  One dozen vertically oriented thin rectangular vanes, 62 cm tall and 15.2 cm
wide, were placed 27 cm from the center of heptane and ethanol pool fires in
continuously fed, floor-flush pans 3.2 cm and 5.1 cm in diameter in the
laboratory. The vanes were all oriented at the same fixed angles from the
radial direction, for 9 different angles, ranging from 0 to 85 degrees, thereby
imparting 9 different levels of circulation to the air entrained by each pool
fire. The different swirl levels were observed to engender dramatically
different pool-fire structures. Moderate swirl suppresses the global puffing
instability, replacing it by a global helical instability that generates a tall
fire whirl, the height of which increases with increasing circulation. Except
for the largest heptane pool, higher swirl levels produced vortex breakdown,
resulting in the emergence of a bubble-like recirculation region with a ring
vortex encircling the axis. Measured burning rates increase with increasing
swirl levels as a consequence of the associated increasing inflow velocities
reducing the thickness of the boundary layer within which combustion occurs
right above the liquid surface, eventually forming detached edge flames in the
boundary layer that move closer to the axis as the circulation is increased.
Still higher circulation reduces the burning rate by decreasing the surface
area of the liquid covered by the flame, thereby reducing the height of the
fire whirl. Even higher circulation causes edge-flame detachment, resulting in
formation of the blue whirl identified in recent literature, often meandering
over the surface of the liquid in the present experiments. This sequence of
events is documented herein.
