Investigation of the possible use of sound waves for fire safety in tunnels: a fundamental study on flame dynamics and the extinguishment criteria of gas leak jet fires

The increasing demand for clean, low-carbon energy has led to a growing number of electric vehicles, underground utility tunnels, and natural gas vehicles, further amplifying the potential risks of jet fire accidents caused by gas leaks in tunnels. Research on clean and efficient fire extinguishing technology during the early development stage of tunnel fires will effectively control the fire’s progression and significantly reduce fire losses. This paper aims to investigate the possible use of sound waves as an early fire extinguishing technology in tunnels. Experiments were conducted to investigate the effects of low-frequency sound waves, which are known to destabilize flames, on the diffusion flames of propane jets. The study evaluates the impact of sound waves on flame behavior and blowout limits. The experiment utilized a sound fire extinguishing test system equipped with a 2.5 mm nozzle, installed in sound waves with frequencies of 30 and 70 Hz. During the experiments, we carefully observed and analyzed the variation of the flame’s length at different sound frequencies and sound pressures. We studied the critical sound frequency and sound pressure at the critical condition of flame blowout. The results demonstrate that the lower the frequency, the more pronounced the influence of the sound wave on the flame, leading to a decrease in flame length with an increasing level of sound pressure. A model is proposed to predict the flame length under different sound wave conditions. Through the study of flame blowout behavior under the influence of sound waves, a non-monotonous transition in the extinction limit of non-lifted and lifted jet flames is found. Subsequently, a theoretical model to characterize flame blowout behavior, considering the effect of sound wave fields based on the Damköhler number theory, is developed. This study has confirmed that sound waves can extinguish gas leak jet fires and have the potential to be applied to control fires in confined spaces, such as tunnels. It should also be emphasized as a limitation that the present study is focused on smaller-scale diffusion fires under controlled laboratory conditions, which differ significantly from the high-pressure, large-scale jet fires typically encountered in natural gas or hydrogen vehicle fires. The findings of this study, therefore, should not be directly applied to those types of fires. Further research is needed to extend these findings to high-pressure jet fires, such as those that might occur in vehicle scenarios, where the dynamics and flame behavior are influenced by much higher pressures and different fuel characteristics. Moreover, it should be noted that the present study only investigates the fundamental characteristics of jet flames under controlled conditions, with a particular focus on their stability. By analyzing the effects of factors such as sound waves and jet velocity, this research contributes to the foundational understanding of flame extinguishment of jet flame dynamics with sound waves to provide foundational insights that could inform future research into fire safety technologies, though further work is required to validate their applicability in real-world scenarios.