weaker. Of the two potential culprits for such a difference, namely, the energetic handicap of spray flames due to the latent heat of vaporization of the liquid fuel and droplet inertia, the former was found to be the dominant factor. After extinction occurred, a hole was created in the diffusion flame, confining the combustion process to an annular region. The flame was then able to propagate back toward the centerline, re-establishing itself as a flat diffusion flame. The time interval for this reignition process was investigated as a function of the vortex circulation. It was found that, if the vortex approached the flame from the fuel side, the reignition time was much shorter than when the vortex was injected from the oxidizer side, and decreased for increasing values of the vortex circulation. In contrast, the reignition time increased with the circulation, if the vortex approached the flame from the oxidizer side. Only in the latter case, droplet inertia played a role in the reignition process.

The extinction behavior of methanol counterflow spray diffusion flames was investigated using a combination of formaldehyde planer laser-induced fluorescence (PLIF) and phase Doppler measurements. Extinction was brought about quasi-steadily, by progressively increasing the flow rates of both oxidizer and fuel side, and unsteadily, by generating a vortex on the oxidizer side. The unsteady experiments yielded values of extinction strain rates a factor of 2 larger than the quasi-steady values. The greater robustness of the spray flame under unsteady perturbation, was explained phenomenologically by estimating the timescales involved in the process. It was found that the vortex introduces unsteady effects in the outer diffusiveconvective layer of the flame. The inner reactive-diffusive layer, on the other hand, behaves in a quasisteady manner, since the characteristic chemical time is much smaller than the characteristic unsteady time. As a result, even though the instantaneous strain rate is much larger than the quasi-steady extinction strain rate, the flame is subject to a damped strain rate through the outer layer. An estimate of the thickness of the mixing layer, based on formaldehyde PLIF, provided a convenient means to compare the scalar dissipation rate and the Damkohler number between the two extinction modes, bypassing the need for detailed species measurements for the assessment of the mixture fraction and its gradient. Such a comparison showed that the difference between the two extinction modes was reduced to 25% on the average, consistent with expectations based on flame structure models from asymptotic theory. Spray flames exhibited longer time delays between the onset of extinction and reignition, as compared to gaseous flames. Estimates of the relevant Stokes number suggested that the difference may be attributed to droplet inertia effects.

Using a combination of HCHO planar laser-induced fluorescence and laser Doppler velocimetry measurements, the extinction behavior of methanol counterflow diffusion flames was examined experimentally under conditions in which the extinction was brought about by a vortex generated on the oxidizer side. Comparisons were made with quasi-steady extinction results for the same flames. It was found that the flames can withstand instantaneous strain rates as much as two-and-a-half times larger than the quasisteady ones. The finding was rationalized phenomenologically by comparing the characteristic times of the problem, that is, the mechanical time, the chemical time, and the vortex turnover time. Specifically, estimates of these times yielded the following ordering: sch svort sm. As a result, the vortex introduced an unsteady effect in the outer diffusive-convective layer of the flame, while the inner reactive-diffusive layer behaved in a quasi-steady manner. Consequently, the flame was subject to a damped strain rate through the outer layer. Results from a simple analytical model showed that the difference between vortexinduced extinction and quasi-steady extinction was much more modest in terms of instantaneous scalar