# In Situ Measurements and CFD Numerical Simulations of Thermal Environment in Blind Headings of Underground Mines

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

_{m}), as well as the heat generation from LHDs (denoted as Q

_{L}), has brought significant impacts on the airflow velocity, relative humidity, and temperature distributions in the blind heading. Setting Z

_{m}to 5 m could achieve a relative optimal cooling performance, also indicating that when the LHD is fully operating in the mining face, employing the pure forcing system has a limited effect on the temperature decrease of the blind heading. According to the numerical simulations, a better cooling performance can be achieved based on the near-forcing-far-exhausting (NFFE) ventilation system.

## 1. Introduction

_{m}and Z

_{e}(the distances between the exhausting outlet and the heading face) in PF and PE can be written as Equations (1) and (2), respectively [19]:

^{2}).

_{m}or Z

_{e}based on the Equations (1) and (2).

## 2. Field Measurements

#### 2.1. Measuring Setup

^{2}and a perimeter of 15.5 m. Hence, the hydraulic diameter of the airway was 5.6 m. The heading face was ventilated by PF. The air duct (80 cm diameter) was being installed in the roof of the airway which was producing an airflow velocity of 12.0 m/s with a volume airflow of 7.0 m

^{3}/s. The average airflow velocity can be calculated as 0.3 m/s in the airway. The heat capacity of the airflow per unit time (W/°C) in this airway can be determined as 8442 W/°C by Equation (3):

^{3}/s), ρ

_{a}is air density (kg/m

^{3}), C

_{a}is specific heat capacity of air (J/(kg·°C)). In this paper, ρ

_{a}is 1.2 kg/m

^{3}and C

_{a}is 1005 J/(kg·°C).

_{m}was maintained about 20 m. The blind heading would advance about three meters in each blasting. Data loggers (midi LOGGER GL200, Graphtec Corporation, Yokohama City, Prefecture of Kanagawa, Japan) and thermocouples of Type K (diameter is 0.5 mm, Graphtec Corporation, Yokohama City, Prefecture of Kanagawa, Japan) were used for these measurements. The measuring interval of the data loggers was set at 30 s. The thermocouples were installed in nine locations of the cross-section (Figure 3). One of them was employed to measure the air temperature in the air duct. A blast may cause damage to the data loggers and sensors. Therefore, the sensors had to be installed about one meter from the surface of the airway and the measuring points were 30 m from the face and thus the heat exchange area was 424.6 m

^{2}.

#### 2.2. Results and Discussion

- No mining operation before 20:40;
- Air temperature increased due to the blasting at 20:40; and
- Air temperature increased due to the moving of LHD between 21:50 and 23:30.

#### 2.2.1. Heat Emitted from Surrounding Rock

_{r}is strata heat before the blasting (W), C

_{f}is heat capacity of air in per unit time (W/°C) as shown in Equation (3), Θ

_{f0}is the stable air temperature before blasting (°C), Θ

_{i}is air temperature at the outlet of the air duct (°C), here Θ

_{f0}− Θ

_{i}= 9.7 °C.

^{5}by Equation (5):

_{e}is hydraulic diameter of the airway (m), u

_{m}is average airflow velocity (m/s), v

_{f}is kinematic viscosity (N·m s/kg). In this paper d

_{e}is 5.6 m, u

_{m}is 0.3 m/s and v

_{f}is 15.3 × 10

^{−6}N·m s/kg.

_{p}is isobaric specific heat capacity (J/(kg·°C)). In this paper λ is 0.02 W/(m·°C), C

_{p}is 1005 J/(kg·°C), Pr can be calculated as 0.77, h

_{0}can be calculated as 0.8 W/m

^{2}·°C.

_{0}) of roof, side wall, floor and mining face were 9.7, 5.1, 3.5, and 6.8, respectively, with the average of 5.6 [13]. Hence, the heat transfer coefficient can be determined at 4.5 W/(m

^{2}∙K).

#### 2.2.2. Heat Transferred from LHD

## 3. Numerical Study

#### 3.1. Geometry Model

#### 3.2. Mesh Generation

^{6}elements were generated. Skewness and orthogonal quality are two of the primary quality measures for a mesh. The statistics of the skewness and orthogonal quality is shown in Table 3. The maximal skewness was lower than 0.95 and the minimal orthogonal quality was higher than 0.1, suggesting that the high-quality mesh had been generated as shown in Figure 8.

#### 3.3. Mesh Independence Test

^{6}and 1.75 × 10

^{6}were generated by setting different element size and local inflation mesh controls. The comparison with the element amount of 0.83 × 10

^{6}and 1.28 × 10

^{6}is shown in Figure 9. It can be observed that a finer mesh and the inflation layers were generated in the boundary layers between the airflow and LHD. Figure 10 shows the distribution curves of the airflow velocities and temperatures at the position of Y = 4.1 m, Z = 1.7 m along the airway with three different amount of elements. It can be found that the element amount of 1.28 × 10

^{6}gives about 2% deviation compared to the element size of 1.75 × 10

^{6}. Whereas, the results from the mesh size of 0.83 × 10

^{6}deviate up to 15% as compared to those from the finest one. Therefore, a mesh of 1.28 × 10

^{6}elements was sufficient for the numerical simulation purposes.

#### 3.4. Boundary Conditions

#### 3.5. Turbulence Model

_{k}represents the generation of turbulence kinetic energy due to the mean velocity gradients, C

_{1ε}and C

_{2ε}are model constants, σ

_{k}and σ

_{ε}are the turbulent Prandtl numbers corresponding to the k equation and the ε equation, respectively, and μ

_{t}is turbulent viscosity given by:

_{1ε}, C

_{2ε}, C

_{μ}, σ

_{k}, and σ

_{ε}are 1.44, 1.92, 0.09, 1, and 1.3, respectively.

## 4. Results and Discussion

#### 4.1. Effect of Different Z_{m} on the Thermal Environment

_{m}was set at five different values (specifically 5 m, 10 m, 15 m, 20 m, and 25 m). According to the International Tunnelling Association, the minimum airflow velocity should be greater than 0.3 m/s [27]. Obviously, the airflow velocity was maintained below 0.3 m/s in many regions from Figure 14 when Z

_{m}= 5 m, Z

_{m}= 10 m, and Z

_{m}= 15 m, respectively. On the contrary, when Z

_{m}= 20 m, it can be observed that the regions of low airflow velocity can achieve less coverage. With this consideration, when Z

_{m}= 5 m, Z

_{m}= 10 m and Z

_{m}= 15 m, the forcing duct outlet was too close to the mining face, and the airflow could rapidly enter the relatively closed mining region after free expansion. The LHD greatly obstructed the return airflow, resulting in low reflux velocity.

_{m}= 20 m, the tail of the LHD restricted the development and diffusion of the jet flow and it reflowed back to the airway due to the blocking effect of the LHD. When Z

_{m}= 25 m, the distance exceeded the effective range of the jet flow. The airflow will form a circulating eddy zone before reaching the mining face and, thus, the effective reflux cannot be formed. Therefore, it can be concluded that the blocking effect of ventilation barriers such as LHDs or roadheaders on the airflow velocity distributions should be considered when repositioning the forcing duct.

_{m}was set at five different values as mentioned above. It can be observed that, there are no obvious differences in the temperature distributions in the unoccupied zone under different values of Z

_{m}. It can be considered that the airflow in unoccupied zone is mostly constituted by reflux and similar heat transfer has taken place between the airflow and the LHD as well as the surrounding rock. On the contrary, obvious differences in the temperature distributions are observed in the occupied zone. Obviously, the smaller Z

_{m}has better cooling performance in the occupied zone from Figure 15 considering that the smaller Z

_{m}can encourage more fresh airflow to involve the cooling process.

_{m}is set at five different values at X = 30 m of the airway due to the heat emitted from the LHD. More obvious fluctuations can be seen at Z

_{m}= 5 m, Z

_{m}= 10 m, and Z

_{m}= 25 m.

_{m}was set at five different values. It can be observed that, the smaller Z

_{m}, the higher relative humidity in the occupied zone. The relative humidity gradually decreases along the length of the airway (from the heading face to the outlet of the airway). In the case of the same moisture content of the airflow (the moisture content of the airflow was 13.7 g/m

^{3}), the relative humidity decreases with the increasing dry bulb temperature. If Z

_{m}is set to a higher value, the cooling airflow cannot fully involve in the cooling process in the occupied Zone due to the blocking effect of the LHD. To sum up, the temperature in the occupied zone is about 29 °C and the relative humidity is about 55% when Z

_{m}= 5 m, which is more comfortable for miners. Hence, Z

_{m}= 5 m should be adopted in subsequent analysis. Much care should be taken that the position of the LHD and the heat generated from the LHD are the important factors when optimizing the ventilation and cooling system.

#### 4.2. Effect of Heat Emitted from LHD on the Thermal Environment

_{L}is set at five different values (specifically, 10 kW, 20 kW, 30 kW, 40 kW, and 50 kW) which represent the heat generation from LHD under different working conditions. Obviously, when the Q

_{L}increased from 10 kW to 50 kW, the average air temperature in the occupied zone increased from 30 °C to 34 °C. The airflow in the unoccupied zone is mostly constituted by the reflux. The heat transferred between the airflow and the LHD will result in the reflux temperature increasing to above 37 °C. Moreover, there is a long range and high flow velocity of the forcing system in the ventilation duct outlet that is frequently employed in the hot blind headings but it still has a limited cooling effect. Therefore, great efforts should be made to explore better ventilation and cooling schemes.

_{L}is set at five different values. It can be observed that the raise of Q

_{L}, can decrease the relative humidity in the airway. As mentioned above, the more heat generated by the LHD leads to the higher dry bulb temperature. In the case of the same moisture content of the airflow, the relative humidity decreases with the increasing dry bulb temperature.

#### 4.3. Effect of Auxiliary Ventilation on Thermal Environment of Airway

_{e}= 5 m and the exhaust airflow velocity is 24 m/s. The exhausting duct is located on the airway floor. The third one is FFNE. The diameter of the forcing duct and the exhausting duct are the same as PF and PE, respectively, but Z

_{m}= 15 m and Z

_{e}= 5 m. To prevent recirculation, the total quantity of air exhausted must be at least twice the quantity delivered by the force fan. Therefore, the force airflow velocity was 12 m/s and the exhaust airflow velocity was 24 m/s. The fourth one is NFFE, which is similar to the FFNE but Z

_{m}= 5 m and Z

_{e}= 15 m [7].

_{uz}− t

_{s})/(t

_{oz}− t

_{s}), where t

_{uz}was the average temperature in the unoccupied zone, t

_{s}was the supply air temperature, and t

_{oz}was the average temperature in the occupied zone. Thus, it had conformed to the reasonable distribution of the cold flow [29].

## 5. Conclusions

_{L}, indicating that the airflow temperature distribution is closely correlated with the heat emitted from the mechanical equipment. However, how to further improve the local ventilation cooling performance needs to be further investigated.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 10.**Airflow velocities and temperatures distribution of calculating three times along the airway. (

**a**) Airflow velocities distribution; and (

**b**) airflow temperatures distribution.

**Figure 11.**Airflow temperature distributions in the cross section of X = 14 m without and with LHD: (

**a**) without LHD; (

**b**) with LHD.

**Figure 12.**Comparison between the measured data and the simulation results without and with LHD: (

**a**) without LHD; and (

**b**) with LHD.

**Figure 14.**Airflow velocity distributions in plane A when Z

_{m}= 5 m, Z

_{m}= 10 m, Z

_{m}= 15 m, Z

_{m}= 20 m, and Z

_{m}= 25 m.

**Figure 15.**Airflow temperature distributions in plane A when Z

_{m}= 5 m, Z

_{m}= 10 m, Z

_{m}= 15 m, Z

_{m}= 20 m, and Z

_{m}= 25 m.

**Figure 17.**Airflow relative humidity distributions in plane A when Z

_{m}= 5 m, Z

_{m}= 10 m, Z

_{m}= 15 m, Z

_{m}= 20 m, and Z

_{m}= 25 m.

**Figure 24.**Air temperature distributions in Plane B with different Q

_{L}under the NFFE ventilation system.

Starting Time | End Time | Operation | Heavy-Duty Machinery | Charge Quantity (kg) | Ore Amount (ton) |
---|---|---|---|---|---|

20:40 | — | Blasting | — | 41.8 | 49.6 |

21:50 | 23:30 | Loading and hauling the Ore | LHD | — | — |

Combustion engine | Displacement | 6.7 L |

Output Power | 144 kW | |

Physical dimension | Length | 8.0 m |

Width | 2.5 m | |

Height | 2.0 m |

Min | Max | Average | Standard Deviation | |
---|---|---|---|---|

Skewness | 1.72 × 10^{−4} | 0.84 | 0.30 | 0.17 |

Orthogonal quality | 0.16 | 0.99 | 0.78 | 0.15 |

Boundary | Conditions |
---|---|

Air duct outlet | Supply airflow temperature 25 °C supply airflow velocity 12 m/s, supply airflow relative humidity 70% |

Airway outlet | Pressure outlet |

Wall of airway | The heat thermal conductivity: 2.8 W/m·K [22]; Outside heat flux 192.9 W/m^{2} |

Wall of LHD | Fixed heat: Total power: 27 kW |

Wall of miners | Metabolic rate: 180 W/m^{2} [23] |

**Table 5.**Comparison between the measurement data of airflow velocity and the simulation data for various models.

Measurement Point | Measured Results | Spalart–Allmaras | Standard K-Epsilon | Standard K-Omega | Reynolds Stress Model | |||||
---|---|---|---|---|---|---|---|---|---|---|

with LHD | without LHD | with LHD | without LHD | with LHD | without LHD | with LHD | without LHD | with LHD | without LHD | |

a1 | Unmeasurable | 0.31 | 0.15 | 0.33 | 0.14 | 0.31 | 0.15 | 0.28 | 0.17 | 0.35 |

a2 | 0.32 | Unmeasurable | 0.34 | 0.26 | 0.33 | 0.27 | 0.30 | 0.25 | 0.36 | 0.28 |

a3 | 0..50 | 0.75 | 0.54 | 0.77 | 0.50 | 0.77 | 0.47 | 0.74 | 0.55 | 0.79 |

a4 | 0.85 | 1.18 | 0.86 | 1.19 | 0.86 | 1.17 | 0.83 | 1.15 | 0.87 | 1.21 |

a5 | Unmeasurable | Unmeasurable | 0.16 | 0.31 | 0.16 | 0.29 | 0.18 | 0.26 | 0.19 | 0.30 |

a6 | Unmeasurable | Unmeasurable | 0.29 | 0.30 | 0.29 | 0.30 | 0.29 | 0.28 | 0.31 | 0.30 |

a7 | 0.43 | 0.78 | 0.45 | 0.79 | 0.44 | 0.79 | 0.40 | 0.74 | 0.46 | 0.82 |

a8 | 0.65 | 1.25 | 0.67 | 1.28 | 0.65 | 1.25 | 0.61 | 1.23 | 0.69 | 1.29 |

p | \ | \ | 0.039 | 0.041 | 0.083 | 0.414 | 0.041 | 0.042 | 0.042 | 0.034 |

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## Share and Cite

**MDPI and ACS Style**

Wang, W.; Zhang, C.; Yang, W.; Xu, H.; Li, S.; Li, C.; Ma, H.; Qi, G.
In Situ Measurements and CFD Numerical Simulations of Thermal Environment in Blind Headings of Underground Mines. *Processes* **2019**, *7*, 313.
https://doi.org/10.3390/pr7050313

**AMA Style**

Wang W, Zhang C, Yang W, Xu H, Li S, Li C, Ma H, Qi G.
In Situ Measurements and CFD Numerical Simulations of Thermal Environment in Blind Headings of Underground Mines. *Processes*. 2019; 7(5):313.
https://doi.org/10.3390/pr7050313

**Chicago/Turabian Style**

Wang, Wenhao, Chengfa Zhang, Wenyu Yang, Hong Xu, Sasa Li, Chen Li, Hui Ma, and Guansheng Qi.
2019. "In Situ Measurements and CFD Numerical Simulations of Thermal Environment in Blind Headings of Underground Mines" *Processes* 7, no. 5: 313.
https://doi.org/10.3390/pr7050313