Effects of Ambient Inert Gas Properties on Cool-Flame Combustion of Normal-Alkane Droplets in Microgravity
Vedha Nayagam, Daniel L Dietrich, Forman A Williams
Experiments: CFI, FLEX, FLEX-2
Cool-flame-supported quasi-steady droplet combustion was first observed during droplet combustion experiments conducted onboard the International Space Station (ISS) in the Combustion Integrated Rack (CIR) with n-alkane fuels. These cool flames, not visible to the naked eye, result from the existence of the negative-temperature-coefficient (NTC) region during the combustion n-alkane fuels when the chemical-kinetic pathways experience a transition from the low-temperature chemistry to the normal, high-temperature-controlled chemistry. In this study, we present results for the cool-flame-supported droplet burning rates and extinction diameters for n-alkane fuels (n-heptane, n-octane, n-decane, and n-dodecane) in inert-diluent-substituted ambient environments. In these ISS experiments, some of the nitrogen in the ambient gas mixture is replaced by an inert gas, namely, carbon dioxide, helium, or xenon. Different diluents were observed to exert remarkably different influences on the cool-flame combustion of fuel droplets. The results show that the cool-flame extinction diameter is drastically increased with helium dilution compared to N2, CO2 or Xe, and Xe dilution leads to longer burning with a smaller extinction diameter. Unlike hot flames, the cool-flame burning rates are controlled by the cool-flame temperature established in the NTC region by the chemical kinetics, rather than by equilibrium thermodynamics, and the thermo-physical properties of the ambient gases, which control the heat feed back to the liquid droplet. The cool-flame extinction is triggered by the instability that occurs at the low-temperature end of the NTC region, below which cool diffusion flames become statically unstable. A quasi-steady theoretical model, presented earlier, which involves a six-step reduced-chemistry mechanism, valid around the lower end of the NTC region, is shown to explain the observed experimental trends qualitatively.
Test conditions.
Extracted from the paper by AI · “Verified” means a person checked it
| Fuel | O₂ | Pressure | Airflow | Sensors | Source | AI conf. | Status |
|---|---|---|---|---|---|---|---|
| n-octane droplet |
0.17 | — | — | Video images, Narrow-band radiometers, Wide-band radiometers | p. 2 | 95% | Pending |
| n-alkane droplet |
21% | — | — | Video images, Radiometers | p. 6 | 90% | Pending |
| n-alkane droplet |
varies | — | — | Video images, Radiometers | p. 6 | 90% | Pending |
| n-alkane droplet |
— | — | — | Video images, Radiometers | p. 6 | 90% | Pending |
Findings.
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01
Quasi-steady cool flames were discovered during normal-alkane fuel droplet combustion onboard ISS in the FLEX experiments.
Detection · p. 1 · AI confidence 95% · Reported · Pending
“Quasi -steady cool flames were discovered during normal -alkane fuel droplet combustion onboard ISS in the FLEX experiments.”
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02
Cool flames can extend the flammability of fuels in a microgravity environment.
Other · p. 1 · AI confidence 95% · Reported · Pending
“Cool flames can extend the flammability of fuels in a microgravity environment”
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03
Inert gases are used to extinguish fires in microgravity, with CO2 currently used in the US module of ISS.
Suppression · p. 1 · AI confidence 95% · Reported · Pending
“Inert gases are used to extinguish fires in microgravity. Currently CO 2is used in the US module of ISS.”
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04
Unlike hot-flames, cool flames are controlled by chemical kinetics.
Other · p. 1 · AI confidence 95% · Reported · Pending
“Unlike hot -flames, cool flames are controlled by chemical kinetics.”
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05
Inert diluents affect cool-flame-supported n-alkane droplet combustion substantially through heat-transfer and diffusion processes, not by influencing chemical-kinetic rates.
Other · p. 10 · AI confidence 95% · Reported · Pending
“Inert diluents affect the cool -flame -supported n-alkane droplet combustion substantially, through heat -transfer and diffusion processes, and not by influencing chemical -kinetic rates.”
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06
Xenon decreases burning rates substantially but can sustain cool flames at much lower oxygen concentrations compared to helium.
Other · p. 10 · AI confidence 95% · Reported · Pending
“Xenon decreases burning rates substantially, but can sustain cool flames at much lower oxygen concentrations compared to helium.”
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07
For case (c) with Helium, the burning becomes highly transient and quasi-steady theory is not valid.
Other · p. 6 · AI confidence 90% · Reported · Pending
“For case (c) with He, the burning becomes highly transient –QS not valido”
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08
Extinction-diameter correlations suggest currently available chemical-kinetic descriptions are adequate for lower normal alkanes, while further study is needed for higher alkanes.
Other · p. 10 · AI confidence 90% · Reported · Pending
“Extinction -diameter correlations suggest that the currently available chemical -kinetic descriptions are adequate for lower normal alkanes, while further study is needed for higher alkanes.”