EFFECT OF UNSTEADY STRETCH RATE ON OH CHEMISTRY DURING A FLAME VORTEXINTERACTION - TO ASSESS FLAMELET MODELS

Some basic assumptions of flamelet models are assessed by comparing pr ofiles of OH mole fraction measured during an unsteady flame-vortex in teraction to the OH profiles computed for a steady, planar counterflow flame (SPCF) with full chemistry. It is important to make such compar isons for both the same local three-dimensional stretch rte, which is measured instantaneously at locations along the flame front, and the s ame heat loss, as characterized by the product temperature T-2. The fu ndamental experimental procedure consists of interacting a laminar, pr emixed 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 we re made possible by the use of particle-imaging velocimetry (PIV) diag nostics on the repeatable, axisymmetric experiment. Whereas previous c omparisons have been limited to steady counterflow flame experiments, the present study considers a flame which is unsteady, freely-propagat ing, curved, far from walls, and has realistic heat losses; thus, it c ontains the physical processes present in turbulent premixed flames. I t was found that there are significant (25%) differences between measu rements and steady counterflow flame computations of peak OH mole frac tions and OH reaction zone widths. Even where the stretch rate was con stant along the flame, the OH profiles showed variations, indicating t hat the OH profile is not a unique function of the instantaneous local stretch, but depends on the time history of the flowfield. Such histo ry effects may be better modeled using unsteady counterflow flame simu lations. Large differences (a factor of two on centerline) occur betwe en the measured three-dimensional stretch rate for meaningful comparis ons with models. Sensitivity analysis shows that heat losses must be r ealistically modeled, especially if flame extinction is to be simulate d. The present type of comparison represents a first step in the asses sment of flamelet models.