Smoke Blocking by Water Mist Curtains in Tunnel Fires: A Parametric Study and Momentum-Ratio Criterion

Controlling fire-induced smoke in tunnels remains a critical issue in tunnel fire safety. This study investigates the use of a water mist curtain to contain fire-induced heat and smoke through small-scale experiments and Fire Dynamics Simulator (FDS) modeling. Reduced-scale tunnel tests first show that a properly arranged water mist curtain can limit the downstream spread of hot smoke under natural ventilation. Full-scale simulations are then used to examine the effects of the number of nozzles per row, row spacing, and total water flow rate for heat release rates (HRRs) from 1000 to 5000 kW. The results indicate that, when the total water flow rate is low, i.e., the water mist curtain is insufficient to block smoke, changing the nozzle arrangement has only a limited effect on downstream temperature, although the upward flow becomes stronger with more nozzles per row. Increasing the total water flow rate enhances the cooling effect and induces a reverse-flow region near the water mist curtain, making it more difficult for fire-induced smoke to pass through the water mist zone. Based on the theoretical analysis and simulation results, a dimensionless momentum ratio (the water-mist momentum to the smoke momentum), R=MwMs, is proposed to evaluate the smoke blocking effect of water mist curtains. A critical value of R = 24 is identified for the tested HRR, water mist curtain and tunnel geometry.