Fire Behavior
The chemistry and physics of fire
The fire tetrahedron - fuel, oxygen, heat, and chemical chain reaction - replaced the older triangle because it explains how clean agents extinguish flame. Understand states of matter and how fuel geometry drives burning: solid fuels pyrolyze into flammable gases, liquids give off vapors above their flash point, and gases must fall between their lower and upper explosive limits to ignite. Vapor density below 1.0 means the gas rises (methane); above 1.0 it settles into low areas (propane). Specific gravity applies to liquids relative to water.
Heat transfer and the stages of fire
Conduction, convection, and radiation move heat; convection drives the hot gas layer that controls the modern fireground. The stages - incipient, growth, fully developed, decay - are defined by heat release and oxygen, not by clock time. Thermal layering describes the hot gas layer forming over cooler air and is the reason door control and ventilation discipline matter.
Traps
Questions love to blur the phenomena: flashover is heat-driven transition to simultaneous ignition of all exposed fuel in a compartment (roughly 1,100 °F at the ceiling) during growth; backdraft is oxygen-starved, pressurized smoke that explodes when air is introduced; rollover is flames rolling in the hot layer; a smoke explosion ignites a pre-mixed accumulated cloud. BLEVE is a pressure-vessel failure, not a fuel-air explosion. Remember water expands about 1,700 times into steam.
Rule of thumb
When you must predict where a gas will accumulate, check vapor density first: less than 1.0 rises and collects high; greater than 1.0 sinks into basements, pits, and low spots.