Experimental Research on Burning Rate, Vertical Velocity and Radiation of Medium-Scale Fire Whirls

Abstract

Fire whirls are often reported in forest and urban fires with devastating damage to life and property. This work conducted experiments using a medium-scale facility to study the initiation mechanism, the vertical velocity and radiative heat transfer of fire whirls. Heptane was used as the fuel in the experiments. The variations of burning rates versus time indicate that the drag force on the root of the flame plays an important role in the initiation and decay of a fire whirl. Analyses show that the pressure difference due to whirling, characterized by the circulation  , significantly affects the vertical centerline velocity. The variation of the vertical centerline velocity is correlated by , where m is nearly 1.1 for both fire whirls and general pool fires. It is also found that the radiative fraction of fire whirls is nearly 42 %, 1.4 times as large as that for general pool fires. The radiative heat feedback fraction is shown to increase with pool diameter, similarly as for general pool fires. KEYWORDS: fire whirl, radiation, centerline velocity, burning rate, pool fires. NOMENCLATURE LISTING

Fire Whirls

“…However, this is not the case for fire whirls, as Zhou et al (2011) argued that both buoyant plume theory and the circulation-induced vortex should be included in the dimensional analysis. According to Zhou et al (2011), Fr is approximately ( / gz 3 ) η , where η is found to be 0.22 through the continuous flame region and 0.77 through the plume. However, this scaling seems to be inappropriate because the circulation in the free vortex region is typically constant along the vertical direction (Lei et al 2011.…”

“…Following the discussion above, heat transfer at the fuel surface was thought to increase significantly due to the flow structure (presence of swirl), effectively increasing the heat transfer coefficient for fire whirls (Muraszew et al 1979). In larger fires, the radiative heat feedback fraction was found to increase for fire whirls compared to that of pool fires (Zhou et al 2011). However, Snegirev et al (2004) found that this fraction decreases slightly with circulation.…”

“…To evaluate the radiative heat flux from fire whirls to the external surroundings, researchers have often assumed the whirl to be a homogeneous black body emitter (Zhou et al 2011), similar to studies from pool fires (Hamins et al 1996). Zhou et al (2011Zhou et al ( , 2014 have proposed two models that show that there is a considerable variation in the radiant heat flux profile as height increases.…”

“…Zhou et al (2011Zhou et al ( , 2014 have proposed two models that show that there is a considerable variation in the radiant heat flux profile as height increases. In fire whirls with various pool sizes, the radiative heat flux increases throughout the continuous flame zone up to z/H = 0.4 and then decreases rapidly beyond it (Zhou et al 2011, Wang et al 2015.…”

“…Zhou et al (2011Zhou et al ( , 2014 have proposed two models that show that there is a considerable variation in the radiant heat flux profile as height increases. In fire whirls with various pool sizes, the radiative heat flux increases throughout the continuous flame zone up to z/H = 0.4 and then decreases rapidly beyond it (Zhou et al 2011, Wang et al 2015. The decline of radiative heat flux in the plume region is faster than in the intermittent zone, similar to nonswirling pool fires, where the corresponding height through which the radiative heat flux increases is up to z/H = 0.5 (Hamins et al 1996).…”