An ultrasonic imaging technique has been developed and applied to exam
ine smoldering combustion within a permeable medium. The technique pro
vides information about local permeability variations within a smolder
ing sample, which can, in turn, be interpreted to visualize the propag
ation of the smolder reaction. The method utilizes the observation tha
t transmission of an ultrasonic signal through a porous material incre
ases with increasing permeability. Since a propagating smolder reactio
n leaves behind a char that is higher in permeability than the origina
l (unburnt) material, ultrasonic transmission can be employed to monit
or smolder progress. Additionally, the technique allows observation of
the evolution of the char (i.e., material left by the smolder reactio
n), which: in certain circumstances, can continue to increase in perme
ability, due to secondary reactions (either oxidative or pyrolytic in
nature). Experiments have been conducted, applying the technique to sm
oldering combustion in a two-dimensional geometry with line-of-sight i
maging. For axisymmetric configurations. tomographic techniques have b
een implemented, providing three-dimensional mappings of the smolder f
ront, as well as visualization of the smolder process itself. The resu
lts have furthered the understanding of two-dimensional smolder, and h
ave been especially informative in identifying the controlling mechani
sms leading to the transition from smoldering to flaming combustion. (
C) 1998 by The Combustion Institute