Experimental study of transitional behavior of fully developed under-ventilated compartment fire and associated facade flame height evolution

Abstract

For compartment fires with an opening, two distinct states are widely recognized: wellventilated (oxygen is sufficient for combustion inside the compartment) and underventilated (oxygen is completely used inside the compartment). And as a consequence, the flame ejects out through the opening with excess fuel burning outside when reaching under-ventilated condition. However, in this work, a further transitional behavior of under-ventilated compartment fires with increasing fuel supply is revealed and quantified. Experiments were carried out using an under-ventilated fire compartment (0.4 m cubic) with a fixed opening width (0.25 m) and various opening heights (0.0125 m to 0.15 m) (corresponding ventilation factors, , 3.49×10-4 to 1.45×10-2 m2.5). The temperature inside the compartment and the facade flame ejected through the opening were recorded with increasing fuel supply (or namely total heat release rate) for under-ventilated conditions. The results showed that when the under-ventilated compartment fire reached a transitional state, the temperature inside the compartment experienced a sudden drop associated with a sudden increase of facade flame height outside the opening, posing a more severe impact on the building facade. The critical fuel supply rate for reaching the transitional state could be divided into two different mechanisms by a critical value of the opening ventilation factor (kg/s), based on the mass balance and flame stoichiometric extinction analysis inside the compartment. The formula for describing the critical fuel supply rate by considering the evolution of the critical air-fuel equivalence ratio () for both mechanisms was proposed. The classic model on the facade flame height for common under-ventilated compartment fires was found to be not applicable when the transitional state of underventilated condition had been reached. A new model was proposed to correlate the facade flame height for the transitional state of under-ventilated condition, which was shown to have a 2/3 power dependence on the total heat release rate taking the opening width as the characteristic length.

Numerical Modeling of Compartment Fires: Ventilation Characteristics and Limitation of Kawagoe’s Law

“…8a), the compartment fire can even develop from the overventilated stage the jet-flame stage directly. Similar fire behaviors had been observed in reducedscale compartment fire experiments [33]. As the ventilation factor decreases, it becomes easier to form a jet flame under a lower fuel supply rate (𝑚̇𝑗 𝑒𝑡 ).…”

“…compartment fire [33]. Nevertheless, the high-temperature region still exceeds 600 °C, so that it is still reasonable to use a smoke layer temperature of above 600 °C to judge the occurrence of flashover.…”

A Critical Appraisal of the UK’s Regulatory Regime for Combustible Façades

“…In the case of a timber crib, wind will have a strong accelerating effect in the initial fire growth rate. Delichatsios [36] describes the classic model of fac¸ade flame height as needing revision. When a compartment undergoes transition to under-ventilated flaming, this reduces the temperature within the compartment but increases the temperature outside leading to more severe fac¸ade fires.…”

Numerical study of the combustion regimes in naturally-vented compartment fires

“…A similar behavior was also reported by Lock et al [12] during their full-scale experimental study with a gas burner. Recently, Ren et al [13] investigated experimentally the transitional behavior of under-ventilated compartment fires. In this study, the fire source also consisted of a gas burner.…”