Effects of combustible dust clouds on the extinction behavior of strained, laminar premixed flames in normal gravity

An experimental and numerical study was performed on the interaction of combustible solid particles with atmospheric, strained, laminar premixed methane/air and propane/air flames in normal gravity. The study was conducted in the opposed-jet configuration in which a single flame was stabilized below the gas stagnation plane by counterflowing a mixture against an air jet. Into the flame were seeded spherical 50 mum, combustible glassy-carbon particles. Flame extinction data were analyzed to provide insight into the effects of fuel type, gas-phase composition and temperature, flame thickness, strain rate, and particle number density It was found that at low strain rates, the particles could effectively burn within the gaseous flame zone and thus enhance the overall reactivity of the system and resistance to extinction. At high strain rates, however, the particles are rapidly transported through the flame and their ignition is delayed until they are well downstream of the flame and so have no effect or at best a minor one on the gaseous flame. If no ignition occurs, the combustible particles act simply as heat sinks, promoting extinction. The effects of Lewis number and flame thickness were also studied. It was found that the enhancement of the gas-phase reactivity by the particles is more profound for Le < 1 and for thick flames. Finally, at the same flame temperature, fuel-lean flames were determined to be more resistant to extinction compared to fuel-rich flames in the presence of combustible particles. This was attributed to the higher concentrations of O-2, O, and OH species, which are largely responsible for the consumption of carbon.