Numerical simulations of the extinguishment of downward propagating flames

Abstract

Detailed multidimensional numerical simulations that include the effects of wall heat losses have been performed to study the dynamics of downward flame propagation and extinguishment in lean hydrogen-air mixtures. The computational results show that a downward propagating flame in an isothermal channel has a flammability limit of around 9.75%, in good agreement with experimental results. Also in good agreement are the detailed observations of the flame behavior at the point of extinguishment. The flame is quenched at the walls and tongues of colder gases, comprised mainly of burnt products flow down the sides. At the same time, there is an upward motion in the gases at the center of the channel, causing the flame to rise up into the burnt products. The primary conclusion of this work is that detailed numerical simulations that include wall heat losses can adequately simulate the dynamics of the extinguishment process in downward-propagating hydrogen-air flames. Additional simulations indicate that both heat losses and gravity are simultaneously required to cause the observed limit. The simulations also show that the dilution of the unburnt mixture with the products of combustion is an essential step in the extinguishment process.