Where does backdraft collide?

In fire dynamics, a backdraft is the sudden, often explosive, combustion that occurs when fresh air is admitted to a compartment that has been burning in a severely oxygen-depleted state, leaving the space full of hot unburnt fuel gases. When a door or window is opened, incoming air forms a gravity current that mixes with those gases until the mixture reaches its flammable range and ignites as a rapid deflagration, driving a fireball and pressure wave back out the opening. Fleischmann, Pagni, and Williamson's 1994 Quantitative backdraft experiments established the controlled conditions and gravity-current behaviour behind the phenomenon. The referent is a transient combustion event triggered by ventilation, measured by the delay to ignition and the overpressure generated.[1]

Unit

The overpressure and flame extent produced when air reignites the trapped fuel gases

Recovery means

Not applicable

Measured by

Characterised in compartment experiments by fuel-gas concentration, the gravity-current mixing time, ignition delay after the opening, and the resulting overpressure

In building science and indoor-air-quality work, backdrafting (British backdraught) is the reverse flow of combustion products from a naturally vented appliance such as a furnace, water heater, or fireplace, which escape into the occupied space instead of rising up the flue. It is driven by depressurisation of the building envelope, created by exhaust fans, other appliances, or stack effect, that overcomes the chimney's weak draught. The ASTM E1998 guide for assessing depressurization-induced backdrafting and spillage from vented combustion appliances sets out the worst-case depressurisation test used to assess the hazard. The referent here is a steady or intermittent spillage condition, measured by the house depressurisation at which the flue fails to draw, and it threatens occupants through carbon-monoxide exposure rather than through an explosion.[2]

Unit

The envelope depressurisation at which flue gases spill back indoors

Recovery means

Not applicable

Measured by

Assessed by worst-case depressurisation testing, spillage duration at the draught hood, and flue-gas or carbon-monoxide concentrations in the space