Combustion Characteristics in a Non-Premixed Cool-Flame Regime of n-Heptane in Microgravity
Takahashi, Fumiaki, Katta, Viswanath R., Hicks, Michael C.
A series of distinct phenomena have recently been observed in single-fuel-droplet combustion tests performed on the International Space Station (ISS). This study attempts to simulate the observed flame behavior numerically using a gaseous n-heptane fuel source in zero gravity and a time-dependent axisymmetric (2D) code, which includes a detailed reaction mechanism (127 species and 1130 reactions), diffusive transport, and a radiation model (for CH4, CO, CO2, H2O, and soot). The calculated combustion characteristics depend strongly on the air velocity around the fuel source. In a near-quiescent air environment (< or = 2 mm/s), with a sufficiently large fuel injection velocity (1 cm/s), a growing spherical diffusion flame extinguishes at ≈1200 K due to radiative heat losses. This is typically followed by a transition to the low-temperature (cool-flame) regime with a reaction zone (at ≈700 K) in close proximity to the fuel source. The 'cool flame' regime is formed due to the negative temperature coefficient in the low-temperature chemistry. After a relatively long period (≈18 s) of the cool flame regime, a flash re-ignition occurs, associated with flame-edge propagation and subsequent extinction of the re-ignited flame. In a low-speed (≈3 mm/s) airstream (which simulates the slight droplet movement), the diffusion flame is enhanced upstream and experiences a local extinction downstream at ≈1200 K, followed by steady flame pulsations (≈0.4 Hz). At higher air velocities (4-10 mm/s), the locally extinguished flame becomes steady state. The present axisymmetric computational approach helps in revealing the non-premixed 'cool flame' structure and 2D flame-flow interactions observed in recent microgravity droplet combustion experiments.
Test conditions.
Extracted from the paper by AI · “Verified” means a person checked it
| Fuel | O₂ | Pressure | Airflow | Sensors | Source | AI conf. | Status |
|---|---|---|---|---|---|---|---|
| n-Heptane Gaseous fuel source from three superimposed porous cylinders (4 mm dia. × 0.67 mm ht., 2.5 mm dia. × 2 mm ht., and 1 mm dia. × 4 mm ht.) |
— | — | Near-quiescent air (≤ 2 mm/s) | Numerical simulation (UNICORN code) | p. 1 | 95% | Pending |
| n-Heptane Gaseous fuel source from three superimposed porous cylinders |
— | — | Low-speed airstream (≈ 3 mm/s) | Numerical simulation (UNICORN code) | p. 1 | 95% | Pending |
| n-Heptane Gaseous fuel source from three superimposed porous cylinders |
— | — | Higher air velocities (4 – 10 mm/s) | Numerical simulation (UNICORN code) | p. 1 | 95% | Pending |
Findings.
-
01
The cool flame regime is formed due to the negative temperature coefficient in the low-temperature chemistry.
Other · p. 1 · AI confidence 95% · Reported · Pending
“The ‘coo l flame’ regime is formed due to the negative temperature coefficient in the low - temperature chemistry.”
-
02
After a relatively long period (≈ 18 s) of the cool flame regime, a flash re-ignition occurs, associated with flame-edge propagation and subsequent extinction of the re-ignited flame.
Other · p. 1 · AI confidence 95% · Reported · Pending
“A fter a relatively long p eriod ( ≈ 18 s) of the cool flame regime, a flash re - ignition occurs, associated with flame - edge propagation and subsequent exti nction of the re - ignited flame .”
-
03
The transition from the high-temperature to low-temperature combustion regime occurs discretely because oxygen must diffuse through the relatively low temperature zone to reach the burner surface.
Other · p. 5 · AI confidence 90% · Reported · Pending
“Thus, the transition from the high - temperature to low - temperature combustion regime occurs discretely, instead of a propagation of a flame front , because the oxygen must diffuse through the relativel y low temperature zone to reach the burner surface .”
-
04
In the cool flame regime, the peak heat release rate occurs at a constant temperature of 700 K in close proximity (≈ 1 mm) to the fuel source.
Other · p. 4 · AI confidence 95% · Reported · Pending
“The peak HRR (2.1 ± 0.1 W/cm 3 ) , which is comparable to that of the high - temperature flame, occurs at a constant temperature of 700 K in close proximity ( ≈ 1 mm) to the fuel source .”