Active isolation of methane–coal dust deflagration by expandable graphite through restructuring of wave-induced reactive flow

Methane–coal dust deflagrations involve coupled flame acceleration, pressure/compression-wave propagation, dust entrainment, and transient flow-field evolution. This study investigates active isolation of methane–coal dust deflagrations in a duct apparatus using expandable graphite (EG), N2, NH4H2PO4, and an EG/NH4H2PO4 composite suppressant. EG and the composite system both achieved near-field flame termination, with the flame extinguished within 0.9–2.5 m downstream of the isolator, whereas N2 mainly reduced propagation intensity without rapid quenching. Schlieren-based image velocimetry showed that EG reduced the apparent velocity of the wave-induced flow and formed a particle-rich low-velocity region near the duct bottom, spatially coinciding with the flame-restricted zone. Proper orthogonal decomposition analysis showed that EG decreased the first-mode energy ratio from 55.56% to 13.93% and the cumulative energy of the first three modes from 76.21% to 25.47%, indicating weakened large-scale coherent structures. Spectral analysis further showed that the integrated spectral energy under EG decreased to 2.574 × 10−3 of the baseline value, corresponding to a 99.74% reduction. These results indicate that EG-based active isolation arises from coupled flame-retardant action and gas–solid flow restructuring, which disrupts the hydrodynamic support for sustained flame propagation.