Critical Drip Size and Blue Flame Shedding of Dripping Ignition in Fire

Dripping of molten fuels is a widely observed fire phenomenon, and, by igniting other fuels, it can promote fire spread and increase fire hazards. In this work, dripping phenomena from fires of horizontally oriented wires, coated with polyethylene (PE), are investigated in the laboratory. It is found that as long as a flame is attached to the drip, thin tissue paper can be ignited by a single drip. Below a minimum diameter (D_min = 0.63 mm), the drip floats up. Above a critical diameter (D_crt = 2.3 mm), a flame can remain attached to the drip and ignite tissue paper as it falls through a distance of at least 2.6 m, thereby posing a significant fire hazard. A falling burning drip appears to the eye to be a blue chain of flame as a result of persistence of vision. Photographic evidence identifies a flame-shedding process, most likely associated with continual sequential ignition of fuel vapor within a von Karman vortex street generated behind the falling burning drip. The frequency of flame shedding agrees with both the frequency of modeled vortex shedding and the frequency of the unexpected sound that is heard during the process. This is the first time that combustion characteristics of dripping fire phenomena have been studied in detail, and this helps to better evaluate the risk and hazards of wire and façade fires.