Experimental and computational study of fluid dynamics-combustion coupling in a diffusion flame-vortex ring interaction

Flame-vortex interaction is a canonical configuration that contains the fundamental element of flow, transport and combustion phenomena found in turbulent reacting flows, and provides a means for testing existing models and developing new models for diffusion flames. The configuration used in this study is the first to examine an exothermic reacting vortex ring under microgravity conditions, formed from the rollup of a diffusion flame during formation of a vortex ring composed of both fuel and entrained ambient oxidizer.; Experiments and computational studies were conducted to shed light on the effects of chemistry and hydrodynamic parameters on the flame structure and ring dynamics. The dominant effect of combustion heat release was found to be volumetric expansion (dilatation) over enhanced diffusivities due to temperature. Dilatation had a dramatic effect on the ring trajectories which initially provided an increase in ring speed during the early phase and a large reduction in speed in the latter phase of the diffusion flame-vortex ring interaction. Fuel volume was also determined to play a key role in the amount of heat released during the initial phase of interaction. An increase in fuel volume beyond a critical limit set by stoichiometric requirements, while keeping the circulation unchanged, led to a decrease in heat released which resulted in a decrease in ring speed. The effect of nitrogen dilution of propane was mainly a reduction in flame luminosity with little change in the flame structure. In addition, comparisons between propane and ethane cases reveal that radiative heat loss can significantly affect the flame structure and dynamics of reacting vortex rings. Numerical simulations of methane-air chemistry with flamelet chemistry revealed that the OH layer was thicker than the CH and HCO layers. Increasing the ring circulation while keeping the fuel volume constant resulted in an increase of these minor species along the forward stagnation point of the reacting vortex rings. These results should provide the basis for future laser diagnostic studies of certain key aspects of this flame-vortex configuration.