A Zone Model Represents a Room Fire as Two Uniform Layers
Fire inside a building is modelled differently from fire on a landscape. A zone model, of the CFAST type, makes one bold simplification: each room contains exactly two zones, a hot upper layer of smoke and combustion products and a cool lower layer of relatively clear air, each treated as uniform in temperature and composition. The model then solves conservation of mass and energy for those layers, plus a plume that pumps mass from the lower layer into the upper one, and vent flows between rooms. What comes out is the upper-layer temperature, the height of the interface between the layers, and the smoke conditions at any point in a multi-room building, in seconds of computer time. The two-layer assumption is also the model's boundary: it is a good description of a small-to-medium room well after the plume has established, and a poor one for a large space, a corridor with a long travelling smoke wave, or the first seconds of a fire. After this Concept you can state the two-layer assumption and name a room geometry that breaks it.
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Fire inside a building is modelled differently from fire on a landscape. A zone model, of the CFAST type, makes one bold simplification: each room contains exactly two zones, a hot upper layer of smoke and combustion products and a cool lower layer of relatively clear air, each treated as uniform in temperature and composition. The model then solves conservation of mass and energy for those layers, plus a plume that pumps mass from the lower layer into the upper one, and vent flows between rooms. What comes out is the upper-layer temperature, the height of the interface between the layers, and the smoke conditions at any point in a multi-room building, in seconds of computer time. The two-layer assumption is also the model's boundary: it is a good description of a small-to-medium room well after the plume has established, and a poor one for a large space, a corridor with a long travelling smoke wave, or the first seconds of a fire. After this Concept you can state the two-layer assumption and name a room geometry that breaks it.
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