Abstract: High-temperature smoke reverse flow during tunnel fires hinders personnel evacuation and emergency rescue operations. In spiral tunnels, smoke-wall collision complicates high-temperature smoke discharge. Smoke control remains a key issue in tunnel fire prevention. Therefore, numerical simulations and theoretical analysis are combined to investigate the impact of curvature, slope, and water mist (WM) on critical wind speed in spiral highway tunnels. Seven curvatures, six slopes, five WM nozzle flow rates, and four atomization angles are examined to determine the variation patterns of critical wind speed under various conditions. The results show that: (1) Critical wind speed increases as curvature decreases. Critical wind speed exhibits different trends with varying slopes, where an uphill slope decreases the critical wind speed and a downhill slope increases it. (2) The WM nozzle flow rate substantially affects critical wind speed: as the flow rate increases, critical wind speed gradually decreases, but the rate of change slows once the flow rate reaches a certain value. (3) The atomization angle of the WM has a nonlinear effect on critical wind speed, with an initial increase followed by a decrease. Considering both economic factors and performance, the optimal atomization angle is 120° and the optimal nozzle flow rate is 15 L/min. (4) Based on the observed effects of WM on critical wind speed, a theoretical predictive model for critical wind speed under WM conditions in spiral highway tunnels is proposed. The model reveals a one-third power-law relationship among critical wind speed, fire heat release rate, and WM heat absorption.

Key words: highway tunnel, spiral tunnel, water mist, smoke control, fire, critical speed, curvature, gradient