Experimental study and physical analysis of flame geometry in pool fires under relatively strong cross flows

Abstract

This paper investigated the influence of horizontal cross flows on the flame geometry in pool fires. Four square gaseous burners with dimensions of 8 cm, 10 cm, 15 cm, and 20 cm were used employing propane as fuel with various heat release rates. The cross flows were provided by a wind tunnel with air speed ranging from 0 m/s to 6.0 m/s in 0.5 m/s intervals. Five basic quantities of the flame geometry were measured comprehensively, namely, the horizontal flame length Lx, the vertical flame height H and the flame base drag Ldrag as well as two definitions of the flame tilt angle θ1 (originating from pool center) and θ2 (originating from flame base center). The horizontal flame length Lx and the flame base drag Ldrag are found to firstly increase then decrease, while the vertical flame height H decreases and the flame tilt angles θ1, θ2 increase monotonically, with increasing of cross flow air speed. The flame is characterized into two regions, i.e., one similar to boundary layer near the pool surface followed by the other similar to free flow rising above ground. This explains the aforementioned different behaviors of horizontal flame length Lx and vertical flame height H under different cross flow air speeds. In addition, none of correlations previously proposed describes all the flame geometry parameters comprehensively in both horizontal- and vertical directions under relatively strong cross flows. A physical model is developed based on the characteristic length scale derived by combining cross flow air speed, the characteristic volumetric air entrainment, the turbulent flame buoyancy and the air required for stoichiometric combustion. The basic new element is the flame buoyancy (ΔTfTag=g′)in combination with the cross flow air speed, which determines a characteristic length scale, Uw2/g. The proposed model correlates well the above five basic quantities of flame geometry in pool fires under cross flows. This approach allows us predicting the flame geometry in both horizontal- and vertical directions of pool fires for relatively high cross flow air speeds having cross flow (wind) Froude number Uw2gD>0.13.