2. Model Establishment
2.1. Basic Assumptions
- The temperature of other adjacent surfaces except the cold wall adjacent to the airflow is the original ground temperature at the mining depth;
- The original ground temperature at the mining depth remains unchanged and heat is released uniformly along the working face;
- The cold wall temperature remains unchanged and the cold is released uniformly along the working face;
- After the goaf is backfilled, the heat brought by air leakage to the working face is not considered, and the internal heat source of the working face is also not considered;
- The moisture exchange among the airflow and cold wall, coal wall, surrounding rock, etc., is ignored.
2.2. Mathematical Model
2.3. Geometric Model and Meshing
2.4. Independence Analysis
2.5. Fluent Parameters and Solution Settings
2.6. Model Validation
3. Result and Analysis
3.1. Analysis of Airflow Temperature Variation along the Working Face before and after Cooling
3.2. Analysis of Airflow Temperature Variation at Measuring Points of Working Face before and after Cooling
3.3. Effect of Air Supply Temperature on Airflow Temperature
3.4. Effect of Air Supply Velocity on Airflow Temperature
3.5. Effect of Surrounding Wall Temperature on Airflow Temperature
3.6. Airflow Temperature Distribution in Working Face under the Most Unfavorable Condition
- When cooling measures are not taken, the airflow continuously absorbs heat from the surrounding wall along the working face, and the temperature gradually rises. After cooling by cold wall, the trend of the airflow temperature rising along the working face is slowed down. Compared with before cooling, the temperature increment of Z = 50 m, 100 m, 150 m, 200 m, 250 m and 300 m sections decreases by 0.67 °C, 0.48 °C, 0.40 °C, 0.36 °C, 0.33 °C and 0.29 °C, respectively.
- The airflow temperature at measuring points increase with the ventilation time, and the time to reach the stable value is obviously different. The closer to the exit of the working face, the longer the time it takes to reach the stable temperature and the more obvious the cooling effect is. When the air supply temperature is 26 °C, the temperature of the measuring point can reach 31.97 °C without cooling measures, and the temperature of the measuring point is 29.19 °C after adopting cooling measures. Compared with before cooling, the temperature increment is reduced by 2.78 °C.
- The higher the air supply velocity, the faster measuring point 3 reaches the maximum temperature, the lower the maximum temperature of the airflow. When the air supply velocity increases from 1.5 m/s to 3.5 m/s, the temperature of measuring point 3 increases to the maximum at 210 s, 165 s, 135 s, 120 s, 90 s respectively. After using cooling measures, the temperature increment decreases to 2.93 °C, 2.84 °C, 2.75 °C, 2.68 °C and 2.50 °C successively.
- When cooling measures are not taken, the airflow temperature of the measuring points increase by 1.57 °C for every 5 °C increase in the surrounding wall temperature after reaching stability. After cooling measures are taken, the surrounding wall temperature is between 35–50 °C, the airflow temperature of measuring point 3 is lower than 28 °C.
- When cooling measures are not taken, airflow temperatures of measuring points exceed the limit rapidly, except for the measuring points near the airflow inlet. After cooling measures are taken, the temperature increment of measuring points 1–6 decreases by 0.01 °C, 0.13 °C, 1.25 °C, 2.54 °C, 2.44 °C, 2.51 °C respectively at 240 s.
- In this study, the radiation refrigeration and ventilation combined with operation cooling method not only reduces the temperature of the working face but it also provides a suitable working environment for the staff, realizes the integration of mine cooling and thermal energy utilization, as well as provides a utilization for geothermal resources.
Conflicts of Interest
|airflow density (kg/m3)||average temperature of the heating surface (K)|
|time (s)||average temperature of the cooling surface (K)|
|velocity vector||radiative heat transfer between walls (W·m−2)|
|pressure (N)||convective heat transfer between air flow and wall surface (W·m−2)|
|viscous stress components||Stephan Boltzmann constant (W·m−2·K−4)|
|volumetric forces on elements (N)||comprehensive factor of wall radiation heat transfer|
|total energy of the fluid (J)||average airflow temperature (K)|
|effective heat transfer parameter||heat transfer coefficient between wall surface and airflow (W·m−2·K−1)|
|diffusion flow rate of component j (kg·m −2·s−1)||equivalent diameter of surrounding wall (m)|
|heat source term||area (m2)|
|average wall temperature of the working face (K)||perimeter (m)|
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|Airflow Temperature/°C||before cooling||24.41||26.19||27.71||29.11||30.42||31.62|
|Before Cooling||After Cooling|
|90 s||180 s||90 s||180 s|
|Air Supply Temperatures/°C||18||20||22||24||26|
|Airflow Temperature/°C||Before cooling||26.49||27.86||29.23||30.60||31.97|
|Air supply Velocities/(m/s)||1.5||2.0||2.5||3.0||3.5|
|Airflow Temperature/°C||Before cooling||29.76||29.45||29.19||29.00||28.65|
|Surrounding Wall Temperatures/°C||35||40||45||50||55|
|Airflow Temperature/°C||Before cooling||90 s||24.89||26.00||27.11||28.25||29.34|
|After cooling||90 s||23.56||24.44||25.32||26.19||27.07|
|Airflow Temperature/°C||Before cooling||26.09||26.77||29.38||31.98||32.98||34.06|
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