Flame Spread Transition to Regression of Thick Fuel in Oxygen-Limited Concurrent Flow

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.