## 1 Introduction  
A large amount of harmful gases and dust are generated due to blasting and mechanical operation during the excavation process of tunnels. The pollutants seriously endanger the health of workers and the normal operation of equipment. These harmful gases are mainly CO, and the concentration must be reduced to below the safe concentration standard before proceeding with the next construction operation [1; 2; 3]. Ventilation is one of the most effective methods for removing harmful gases [4]. In order to quickly eliminate pollutants and accelerate construction progress, the optimization of the ventilation scheme is extremely important [5; 6]. Mastering the transport and diffusion mechanism of pollutants under ventilation conditions is a prerequisite for optimizing the scheme. The ventilation system of the tunnel is composed of axial fans and ventilation ducts [7]. Fans are usually installed at tunnel outlets to deliver fresh air to the working face through ventilation ducts. Fresh air dilutes and discharges pollutants generated by work in the working area outside the tunnel [8; 9]. During the ventilation scheme design phase, full consideration will be given to the number of workers, the power of diesel engines operating simultaneously, and the quantity and type of explosives used in blasting operations [10]. Ventilation engineering needs to ensure that the working area has sufficient airflow power to expel pollutants and purify the working area. In the construction of tunnels, due to poor duct quality (such as breakage caused by collisions), it is more common for the joints between ducts to cause air leakage [11]. However, air leakage in ducts can directly lead to insufficient airflow in the working face, making it difficult to achieve the preset purification effect [12]. The phenomenon of air leakage in ventilation ducts makes the design of ventilation systems more complex [13]. Therefore, it is necessary to conduct a thorough analysis of the ventilation system of the construction tunnel to ensure that the working face meets the required air volume. Mastering the ventilation laws can achieve purification of the workplace and accelerate construction progress [14; 15; 16]. Numerical simulation methods are commonly used to solve complex engineering problems. The results of numerical simulation can further deepen the theoretical research. For ventilation research in tunnels, the analysis of flow field characteristics is the basis for solving the diffusion laws of pollutants [17; 18]. The blocking rate and blocking distance of tunnels are one of the reasons for the decrease in ventilation performance due to the resistance of ventilation airflow [8; 19]. Due to the blockage of tunnel equipment, collisions between airflow and tunnel walls and equipment can lead to the generation of the vortex. The pitch angle variation of the fan can accelerate the dispersion of pollutants in the vortex area [20]. This is due to the instability of fluid motion caused by the shear force generated by changes in flow velocity during airflow movement. The motion of the vortex is influenced by the inertial and viscous forces of the fluid. Inertial force keeps the vortex rotating, while viscous force gradually weakens and causes the vortex to disappear. In addition, the installation spacing of fans also has a certain impact on the stability of air velocity and turbulence. [PERSON] [21] derived a function-fitting formula for the effect of fan installation spacing on fan airflow through numerical modeling, in order to obtain the optimal ventilation effect. [PERSON] et al. [22] obtained the optimal position of the vertical shaft by analyzing the influencing factors of the flow field. The transport process of pollutants is based on the fluid movement inside the tunnel. Mastering the spatiotemporal evolution and distribution patterns of pollutants is the basis for formulating ventilation control strategies [23; 24]. [PERSON] [19] discovered the diffusion law of CO concentration through experiments.