Study on Smoke Control Strategies for Tunnel Fires Under Near-Critical Velocity Conditions

High-temperature toxic smoke from tunnel fires poses a serious threat to safe evacuation, while direct experimental support is still lacking for the performance transition and parameter matching resulting from the combined action of centralized smoke extraction and a water curtain under near-critical longitudinal ventilation. To investigate their combined smoke control performance under near-critical longitudinal ventilation, this study combines FDS numerical simulations, 1:10 reduced-scale experiments, and dimensionless analysis to examine the effects of the smoke extraction rate and water curtain parameters on temperature and CO concentration in an 8 MW tunnel fire. The results show that, without a water curtain, 15 m3/s is a representative suitable smoke extraction rate among the discrete cases; with 500 μm droplets and a flow rate of 40 L/min per nozzle, the 18 m3/s case exhibits comparatively favorable smoke control performance after the water curtain is introduced. As the flow rate per nozzle increases from 30 to 60 L/min, the smoke extraction performance transition interval shifts from 17–18 to 20–21 m3/s, and a dimensionless interval-constrained relation is established: Πe = 3.90596Πw + 0.32834. This relation applies to the fixed geometry, fire, ventilation, and spray conditions in this study and is not for direct use in engineering design.