Methanol Droplet Combustion in Oxygen-Inert Environments in Microgravity
Nayagam, Vedha, Dietrich, Daniel L., Hicks, Michael C., Williams, Forman A.
The Flame Extinguishment (FLEX) experiment that is currently underway in the Combustion Integrated Rack facility onboard the International Space Station is aimed at understanding the effects of inert diluents on the flammability of condensed phase fuels. To this end, droplets of various fuels, including alkanes and alcohols, are burned in a quiescent microgravity environment with varying amounts of oxygen and inert diluents to determine the limiting oxygen index (LOI) for these fuels. In this study we report experimental observations of methanol droplets burning in oxygen-nitrogen-carbon dioxide and oxygen-nitrogen-helium gas mixtures at 0.7 and 1 atmospheric pressures. The initial droplet size varied between approximately 1.5 mm and 4 mm to capture both diffusive extinction brought about by insufficient residence time at the flame and radiative extinction caused by excessive heat loss from the flame zone. The ambient oxygen concentration varied from a high value of 30% by volume to as low as 12%, approaching the limiting oxygen index for the fuel. The inert dilution by carbon dioxide and helium varied over a range of 0% to 70% by volume. In these experiments, both freely floated and tethered droplets were ignited using symmetrically opposed hot-wire igniters and the burning histories were recorded onboard using digital cameras, downlinked later to the ground for analysis. The digital images yielded droplet and flame diameters as functions of time and subsequently droplet burning rate, flame standoff ratio, and initial and extinction droplet diameters. Simplified theoretical models correlate the measured burning rate constant and the flame standoff ratio reasonably well. An activation energy asymptotic theory accounting for time-dependent water dissolution or evaporation from the droplet is shown to predict the measured diffusive extinction conditions well. The experiments also show that the limiting oxygen index for methanol in these diluent gases is around 12% to 13% oxygen by volume.
Test conditions.
Extracted from the paper by AI · “Verified” means a person checked it
| Fuel | O₂ | Pressure | Airflow | Sensors | Source | AI conf. | Status |
|---|---|---|---|---|---|---|---|
| Methanol Droplet (freely floated or tethered) |
12% to 30% by volume | 0.7 and 1 atmospheric pressures | Quiescent microgravity environment | Black-and-white back-lit droplet image capture camera, UV-sensitive flame imaging camera (OH* chemiluminescence at 310 nm), CCD color camera | p. 1 | 95% | Pending |
Findings.
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01
The limiting oxygen index (LOI) for methanol in oxygen-nitrogen-carbon dioxide and oxygen-nitrogen-helium diluent gases is approximately 12% to 13% oxygen by volume.
Prevention · p. 1 · AI confidence 95% · Reported · Pending
“The experiments also show that the limiting oxygen index for methanol in these diluent gases is around 12% to 13% oxygen by volume.”
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02
Methanol droplet extinction occurs via two mechanisms: diffusive extinction due to insufficient residence time at the flame, and radiative extinction due to excessive heat loss from the flame zone.
Suppression · p. 1 · AI confidence 90% · Reported · Pending
“The initial droplet size varied between approximately 1.5 mm and 4 mm to capture both diffusive extinction brought about by insufcient residence time at the ame and radiative extinction caused by excessive heat loss from the ame zone.”
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03
For small methanol droplets burning in air, radiation is unimportant, and flames extinguish by diffusion-controlled mechanisms.
Suppression · p. 2 · AI confidence 85% · Extracted · Pending
“Marchese et al. [7] show that for small methanol droplet burning in air, radiation is unimportant, and the ames extinguish by diffusion-controlled mechanisms”
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04
When the initial methanol droplet diameter is greater than roughly 4 mm, the droplet flame extinguishes through excessive radiative heat loss.
Suppression · p. 2 · AI confidence 85% · Extracted · Pending
“They also show that when the initial droplet diameter is greater than roughly 4 mm, the droplet ame extinguishes through excessive radiative heat loss”
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05
Water absorption into the methanol droplet during combustion leads to a gradual increase in water concentration; when water mass reaches about 30%, outward water transport begins, lowering the adiabatic flame temperature below the range required for chemical-kinetic gas-phase flame extinguishment.
Suppression · p. 3 · AI confidence 90% · Reported · Pending
“This occurs when about 30% of the mass of the droplet is water... The adiabatic ame temperature of this diluted mixture is, however, signicantly less than that of pure methanol, actually below the temperature range, 1200 - 1700 K, that results in chemical-kinetic gas-phase ame extinguishment.”