# Study on the Law of Diffusion of Sudden Pollutants in Subway Ventilation Shaft

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## Abstract

The underground subway stations have become the targets of terrorism due to the features of staff-intensive and poor ventilation. In the present study, the dynamic mesh model was used to predict the dynamic change of the flow field in the tunnel induced by the train motion. The DPM model was employed to simulate the interaction between the continuous phase of the air and the discrete phase of pollution particles. Accordingly, the dispersion of sudden pollutants released in the ventilation shaft was investigated. The results show that the submicron particles have a good following performance with the air. The dispersion characteristics of the particles highly depend on the unsteady flow field in the tunnel, which is significantly affected by the piston effect induced by the train motion. It can be concluded that particle diffusion experiences three stages: vertical forward and reverse diffusion in the wind shaft, vertical positive diffusion in the wind shaft, and longitudinal stable diffusion in the interval tunnel. It hopes that the study can provide guides for future biochemical protection in the subway system.

## Keywords

Subway system Discrete phase model Diffusion law## 1 Instruction

Now, terrorists are increasingly rampant and the means of attack are becoming more diverse. Large-scale public buildings, where people spend the longest time, have the highest density of personnel. Sextro et al. [1] simulated the transmission process of the terrorist attack by mail anthrax bacillus in the building; Li et al. [2], using multi-zone airflow model, found that natural ventilation would help SARS virus spread in the building. In the sudden pollution of the ventilation system, Hao et al. [3] analyzed the harm of biochemical attacks in various scenarios and emphasized the possibility and destructiveness of the air-conditioning ventilation system to become the propagating power of biochemical pollution.

In this paper, CFD simulation method is used to reproduce the airflow field caused by piston effect in tunnel, and discrete phase model (DPM model) is used to simulate the propagation and diffusion of pollutant particles and airflow under the coupling action, so as to analyze the diffusion law of sudden pollutants in ventilation shaft.

## 2 Method Validation

The simulation study in this paper uses two complex models: the dynamic mesh model and the DPM model. This paper verifies the two models separately.

### 2.1 Dynamic Mesh Model Verification

In this paper, numerical simulation is carried out by establishing the same model as the literature [4]. The train runs at an acceleration of 1 m/s^{2}. When the train runs at a speed of 3 m/s, it runs at a constant speed of 8 s. Finally, the train runs to stop at a deceleration of 1 m/s^{2}.

The model is divided into structured grids. The motion state is given to the whole fluid domain and the wall, and the tunnel surface is set as a non-slip boundary condition. The pressure inlet and the pressure outlet are used at the tunnel entrance and the tunnel exit respectively, and the calculation time step is set to 0.1 s.

*u*/

*u*

_{max}is defined, where u is the wind speed, 0.5 m from the tunnel entrance, and

*u*

_{max}is the maximum speed of the train. Figure 1 is a comparison of the simulation results with the experimental results of the literature. The simulation results are in good agreement with the experimental results. Therefore, it can be explained that the CFD dynamic mesh technology can well simulate the natural ventilation of the subway tunnel under train movement.

### 2.2 DPM Model Verification

At the initial time, the particles are evenly distributed in zone-1 space, and the density of the particles is 865 kg/m^{3}. In order to achieve uniform distribution of particles, 800 points were uniformly taken in zone-1 to synthesize a parcel as a source of emission.

The average particle concentration comparison of the two zone-1 regions is shown in Fig. 2. As can be seen from the figure, there is a high degree of consistency between the simulated data and the experimental data. Therefore, the DPM model can be applied for further research.

## 3 Models and Boundaries

### 3.1 Introduction to the Model

^{2}and runs at a uniform speed for 28 s.

### 3.2 Force Analysis of Particles

*F*is the force of particles. In this paper, the molecular forces that need to be considered in this case include: Saffman lift, drag force, gravity, and Brownian force.

### 3.3 Boundary Conditions

The particle source is a surface source, and the particle size distribution of the particle source adopts a double *R* distribution. The minimum particle diameter is 1 × 10^{−6} m, the maximum particle diameter is 5 × 10^{−6} m, the average diameter is 3 × 10^{−6} m, the distribution index *n* is 4, and the number of diameters is 10. When the train head moves to the position near the ventilation shaft, the particles are released. The particle release rate is 0.02 kg/s, and the initial particle velocity is 0.1 m/s perpendicular to the downward direction of the ventilation shaft.

Boundary condition settings

Boundary | Ventilation shaft | Tunnel entrance | Tunnel exit | Tunnel wall | Tunnel ground |
---|---|---|---|---|---|

Condition setting | Pressure outlet | Pressure inlet | Pressure outlet | Collision and reflection with particles | Collision capture with particulate matter |

## 4 Results Analysis

In summary, submicron particles have good airflow follow-up, and the change of airflow field in the tunnel has a great impact on particle diffusion.

## 5 Conclusion

- (1)
When the train runs in a subway tunnel, the airflow in the tunnel is greatly affected by the “piston effect.” The direction of airflow in the ventilation shaft tuyere varies with the change of train position. According to the direction of the airflow, the airflow variation can be divided into two stages: the exhaust stage when the train is upstream or directly below the ventilation shaft, and the supply stage when the train is downstream.

- (2)
Submicron particles have good airflow followability, and their propagation and diffusion are greatly affected by airflow. Where the airflow velocity is high, the coupling force between particulate matter and air is large, and the diffusion velocity increases significantly. In the vortex region where the velocity of airflow is small, the coupling force between particulate matter and air is small, and the diffusion velocity is relatively small.

- (3)
Due to the influence of the tunnel airflow over time, the diffusion of submicron particles also changes with time. According to the diffusion direction and velocity of the particles in the tunnel, the diffusion of the particles can be divided into three stages:

Positive and negative diffusion of particulate matter in the ventilation shaft; positive diffusion of particulate matter in the ventilation shaft; and particulate matter diffuses longitudinally in interval tunnels.

## References

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