Abstract: In order to study the effect of nitrogen injection distance on the fire extinguishing efficiency of liquid nitrogen in an underground enclosed space, an underground pipe gallery was taken as an example to establish a liquid nitrogen fire extinguishing efficiency evaluation system. Through liquid nitrogen fire extinguishing tests, quantitative analysis and evaluation of liquid nitrogen fire extinguishing efficiency based on flame, temperature and oxygen volume fraction were made. The results show that the increase of the horizontal nitrogen injection distance will lead to the weakening of the instantaneous inhibitory effect of liquid nitrogen. The fire extinguishing time increases almost uniformly and linearly with the increase of the nitrogen injection distance, but the influence degree of the time when the temperature drops to the controllable stage gradually decreases. The fire extinguishing efficiency will change regularly with the change of the nitrogen injection distance. However, when the nitrogen injection distance is greater than 3 m, the influence of the change of the nitrogen injection distance on the fire extinguishing efficiency is weakened.

Key words: underground enclosed space, underground pipe gallery, liquid nitrogen, fire extinguishing efficiency