2016 Volume 3 Issue 6 Pages 16-00277
Flame spread in the kinetic regime and eventual extinction have been studied for more than four decades for the implications on fire safety and flame instabilities. It is well known that the ratio between the residence time and the combustion time at the leading edge, the so called Damköhler number, plays a fundamental role in the blow-off extinction of a spreading flame. However, the role of the boundary layer, which may significantly affect the residence time at the flame leading edge, on the blow-off extinction has not been thoroughly studied. In this work we present new experimental data on blow-off extinction of PMMA (polymethyl methacrylate) fuels and establish an empirical relation between the boundary layer development length and the extinction flow velocity. Using a vertical wind tunnel it has been possible to carry on a large number of experiments over thin PMMA samples, for an opposed flow velocity range from 0 cm/s up to 100 cm/s. Furthermore, it was possible to rotate the wind tunnel to obtain results with a horizontal configuration, reducing the effect of buoyancy on the flame spread. The experimental data reveal that the extinction length, the distance from the sample leading edge at which the blow-off extinction occurs, is directly related to the opposing flow velocity. Using a simplified scale analysis previously proved to be reliable, the blow-off extinction appeared to occur at a constant effective velocity (defined inside of the boundary layer). This conclusion can have important implications in the definition of the kinetic regime and the quantification of an extinction limit for thin fuels.