Abstract: Fire suppression systems commonly used in narrow and confined spaces have limitations when applied in urban underground logistics systems, making them difficult to implement directly. To investigate the improvement in liquid nitrogen extinguishing efficiency under the influence of enhanced longitudinal ventilation in urban underground logistics systems, this study analyzes the changes in oxygen volume fraction and ceiling temperature under different working conditions by varying wind speed and nitrogen injection position. The results indicate that nitrogen injection can effectively extinguish fires in underground logistics tunnels, and enhanced longitudinal ventilation significantly improves the extinguishing efficiency of liquid nitrogen. When the nitrogen injection distance is 50 m, the extinguishing efficiency under a wind speed condition of 2 m/s is increased by 41.86% compared to a wind speed condition of 0 m/s. Additionally, at a fixed nitrogen injection distance, enhanced longitudinal ventilation significantly advances the time when the temperature and oxygen volume fraction near the fire source begin to decline. Moreover, as the nitrogen injection distance increases, the time for the temperature and oxygen volume fraction near the fire source to decline is delayed; however, enhanced longitudinal ventilation can mitigate the lag in cooling and asphyxiation effects caused by the increased injection distance. These findings provide important theoretical support for fire suppression research in urban underground logistics systems.

Key words: underground fire, urban underground logistics systems, liquid nitrogen extinguishing, enhanced longitudinal ventilation