The Two-Zone Model
When a fire starts in an enclosed space, the buoyant plume carries hot combustion products to the ceiling, where they spread laterally and form a distinct hot upper layer. Below remains a cooler, clearer lower layer where occupants can still breathe and see. This two-zone approximation — a well-mixed hot upper layer and a cool lower layer separated by a sharp interface — is the foundation of compartment fire modeling and smoke management design.
Rate of Smoke Descent
The smoke layer descends as the plume continuously feeds hot gas into the upper layer. The descent rate depends on the balance between the plume's mass flow rate (which grows with height above the fire) and the room's cross-sectional area. In the early stages with high ceilings, the plume entrains significant air and the layer descends quickly. As the layer thickens and the plume height decreases, entrainment drops and descent slows — a self-regulating but ultimately insufficient mechanism.
Tenability and Evacuation
The smoke layer represents multiple hazards: elevated temperature (burns and heat stroke), toxic gases (CO and HCN), and reduced visibility (disorientation and panic). Fire engineers define tenability limits — typically 60°C temperature, 1400 ppm CO, and 10 m visibility — and calculate when the smoke layer descends past the critical height where these limits are exceeded. The Available Safe Egress Time (ASET) must exceed the Required Safe Egress Time (RSET) with a safety margin.
Smoke Management Systems
Mechanical smoke exhaust, natural smoke vents, and pressurization systems are designed to maintain tenable conditions during evacuation. The design process uses zone models or CFD simulations to predict smoke layer behavior and specify exhaust rates. Standards like NFPA 92 and EN 12101 provide frameworks for smoke control design in atriums, shopping malls, tunnels, and high-rise buildings.