Experimental investigation on extinguishing characteristics of liquid nitrogen in underground long and narrow space

Abstract As an efficient and environmentally friendly cryogenic extinguishing agent, liquid nitrogen (LN2) is highly promising for fire extinguishing in narrow and long underground confined spaces. It is difficult to tackle the urban utility tunnel fire due to its complex and narrow structural characteristics. Thus work aims to study the feasibility and effectiveness of LN2 fire suppression in urban utility tunnel. An experimental study on the fire extinguishing characteristics of LN2 in urban underground utility tunnel was carried out in a 1/3.6 reduced-scale model tunnel with different injection parameters, such as the direction of the LN2 nozzle and the distance between the LN2 nozzle and the fire source. The results show that LN2 can quickly extinguish a utility tunnel fire. The rapid extinguishing mechanism of LN2 involves the combined effect of cooling and inerting. When the LN2 release distance is 2 m and 4 m, the fire extinction time increases by 8.7% and 27.5% respectively for the horizontal injection. On the contrary, the vertical injection is more effective when the LN2 nozzle is far from the fire source. By injecting LN2 into the utility tunnel, an abrupt evaporation occurs, followed by a thermal expansion. The vaporizing cryogen forms a low temperature nitrogen cloud, which rapidly cools the gas. As the cloud spreads, the pyrolysis gas is rendered inert, and the fire is isolated from oxygen and finally extinguished.