Construction of multiphysics coupling equations and study on flow field migration patterns in shallow buried goaf under air leakage conditions

Shallow depth, near-level coal seams in the mining process mining fissures directly to the surface, by the atmospheric pressure, negative pressure ventilation and repeated mining and other factors, resulting in serious surface air leakage. Taking the 45,206 mining face in Sandaogou Coal Mine as the research object, this study aims to understand the migration patterns and influencing factors of flow fields in shallow buried goaf. The existence of air leakage channels was verified through SF₆ gas release experiments conducted at both the surface and working face. A CFD numerical simulation model of the goaf was established using COMSOL Multiphysics software. Coupled equations were constructed based on physical field interaction analysis and modified according to field conditions. Multiple physical fields including Porous Media Fluid Flow (FP), Heat Transfer in Porous Media (HT), Chemical Reactions (CHEM), and Transport of Diluted Species (TDS) were coupled to simulate the migration patterns of characteristic gases within the goaf under both sealed and surface leakage conditions. The research analyzes the variation patterns of temperature, pressure, and gas composition within the goaf under multi-parameter coupled physical field conditions. Through comparative analysis of goaf simulations under conditions with and without surface air leakage, the research findings reveal that surface air leakage not only triggers abnormal gas emission in the goaf but also frequently induces hypoxic conditions at the return airway corner. Driven by the pressure differential between surface and underground environments, atmospheric air infiltrates the goaf through fissures, elevating oxygen concentrations within the goaf. Concurrently, the leakage airflow causes heat accumulation in deeper goaf regions, heightening the risk of spontaneous combustion of residual coal. Localized pressure peaks emerge within leakage zones, where the pressure distribution shifts from unidirectional competition to multivariate competition, generating airflow vortices that disrupt fresh airflow circulation in the goaf. The research results can provide a theoretical basis for low-oxygen management and spontaneous combustion prevention in mining faces with similar geological conditions.