# Parameter Optimization on the Forced Ventilation of Symmetric Tunnel Construction Based on the Super-Short Bench-Cut Method

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## Abstract

**:**

## 1. Introduction

## 2. The Engineering Situation and Ventilation Calculation

^{3}/min.

## 3. The Numerical Simulation Validation

^{3}/min) required by the main cave construction uses an air duct with a diameter of 2 m, where the wind velocity in the outlet is 8.63 m/s. As the left line of the main cave is 420 m, the monitoring points of the velocity are set at 30 m, 100 m, 150 m, 200 m and 300 m from the tunnel face. The finite element software, Fluent, is used to establish the corresponding numerical model. In the numerical simulation, the air in the duct is regarded as an ideal gas. Using the k-epsilon turbulence model, the outlet of the tunnel is set as a free outlet, the outlet of the air duct is set as the velocity outlet, and the sidewalls of the air duct and tunnel are set as the wall with no energy exchange. Additionally, within Fluent, monitoring points are set at distances of 30 m, 100 m, 150 m, 200 m and 300 m from the tunnel face. The comparison between the measured wind velocity and the velocity in the numerical simulation is shown in Figure 3.

## 4. The Numerical Model and Related Parameters Calculation

#### 4.1. Establishment of the Numerical Model

#### 4.2. Calculating Operating Conditions

#### 4.3. Setting Boundary Conditions

^{3}, $v$ is the flow velocity of the air medium in the air duct, $d$ is the equivalent diameter of the air duct, $l$ is the turbulent scale, and $c$ is the empirical constant (0.09).

## 5. Analysis of the Numerical Simulation Results

#### 5.1. The Influence of the Distance between the Orifice and the Tunnel Face on the Wind Velocity Flow Field

- When L is 5 m, the wind velocity flow field in the upper bench is very uniform without backflow. This is because, with the close distance between the air duct and the tunnel face of the lower bench, the air sent by the air duct tends to have a downward trend and clings to the tunnel face without an obvious backflow. However, the uniformity of the airflow field behind the tunnel face is significantly poor.
- When L is 8 m, the wind velocity flow field in the upper step is relatively uniform without backflow. At this moment, the distance between the orifice and tunnel face is far and the differential flow field of the lower step improves largely without backflow. The uniformity of the air flow field in the upper and lower tunnel faces is good.
- When L is 10 m and 12 m, the backflow appears below the upper tunnel face. When L = 10 m, the ground behind the lower tunnel face appears to be an obvious vortex area and when the distance between the air duct and tunnel face increases to 12 m, the vortex area enlarges and new vortex areas appear behind it.
- Calculation results show that when the distance between the orifice and tunnel face is 8 m, the wind velocity flow field near the upper and lower tunnel face is the most uniform without obvious backflow.

#### 5.2. The Influence of the Air Duct Diameter on the Wind Velocity Flow Field

- After comparing the distribution of the wind velocity flow field in the tunnel when the value of air duct diameter is different, as shown in Figure 4, it is clear that when D is 1.0 m, 1.1 m and 1.2 m, due to the large velocity when the pipe diameter is small, a lot of energy is rebounded after touching the wall, leading to a less than ideal ventilation effect occurring in the corner of the wall.
- When the air duct diameter is 1.6 m and 1.8 m, due to the further reduction of the velocity of the orifice, an obvious backflow appears at the tunnel face of the upper step near the ground. The ventilation effect is also not ideal at this moment.
- When the air duct diameter is 1.4 m, there is not only obstructed ventilation in the corner because of the large velocity when the air duct diameter is small, but the backflow is also prevented from forming near the tunnel face when the air duct diameter is large.

## 6. Conclusions

- After obtaining the related parameters in the first stage of the construction ventilation from a tunnel in the northwestern area of China, a numerical model was established to achieve the simulated value of the velocity of each measuring point and compare them with the measured values found in order to ascertain whether the numerical simulation applied in this study was entirely feasible.
- The ventilation effect is better when the wind pipe is arranged on one side of the tunnel wall (an asymmetrical layout), although the space in the tunnel is axisymmetric.
- When the air duct diameter is 1 m, the calculation results are as follows: when the distance between the air duct and tunnel face is 5 m, the uniformity of the air flow field near the tunnel face is poor compared to when the distance between them is 8 m, even though there are no obvious vortex zones. However, when the distance is 10 m or 12 m, vortex zones appear in the tunnel. Thus, it is more reasonable when the distance between the air duct and the tunnel face is 8 m.
- When the distance between the air duct and the tunnel face is 8 m and the air duct diameter is less than or equal to 1.2 m, the velocity is high as the pipe diameter is small and a large amount of energy is rebounded after touching the wall, which leads to the non-ideal ventilation effect outputting from the corner of the wall. However, when the pipe diameter is more than or equal to 1.6 m, the upper tunnel face near the ground appears to show more obvious backflow for the small velocity of the orifice.
- When the air duct is 1.4 m and the distance between the orifice and tunnel face is 8 m, there is no obvious backflow in the tunnel and the uniformity of wind velocity flow field is good, which can be regarded as the best combination for application in practical engineering.
- We can obtain better ventilation effect when the distance between the nozzle of the ventilator and the tunnel face is 6 m–9 m and the wind speed of the nozzle is 6 m/s–8 m/s. In practical engineering, the wind speed and the required air volume should be taken into consideration to determine the diameter of the ventilator.

## Author Contributions

## Conflicts of Interest

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**Figure 2.**The first stage of the ventilation construction of a tunnel in the northwest regions of China.

**Figure 5.**The wind velocity flow field when the air duct diameter is 1.0. (

**a**) L = 5 m; (

**b**) L = 8 m; (

**c**) L = 10 m; and (

**d**) L = 12 m.

**Figure 6.**The velocity of each measuring point when the distance between the blast pipe and the tunnel face is different.

**Figure 7.**The wind velocity flow field of the tunnel when the blast pipe diameter is different. (

**a**) D = 1.0 m; (

**b**) D = 1.1 m; (

**c**) D = 1.2 m; (

**d**) D = 1.4 m; (

**e**) D = 1.6 m; and (

**f**) D = 1.8 m.

**Figure 8.**The velocity of each measuring point in the tunnel when the air duct diameter is different.

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**MDPI and ACS Style**

Niu, X.; Zhang, D.; Su, J.; Guo, H.
Parameter Optimization on the Forced Ventilation of Symmetric Tunnel Construction Based on the Super-Short Bench-Cut Method. *Symmetry* **2018**, *10*, 49.
https://doi.org/10.3390/sym10020049

**AMA Style**

Niu X, Zhang D, Su J, Guo H.
Parameter Optimization on the Forced Ventilation of Symmetric Tunnel Construction Based on the Super-Short Bench-Cut Method. *Symmetry*. 2018; 10(2):49.
https://doi.org/10.3390/sym10020049

**Chicago/Turabian Style**

Niu, Xiaokai, Dingli Zhang, Jie Su, and Hong Guo.
2018. "Parameter Optimization on the Forced Ventilation of Symmetric Tunnel Construction Based on the Super-Short Bench-Cut Method" *Symmetry* 10, no. 2: 49.
https://doi.org/10.3390/sym10020049