The Cool Flames Experiment: Recent Results at Reduced and Partial Gravity

Abstract

Cool flames at Earth (1g), Martian (0.38g), Lunar (0.18g) and reduced-gravity (10g) 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, Ra≡βg∆TR3/να, exceeds 600 [2,3,6]. At 1g, typical values of the Ra are 10-10. 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. HARDWARE DESCRIPTION AND OPERATION The static, unstirred reactor and support apparatus are described in detail in reference 5. The essential hardware components include a furnace (0-600C), a fused-silica spherical vessel (i.d.=10.2 cm) and a premixed gas mixing and delivery system. The temperature uniformity within the furnace is ± 10C. The pressure in the reaction vessel is recorded with a Setra Model 204 0-25 psia transducer (accuracy:±1.4 Torr) mounted on the gas inlet to the reactor. The radial gas temperature profile is measured at 5 discrete radial locations, using a horizontally-mounted, type-K thermocouple rake in the vessel. A Hamamatsu C5909 ICCD camera operating at maximum gain records the emitted light through an optical port on the side of the furnace and a Xybion intensified camera mounted on the top of the furnace records an orthogonal view. EXPERIMENTAL RESULTS The slow heat release that occurs during the initial induction period is expected to induce a slow toroidal convective flow in the closed vessel for Ra>600 (1g, 0.38g, and 0.18g cases) as the hot, less dense gas rises in the bulk and recirculates downward along the internal vessel wall as it cools [3,6]. The rise speed of the hot gas is expected to scale as g [5] and thus decrease as the effective gravitational acceleration (g) decreases. Relative to the 1g case, the manifestation of the slower convective flows at 0.38g and 0.18g on the flame shape and propagation is observed indirectly through the observed modifications in the flame shape and its evolution (see Fig. 1). To date, however, this toroidal flow field has not been quantified experimentally.