Combustion limit has been studied for long time, more than 150 years. In early period, the safety in coal mines was primary motivation of the studies. In 1940’s and 1950’s, the first theoretical descriptions on combustion limits were presented and various experiments under microgravity have been conducted to date.
In the investigation process, various combustion experiments have been contributing to the understandings of fundamental physics of near-limit flames. Among them, contribution by the low-speed counterflow flame method is the most significant to elucidate the limit mechanism of deflagration wave, i.e., propagating premixed flame. In late 90’s, comprehensive and complex natures of the combustion limits for deflagration wave were clarified by low-speed counterflow flame experiments under microgravity which realizes the ideal condition for flame extinction experiments in which no interference due to buoyant flows is being induced by gravity effect.
Apart from the study on the limit of deflagration wave, the phenomenon termed “flame ball” was first predicted by Zel’dovich in 1940’s and its existence was proved through microgravity experiments in drop towers in U.S. and Japan and eventually, space experiments in the Space Shuttle at the beginning of the 2000’s.
Our final goal of the present study is to construct comprehensive combustion limit theory which comprehensively describes the limits of both deflagration wave and flame ball. Our proposal to Japanese space program for International Space Station was accepted in 2010, and currently, preliminary microgravity experiments using airplane are being conducted and some interaction between deflagration wave and flame ball near the limits were successfully observed. Overviews of the hypothesis of comprehensive combustion limit theory based on the low-speed counterflow method and current understandings on the limit interaction will be discussed.