A Compartment Fire Builds a Hot Gas Layer Beneath the Ceiling
In an enclosure, the plume from a fire hits the ceiling, spreads outward and, having nowhere else to go, accumulates as a hot smoky layer that thickens downward over time. This creates the two-zone picture that most compartment fire analysis uses: a hot, smoke-filled, oxygen-depleted upper layer, and a relatively cool clear lower layer, separated by a reasonably sharp interface. The layer grows because the plume keeps pumping entrained air upward, so the descent of the interface depends on the room's area, the ceiling height and the fire's heat release rate rather than on the fuel alone. As the layer deepens and heats it radiates downward onto everything in the room, which is what eventually drives the whole space towards ignition. The confusion this resolves is imagining a room filling uniformly with smoke like a glass filling with water; it fills from the top down, which is why the air near the floor stays breathable and visible for a while. After this Concept you can describe how smoke and heat accumulate in a room and what sets how fast the interface descends.
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In an enclosure, the plume from a fire hits the ceiling, spreads outward and, having nowhere else to go, accumulates as a hot smoky layer that thickens downward over time. This creates the two-zone picture that most compartment fire analysis uses: a hot, smoke-filled, oxygen-depleted upper layer, and a relatively cool clear lower layer, separated by a reasonably sharp interface. The layer grows because the plume keeps pumping entrained air upward, so the descent of the interface depends on the room's area, the ceiling height and the fire's heat release rate rather than on the fuel alone. As the layer deepens and heats it radiates downward onto everything in the room, which is what eventually drives the whole space towards ignition. The confusion this resolves is imagining a room filling uniformly with smoke like a glass filling with water; it fills from the top down, which is why the air near the floor stays breathable and visible for a while. After this Concept you can describe how smoke and heat accumulate in a room and what sets how fast the interface descends.
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