Dynamic Formation and Coupled Evolution of Oxygen-Deficient Conditions at the Return-Air Corner in a Shallow-Buried Coal Seam Working Face.

To elucidate the dynamic formation and evolution of oxygen-deficient conditions at the return-air corner during the advancement of the No. 45207 working face in the Sandaogou Coal Mine, programmed temperature-rise experiments, field tube-bundle monitoring, and three-dimensional numerical simulations were conducted. The programmed temperature-rise experiments characterized O2 consumption and CO generation during the low-temperature oxidation of residual coal and provided the experimental basis for defining the species source terms in the numerical model. To overcome the limitation of conventional static models in representing the continuous evolution of the working-face position and goaf geometry, a dynamic goaf model was developed in Fluent using the dynamic-mesh layering method. The model continuously updated the working-face position and goaf extent during advancement and was compared with a conventional static model. Field-monitoring results showed that the O2 concentration at the return-air corner decreased from approximately 20% to 3.2% as the working face advanced, with pronounced oxygen-deficient conditions developing at an advancing distance of approximately 110 m. The numerical results further indicated that the oxygen-deficient zone migrated from the deep goaf toward the return-air side and expanded rapidly when the advancing distance reached approximately 110 m. Meanwhile, the region with O2 concentrations of 5%-15% gradually migrated toward the working face, accompanied by CO transport and accumulation along the return-air side. Comparison with field-monitoring data showed that the dynamic-model results agreed more closely with the measured O2 and CO concentrations along the return-air side than those of the static model, reducing the corresponding mean absolute errors by 45.94% and 52.16%, respectively. These findings demonstrate that the dynamic model can more effectively represent the evolution of goaf geometry, leakage airflow, and gas-transport conditions induced by working-face advancement, thereby providing a basis for identifying oxygen-deficient zones in the goaf and controlling abnormal CO accumulation at the return-air corner.