Quantitative scalar dissipation rate measurements in vortex-perturbed counterflow diffusion flames

Abstract

Even though the scalar dissipation rate at the stoichiometric surface, v 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 v 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 v 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 v stoich using estimates of the thickness of mixing layer from temperature profiles, with significant simplifications in the required measurements, is investigated.

Nitric oxide formation during flame/vortex interaction

“…[6], and to obtain the scalar dissipation rate at the stoichiometric surface, as in Ref. [3]. The latter was defined as v s ‫ס‬ where D is the 2 2D|ٌZ| s thermal diffusivity of the mixture, and Z ‫ס‬ (b ‫מ‬ b ox )/(b f ‫מ‬ b ox ) the mixture fraction (ox and f indicate the conditions on the oxidizer side and the fuel side, respectively).…”

“…The conserved scalar b was defined as b ‫ס‬ for reasons explained in Ref. [3]. Evidently, Y , N 2 this definition is based on the presumed inertness of N 2 and does not take into account the NO x chemistry.…”

“…[1]. Specifically, this approach proved to be useful for the quantitative study of extinction phenomena in diffusion flames [2][3][4]. This work showed that extinction is not dependent on the timescale of the perturbation when described in terms of the scalar dissipation rate at the stoichiometric surface, because of the fast chemistry associated with the heat release.…”

“…A detailed description of this technique in this experimental configuration was presented in Ref. [3]. A Nd:YAG laser delivering 100 mJ/pulse at 532 nm was used as the excitation source.…”

On the dynamics of flame edges in diffusion-flame/vortex interactions

“…Thus, for values of the Damköhler number larger than a critical value, Da > Da* (>Dae), the flame edges propagate along the stoichiometric surface toward the unburned mixture as ignition fronts (triple or edge flames with positive velocity, f/ F > 0, relative to the upstream flow), while for Da* > Da (>Da e ) they behave as failure waves (edge flames with negative velocity, f/ F < 0) that recede from the unburned mixture. The analysis of the scalar dissipation rate at the stoichiometric surface, which yields the local value of Da at the flame front, is therefore of interest for the description of the evolution of extinguished holes [44]. After local flame extinction, the holes grow rapidly in size if the stretch rate is maintained, because close to the extinction value of Da, of order unity, the dimensionless edge front velocity E/F/SL is negative and, more importantly, the flame-edge displacement is soon assisted by the flow velocity components associated with the stretch.…”