A fire in a confined space can be divided into three distinct stages based on fuel and oxygen consumption, heat release and variation of average gas temperature. These stages are usually described as the fire growth period, fully developed fire and decay period. Flashover is a short period of transition from a localized initial fire to the fully developed fire where all fuel surfaces within the compartment start to burn.
From the fire-fighting perspective, flashover is a critical stage of fire growth. Namely, when flashover takes place, the probability of survival of occupants rapidly decreases. As the transition from the initial localized fire to the general conflagration takes usually less than a minute, fatalities are very likely to occur. Also, flashover creates a large increase in the rate of combustion; therefore, significantly greater effort is needed to reduce the burning material below its ignition temperature.

A Computational Fluid Dynamics (CFD) model was developed to simulate flashover behaviour. It describes transport of mass, momentum and energy in a gaseous environment. It also includes combustion and thermal radiation. At the same time, degradation of wooden samples is described with a pyrolysis model. To validate the numerical model, experimental results of Pearson et al. [1] were used. The figure above shows their experimental setup.
The primary source of fire was a propane burner with thermal power of 55 kW. Square beech wood samples with the edge length of 30 mm and 5 mm thickness were used as fire spread targets. Exposed to the heat fluxes from the fire, the temperature in the wood samples rises; at some stage the wood undergoes pyrolysis. Time to ignition of the wooden samples and the sample temperature variations were used for comparison of the numerical and experimental results.

The developed numerical model allows simulation of fire spread and the subsequent flashover occurrence for large scale fires with full geometrical complexity (i.e. potential fuel distribution and variety of ventilation conditions) of a realistic residential environment.