Neutral Plane
Definition
The interface between the hot gas layer above and the cooler ambient air layer below in a compartment fire. Above the neutral plane, pressure is positive relative to outside; below it, pressure is negative, driving air inflow at low openings.
- Location
- Interface between hot gas and cool air layers
- Field
- Fire dynamics, compartment fires
- Above plane
- Positive pressure relative to outside
- Below plane
- Negative pressure, drives air inflow
Common questions
Why does the neutral plane matter for reconstructing ventilation?+
Its height indicates how a compartment was venting during the fire, helping explain burn patterns and how fresh air fed combustion at low openings.
How does opening a door or window affect the neutral plane?+
A new opening changes the balance of inflow and outflow, shifting the neutral plane's height and altering fire growth and smoke movement in the compartment.
Related terms
- Backdraft
- A deflagration event triggered when air is suddenly admitted to a compartment where a fire has been oxygen-depleted but has continued to...
- Deflagration
- A subsonic combustion front propagating through an explosive by thermal conduction. Characteristic of low explosives such as black powder and smokeless powder;...
- Equivalence Ratio (Phi)
- The ratio of actual fuel supply rate to the stoichiometric fuel supply rate for the available air. Values below 1 indicate fuel-controlled...
- Flashover
- The near-simultaneous ignition of all exposed combustible surfaces in a compartment, occurring when the upper gas layer reaches approximately 600°C and radiative...
- Fuel-Controlled Fire
- A compartment fire in which the rate of energy release is limited by the fuel supply (surface area, arrangement, reactivity) rather than...
- Hot Gas Layer
- The stratified layer of buoyant combustion gases that accumulates at ceiling level in a compartment fire. Its temperature rise drives flashover when...
- Low Burn Pattern (LBP)
- Fire damage at or near floor level in a compartment. Historically misattributed solely to poured accelerants; now recognised as producible by post-flashover...
- Ventilation-Controlled Fire
- A compartment fire in which the rate of energy release is limited by air inflow through ventilation openings. Fuel-rich conditions produce high...
- Ventilation Factor
- A parameter used in fully developed fire characterisation, calculated as Av × Hv^0.5, where Av is the area of ventilation openings (m²)...