Applicability of liquid nitrogen fire extinguishing in urban underground utility tunnel

This paper built the experimental platform in the shrinkage-sized circular utility tunnel. A series of experiments were conducted by changing the parameters of liquid nitrogen injection. The feasibility of liquid nitrogen fire extinguishing in utility tunnel was studied. It is found that the main mechanism of liquid nitrogen fire extinguishing is cooling and asphyxiation. Cooling during vaporization can suppress fire rapidly, and the asphyxiation completely extinguishes the flame. During the liquid nitrogen fire extinguishing process, the temperature and the heat flux showed a decreasing trend of fast first and then slow. By increasing the injection rate and time of liquid nitrogen, the process of fire extinguishing can be accelerated and the extinguishing time would be shortened significantly. In addition, it is also found that the critical temperature at the flame of liquid nitrogen fire extinguishing is 230.0 degrees C, and the critical heat flux at 0.2 m from the flame is 0.20 kW/m(2). Not considering oxygen consumption and nitrogen leakage, the critical oxygen volume fraction of liquid nitrogen extinguishing cable fire in utility tunnel is about 15-16%.