Unilateral Blow-Off and Periodic Smoldering Holes in Upward Flame Spread Over Thin Charring Material

In this study, upward flame spread over thin charring material under varied ambient pressures and oxygen concentrations diluted with different inert gases was investigated. After symmetric ignition, a small systematic disturbance at the flame base region triggers asymmetric two-sided flames, while the flame tips remain aligned. Afterward, one branch of flame dwells and speeds up in a cycle, associated with periodic smoldering holes, mainly ascribed to the secondary pyrolysis and surface oxidative degradation (smoldering) of the remaining char, while the other branch of flame extinguishes eventually, primarily attributed to the self-induced buoyant blow-off at a low critical Damköhler number associated with finite-rate kinetics. The temperature distribution over the sample surface is obtained by infrared thermal imagers, and six stages of the whole pyrolysis process are roughly divided, which gives a better perspective on the underlying mechanism of these unique combustion behaviors. With the burnout (end) of the primary or secondary pyrolysis processes, the local fuel/oxidant mixture ratio at the flame base region approaches that of the inflammability limit (neutral stable point) where the near-limit flames are extremely sensitive to the environmental disturbances, and thus the flame base instability appears. With the increase of the oxygen concentration, the pressure range of the existence of the unique combustion behaviors narrows down due to intensified chemical reactions and thus the stability of the flame base. Furthermore, compared with the environments diluted in N2, the unique combustion behaviors are more conspicuous in that of CO2 (radiative intervening medium) due to larger molecular weight and lower Lewis number. This investigation into the upward flame propagation will enrich the combustion science at transition regimes such as the fire scenarios after the application of the carbon dioxide fire extinguisher.