## 5 Conclusions  
This study uses the method of combining on-site monitoring and numerical simulation to establish a three-dimensional numerical model of the goat, master the collapse law of the overlying strata and the evolution law of porosity during the mining process of the ultra-long working face, elaborate the space-time evolution law of the three-dimensional flow field in the goat of the ultra-long working face, and combine the distribution of oxygen and residual coal to accurately divide the risk area of spontaneous combustion in the goat of the ultra-long working face.  
1. The oxygen concentrations on the intake airway and return airway sides and the middle gob were measured on site using the high-pressure rubber hoses embedded in the working face. The results show that 1# and 2# measuring points enter from the heat dissipation zone to the oxidation zone as they advance to 136.4 m and 158.1 m, respectively. The 3# measuring point does not enter the oxidation zone until advancing to 210.2 m because it is close to the return airway. The 4# measuring point evolves to the oxidation zone at the depth of 160.1 m, where the oxygen concentration drops from 19.29% to 17.85%.
2. The collapse of overlying stratum and the evolution distribution of the porosity in the gob were explored by numerical simulation using the PFC3D particle flow software. The results suggest that the growth of fractures in each rock formation slows down after caving but the number of fractures continues to increase. After the stratum collapse is completed, the collapsed rocks continue to accumulate and, eventually, the number of fractures is reduced to a stable value. The porosity becomes stable at \(\sim\)0.45, \(\sim\)3.5 times that of the unmined coal seam. The gob enters the spontaneous combustion hazardous zone at 160 m behind the working face.
3. The simulation using FLUENT software reveals the distribution law of gob flow field. The wind flow field shows an arch-shaped distribution with a swirl shape on the intake airway side. As the working face advances, the wind flow gradually becomes  
Figure 9: Distribution of coal spontaneous combustion hazardous zone in the gob (own elaboration).  
weaker and the air seepage range declines in the strike direction. In the dip direction, the air seepage range on the return airway side is obviously higher than that on the intake airway side. In the vertical direction, the flow field range in the upper gob is larger than those in the middle and lower gob.
* Residual coals are mainly distributed in the two consecutive roadways and the points of slope gradient change. The oxidation zone is determined to be in the range of 140.4-313.3 m on the intake airway side, 201.2-351.6 m in the middle of the gob, and 153.2-328.1 m on the return airway side. The hazardous zone of residual coal spontaneous combustion is then drawn by superimposing the oxygen concentration distribution and residual coal distribution, with the former as the main factor.  
All authors contributed to this paper. Conceptualization, S.X. and G.W.; methodology, E.W.; validation, S.X., G.W. and E.W.; formal analysis, Q.H. and M.X.; investigation, S.X.; writing--original draft preparation, S.X.; writing--review and editing, G.W.; supervision, Q.H.; funding acquisition, G.W. All authors have read and agreed to the published version of the manuscript.  
The authors would like to acknowledge the support of the project of National Natural Science Foundation of China (No. 51974176, No. 52174194, No. 51934004), Taishan Scholars Project (TS20190935), Shandong outstanding youth fund (ZR2020 JQ22).  
The data used for conducting classifications are available from the corresponding author upon request.  
The authors declare no potential conflict of interest with respect to the research, authorship, and publication of this article.