Flashover screening calculator: Thomas and Babrauskas.

Three numbers from the room alone, and what they can and cannot say.

A compartment fire is limited by one of two things: the fuel, or the air. Kawagoe showed that the air a fire can draw through its openings scales with the ventilation factor, the opening area times the square root of its height, at about half a kilogram of air per second for every unit of it. Huggett's rule that burning releases about 3 megajoules per kilogram of air consumed turns that into a heat release rate: the most the room can burn at, however much fuel is inside, until a window fails or a door opens. That is the ventilation-limited maximum.

Flashover is the transition from a fire in a room to a room on fire, and two correlations estimate the heat release it takes. Thomas's, from a heat balance on the hot layer, needs the room's internal surface area as well as its openings, since the walls and ceiling absorb heat; Babrauskas's, fitted to test fires, reads the openings alone and usually gives a lower figure. When the ventilation-limited maximum sits at or above the Thomas threshold, the openings could have fed a flashover fire and the question passes to the fuel; when it sits below, the room as vented could not have reached the threshold, and an investigator asking how the room flashed over has to find the opening that changed.

These are screening tools, and NFPA 921 says so at length. They assume a single compartment with the layer at or above the openings, no forced ventilation, and openings whose state during the fire is known. A room with an unknown opening is a room with an assumption in it, which is why the calculator names every one it makes. What they are good for is the first question at a scene: is the flashover the witnesses describe consistent with the openings the investigator found?