Near Quenching Limit Instabilities of Concurrent Flame Spread over Thin Solid Fuel

ABSTRACT

Near quenching limit instabilities of concurrent flame spread over thin cellulosic fuel are experimentally studied by employing a narrow channel apparatus. Depending on the oxygen concentration of the imposed flow, two different kinds of instabilities have been identified. Specifically, for oxygen concentration below a critical value, the instability is of fingering or cellular type, whereas for supercritical oxygen concentrations, traveling wave instability prevails, characterized by transverse creeping motion of the flamelets across the fuel edge. Both instabilities are usually accompanied by recurrent flamelet growing and splitting during the flame spread processes. It is justified that the two kinds of instabilities herein identified are diffusive-thermal in nature and may be classified into the category of near quenching limit instability of non-adiabatic diffusion flames. Further, an attempt has been made to gain insight into the physical mechanisms of the flame instabilities by exploiting the similarities between flame spread and smolder wave propagation.

KEYWORDS:

• Concurrent flame spread
• Fingering instability
• Near quenching limit
• Smolder wave
• Thin solid fuel
• Traveling wave instability

Notes

¹ Strictly speaking, the terms ‘reaction trailing’ and ‘reaction leading’ are meaningful only in the context of adiabatic forward smolder combustion, wherein a persistent thermal wave structure is developed in front of or behind the reaction front (Schult et al., 1996). Nevertheless, we will retain these terms in the description of nonadiabatic forward smolder combustion, in the sense that the smolder wave would recover the corresponding adiabatic structure if heat loss were purged.

Additional information

Funding

This work is supported by the Strategic Pioneer Program on Space Science, Chinese Academy of Sciences (Grant No. XDA04020410), the National Science Foundation of China (Grant Nos. 50706024 and 11472167), and the Shanghai Leading Discipline Project. The support from the Key Laboratory of Microgravity, Institute of Mechanics, Chinese Academy of Sciences, is also gratefully acknowledged.