Abstract
The flame behaviors in a narrow gap with low-velocity airflow are significantly different from buoyancy-controlled flames in open areas. The conditions experienced by microgravity flame may be reproduced in a narrow gap environment where the buoyancy is limited. This work studies the behaviors of near-limit concurrent flame spread over a thick solid fuel in an oxygen-limited narrow channel with 3 mm and 5 mm heights. As the concurrent airflow and oxygen concentration decrease below a critical value, the flame spread transitions to the fuel-regression mode, burning like a candle flame. Further reducing the oxygen, the flame tip tilts towards the inflow like the flame in the opposed flow. A flammability map is found to define three regimes (1) concurrent flame spread, (2) fuel regression, and (3) extinction. The fuel-regression regime is characterized by a fuel regression angle of over 30° and a global flame equivalence ratio of over 1.9. The existence of the fuel-regression mode extends the low-flow flammability limit in the concurrent flow. The ‘round-trip’ flame phenomenon is observed where the 1st-stage near-limit opposed flame spread transitions to the 2nd-stage fuel regression in the concurrent flow. This work provides new insights into the concurrent flame-spread and extinction behavior under oxygen-limited and microgravity environments.
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Acknowledgements
This work is supported by the National Key R&D Program of China (Grant No. 2021YFA0716203), the Opening Fund of State Key Laboratory of Fire Science (SKLFS) under Grant No. HZ2021-KF12, and the National Natural Science Foundation of China under Grant No. U1738117.
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Supplementary file1 Near-limit flames in concurrent airflow (MP4 659 KB)
Supplementary file2 Near-limit flames in the concurrent flow with 18.1% oxygen concentration (MP4 827 KB)
Supplementary file3 Two-stage “round-trip” flame spread process in the concurrent airflow (MP4 603 KB)
Appendix
Appendix
The velocity profiles at the outlet of the narrow channel with a height of 5 mm are shown in Figure
12. The gas flow velocity near the wall is about 2 cm/s, slightly slower than that in the central region, mainly due to the effect of the boundary layer. The average velocity is close to the forced flow velocity. In general, it can be considered that the flow field around the sample is uniform.
The position of the pyrolysis front as a function of time is measured at various flow velocities under an air environment, as shown in Figure
13. It is seen that the pyrolysis front has a linear relationship with time, implying that the flame in the concurrent assistant flow environment is in a steady state.
Images of flame under 18.1% O2 with a flow velocity of 8 cm/s from the top view are shown in Figure
14. The time evolution of the flame base position is measured from the top-view images. The yellow line depicts the flame boundary.
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Zhu, F., Huang, X., Chen, X. et al. Flame Spread Transition to Regression of Thick Fuel in Oxygen-Limited Concurrent Flow. Fire Technol 59, 827–845 (2023). https://doi.org/10.1007/s10694-023-01369-9
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DOI: https://doi.org/10.1007/s10694-023-01369-9
Keywords
- Fire spread
- Near-limit
- Fuel regression
- Flammability
- Flamelet
- Thick PMMA