Influence of injection method on the fire extinguishing efficiency of liquid nitrogen in urban underground utility tunnel

Liquid nitrogen has excellent ability of cooling and asphyxiating, which is suitable for fire extinguishing in urban underground utility tunnel. In order to obtain the influence of injection method on the fire extinguishing efficiency, a medium-sized square utility tunnel was built. Fire extinguishing experiments with different liquid nitrogen injection methods and distances were carried out. The distribution and the decrease law of oxygen volume fraction and temperature are analyzed. The result shows that the time to extinguish the fire and enter the controllable stage are greatly reduced under both conditions. Among them, the fire extinguishing efficiency of vertical injection is higher. Liquid nitrogen accumulates near the injection port first, which has a great cooling effect on this area, and acts on the fire in two stages. As the distance increases, the cooling effect is weakened and the oxygen volume fraction increases stepwise. Liquid nitrogen shows continuous cooling and asphyxiating effect during horizontal injection. It moves fast in this mode, so the difference in oxygen volume fraction is small. The initial cooling effect of vertical liquid nitrogen injection is better than that of the horizontal injection. However, the temperature at the end recovers earlier after fire extinguishing in the latter condition.