Cool-Flame Burning and Oscillations of Envelope Diffusion Flames in Microgravity
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The two-stage combustion, local extinction, and flame-edge oscillations have been observed in single-droplet combustion tests conducted on the International Space Station. To understand such dynamic behavior of initially enveloped diffusion flames in microgravity, two-dimensional (axisymmetric) computation is performed for a gaseous n-heptane flame using a time-dependent code with a detailed reaction mechanism (127 species and 1130 reactions), diffusive transport, and a simple radiation model (for CO2, H2O, CO, CH4, and soot). The calculated combustion characteristics vary profoundly with a slight movement of air surrounding a fuel source. In a near-quiescent environment (≤ 2 mm/s), with a sufficiently large fuel injection velocity (1 cm/s), extinction of a growing spherical diffusion flame due to radiative heat losses is predicted at the flame temperature at ≈ 1200 K. The radiative extinction is typically followed by a transition to the “cool flame” burning regime (due to the negative temperature coefficient in the low-temperature chemistry) with a reaction zone (at ≈ 700 K) in close proximity to the fuel source. By contrast, if there is a slight relative velocity (≈ 3 mm/s) between the fuel source and the air, a local extinction of the envelope diffusion flame is predicted downstream at ≈ 1200 K, followed by periodic flame-edge oscillations. At higher relative velocities (4 to 10 mm/s), the locally extinguished flame becomes steady state. The present 2D computational approach can help in understanding further the non-premixed “cool flame” structure and flame-flow interactions in microgravity environments.
KeywordsLocal extinction Pulsating diffusion flame Microgravity droplet combustion Negative temperature coefficient Cool flame
This work was supported by the NASA Space Life and Physical Sciences Research and Applications Division (SLPSRA). Initial versions of this paper were presented at the 9th U. S. National Combustion Meeting, Cincinnati, Ohio, May17-20, 2015, and the 11th Asian Microgravity Symposium, Sapporo, Japan, October 25–29, 2016. The authors would like to thank Daniel Dietrich, Vedha Nayagam, and Forman Williams for their fruitful discussions and Tanvir Farouk and Frederick Dryer for providing us with the reduced n-heptane reaction mechanism.
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