Role of acoustic wave on extinguishing flames coupling with water mist

Abstract

Water mist plays an indispensable role in various fire extinguishing scenarios, but its efficiency is restricted by the interaction area of droplets and the flame. With the characteristics of simple operation and limited environmental pollution, acoustic waves can deliver water droplets to flame and have the potential to enhance the fire extinguishing efficiency of water mist. However, our understanding of the synergistic effect of acoustics and water mist on suppressing flame is still limited. In this study, the enhancement mechanisms of acoustic waves in extinguishing ethanol fueled flames are experimentally investigated. The effects of critical parameters related to sound pressure and acoustic-induced air speed on flame behaviors are comprehensively discussed. Initially, the flames cannot be extinguished by the pure water mist. Coupling with acoustic waves, the response of the flames significantly changes. For the acoustic wave of a sine signal, the frequency of 30 Hz induces a greater air speed, which is superior to the frequency of 40 Hz and 50 Hz in suppressing the flame. The acoustically enhanced mechanism is perhaps related to the acoustic-induced wind, which causes the flame oscillation. The flame and the wick expose the larger area to be cooled directly by the water mist when the flame is “pulled” and “pushed” by the acoustic-induced wind. A dimensionless number θ is finally estimated for determining the critical flame extinction condition for this collaborative system. These results could advance our understanding of the underlying enhancement mechanisms in coupling acoustics with water mist and provide scientific insights into applying this novel suppression strategy.