Equivalence Ratio (Phi)
Definition
The ratio of actual fuel supply rate to the stoichiometric fuel supply rate for the available air. Values below 1 indicate fuel-controlled (lean) combustion; values above 1 indicate ventilation-controlled (rich) combustion.
- Symbol
- phi
- Below 1
- Fuel-controlled, lean combustion
- Above 1
- Ventilation-controlled, rich combustion
- Forensic use
- Distinguishing flashover from backdraft conditions
Common questions
How does equivalence ratio relate to the risk of a backdraft?+
A backdraft occurs when a ventilation-controlled, fuel-rich compartment accumulates unburned pyrolysis products and superheated gases, and introducing fresh air suddenly shifts the local equivalence ratio into the flammable range, causing rapid ignition.
Why do investigators care whether a fire was fuel-controlled or ventilation-controlled?+
The two regimes leave different burn patterns, soot distributions and flashover timing, so identifying which regime applied helps reconstruct the fire's development and supports or rules out specific ignition scenarios.
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;...
- 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...
- Neutral Plane
- The interface between the hot gas layer above and the cooler ambient air layer below in a compartment fire. Above the neutral...
- 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)...