Effect of unsteady stretch rate on OH chemistry during a flame-vortex interaction: To assess flamelet models

Some basic assumptions of flamelet models are assessed by comparing profiles of OH mole fraction measured during an unsteady flame-vortex interaction to the OH profiles computed for a steady, planar counterflow flame (SPCF) with full chemistry. It is important to make such comparisons for both the same local three-dimensional stretch rate, which is measured instantaneously at locations along the flame front, and the same heat loss, as characterized by the product temperature T₂. The fundamental experimental procedure consists of interacting a laminar, premixed flame with an impinging vortex ring of reactants. The OH flame chemistry was quantified using planar laser-induced fluorescence (PLIF) techniques, while the three-dimensional stretch rate measurements were made possible by the use of particle-imaging velocimetry (PIV) diagnostics on the repeatable, axisymmetric experiment. Whereas previous comparisons have been limited to steady counterflow flame experiments, the present study considers a flame which is unsteady, freely-propagating, curved, far from walls, and has realistic heat losses; thus, it contains the physical processes present in turbulent premixed flames. It was found that there are significant (25%) differences between measurements and steady counterflow flame computations of peak OH mole fractions and OH reaction zone widths. Even where the stretch rate was constant along the flame, the OH profiles showed variations, indicating that the OH profile is not a unique function of the instantaneous local stretch, but depends on the time history of the flowfield. Such history effects may be better modeled using unsteady counterflow flame simulations. Large differences (a factor of two on centerline) occur between the measured three-dimensional and two-dimensional stretch rates, indicating the importance of experimentally determining the full three-dimensional stretch rate for meaningful comparisons with models. Sensitivity analysis shows that heat losses must be realistically modeled, especially if flame extinction is to be simulated. The present type of comparison represents a first step in the assessment of flamelet models.