A new perspective on the sound extinction mechanism and the triggering parameters for jet fires in tunnels

Fire has become one of the most serious disasters in tunnels and underground spaces, and sound waves for firefighting (SWFg) are expected to become an effective technology for tunnel firefighting. To investigate the sound extinction mechanism and the triggering parameters for jet fires in tunnels, a testing platform was constructed using a methane jet fire as the ignition source. In the experiment, the frequency range of sound waves was 20–200 Hz, the pressure range was 0.4–3.0 Pa, and the corresponding local flow velocity range was 0.2–1.6 m/s. The results indicate that the periodic motion of the diaphragm of the sound source causes intense fluid flow characteristics. The fluid velocity induced by the diaphragm increases exponentially with the increase of sound source power. Through theoretical analysis, it is found that the fluid flow in the near-sound source sound field (NSSF) can be decoupled into periodic flow and time-averaged flow. Based on experiments and theoretical analysis, it has been found that the fluid flow velocity induced by the sound source is 100 times greater than that induced by sound waves. Jet fires above 0.43 kW exhibit intense lateral motion in the NSSF, while jet fires in the far-sound source sound field (FSSF) have little obvious response characteristics. Periodic flow will cause the periodic movement of the flame, and the effect of time-averaged flow and crosswise is similar to that of flame tilting. Unlike the near-steady-state characteristics of lateral wind, under NSSF conditions, the flame is subjected to non-steady-state periodic effects and is more prone to extinction. The critical fire extinguishing velocity is linearly correlated with the fuel outlet velocity. Based on feature dimensionless numbers, a global model for the extinction of diffusion flames under NSSF conditions was constructed. This study explores the basic mechanism of SWFg technology, which can provide a basis for the research and development of fire extinguishing technology in tunnels.