Experimental evidence now shows that flame impingement is required for the ignition of fine fuel particles responsible for the spread of wildland fires. However, the characteristics of the non-steady flame zone that produce convective heating of fuel particles has not been studied. It is not known how to describe - qualitatively or mathematically - the flame dynamics that allow forward spread of wildland fires.
Flame structure studies were conducted and particle heating signals were evaluated by Fire, Fuel and Smoke Science Program Research Forester Mark Finney and collaborators from the University of Maryland, University of Kentucky, and Los Alamos National Laboratory. These studies were done using laboratory fires that spread through laser-cut cardboard fuel beds and also using stationary flames from gas-fed burners. High-speed video and infrared imagery were used to capture flame dynamics to compare with thermocouple measurements.
Flame structure and dynamics were found to be remarkably similar to heated boundary layers that are well known from fluid dynamics research. The flame fronts in stationary and spreading fires divided into “peak and trough” patterns that were produced by instabilities of air inflow to the flame zone produced by the upward buoyant force. These instabilities result in paired-streamwise vortices, otherwise known as Görtler vortices, that force flames down into the fuel bed at convergence zones. Secondary instabilities resulted in an apparent forward pulsing that extends flames into the unburned fuels at frequencies that appear to scale inversely with flame length but proportionally to wind speed.
Results indicated that the buoyant dynamics of the flame zone appear to govern the time-varying heating of fuel particles, and the distance ahead of the front where heating takes place.