Energy distribution analysis on suppressing a shielded fire with water mist in a tunnel rescue station

Abstract

In real tunnel fire scenarios, water mist has little opportunities to directly apply to train fires shielded by a carriage body. However, most models for heat transfer analysis assume direct contact between fire source and mist droplets. It is of interest to find a method to evaluate the energy exchange between water mist and a shielded fire. To investigate the water mist suppression performance on a shielded fire, a full-scale experimental and theoretical study was performed by varying the activation time of water mist system, working pressure, and the diameter of mist droplets. The suppression performance is found to be dependent on the working pressure for the small mist droplets, and sensitive to the size of mist droplets at high working pressure. Moreover, the suppression performance is also influenced by the water mist activation time due to the competition of fuel cooling and the inhibition effect of the smoke layer. A theoretical model was developed to predict the energy exchange among smoke, water mist, and surroundings. It was found that in comparison to the indirectly restricted effect of cooling the carriage and fire, the direct heat loss between water mist and smoke is the main controlling mechanism. This study provides an important reference for the design of fire water system in the tunnel rescue station and is beneficial to the environmental protection of fire extinguishing sites.