Abstract: Due to constraints imposed by geological and topographical conditions, urban longdistance immersed tunnel projects may exhibit structural characteristics with multiple slopes forming a W shape, without the installation of wind towers. For such urban tunnels without wind towers, their opening strategy of fans and pollutant distribution patterns need to be further studied. In this paper, numerical simulation is employed, mainly considering different segments with obstruction (entrance section, middle section, exit section and full blockage) and varying numbers of fans (3, 5, 6 and 7 groups) being activated, to investigate the flow field and pollutant distribution patterns. The results indicate that the ceiling tunnel wind speed inside the tunnel increases with the growing number of activated fans. When considering blockages, it becomes evident that the obstructed section exerts a more pronounced impact on the flow field distribution within the expanding fan jet region. The increase in the number of tunnel fans has a minor impact on the pollutant concentration in the obstructed segment, primarily affecting the pollutant concentration in the non-obstructed segment. Fans located at the slope change point in the center of the tunnel have limited effects on the distribution of the flow field inside the tunnel and the dilution of CO pollutants. This is because the jet is prematurely impinged on the tunnel wall due to the influence of slope changes, affecting the jet's development. Accumulation of pollutants is anticipated in the latter portion of the tunnel, suggesting the need to contemplate the incorporation of supplementary jet fans within this segment during the design. After comprehensively analyzing the exported CO concentration of similar projects, it was found that the design criteria for CO concentration adopted in the current domestic specifications are conservative, which can lead to a waste of resources.

Key words: tunnel ventilation, immersed tunnel, jet fan, opening strategy of fan