Full-Scale Experimental Investigation on the Thermal Control of a Water Mist System in a Road Tunnel Under Single-Source and Double-Source Fire Scenarios

This study investigates the thermal control effect of a water mist fire-extinguishing system in road tunnels under both single-source and double-source fire scenarios. A total of eight full-scale fire tests were executed in a physical tunnel, and the double-source fire scenarios were further subdivided into two spatial configurations, including fire sources close together and fire sources with a center-to-center distance of 2 m. During the fire tests, the evolution of fire, temporal and spatial temperature distributions of the tunnel ceiling, longitudinal and vertical temperature gradients, and smoke behavior within the tunnel were systematically recorded and interpreted. The results demonstrate that early activation of the water mist system optimizes its physicochemical mechanisms by preventing the transition from the growth phase of fire to a stable phase. In single-source fire scenarios, the water mist directly suppresses the flame and eliminates the high-temperature core, leading to a significant alteration in the vertical temperature gradient. For double-source fire scenarios, the high-temperature region on the ceiling is reduced upon the application of the water mist. However, when the fire sources are positioned in close proximity, they tend to merge into a larger fire source, with the water mist proving insufficient to prevent this fusion. Conversely, when the center-to-center distance between the fire sources is 2 m, the water mist effectively separates the sources, blocking thermal feedback between them and forcing the flames to develop vertically. This, in turn, accelerates the attenuation of the fire and the recovery of the ambient temperature. Additionally, within the effective coverage of the water mist, the longitudinal temperature distribution on the tunnel ceiling still follows an exponential attenuation pattern, with a significantly high rate of temperature decline.