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Asymptotic analysis of ignition in a laminar supersonic mixing layer reveals that, for small viscous heating values, the ignition distance scales linearly with the free stream Mach member. For larger viscous heating values, the ignition distance decreases rapidly due to temperature-sensitive reaction rate. Local flow retardation can be used to enhance ignition. Flow non-similarity effects are significant for mixing layer flow.

Abstract: Radiative extinction of spherical diffusion flames was investigated experimentally and numerically. The experiments involved microgravity spherical diffusion flames burning ethylene and propane at 0.98 bar. Both normal (fuel flowing into oxidizer) and inverse (oxidizer flowing into fuel) flames were studied, with nitrogen supplied to either the fuel or the oxygen. Flame conditions were chosen to ensure that the flames extinguished within the 2.2 s of available test time; thus extinction occurred during unsteady flame conditions. Diagnostics included color video and thin-filament pyrometry. The computations, which simulated flow from a porous sphere into a quiescent environment, included detailed chemistry, transport, and radiation and yielded transient results. Radiative extinction was observed experimentally and simulated numerically. Extinction time, peak temperature, and radiative loss fraction were found to be independent of flow rate except at very low flow rates. Radiative heat loss was dominated by the combustion products downstream of the flame and was found to scale with flame surface area, not volume. For large transient flames the heat release rate also scaled with surface area and thus the radiative loss fraction was largely independent of flow rate. Peak temperatures at extinction onset were about 1100 K, which is significantly lower than for kinetic extinction. An important observation of this work is that while radiative heat losses can drive transient extinction, this is not only because radiative losses are increasing with time but also because the heat release rate is falling off as the flame expands away from the burner and the reactant supply to the flame decreases. [Copyright &y& Elsevier]

This study is an examination of the sooting behavior of spherical microgravity diffusion flames burning ethylene at atmospheric pressure in a 2.2-s drop tower. In a novel application of microgravity, spherical flames were employed to allow convection across the flame to be either from fuel to oxidizer or from oxidizer to fuel. Thus, spherical microgravity flames are capable of allowing stoichiometric mixture fraction, Zst, and direction of convection across the flame to be controlled independently. This allowed for a study of the phenomenon of permanently blue diffusion flames—flames that remain blue as strain rate approaches zero. Zst was varied by changing inert concentrations such that adiabatic flame temperature did not change. At low Zst, nitrogen was supplied with the oxidizer, and at high Zst, it was provided with the fuel. Flame structure, quantified by Zst, was found to have a profound effect on soot production. Soot-free conditions were observed at high Zst and sooting conditions were observed at low Zst regardless of convection direction. Convection direction was found to have a smaller impact on soot inception, suppressing formation when convection at the flame sheet was directed towards the oxidizer. A numerical analysis was developed to simulate steady state conditions and aided the interpretation of the results. The analysis revealed that steady state was not achieved for any of the flames, but particularly for those with pure ethylene or oxygen flowing from the porous burner. Furthermore, despite the fact that all flames had the same adiabatic flame temperature, the actual peak temperatures differed considerably. While transient burner heating and burner radiation reduced flame temperature, gas-phase radiative heat loss was the dominant mechanism accounting for these differences. [Copyright &y& Elsevier]