Low-speed Counterflow Flame Experiment in Space toward Understanding of Combustion Limit Mechanism Comprehending Flame Ball and Propagating Flames

Abstract

Flammability limit is a concentration limit for sustaining steady combustion. Measuring an intrinsic flammability limit, which is independent of experimental apparatus, has been a fundamental interest in combustion science for over a century. This report reviews pivotable studies and presents a comprehensive viewpoint on flammability limits. A series of microgravity experiments and numerical simulations with counterflow premixed flame have revealed that the factors like Lewis number, flame stretch, and radiation heat loss significantly influence near-limit combustion dynamics, influencing the flammability limit. Extensive researches have also been conducted on the flame ball, a steady spherical flame observed in a quiescent premixture at leaner condition than the flammability limit of conventional flames under microgravity. The final goal is to unify the intrinsic flammability limit that comprehending propagating flame and flame ball, which also covers recent observations on several peculiar combustion behaviors using low speed counterflow field under microgravity.