Research on the distribution law of flow field in goaf before and after injecting low temperature nitrogen of layered mining of near-vertical coal seams

To understand the flow field evolution in a composite goaf under layered mining of near - vertical coal seam conditions,this study investigates the impact of upper layer mining on the distribution of gas and temperature fields in the goaf of the lower layer working face. Researchers first established a three - dimensional numerical model and used PFC particle flow simulation software to simulate the collapse of the overlying rock and the distribution of porosity in the goaf. Subsequently,the porosity distribution data were imported into CFD using UDF functions to perform numerical simulations of the flow field evolution in the composite goaf under non - nitrogen injection and low - temperature nitrogen injection conditions. The study predicted the distribution of spontaneous combustion “three zones”,analyzed air leakage conditions,and examined the dynamic evolution of the flow field and temperature field under low-temperature nitrogen injection. Finally,the simulation results were validated against field measurements of CO concentration and temperature variations in different areas. The results show that areas with higher porosity were mainly concentrated at both ends of the original intake and return airways in the horizontal sectional goaf. High-oxygen concentration regions in the composite goaf were mainly located near the intake and return airways, with a general trend of decreasing oxygen concentration towards the deeper parts of the goaf. In the upper layer goaf,high-oxygen concentration regions were mainly located near the stop line and the upper part of the intake and return airways. After injecting low-temperature nitrogen,the width of the heat dissipation zone in the goaf decreased by 1. 7 m to 6. 4 m,and the width of the oxidation zone decreased by 9. 6 m to 12. 2 m. The closer to the nitrogen injection point,the greater the temperature reduction. Field observations show that the width of the oxidation zone in the current layer significantly decreased,the temperature at the return air corner dropped by 16. 6 ℃,and the CO volume concentration gradually decreased from a maximum of 16. 5 μ L / L to 0 μ L / L. In the upper layer return air corner,the temperature decreased from an average of 32. 1 ℃ to 20. 1 ℃,and the CO volume concentration dropped from 14 μ L / L to 0 μ L / L. The error between the field test results and the simulation results was less than 5%,verifying the feasibility of the research method.