Self-extinction characteristics of fire extinguishing induced by nitrogen injection rescue in an enclosed urban utility tunnel

Extensive studies have been conducted on effect of sealing ratio on combustion process and temperature field of pool fire, while still lacking emergency rescue research after the failure of the original fire extinguishing system. Therefore, a series of tests were carried out in a 1/3 reducedscale tunnel to explore the nitrogen injection into the closed tunnel on the effectiveness of firefighting. The results show that, the presence of nitrogen not only significantly lowers the oxygen concentration and mass loss rate of the air drawn into the combustion area, but the flame continues to entrap the polluted low -oxygen air, resulting in the continuous inadequate combustion of the fire and a large amount of asphyxiating smoke, thus shortening the self -extinguishing time. The concentration range of oxygen concentration of flame extinguishing during nitrogen injection is 11.13 %-14.98 %. Simultaneously, an analytically derived model to estimate the extinction time of nitrogen injection into the closed utility tunnel was developed, which is found that the deviation between the improved classical model and the experimental data within 10 %. The present work positively provides a tool for the assessment of thermal environment and estimation of fire detection time in engineering practice.