Backdraft is one of the most hazardous events related to under-ventilated fires. It is caused by fuel vapour being generated after a fire is extinguished, or reduced in intensity by oxygen starvation, and the subsequent introduction of fresh oxygen, for example by opening a door. Following the mixing of fresh air with the fuel rich environment, concentrations can return to the combustible range, and since ignition sources are likely to exist, flaming combustion may be initiated and can develop into a deflagration.

CFD modelling of a full scale backdraft experiment was conducted. The numerical simulations covered the initial gravity current, the ignition, the spreading of flame in the enclosure, the external fireball and the subsequent decay.

The Detached Eddy Simulation (DES) approach was used to model turbulence. In order to describe the combustion process of the mixture from the local ignition to progressive deflagration, three separate combustion models were implemented for laminar, low and high intensity turbulence flow regimes.

The calculated ignition time is slightly shorter than the average ignition time observed in the experiments. The fire front progresses through the combustible mixture, generating a cloud of hot gases that are accelerated from the container into the external environment. The velocity increases up to 20 m/s. When the fire front reaches the door, combustion continues outside the enclosure as the fuel has been pushed through the door.

The comparison between the calculated time history of relative pressure and the pressure sensor record shows that the numerical simulations slightly overpredict the flame front speed, with a stronger pressure pulse and higher temperatures than the observations.