## 3 Results
### Simulation of Flow Field Distribution behind Working Face  
Before the numerical simulation of the gob flow field distribution, it is necessary to compile the porosity conditions for each part of the 3D gob model. At present, the porosity conditions of gob numerical simulation models are usually set by two methods. The first method divides the gob and sets the corresponding porosities based on the overlying stratum collapse law and the distribution law of three horizontal zones and three vertical zones. The second method sets gob porosity using UDF. In Section 3.2, the numerical simulation of porosity suggests that the distribution of porosity in the gob is relatively random and the studied coal seam is a near-horizontal coal seam. In addition, the hyperbolic tangent function compiled by the UDF method is symmetrical, which is more reasonable in practical applications. Therefore, this study adopts the UDF method to set the gob porosity. How to import the PFC3D simulation results into FLUENT software is the key to the application of this method. Because the porosity data in PFC3D are unit parameters and do not require interpolation or extrapolation transformation, a data transfer program can be compiled for one-way data communication between PFC3D and FLUENT. The FLUENT software can then obtain the porosity simulation results of PFC3D software. With the FLUENT software used as the main program of the calculation, the porosity data obtained with PFC3D are compiled into UDF files. It is worth noting that the calculations of the two programs are not conducted at the same time as they are connected. Instead, the numerical simulation of the PFC3D particle flow is carried out first to obtain the porosity parameters. The program is then compiled to import the porosity parameters into FLUENT. The gob numerical simulation of the 31,116 working face was aimed to determine the gas distribution law in the gob under variable mining conditions by calculations, to accurately obtain the flow field distribution in the gob, and to evaluate the 3D spontaneous combustion situation in the gob based on the dynamic distribution of oxygen in the gob. The simulation results are shown in Figure 7.  
As shown in Figure 7a, the oxygen concentration distribution in the gob is affected by the wind flow migration. Along the strike, the air seepage range in the gob shows a downward trend with the increase in gob depth. As can be seen from Figure 7b,c, in the dip direction, the seepage range of oxygen in the center of the gob is more obvious than those on the intake airway and return airway sides, and the oxidation zone reaches as deep as 351.6 m. From the view of the working face, the width of the oxidation zone shows a first decreasing and then increasing trend. The air seepage range on the return airway side is significantly higher than that on the intake airway side, contrary to the results of most studies. It can be explained that the GER process builds a flexible concrete formwork wall on the intake airway side in the gob and most of the air flow enters the entry. The air flow entering the intake airway is limited, and thus the air seepage range is significantly reduced. As shown in Figure 7d, in the vertical direction, the closer to the gob, the larger the oxygen seepage range, which can be explained with the simulation results of the overlying stratum collapse law using the PFC3D particle flow software. The overlying stratum collapses as the working face advances, which increases the porosity in the upper gob. Meanwhile, rocks and gangue are accumulated and compacted in the lower gob, resulting in lower porosity in the gob bottom. Therefore, the air seepage range in the upper gob is relatively larger. Figure 7: Oxygen concentration distribution in each axis (own elaboration). (**a**) Oxygen concentration distribution in gob. (**b**) Oxygen concentration distribution in yz-plane. (**c**) Oxygen concentration distribution in xz-plane. (**d**) Oxygen concentration distribution in z-plane.