Abstract
High frequency (30 Hz) two-dimensional particle image velocimetry (PIV)
data recorded during a field experiment exploring fire spread from point
ignition in hand-spread pine needles under calm ambient wind conditions
are analysed in this study. As the flame spreads approximately radially
away from the ignition point in the absence of ambient wind forcing, it
entrains cooler ambient air into the warmer fire core, thereby
experiencing a dynamic pressure resistance. The fire-front, comprising a
flame that is tilted inward, is surrounded by a region of downdraft.
Coherent structures describe the initial shape of the fire-front and its
response to local wind shifts while also revealing possible fire-spread
mechanisms. Vortex tubes originating outside the fire spiral inward and
get stretched thinner at the fire-front leading to higher vorticity
there. These tubes comprise circulation structures that induce a
radially outward velocity close to the fuel bed, which pushes hot gases
outward, thereby causing the fire to spread. Moreover, these circulation
structures confirm the presence of counter-rotating vortex pairs that
are known to be a key mechanism for fire spread. The axis of the vortex
tubes changes its orientation alternately towards and away from the
surface of the fuel bed, causing the vortex tubes to be kinked. The
strong updraft observed at the location of the fire-front could
potentially advect and tilt the kinked vortex tube vertically upward
leading to fire-whirl formation. As the fire evolves, its perimeter
disintegrates in response to flow instabilities to form smaller fire
“pockets”. These pockets are confined to certain points in the flow
field that remain relatively fixed for a while and resemble the behavior
of a chaotic system in the vicinity of an attractor. Increased
magnitudes of the turbulent fluxes of horizontal momentum, computed at
certain such fixed points along the fire-front, are symptomatic of
irregular fire bursts and help contextualize the fire spread. Most
importantly, the time-varying transport terms of the turbulent kinetic
energy (TKE) budget equation computed at adjacent fixed points indicate
that local fires along the fire-front primarily interact via the
horizontal turbulent transport term.