Effect of water mist system settings on fire and upstream smoke control in a longitudinally ventilated tunnel

The effect of Water Mist System (WMS) settings on fire Heat Release Rate (HRR), smoke gas temperature, smoke blocking effectiveness and required critical velocity were investigated in a 3 m (width)x 2.2 m(height) x 30 m (length) tunnel model equipped with longitudinal ventilation (LV). A total of 60 tests were conducted in 12 configurations, varying the nozzles position, nozzle row number and nozzles pressure. Additionally, 5 bucket tests were also carried out to assess the influence of LV on water mist trajectories. The bucket test results revealed a strong correlation between fire HRRs and the "local" water quantity on the ground near the fire source, particularly for smaller fires. The WMS's effectiveness in reducing the maximum smoke gas temperature rise and critical velocity largely depends on the nozzles which are placed closest to the fire source to a great extent. Reducing the distance to the fire source or increasing water flow rates could both enhance the reduction effect. However, activating additional upstream nozzle rows provides minimal benefit, as the interaction between adjacent rows becomes negligible at higher LV velocities. In contrast, increasing the number of nozzle rows can enhance smoke blocking effectiveness, as measured by the confinement velocity reduction rate. The findings demonstrate that the entrainment and blockage effect of WMS are the primary determinants of the smoke back- layering (BL) degree under the confinement velocity conditions, rather than the smoke cooling effect. Furthermore, positioning the WMS upstream of the fire source and maintaining a certain distance can optimize the combined effects of WMS and LV on fire HRR and smoke back-layering control, which is partly due to the water mist deflection under ventilation flow. This preliminary study provides foundational data on fire and smoke control using water mist in longitudinally ventilated tunnels and offers insights for the optimal design of WMSs.