Knowledge of the first principles defining fire behaviour in large enclosures remains limited despite their common use in modern tall buildings. The evolution of a fire in large enclosures can be defined by the relationship between the flame front and burnout velocities (𝑉 " /𝑉 $% ). The mechanisms governing flame spread and burnout are investigated using four full-scale enclosure fire experiments with high porosity wood cribs with similar enclosure geometries. Flame and burnout fronts position and velocity are estimated using video data. Velocities are affected by the heat feedback from the enclosure and smoke layer to the fuel. The spread velocity shows two regimes, a critical heat flux below which there is no spread (𝑞̇( ,*+,-.. ) and a heat flux that defines the onset of very rapid flame spread (𝑞̇+ (,*+,-.. ). A phenomenological model is developed to help identify the underlying mechanisms controlling the transition between the different spread modes. Both the model and data show that 𝑞̇( ,*+,-.. is controlled by the fuel's surface temperature ahead of the flame front, and that 𝑞̇( ,*+,-.. reduces as the surface temperature approximates steady state. The magnitude of 𝑞̇+ (,*+,-.. is constant and is mainly delivered by the flame heat flux. The dependence of the burnout front velocity to the external radiation is found to be weak.
NomenclatureA area b stick thickness C crib burning constant hT total heat transfer coefficient Lv latent heat of vaporisation 𝑚 . mass per unit length 𝑚̇0 .. burning rate per unit area 𝑚̇1 .. free burning rate per unit area 𝑚̇+ .. radiation enhanced burning rate per unit area 𝑞2 ..