Fluctuation and extinction of laminar diffusion flame induced by external acoustic wave and source

Abstract

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.

Displacement and Extinction of Jet Diffusion Flame Exposed to Speaker-Generated Traveling Sound Waves

“…Bennewitz et al [12] studied the extinction mechanism for a single droplet flame in acoustic standing waves and found that extinction was caused by the temporal increase in the local rate of normal flame strain. This extinction mechanism was also supported by Xiong et al [13][14][15][16]; they further modified the Damköhler number (Da), suitable for acoustic extinction, and the flaming firebrand. Furthermore, droplets can be extinguished when the value of Da drops below 1.0.…”

Properties of turbulent non-premixed methane/air flames in a miniature-scale swirl burner under different coaxial airflow swirl numbers

“…To obtain flame fluctuations, the Otsu threshold is used to generate binarized images of 2000 frames for each operating condition (boundaries of all 2000 frames are shown simultaneously in Figure 7 for methane and airflow rates of 0.200 and 3 𝑠𝑙𝑝𝑚, respectively). The mean center of gravity (𝑥 𝐶𝐺,𝑚𝑒𝑎𝑛 , 𝑦 𝐶𝐺,𝑚𝑒𝑎𝑛 ), length (𝐿 𝑚𝑒𝑎𝑛 ), and width (𝑊 𝑚𝑒𝑎𝑛 ) are determined; then, the standard deviation is then used to calculate the flame's pulsating displacement in terms of the flame's center of gravity, length, and width [28]: (5)…”

“…Because of high energy density of hydrocarbon fuels, combustion-based micro-power devices are a more attractive option for portable power generation than rechargeable batteries [18][19][20][21][22]. In these systems, swirling flows are one method for flame stabilization [23][24][25][26][27][28][29][30]. To the best of authors' knowledge, there has never been research on the effects of swirl number on turbulent flame dynamics and characteristics by taking advantage of threshold-based image segmentation method.…”

Tunable self-extinguishing of dripping fire mediated by impacted substrates

“…Xiong et al [15] found that the effectiveness of acoustic wave extinguishment decreased with increasing dripping velocity. They [16] also discovered the use of acoustic waves to extinguish propane flames and found that the frequency and sound pressure of the acoustic source had a significant effect on the flame. Lower-frequency acoustic sources produced larger membrane vibrations and flame flicker displacements, promoting flame extinction.…”