Enhancing mine safety: Experimental validation of friction factor for airflow through broken rocks

Spontaneous combustion, the auto-ignition of coal without an external heat source, is a major challenge in coal mining. Most mine fires result from this process, known as “spontaneous heating” or “auto-oxidation,” where coal reacts with oxygen at normal temperatures. As heat accumulates, oxidation accelerates, potentially leading to fires. Understanding gas and air movement through porous media is crucial but challenging in underground coal mines. Air leakage through broken rocks into gob areas, combined with methane emissions, raises safety concerns. Leakage from ventilation systems exposes coal to oxygen, initiating slow oxidation and heat build-up, which can lead to combustion. Thus, studying airflow through porous zones is essential for the mine safety. However, measuring flow resistance in broken rocks is difficult due to their complex structure and mining safety constraints. In a previous study, one developed a novel friction factor model for airflow through broken rocks using a 3D pore-scale CFD model. This model was integrated into mine ventilation software like Ventsim and successfully validated against experimental data from a packed bed of 25 mm rocks. Current research extends this validation to larger rock diameters. We designed an experimental setup to measure system characteristic curves in a packed bed of 55 mm spherical rocks. Preliminary results confirm that our friction factor model aligns well with experimental data for larger rock sizes. This validated porous friction factor will be incorporated into Ventsim, enabling engineers to calculate airflow and gas leakage in gob areas much faster while keeping the accuracy, enhancing underground mine safety and efficiency. Furthermore, it was observed that air leakage from mine ventilation into the gob area can reach up to 45%, transporting methane gas produced by residual coal and resulting in up to 10% mass fraction of methane accumulated in the exhausted areas. These findings emphasize the importance of accurate modeling to mitigate the risk of gas accumulation and spontaneous combustion.