Limits of flame spread through some forest fuels

Abstract

The flame spread limit moisture content w l of fallen pine needles was investigated in a centrifuge. The w l value was found to decrease with increasing gravitational forces and with increasing air flow velocity ν f when flame spread and centrifuge rotation directions coincided. The w l( ν f) dependence had a maximum in the region of low air flow velocities when the above directions were opposite. The limit moisture content was established to be a function of particle slope angle, particle diameter, and its composition. It was also found that the limit flame spread rate depends strongly on burning conditions. The mathematical model of limit phenomena has been worked out on the basis of the heat theory of diffusion flame extinction by describing the flame spread process along plant particles as the result of interaction between a point source of flammable gases and the surrounding air flow. By comparing the experimental data with the mathematical model, gravitational forces were shown to influence the extinction limit by increasing the velocity of air flow which arises around the flame through buoyancy forces. Also it was shown that extinction by gravitational forces and by air flow occurred mainly due to increasing convective heat loss rate from the flame reaction surface to the surrounding air.