On the structure and stability of ultra-lean flames

The present study discusses features of flame propagation and stability in ultra-lean hydrogen-air mixtures under terrestrial gravity conditions. Transient flame dynamics, flame structure and the temperature of the combustion products are analyzed numerically. Obtained results allowed us to distinguish the role of natural convection in the development of superadiabatic ultra-lean flame structure and its stability. It is shown that at significantly low hydrogen content in the mixture even superadiabatic temperature of the combustion products could not maintain the stability of the flame ball rising under the buoyancy action. The convective flows break the flame ball already on the early stage, so there is no opportunity for stable flame ball to be formed. The corresponding value of hydrogen concentration defines the lean concentration flammability limit which is estimated as approximate to 5.25-5.5% and occurs to be larger than one estimated from spherically symmetric calculations (3.45%).