Effect of microgravity on the formation and geometry of whirling flames*

Fire Whirls (FWs) are structures that frequently occur in wildfires and are formed when a buoyant plume is subjected to ambient circulation. The two primary physical processes controlling FW structure are circulation and buoyancy. Here, we describe experimental investigations performed at the NASA Glenn Research Center's Zero Gravity Research Facility drop tower, which provides 5.18 s of microgravity time to study the effects of normal (1g) and micro gravity ($\mu $g) on FW geometry. The FWs were formed in both 1g and $\mu $g, using a paraffin wax wick in an enclosure formed by two offset half cylinders. Ambient circulation was controlled using a vertical bank of fans at each inlet. Results show that in $\mu $g, where the effects of circulation are higher, the flame height reduces dramatically, and the flame width increases moderately. The burning rate also reduces in $\mu $g, sometimes leading to short-lived blue flames, attributed to the increased importance of diffusion. Elevated gravity (\textasciitilde 35g), due to deceleration at the end of the drop, resulted in a brief transition to a blue-whirl-like regime, that is, a state in which a recirculation zone exists. Finally, a scaling approach to analyze whirling flames in $\mu $g is presented.