Lab-scale observations of flame attachment on slopes with implications for firefighter safety zones

The Coanda effect is the phenomenon in which a jet entering quiescent fluid attaches to a nearby solid object due to inhibited entrainment of ambient fluid near the solid. Little is known about the influence of the Coanda effect on wildland fire behavior. Specifically, there is a lack of knowledge regarding how the flame attachment on slopes influences firefighter safety zone considerations in rugged terrain. This paper presents results for small-scale n-heptane pool fire experiments near a slope, showing how flame shape and heat flux are influenced by slope angle, slope boundary condition and distance from flame base. Flames near slopes leaned toward the slope, and at some slope angles the flames attached to the slope. The average angle at which the effects of slope were noticeable was between 10° and 45°, depending on the criteria used. No difference was observed between a bare slope and an insulated slope. Dimensional analysis showed that important characteristics of flame attachment on slopes in the small-scale experiments were in the range estimated for the large fires. The implication is that the traditional view of safe separation distance as being the distance from the flame base is inadequate for fires near slopes.