Researchers at the Colorado School of Mines have developed a full-scale, 3D longwall ventilation model capable of modeling methane gas explosions analyze explosion hazards in longwall coal mine gobs through computational fluid dynamics (CFD) modeling. One area of concern is the active gob directly behind the longwall face, where high concentrations of methane are likely to accumulate and active roof caving still occurs. Most researchers have represented the entire gob as a porous medium that is governed by Darcy's law. While this assumption may be applicable for the consolidated center of the gob with significantly lower permeability and porosity compared to the surrounding edge area, Darcy-type flow may not apply directly behind the longwall shields and in the unconsolidated fringes of the gob along the gate roads. CFD model studies performed on longwall face airflow indicate that significant amounts of ventilation air leak from the face to the gob. This leakage is governed by gob permeability characteristics and caving conditions, especially near the headgate and tailgate corner areas. The study presented here adopts a hybrid approach to modeling the gob: the outer part of the gob is modeled as discrete objects that simulate coarse rock rubble, while the gob center is modeled as a porous medium. CFD simulations using 3m and 1.5m diameter obstacles in a cubic packing arrangement were found to adequately capture the bulk flow patterns commonly observed in longwall operations. Although both 3m and 1.5m cases have the same porosity, the reduction of the void space with the 1.5m obstacles reduced the amount of face leakage. Modeling results clearly show that merely matching the gob porosity values in the CFD model is not sufficient, as other parameters such as the packing arrangements and obstacle sizes can significantly change the resulting flow resistance.
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