2579 Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 2579–2586 DIFFUSION FLAME STRUCTURE OF A LAMINAR VORTEX RING UNDER MICROGRAVITY CONDITIONS SHIN-JUH CHEN and WERNER J. A. DAHM Laboratory for Turbulence and Combustion (LTC) Department of Aerospace Engineering The University of Michigan 1320 Beal Avenue Ann Arbor, Michigan 48109, USA 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 micro- gravity 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. Introduction Flame-vortex interactions are one of the central components of turbulent combustion. Various ele- mentary forms of such interactions contain many of the fundamental elements of ﬂow, transport, and combustion phenomena present in turbulent ﬂames. These include concentrated vorticity, entrainment and mixing, strain and nonequilibrium phenomena, diffusion and differential diffusion, partial premixing and diluent effects, and heat release effects. More- over, the relative simplicity of these elementary ﬂame-vortex conﬁgurations allows study of the un- derlying transport and reaction processes under far more controllable conditions than is typically the case in direct investigations of turbulent ﬂames. For these reasons, various ﬂame-vortex conﬁgurations play a key role in combustion science and are pro- viding insights into these elementary transport and reaction processes, and into the suitability of current models for these processes, in turbulent ﬂames. The simplest of these elementary ﬂame-vortex conﬁgurations involves the interaction of a premixed or diffusion ﬂame with an isolated vortex. Since the initial study of Marble [1], this problem has received extensive theoretical [2–4] as well as numerical [5– 11] and experimental [12,13] attention. More re- cently, the interaction of a vortex pair or vortex ring with a planar premixed ﬂame [14–17] or diffusion ﬂame [18–26] has also been studied. Much of this work has been reviewed by Rolon et al. [24] and The ´ venin et al. [25], who have conducted detailed experimental and numerical studies of the interac- tion that results when a previously developed vortex ring meets a counterﬂow diffusion ﬂame. Their work has shown a number of key insights that can be gained from the study of such ﬂame-vortex interac- tions. Here we examine a different elementary conﬁg- uration for the interaction between a vortex ring and a diffusion ﬂame, shown schematically in Fig. 1. Fuel is issued impulsively from the exit of a round nozzle and rolls up with air to form a laminar vortex ring. A diffusion ﬂame at the initial air–fuel interface also rolls up with the ring and is subjected to the resulting strain rate ﬁeld of the ring. At the same time, heat release due to combustion affects the vortex ring and the transport properties within it. This produces a strong transient interaction between the vortex and the ﬂame, which continues until all the fuel in the ring has been consumed. This conﬁguration is rather different in detail from that of Rolon et al. [24] and The ´ venin et al. [25], where the ring consists entirely of the fuel or oxidizer constituents when it encoun- ters the ﬂame. The present interaction is similar to