Fire Chemistry, Physics, and Fire Dynamics
Chemistry of ignition
Combustion requires fuel, an oxidizer, heat, and an uninhibited chemical chain reaction (the fire tetrahedron). Solids do not burn directly: heat drives pyrolysis, releasing combustible vapors that mix with air and burn. Know the temperature terms. Flashpoint is the lowest temperature at which a liquid gives off enough vapor to ignite momentarily with a pilot. The fire point is the temperature at which vapor production sustains burning. Autoignition temperature requires no pilot at all. Flammability (explosive) limits bound the vapor-in-air range that propagates; below the LFL the mixture is too lean, above the UFL too rich. Methane's stoichiometric concentration is roughly 9.5 percent by volume.
Heat transfer and fire dynamics
Conduction, convection, and radiation move energy; radiation from the flame and hot gas layer dominates room growth. Classic flashover indicators include floor heat flux near 20 kW/m², upper-layer temperatures around 600 °C (1,100 °F), and flames rolling from openings. Distinguish flashover (transition from fuel- to ventilation-controlled burning), backdraft (oxygen-starved fuel-rich smoke admitted air and ignites, often with a pressure pulse), rollover (flames propagating in the ceiling layer), and smoke explosion (ignition of a pre-mixed accumulated smoke cloud). Heat release rate governs hazard: HRR = mass loss rate × heat of combustion.
TRAP: Items swap flashover and backdraft, or imply a low flashpoint means a low autoignition temperature - they are unrelated properties.
RULE OF THUMB: Flame in the ceiling layer plus a thick hot upper layer suggests rollover progressing to flashover; a dark, pressurized room that gulps air when opened suggests backdraft.