AN APPLICATION OF ULTRASONIC TOMOGRAPHIC IMAGING TO STUDY SMOLDERING COMBUSTION

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