Experimental study on the impact of asymmetric heavy rainfall on the smoke spread and stratification dynamics in tunnel fires

This work examines the impact of heavy rainfall on the smoke spread and stratification dynamics in tunnel fires with a reduced-scale (1:15) experimental platform. Scaled tests vary the rainfall intensity (up to 60 mm/h, equivalent to 232 mm/h in nature), raindrop size (1.0–1.5 mm, equivalent to 4–6 mm in nature), and fire heat release rate (2.1–6.7 kW, equivalent to 2–6 MW in real scale). We found that the asymmetric rainfall on one exit can induce the longitudinal airflow inside the tunnel due to the dissipation of the raindrop momentum and the rain-induced air entrainment. The velocity of such a longitudinal airflow increases with increasing rainfall intensity, and smaller raindrops tend to induce faster longitudinal airflow. The spread and stratification of hot smoke are sensitive to the induced airflow. In the absence of rainfall, there was a symmetrical temperature distribution from the fire source due to the symmetrical air entrainment. Under rainfall, the temperature distribution of tunnel fires gradually becomes less symmetrical, and the smoke stratification interface becomes unclear. With the increase in rainfall intensity, the phenomenon of smoke back-layering appears, and the smoke layer height decreases. Under a small rainfall intensity, raindrop size has a higher impact on the smoke stratification near the fire source. This study aims to attract more attention to tunnel safety under the dual disaster of fire and extreme weather and support the emergency response.