Acoustic wave can destabilize the flame and has a potential in firefighting, but the influences of the sound source and its frequency are still poorly understood. This work applies a loudspeaker to extinguish a laminar diffusion propane flame of 5–25 mm high, where the local sound frequency is 50–70 Hz and sound pressure is 0.8–3.2 Pa (92.0–104.1 dB). Results reveal a constant flame pulsating displacement at the extinction limit, independent of the sound environment used. Such a flame pulsating displacement is found to be caused by the motion of the speaker membrane (or diaphragm) and its induced wind, which could be two orders of magnitude larger than the displacement of the air that transmits acoustic wave. Thus, under the influence of sound source, a critical flame strain rate, stretched by the pulsating airflow, can be formulated to characterize the blow-off limit better than the local sound pressure. The sound source with a lower frequency can produce larger pulsating displacements of both membrane and flame, and thus promoting extinction. This work improves the understanding of flame dynamics under the external sound field and source, and it helps establish a scientific framework for acoustic-based fire suppression technologies.
Shear layer effect was pointed out to be associated with the vortex-flame interaction and leads to the difference in combustion noise. In this work, numerical simulation combining with an acoustic analogy equation was performed to investigate the shear layer effect on the sound generation by jet diffusion flames. To easily identify the cause and effect, the approach that varies the thickness of shear layer by changing the Prandtl number is adopted, which provides deeper physical insight into the characteristics of the acoustic near field. The formation and evolution mechanisms of sound sources in jet diffusion flames are interpreted. It is observed that the sound sources are concentrated in vortex centers and appear as either the sources or the drains for the sound waves. The spectral analysis shows that the low frequency sound is emitted from the flame base, while the high frequency sound is radiated from the downstream region. Moreover, the theoretical analysis based on vortical dynamics shows that the low frequency sound is primarily affected by the combustion-induced buoyancy effect, whereas the high frequency sound is determined by the baroclinic torque.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.