Two-Zone Model
Definition
A compartment fire model that divides the room into an upper hot gas zone and a lower cool ambient zone, computing their temperatures, depths, and compositions from heat and mass balance equations. Implemented in CFAST and FAST.
- Zones
- Upper hot gas zone, lower cool ambient zone
- Outputs computed
- Temperature, depth, composition of each zone
- Basis
- Heat and mass balance equations
- Implementations
- CFAST and FAST
Common questions
What limitation should an investigator keep in mind when relying on two-zone model output?+
The model assumes each zone is internally uniform, which holds reasonably well for a single compartment in the growth stage but becomes less accurate for complex multi-room geometries or once conditions approach flashover, where zone boundaries can break down.
How does two-zone modelling support fire cause and origin analysis?+
By reconstructing when a room reached conditions consistent with flashover or specific damage patterns under a given fuel and ventilation scenario, an investigator can test whether a witness's timeline or a proposed ignition scenario is physically consistent with the observed fire damage.
Related terms
- Alpert Correlation
- An expression derived by Ronald Alpert at FM Global for ceiling jet temperature and velocity as functions of heat release rate, ceiling...
- CFAST
- Consolidated Fire and Smoke Transport model from NIST; a multi-room two-zone fire model used for evacuation time analysis, detector performance prediction, and...
- Fractional Effective Dose (FED)
- A physiological model that combines exposure to CO, HCN, CO2, reduced O2, and heat into a cumulative incapacitation estimate. Used with FDS...
- Heskestad Correlation
- An analytical expression derived by Gunnar Heskestad at FM Global relating fire plume centreline temperature to heat release rate and height above...
- McCaffrey Plume Model
- A three-zone empirical model for fire plume centreline temperature and velocity developed by Brian McCaffrey at NIST, with separate power-law relationships for...
- NIST FDS (Fire Dynamics Simulator)
- A freely available large-eddy simulation CFD code from NIST that models coupled fire, smoke, and heat transfer in three-dimensional geometries. Validated against...
- Soot Horizon
- The visible line on vertical surfaces marking the lowest height reached by the smoke layer during the early fire growth phase, representing...
- T-Squared Fire Growth
- A model in which heat release rate increases as the square of time from established burning (HRR = alpha × t²). The...
- Virtual Origin
- In plume correlation models, the effective point source location below the actual fuel surface from which the plume behaves as if it...