Solid substances with many voids, such as incense sticks and cigarettes, can continue to burn slowly without producing a flame.This phenomenon is called "kun-yaki".Normally, the flame cannot be maintained when a certain amount of oxygen is blocked, so it disappears when the pressure is reduced to about one-third of the atmospheric pressure.However, in the case of kun-yaki, smoldering continues until the pressure is much lower, about 3/1 of the atmospheric pressure.

　Why can we continue to burn under such adverse conditions?This study has tackled this problem, which has not been clarified so far due to the extremely weak combustion intensity of combustible combustion.
Specifically, by making a hole with a diameter of 0.2 mm in the incense stick, embedding a self-made ultra-fine temperature sensor in it, and providing a highly stable space for steady combustion even near the flame extinguishing limit, the temperature distribution just before it disappears can be obtained. It is said that it was the first in the world to succeed in measuring with high accuracy.As a result, it is found that when the pressure is reduced, the heat escape from the preheating part located several nm of the heat generating part to the ambient air is reduced, and the preheating part directly promotes combustion, and there is a possibility that the combustible state is maintained. Was pointed out.

　The low pressure environment is often used as a standard setting condition for closed spaces in extraterrestrial environments such as inside spacecraft and lunar bases.Therefore, research on low-pressure fires is very important for disaster prevention strategies in space.It can be said that the results of this research provide basic information that will be indispensable for disaster prevention strategies in manned missions in the near future.

Paper information:[Proceedings of the Combustion Institute] Near-extinction behavior of smoldering combustion under highly vacuumed environment