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An ultrasonic imaging technique has been developed to examine the propagation of a smolder reaction through the interior of a porous combustible material. The technique provides information about the location of a propagating smolder front, as well as any permeability variations of the smoldering material. The method represents an important tool in the study of smoldering combustion, and has potential application in studies of other heterogeneous combustion processes. The method is based on the fact that, for an ultrasonic frequency defined by the porous medium properties, the transmission of an ultrasonic signal through a porous material increases with increasing permeability. Thus, since a propagating smolder reaction leaves behind char with a higher permeability than the original material, ultrasound transmission can be employed to differentiate virgin material from char. Furthermore, if the char continues to react, the technique can be used to investigate the evolution of the permeability of the char. In this work, experiments are presented where the technique is applied to smoldering combustion in a two-dimensional geometry. The results of applying the technique have furthered the understanding of two-dimensional smolder, especially in elucidating the controlling mechanisms leading to the transition from smoldering to flaming. The applicability of ultrasonic tomography to smoldering combustion has also been successfully applied.
The ultrasonic imaging technique developed here is based on the evidence that, for an ultrasonic frequency defined by the porous medium properties, the degree of attenuation of an ultrasonic signal through a porous material decreases with increasing permeability. Thus, since a propagating smolder reaction leaves behind char with a higher permeability than the original material, the relative attenuation of ultrasonic transmissions can be employed to differentiate virgin material from char and to monitor any permeability variations of a smoldering material.
In the method developed here, an acoustic frequency in the ultrasonic regime is employed because of its spatial resolution and distinction from background noise. In addition, the frequency to be employed had to be the highest frequency (for spatial resolution purposes) that could provide a detectable transmission (at reasonable speaker output powers) through virgin foam samples of the thickness (~ 16 cm) used throughout the present smolder study. Furthermore, noticeable relative magnitudes of transmission between virgin foam and char had to exist such that a smoldering interface could be visualized. The 40 kHz frequency applied in this work performed best under these criteria, with commercially available pressure transducers (speakers/microphones). The spatial resolution is of the order of 1 cm, corresponding to the ultrasonic wavelength and the size of the diaphragm of the microphone. Only a limited number of commercial pressure transducers were tested; and conceivably, pressure transducers with frequency responses higher than 40kHz that provide better resolution and adaptability may be available and should be investigated.
Another design consideration is whether to employ a continuous ultrasonic wave or an ultrasonic wave-train pulse. Although slightly more difficult to employ, an ultrasonic wave-train pulse allows for a more precise transmission measurement because the first peak in the received wave-form identifies the desired transmitted signal through the foam/char, distinguishing it from reflection and other interference signals. In addition, the frequency at which the wave-train pulses are sent can be reduced, permitting the speakers to be driven at higher powers without overheating. Moreover, the time of flight of the wave-train pulse can be used to measure the average temperature along its propagation path. Although this feature is not employed in this work, it is the subject of future work.
Operation procedure, along with design considerations, for the ultrasonic imaging technique for a single set of speaker and microphone are as follow:
1) A speaker emits a 40 kHz ultrasonic sinusoidal wave-train pulse through the porous medium. The wave-train pulse consists of a given number of cycles (6). The duration of the wave-train pulse is experimentally determined to maximize amplitude magnitude versus time spread of the transmitted wave-train pulse due to superposition of diffracted and reflected signals in the porous medium.
2) A microphone receives a wave-form which includes the transmitted wave-train pulse along with reflection and other interference signals. This wave-form is amplified and converted to an RMS signal. Parenthetically, we are only interested in relative magnitudes of transmission amplitude modulation; thus the absolute value of the RMS signal is not critical. Based on the shortest path length through the sample, the first peak in the received wave-form identifies the desired transmitted signal through the foam/char.
3) The received RMS wave-form is digitally sampled by a computer and stored into memory. The entire received wave-form constitutes a single ultrasonic transmission data point, where the attenuation of the transmitted wave-train pulse is deduced in post-processing. Figure 1 shows the wave-form received by a microphone, after amplification and RMS conversion.
4) The next ultrasonic transmission data point is taken. The frequency of wave-train pulses is very important. The time between wave-train pulses must be longer than the time of flight for a single pulse. Enough time must also be allowed to minimize superposition effects from previously sent wave-trains that can be reflected back into the propagation path of interest. The time interval is limited by the desired rate of ultrasonic transmission data points to be taken.
In the present setup , linear arrays of speaker/microphones are employed, to reduce the time needed to scan the sample. Line-of-sight transmission projections are produced by scanning the samples with horizontal arrays of speakers and microphones, mounted to a 2-axis moving assembly. 40 kHz sinusoidal waves in 6 period bursts are produced by a synthesized function generator, triggered by a function generator. The pulsed signals are amplified and are sent sequentially, via relay modules, from an array of 8 ultrasonic speakers. An array of 8 ultrasonic microphones receive the transmitted pulsed signals. These received wave-forms are pre-amplified, multiplexed, amplified by a gain programmable differential amplifier, converted to a RMS/DC signal, sampled at 250 kHz by a high speed A/D board (which is triggered by the function generator), and recorded onto a personal computer.
The resulting set of data points gives a 2-D image of the line-of-sight average of the attenuation of the sample. Even at this point, much information about the sample can be obtained, however, we can go one step further. 3-D tomographic techniques can be applied. Here, we have used the Abel Transform due to its relative simplicity. It requires an axisymetric sample, but the sample need only be scanned in one direction. Phantoms and smoldering sample have been successfully imaged to date.
The innovative application of ultrasonic imaging employed here brings the study of optically inaccessible heterogeneous combustion into a new perspective, where new insight into the mechanisms controlling smoldering combustion is revealed via visualization of evolving material properties. The technique is especially informative concerning char permeability evolution and propagation of the smolder front. Development of this technique into an ultrasonic tomography of non-axisymmetric material permeability and temperature fields for a smoldering fuel in real time is worth pursuing.