Ignition sources surface temperature limits

Experiments [43] with very high flash point fuels (JP, kerosene, Diesel, etc.) revealed that the flame propagation occurred in an unusual manner and a much slower rate. In this situation, at ambient conditions, any possible amount of fuel vapor above the liquid surface creates a gaseous mixture well outside the fuel s flammability limits. What was discovered [44, 45] was that for these fuels the flame will propagate due to the fact that the liquid surface under the ignition source is raised to a local temperature that is higher than the cool ambient temperature ahead of the initiated flame. Experimental observations revealed [45] that this surface temperature variation from behind the flame front to the cool region ahead caused a variation in the surface tension... 

When a combustible material is exposed to an external radiant heat source, its surface temperature starts to rise. The temperature inside the solid also increases with time, but at a slower rate. Provided the net heat flux into the material is sufficiently high, the surface temperature eventually reaches a level at which thermal decomposition begins. The fuel gases and vapors generated emerge through the exposed surface and mix with air in the gas phase. Under certain conditions, this mixture exceeds the lower flammability limit and ignites. 

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