The Effect of Offset on Burner Fire Whirls

Abstract

ABSTRACT Fire whirls studied in the lab are typically generated by centering vorticity directly over a fuel source. While this is often a reasonable representation of fire whirls in nature, in the chaos of a forest fire the vorticity can organize or be pushed away from the direct fuel source. This type of off-source whirl has not been extensively studied. Here we present a detailed comparison of the structure and behavior of both on- and off-source laboratory whirls. The whirls are generated using staggered cylinder halves and a burner. The off-source whirls are formed by displacing the burner from the center of the cylinder halves. The structure of the whirls is investigated using Stereo Particle Image Velocimetry. We present radial velocity fields inside and outside the core of the whirls in planes parallel and perpendicular to the whirl axis. Whirl behavior is studied by generating scaling relationships for circulation, height, and Froude number in terms of heat release rate and cylinder stagger. The on-source whirls are tall and thin, with zero radial velocity in the continuous region of the whirl, while the off-source whirls are short and tapered, and have non-zero radial velocity in the whirl core. The circulation and Froude scaling for the off-source whirls is well-described by a linear factor applied to the scaling of the on-source whirl. The height scaling of the off-source whirl remains ambiguous.