On burner-supported, spherical diffusion flames under micro-buoyancy conditions

The present research endeavor is concerned with gaining fundamental understanding of the configuration, structure, and dynamics of laminar diffusion flames under micro-buoyancy conditions. Of particular interest is the establishment and hence study of the properties of spherically symmetric flames and their responses to external forces unrelated to gravity. Since buoyant forces are proportional to the product, g-Δp, the effects of buoyancy can be minimized by either reducing the gravity acceleration, g, or the density difference, Δp. Both approaches were employed in the present work via experiments in microgravity and experiments in normal gravity with "inverse" flames of small density difference situations. Regarding the former configuration, microgravity experiments have been performed in the NASA-Lewis 2.2-s drop tower facility, with emphases on the experimental transient response, due to an impulsive step from normal- to micro-gravity conditions. Experimental results along with preliminary numerical simulations indicate that steady-state flame behavior cannot be reached within the 2.2-s microgravity duration. Nonetheless, additional experiments indicate that the introduction of convective flows, such as those induced by a rotating burner, can reduce the transient time scale considerably. In terms of the latter configuration, fairly buoyancy-free, large-scale, steady-state, spherical diffusion flames have been obtained in earth gravity by utilizing an "inverted" configuration, where an oxidizing mixture is ejected from a porous burner into a lowdensity fuel atmosphere, at system pressures below 0.25 atm. Thus, the density differential and hence the upward-buoyant force acting on the hot flame sphere are reduced. In the present investigation, an interesting double luminous zone flame structure has been observed for sub-atmospheric diffusion flames of either air or an O₂/N₂ mixture against a mixture of hydrogen with a small quantity of methane. This flame structure consists of a green luminous zone near the fuel side and a blue/violet luminous zone near the oxidizer side, with a dark space between them. Computational simulation reveals that the blue and green luminous zones correspond to the main consumption layers of H,-O₂ and CH₄, respectively, that the breakdown of CH4 is primarily due to its attack by H, leading to the formation of CO, additional H₂, and eventually H₂O and CO₂, and that the electronically-excited CO₂ and C₂ are responsible for the blue and green luminescence, respectively.