The Effect of Pressure on Radiative Losses in Hot, Warm, and Cool Microgravity Spherical Diffusion Flames
Kendyl A Waddell, Daniel L Dietrich, Vedha Nayagam
This work investigates the effects of ambient pressure on radiative heat losses in hot, warm, and cool microgravity spherical diffusion flames using a numerical model and compares the results to observations from microgravity experiments. The experimental data was obtained during droplet combustion experiments, and the numerical model simulates burner-supported flames. The similarities in the gas phase between the two configurations allow these simulations to provide accurate insights into the experimental results. The model uses a statistical narrow-band model that includes emission and reabsorption. The effect as pressure was varied between 0.5 and 10 atm on the relative radiation contributions from CO<sub>2</sub> and H<sub>2</sub>O, the primary radiating species, was examined. As the pressure increased H<sub>2</sub>O contributed more to radiative loss than CO<sub>2</sub> in both the experiments and computations. This was determined to be caused by increased absorption coefficients leading to increased reabsorption at higher pressures. Calculations performed without reabsorption did not see this effect, confirming it is due to the optically thick nature of the system. Reabsorption was higher for CO<sub>2</sub> due to the prominence of the absorption band at 4.3 μm, which became saturated at higher pressures. These results demonstrated the importance of using the optically thick assumption to accurately account for radiative losses in these flames, which are crucial to capture flame behaviours such as radiative extinction and reignitions.
Test conditions.
Extracted from the paper by AI · “Verified” means a person checked it
| Fuel | O₂ | Pressure | Airflow | Sensors | Source | AI conf. | Status |
|---|---|---|---|---|---|---|---|
| — spherical diffusion flame (burner-supported simulation) |
— | 0.5 to 10 atm | — | radiometers | p. 1 | 95% | Pending |
| — spherical diffusion flame (burner-supported simulation) |
— | 1 atm | — | numerical model (SphDiff) | p. 4 | 90% | Pending |
| — spherical diffusion flame (burner-supported simulation) |
— | 4 atm | — | numerical model (SphDiff) | p. 4 | 90% | Pending |
| — spherical diffusion flame (burner-supported simulation) |
— | 10 atm | — | numerical model (SphDiff) | p. 4 | 90% | Pending |
Findings.
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01
As ambient pressure increased, H2O contributed more to radiative loss than CO2 in both experiments and computations.
Other · p. 1 · AI confidence 95% · Reported · Pending
“As the pressure increased H 2 O contributed more to radiative loss than CO 2 in both the experiments and computations.”
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02
The shift in radiative contribution from CO2 to H2O with increasing pressure is caused by increased absorption coefficients leading to increased reabsorption.
Other · p. 1 · AI confidence 90% · Reported · Pending
“This was determined to be caused by increased absorption coefficients leading to increased r”
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03
Hot flames radiatively extinguish at pressures less than 4 atm, while at 4 atm or higher they transition to warm flames.
Suppression · p. 4 · AI confidence 95% · Reported · Pending
“As hot flame peak temperature reaches ~1200 K, the hot flame either radiatively extinguishes if the pressure is less than 4 atm or transitions to a warm flame if the pressure is 4 atm or higher.”
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04
Warm flames established at pressures of 4 atm or higher radiatively extinguish before a cool flame is established.
Suppression · p. 4 · AI confidence 95% · Reported · Pending
“Once a warm flame is established it burns in a similar manner to a hot flame and radiatively extinguishes before a cool flame is established.”
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05
In microgravity droplet experiments, the fraction of total radiation coming from water vapor increased as ambient pressure increased during the hot flame.
Detection · p. 2 · AI confidence 95% · Reported · Pending
“A trend observed during these experiments was that, during the hot flame, as ambient pressure increased and as the flame burned the fraction of total radiation that came from water vapor increased.”
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06
H2O radiance increases with pressure while CO2 radiance remains nearly constant, causing the H2O radiance ratio to increase.
Other · p. 6 · AI confidence 90% · Reported · Pending
“This same behavior was also observed in microgravity droplet experiments [16] and is caused by H 2 O radiance increasing as pressure increases while CO 2 radiance remains nearly constant.”
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07
Increasing pressure increases molecular density and absorption coefficients, limiting total radiative intensity by the Planck blackbody function.
Other · p. 6 · AI confidence 90% · Reported · Pending
“Increasing pressure while holding other conditions constant increases the molecular density of the system, which increases absorption coefficient. A high absorption coefficient indicates that radiation is likely to be reabsorbed before it escapes the region where it is emitted, with total radiative intensity limited by the Planck blackbody function.”