Cool flames at Earth (1g), Martian (0.38g), Lunar (0.18g) and reduced-gravity (10–2g) have been studied experimentally in a closed, unstirred, static reactor to better understand the role of natural convection and diffusive transport on the induction period(s), flame shape, flame propagation speed, pressure history and temperature profile. Natural convection is known to play an important role in all terrestrial, unstirred, static reactor cool flame and auto-ignition experiments when the Rayleigh number exceeds 600 [2,3,6]. At 1g, typical values of the Ra are 10^4-10^6.
In this paper, experimental results from static, unstirred reactor studies conducted at four different gravitational acceleration levels are reported for an equimolar propane-oxygen premixture. At 1g, the effects of natural convection dominate diffusive transport, the cool flame starts near the top of the vessel and subsequently propagates downward through the vessel. The flame is inherently two-dimensional. As the effective gravitational acceleration decreases, the associated Ra decreases linearly, convective transport weakens relative to diffusive fluxes of heat and species. At reduced-gravity, cool flames are observed to propagate radially outward from a centrally-located kernel without distortion owed to convective flow at a velocity that depends on the flame radius.
Foster, Michael R., "The Cool Flames Experiment: Recent Results at Reduced and Partial Gravity" (2004). Faculty Publications - Department of Mechanical and Civil Engineering. 30.