Diffusion flame structure of a laminar vortex ring under microgravity conditions

Experimental results are presented for the roll-up and burning of a diffusion flame in a laminar vortex ring formed by impulsively issuing fuel from a round nozzle into an oxidizer environment under microgravity conditions. The resulting elementary flame-vortex interaction produces very strong coupling, in both directions, between the flame and the flow. Many fundamental features normally associated with nonburning vortex rings are radically altered by the effects of combustion heat release. Results for pure propane fuel over a range of ring circulations and fuel volumes show a clear effect of the fuel volume on the flame structure and burnout time for rings that do not exceed the overfill limit. The ring overfill limit agrees well with results from nonburning rings. Nitrogen dilution of the propane leads to a reduction in the flame luminosity and burnout time, as well as changes in details of the formation and dissipation of the luminous cap, with little change in the primary structure or dynamics of the interaction. A simple diffusion-limited estimate for the burnout time correctly predicts the effects of fuel volume and dilution. The present results are compared with the analytical work of Karagozian and Manda (1986) and Manda and Karagozian (1988) for a vortex pair. The highly symmetric results obtained in microgravity are well suited for comparison with detailed numerical simulations.