Aspects of Cool-Flame Supported Droplet Combustion in Microgravity
Nayagam, Vedha, Dietrich, Daniel L., Williams, Forman A.
Droplet combustion experiments performed on board the International Space Station have shown that normal-alkane fuels with negative temperature coefficient (NTC) chemistry can support quasi-steady, low-temperature combustion without any visible flame. Here we review the results for n-decane, n-heptane, and n-octane droplets burning in carbon dioxidehelium diluted environments at different pressures and initial droplet sizes. Experimental results for cool-flame burning rates, flame standoff ratios, and extinction diameters are compared against simplified theoretical models of the phenomenon. A simplified quasi-steady model based on the partial-burning regime of Lin predicts the burning rate, and flame standoff ratio reasonably well for all three normal alkanes. The second-stage cool-flame burning and extinction following the first-stage hot-flame combustion, however, shows a small dependence on the initial droplet size, thus deviating from the quasi-steady results. An asymptotic model that estimates the oxygen depletion by the hot flame and its influence on cool-flame burning rates is shown to correct the quasi-steady results and provide a better comparison with the measured burning-rate results.This work was supported by the NASA Space Life and Physical Sciences Research and Applications Program and the International Space Station Program.
Test conditions.
Extracted from the paper by AI · “Verified” means a person checked it
| Fuel | O₂ | Pressure | Airflow | Sensors | Source | AI conf. | Status |
|---|---|---|---|---|---|---|---|
| n-decane droplet |
25% | 0.5 atm | null | narrow-band radiation, wide-band radiation, visible flame extinction | p. 1 | 95% | Pending |
| n-decane binary droplet array |
17-83% | 1 atm | null | narrow-band radiation, wide-band radiation, visible flame extinction | p. 4 | 95% | Pending |
| Decane/Hexanol mixture droplet |
air | 1 atm | null | null | p. 4 | 90% | Pending |
| n-Decane droplet |
air | 1 atm | null | visual imaging (frame averaged) | p. 10 | 95% | Pending |
Findings.
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01
Cool flames are low-temperature diffusion-controlled flames that can support quasi-steady droplet combustion in microgravity.
Other · p. 1 · AI confidence 95% · Reported · Pending
“More recently diffusion - controlled, quasi - steady cool flames supporting droplet combustion were discovered the FLEX team”
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02
Cool flame combustion is relevant to spacecraft fire safety.
Prevention · p. 1 · AI confidence 90% · Reported · Pending
“Important implications with regard to space craft fire safety”
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03
Liñán’s partial burning regime accurately predicts cool flame burning rates and flame standoff ratios.
Other · p. 17 · AI confidence 90% · Reported · Pending
“Cool flame burning rates and flame standoff ratios are reasonably well predicted using Liñán’s partial burning regime”
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04
Cool flame extinction diameters correlate with oxygen concentration to the power of -3/2, but this correlation fails at higher pressures.
Other · p. 17 · AI confidence 90% · Reported · Pending
“Cool flame extinction diameters are correlated against C o2 - 3/2 o At higher pressures correlation does not work well”
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05
The initial hot flame depletes oxygen in the far field, requiring a correction to the oxygen concentration for cool flame modeling.
Other · p. 13 · AI confidence 90% · Reported · Pending
“The initial hot flame depletes oxygen in the far field prior to cool flame start o A correction to the oxygen concentration at the start of the cool flame can be estimated”
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06
Adding alcohol (hexanol) to decane slows the cool flame burning rate and increases the extinction diameter.
Prevention · p. 4 · AI confidence 90% · Reported · Pending
“Alcohol slows the CF burning rate and increases the extinction diameter”