Abstract: The authors examine the effects of natural wind in tunnels on fire smoke diffusion characteristics, with the aim of providing a scientific basis for optimizing tunnel fire ventilation and smoke exhaust systems. The natural wind conditions in the Yunshan extra-long highway tunnel are monitored on site to determine its distribution pattern. In addition, a three-dimensional fire dynamics model is developed through a numerical simulation to evaluate the temperature and smoke fields under the influence of smoke exhaust through a shaft, where the effects of natural wind blowing from both the same and opposite directions are analyzed. Key findings include: (1) The ambient temperature in the tunnel maintains a substantial and stable thermal potential difference between the inside and outside, promoting the formation of natural wind, with a maximum speed exceeding 5.0 m/s. (2) At a longitudinal wind speed of 3.0 m/s, natural wind from the same direction increases the range of temperature rise within the tunnel while lowering the temperature in the lower section between the fire source and exhaust outlet. (3) The natural wind from opposite directions inhibits the diffusion of the high-temperature field, with temperatures between the fire source and smoke outlet rising as natural wind speed increases. (4) When the natural wind speed from the same direction does not exceed 2.5 m/s, smoke concentration peaks between the fire source and the smoke outlet, with higher concentrations in the upper part of the tunnel as compared to the lower part. (5) At wind speeds exceeding 3.5 m/s from the same direction, the stratification of downstream flue gas is disrupted, leading to a more uniform distribution of flue gas. (6) Natural wind from opposite directions helps to limit the downstream spread of smoke from the exhaust port: as wind speed increases, the distance that smoke spreads downstream decreases and approaches a constant value. (7) Both natural wind from the same and opposite directions reduce smoke exhaust efficiency. Specifically, at a wind speed of 4.0 m/s, the smoke exhaust efficiency of the shaft decreases by 22.43% and 21.55%, respectively, compared to conditions without natural wind. The effects of natural wind from the same direction on the smoke exhaust efficiency are greater than those of wind from opposite directions.

Key words: extra-long highway tunnel, natural wind, tunnel fire, smoke exhaust through shaft, smoke spread, on-site test, numerical simulation