Effects of Natural and Forced Entrainment on PM Emissions from Fire Whirls

The influence of different air entrainment conditions on the emissions of particulate matter from fire whirls was investigated by igniting a diesel fuel pool, 0.7 m in diameter, within a four-walled enclosure. Four different natural entrainment conditions resulted when gap sizes in the walls were varied between 0.35 and 0.65 m. In addition, three forced-entrainment conditions were created by holding the gap width constant and varying the air-entrainment velocity using fans positioned at each of the four gaps. The concentration of particulates was measured for these seven conditions, and one pool fire condition for comparison. For fire whirls under natural entrainment, the emission factor of total particulate matter, EF_PM, was lower than that for pool fires, and decreased with increasing gap size. Fire whirls under mild levels of forced entrainment showed the lowest values of EF_PM, but as the level of forced entrainment was increased, EF_PM increased steadily to a value higher than that of pool fires. The reduction in EF_PM is attributed to a combination of leaner stoichiometry and the interaction between the entrainment and the instantaneous burning rate. Thus, for a given pool diameter, an optimum value of entrainment velocity exists where the EF_PM is lowest. Considerations for utilizing whirling flames to reduce airborne particulate emissions from in situ burning are discussed.