*Article* **Characteristics of Airflow Reversal of Excavation Roadway after a Coal and Gas Outburst Accident**

**Junhong Si 1, Lin Li 1,\*, Jianwei Cheng 2, Yiqiao Wang 1, Wei Hu 1, Tan Li <sup>1</sup> and Zequan Li <sup>1</sup>**


**Abstract:** Determining the influence scope of the airflow disorder is an important problem after coal and gas outburst accidents in ventilation systems. This paper puts forward the indexes of airflow disorder, including the length of the excavation roadway, the outburst pressure, the pressure difference of the air door, and the air quantity of the auxiliary fan. Using the orthogonal table of L9 (34) and numerical simulation method, the characteristics of airflow reversal are studied, and the outburst airflow reversal degree is calculated should the ventilation facility fail. Furthermore, on the basis of fuzzy comprehensive optimization theory, the comprehensive evaluation model of the airflow disorder is established. The results show that the length of the excavation roadway is the most important factor affecting the stability of the ventilation system, followed by the outburst pressure, pressure difference of the air door, and air quantity of the auxiliary fan. The influence of a gas outburst accident on the return air system is greater than that on the inlet air system, and a larger air velocity has a greater impact on the ventilation system, especially the air inlet part. Moreover, the airflow reversal degree of the ventilation system increases with the increase of the outburst pressure or decreases with the length of the excavation roadway. This paper provides a basis for the prevention of gas outburst accidents.

**Keywords:** airflow reversal; gas outburst; mine ventilation system; orthogonal experiment; numerical simulation

## **1. Introduction**

A coal and gas outburst is an accident that is caused by a large amount of coal and gas being ejected into the underground roadway in a single moment [1]. In an outburst accident, the high-energy shock wave destroys the roadway facilities instantly and changes the roadway resistance and the structure of the ventilation network [2]. After the dynamic effect of the shock wave disappears, the gas continues to flow and diffuse unsteadily [3], generating ventilation pressure and additional force [4], causing airflow disorder [5], casualties, or secondary gas explosion accidents.

A coal and gas outburst is studied from three aspects: shock wave [6–8], outburst gas [9–12], and outburst coal rock [13,14]. The theoretical analysis [15] and numerical simulation are popular methods [16,17] in a single roadway. After the outburst power disappears in the upwind roadway, Feng et al. [18] concluded, the influencing factors of the airflow reversal in the parallel branch include the pressure of the main fan, the height difference of roadway, the initial air velocity, length, and the sectional area of roadway. Yu et al. [19] pointed out that the airflow is affected by the seam inclination in the inlet roadway. However, the underground mine ventilation network is formed by connected roadways, and the study is often not limited to a single roadway. Zhou et al. [20] established 45◦ and 135◦ crossover tunnels to simulate the flow and attenuation rules of shock waves

**Citation:** Si, J.; Li, L.; Cheng, J.; Wang, Y.; Hu, W.; Li, T.; Li, Z. Characteristics of Airflow Reversal of Excavation Roadway after a Coal and Gas Outburst Accident. *Energies* **2021**, *14*, 3645. https://doi.org/10.3390/ en14123645

Academic Editors: Sheng-Qi Yang, Min Wang, Qi Wang, Wen Zhang, Kun Du and Chun Zhu

Received: 17 May 2021 Accepted: 15 June 2021 Published: 18 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and gas in a gas outburst accident. The influence of air pressure increases with the increase of the angle between the excavation roadway and the adjacent excavation roadway, but the intersection of the roadway has no effect on the airflow reversal. They also found that the outburst pressure could not only lead to the airflow reversal in the downward ventilation roadway, but also may lead to the airflow reversal in parallel branches of the upward ventilation roadway [21,22]. The phenomenon of gas flow restriction and retention appears in the roadways with the air door and windshield, so draining the gas immediately is the key to prevent secondary disasters in the underground roadways.

In recent years, scholars have conducted a series of studies on the disaster process of the whole mine ventilation system during the outburst period. Due to the complexity of the ventilation system after an gas outburst accident, the tunneling face is usually seen as a unidirectional burst source, and a three-dimensional simulation method is adopted to simulate the flow process of air and gas in the ventilation system during the disaster period [23]. The result shows that the disturbance of the ventilation system is affected by the gas force, natural wind pressure, and ventilation power. Then, the characteristics of bidirectional gas outburst in the coal mining face are studied [24]. Different from the unidirectional outburst mode, the impact strength of this type is reduced. Ventilation facilities are the weak point of the ventilation system. The research mainly focuses on the characteristics of air door failure [25,26], the reasonable position, the strength of the air door [27], as well as the automatic air door [28,29]. There is still a lack of research on the influencing factors of airflow disorders in ventilation systems when ventilation facilities fail. There are many factors that affect the ventilation system of a coal and gas outburst [30–32], it is particularly important to establish the index system of the airflow disorder and determine relatively important influencing factors when ventilation facilities fail, which could provide theoretical support for disaster prevention.

The whole paper is structured as follows: Section 2 introduces the ventilation system. In Section 3, the influencing factors of the airflow disorder induced by gas outburst accident are put forward. Using the orthogonal experiment analysis and numerical simulation method, the sensitivity factors of the airflow disorder when the ventilation facility fails are analyzed in Section 4. Section 5 conducts a comprehensive evaluation of the airflow disorder on the basis of fuzzy comprehensive optimization theory.

#### **2. Ventilation System and Its Simplification**

Most of the coal and gas outburst accidents occur in the heading face [24]. The ventilation system is formed by the excavation roadway, heading face, auxiliary fan, and air ducts. Figure 1 illustrates a forced ventilation system, which is the most commonly used ventilation form. It has the advantages of a small air leakage and large air supply. The fresh air is transported from the intake airway to the heading face through the auxiliary fan and air duct, and the polluted air enters the return airway through the excavation roadway. The air door obstructs fresh air and polluted air, and the air duct needs to pass through the air door. The blue arrow indicates the airflow direction of the fresh air, and the red one indicates the polluted air.

In general operation conditions, the order of the air gauge pressure is P20 > P21 > P22, P10 > P11 > P12 > P13, P11 > P31 > P21.

According to graph theory, the air duct can be considered as a separate roadway for the air supply on the heading face, so it is an independent branch. There are nine branches and nine nodes in a general ventilation system. The ventilation system is simplified, as shown in Figure 2.

In a gas outburst accident, the air pressure of the roadway is redistributed as the gas concentration increases rapidly in the ventilation system, and the shock wave usually destroys the ventilation facilities. As a result, if the air door is invalid in a tunneling ventilation system, the outburst gas will flow into the intake airway. Then, the airflow will be reversed if the air pressure at the upwind side is lower than the outburst pressure.

**Figure 1.** Sketch map of the ventilation system.

**Figure 2.** Diagram of the simplified ventilation network in a general ventilation system.

The outburst airflow reversal degree is defined as the influence of outburst gas on the airflow disorder in an airway, which is the ratio of the air quantity variation after the gas outburst accident and the original value. A positive reversal degree means that the airflow direction of the outburst gas is the same as the original airflow, and a negative reversal degree means that the airflow is reversed.

#### **3. Indexes of Airflow Disorder**

There are 3 factors that affect the safety and stability of the mine ventilation system, as shown in Figure 3.

**Figure 3.** Hierarchical structure of factor indicators.

#### *3.1. Parameters of Outburst Accident*

The outburst intensity indicates the amount of coal and rock thrown into the roadway and the amount of gas emissions in a coal and gas outburst accident. Because it is difficult to calculate the amount of gas emission, the outburst intensity is represented by the amount of outburst coal and rock. Usually, an outburst accident is divided into a small outburst accident (<100 t), medium outburst accident (<500 t), large outburst accident (<1000 t), and extra-large outburst accident (≥1000 t).

The outburst position refers to the place where the outburst accident occurs. If the air pressure of the outburst position is low, the outburst gas can be faster exhausted to the ground, as the outburst position is closer to the air shaft. On the contrary, it has a great impact on the ventilation system, especially the air intake roadway. Therefore, we can reduce the impact of the outburst accident on the ventilation system by reducing the air resistance of the return airway or increasing the intake airway.

#### *3.2. Physical Conditions of Roadway*

The characteristics of the roadway are determined by the physical conditions, including the section and length of roadway. Specifically, the outburst gas is easy to exhaust from the large section or the shorter length of roadway.

#### *3.3. Safety and Stability of Mine Ventilation System*

The high-pressure outburst gas suddenly flows into the roadway space in a gas outburst accident, which has a great impact on the mine ventilation system. It is easy to cause a large range of airflow disorders in the mine ventilation system after the shock wave destroys the air doors.

Among the above factors, the outburst pressure, length of roadway, pressure difference of the air door, and supply air quantity of the roadway are chosen to analyze the laws of the airflow disorder and the degree of influence in a mine ventilation system.

#### **4. Orthogonal Experiment**

#### *4.1. Orthogonal Design*

The orthogonal experiment is used for the analysis of the mutual influence of multiple factors [33]. Using the orthogonal table, an appropriate amount of representative points is selected from a large number of test points. Through the overall design, comprehensive comparison, and statistical analysis, the balanced sampling within the range of factor change is carried out. Then, the analysis of the representative results is achieved by a small number of tests.

In the orthogonal design, the table of L9 (34) is adopted to study the effect of 4 factors, i.e., A is the length of roadway, B is the outburst pressure, C is the pressure difference of air door, and D is the air quantity of the auxiliary fan on the airflow disorder in the mine ventilation system. The specific parameters of each test are illustrated in Table 1.


**Table 1.** Initial parameters of the orthogonal experiment.

From Table 1, the level of each factor is in the common value range, such that the length of roadway is 200 m, 1000 m, and 2000 m, the outburst pressure is 0.1 MPa, 0.2 MPa, and 0.3 MPa respectively, the pressure difference of the air door is 500 Pa, 1000 Pa, and 1500 Pa, the air quantity of the auxiliary fan is 240 m3/min, 600 m3/min, and 1200 m3/min. A total of nine combined schemes are designed.
