Mechanisms of flame spread and burnout in large enclosure fires

Abstract

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 ..

Burning dynamics and in-depth flame spread of wood cribs in large compartment fires

“…The illustrated profile of the burning zone in Fig. 1 is only intended to be indicative and it is assumed from observations of the reported experiments in the literature [15,19]. An analysis of the heat and mass transfer mechanisms for a crib must treat the in-depth and external processes independently.…”

“…Therefore, the majority of the energy released from the flames is through convection, and the resultant thermal feedback from the flame onto the fuel is negligible ( F ≈ 0) assuming the flame volume is not substantially large. In an enclosed environment such as a compartment, if the geometry permits, hot gases and smoke can accumulate along the ceiling and enable a thermal feedback mechanism onto the top surface of the fuel by an external heat flux, F [19].…”

“…A push for open floor plans in the built environment has motivated experimental fire research into fuel-controlled fires at very large-scale geometries recently [15][16][17][18][19][20][21][22]. Common to these experiments are porous wood crib fuel beds that are ignited at one end and allowed to freely propagate.…”

“…It was found that each mode of # ?, /# $% induced a characteristic distribution of the energy within the compartment. The mechanisms controlling # ?, /# $% for spreading wood crib fires has recently been studied by Gupta et al [19] using experimental data collected from the MFT [18], Edinburgh Tall Building Fire Tests campaign [37], and the Guttasjön Fire Tests campaign [17]. # ?, was considered as a surface spread property and was found to be controlled by external radiation from the compartment ( F ) that preheats the fuel and the forward flame heat flux ( , F ).…”

Development of an Analytical Model to Determine the Heat Fluxes to a Structural Element Due to a Travelling Fire

“…Hidalgo et al [32] proposed a three modes classification of the fire dynamics based on the ratio of flame spread velocity and burnout front velocity: "fully developed", "growing" and "travelling". Gupta et al [33] precise heat fluxes criteria allowing to distinguish between them; the "travelling" mode occurs when enclosure heat losses are important enough such that the pre-heating by the external heat flux ahead of the flame front is minimal. Several modifications can therefore be brought to improve the proposed analytical procedure.…”