Surface flame spread modes

Figure 8.4 displays a thermally thin solid of thickness d, insulated at its back face and undergoing surface flame spread. Any of the modes shown in Figure 8.1 are applicable. We consider the case of steady flame spread, but this constraint can be relaxed. The surface flame spread speed is defined as... 

Flames can spread over a solid surface in two modes. In the wind-aided flame spread mode, flames spread in the same direction as the surrounding airflow. The second mode is referred to as opposed-flow flame spread, which occurs when flames spread in the opposite direction of the surrounding airflow. These two modes are illustrated for flame spread over a flat surface in Figure 14.2. 

This follows by a steady state energy balance of the surface heated by qe, outside the flame-heated region S. It appears that a critical temperature exists for flame spread in both wind-aided and opposed flow modes for thin and thick materials. Tstmn has not been shown to be a unique material property, but it appears to be constant for a given spread mode at least. Transient and chemical effects appear to be the cause of this flame spread limit exhibited by 7 smln. For example, at a slow enough speed, vp, the time for the pyrolysis may be slower than the effective burning time ... 

For concurrent spread, the growth rate can be much faster, and therefore the critical condition can be reached at lower compartment temperatures. The dependence of the concurrent flame spread area on both Q and the surface temperature of the material make this spread mode very feedback sensitive. 

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