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

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 N₂-mass fraction is the most appropriate, provided that N₂ 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.