Transient transport of air ingress and methane dilution in negative-pressure coal mine methane drainage pipelines

Pipeline defects in negative-pressure coal mine methane (CMM) drainage systems permit ambient air to enter under the pressure differential, causing flow redistribution, localized jet mixing, and methane dilution. To characterize the transient flow and species-transport behavior associated with air ingress, a three-dimensional unsteady, compressible, multicomponent computational fluid dynamics (CFD) model was developed for the CMM drainage network in the No. 16 mining district of the Xin’an Coal Mine. Model reliability was assessed through grid- and time-step-independence analyses, global mass-conservation checks, and comparison with field pressure and flow measurements under the baseline no-ingress condition, with maximum relative errors of 4.31% and 4.51% for static pressure and volumetric flow rate, respectively. Increasing the ingress-hole diameter from 50 to 120 mm intensified jet-induced momentum exchange and accelerated downstream propagation of the methane-depleted region. A greater local baseline pressure deficit produced a larger air-ingress flow rate, stronger methane dilution, and a shorter concentration–response time. Under a fixed 80 mm opening, larger pipes admitted a greater absolute air flow rate, whereas smaller pipes, associated with larger relative opening ratios d/D , exhibited stronger local mixing and faster concentration responses. Methane-concentration disturbances propagated predominantly along the main-flow direction, while upstream propagation was suppressed. These results indicate greater concentration–response sensitivity at downstream virtual monitoring locations under the investigated numerical conditions. Future work should extend the present framework to irregular and multiple ingress defects, actual sensor dynamics, and more complex pipeline-network configurations.