Near-extinction behavior of smoldering combustion under highly vacuumed environment

Abstract

The smoldering behavior of porous combustibles near the extinction limit was examined under a highly-vacuumed environment at various oxygen concentrations. The considered ambient conditions of the present work is 1.0–90 kPa in absolute pressure filled with a nitrogen–oxygen mixture. A 2-mm-rod-shaped incense stick is considered as the burning specimen. The specimen was located vertically and forced ignition was made at the top of the specimen to complete the following downward propagation. Two well-known burning modes were identified; such as flame spread (flaming) and smoldering (non-flaming) depending on the imposed ambient conditions. The results showed that the smoldering rate near the extinction limit decreased dramatically as the ambient pressure decreased. Temperature profiles inside the smoldering specimen were successfully measured by fine thermocouple even at the near-extinction limit. The results imply that there is a peak located slightly below the top surface. Based on the simple radiative-conductive heat transfer model, the role of radiation heat transfer for a forward temperature profile is discussed within the range for which oxidative reaction is neglected. It is implied that the radiant heat transfer may play a role on the preheating ahead of the burning zone, especially when the peak temperature is relatively higher (> 1000 °C). To fully understand the whole temperature profile and extinction behavior, further continuous study is recommended.