A fire in the open burns as fast as its fuel can gasify, which is called fuel-controlled burning. In a closed compartment the fire eventually consumes oxygen faster than openings can supply it, and the burning rate is then set by how much air the openings admit, not by how much fuel is present; this is ventilation-controlled burning. The air supply through a single opening scales roughly with its area times the square root of its height, often written \( \dot{m}_{air} \approx 0.09 \, A \sqrt{H} \) in kilograms per second, which is why a doorway's dimensions predict a post-flashover fire's size better than the furniture does. Starved of oxygen, the fire produces far more carbon monoxide and unburned fuel gas, and those gases burn as soon as air arrives, which is why opening a window or door on such a fire can make it grow violently. The confusion this resolves is assuming more fuel always means a bigger fire. After this Concept you can decide whether a compartment fire is limited by its fuel or by its air, and predict what changing an opening does.
In firefighting fire dynamics, a compartment fire's behavior is governed by its intake (inlet) and outflow (exhaust) openings, which together define its flow path; a fire lacking sufficient outlets f…