Vortex Ring/Diffusion Flame Interactions in Microgravity Conditions

Turbulent diffusion flames involve mixing and reaction between initially non-premixed or partially premixed fuel and oxidizer streams, and represent by far the largest class of combustion problems, with applications spanning an enormous range of critical technologies ranging from aerospace propulsion systems, to industrial combustion processes, to utility power generation systems. Certain aspects of such combustion in turbulent shear flows have been accessible to experimental study under controlled conditions, often by isolating the particular phenomenon or interaction of interest in an idealized canonical configuration. There remain, however, many fundamental issues that are essential to the subject, but which have eluded study to date owing to the dominating influence of buoyancy under normal-gravity conditions. The vortex ring/diffusion flame interaction is among such problems, and is arguably the simplest of all configurations that contain the fundamental elements of flow, transport, and combustion phenomena present in turbulent diffusion 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. All of these phenomena combine in this comparatively simple canonical configuration to permit study of their interactions under carefully controllable conditions. However despite its importance in combustion science, this configuration has not previously been studied owing to the fact that it can be brought within experimental reach only under microgravity conditions; Chen & Dahm (1996). The present paper reports the first experimental examination of this important problem in combustion science, where we examine hydrocarbon combustion in a vortex ring burning in air. Previous theoretical studies have been conducted under somewhat idealized conditions for the related case of a chemically reacting vortex pair by Karagozian & Manda (1986) and Manda & Karagozian (1989), as well as an experimental study of aqueous acid-base reaction in a vortex pair by Karagozian et al (1988). These share many features with the present problem, but differ in important ways from gaseous hydrocarbon combustion in a vortex ring, for which a broad range of nonequilibrium interactions between flow, diffusion, and reaction occur over the range of conditions examined here. The present problem is also the subject of an ongoing direct numerical simulation (DNS) study by Hewett & Madnia (1996), though owing to the scale of the calculations the range of conditions that can be examined is limited.