The Seepage Characteristics of Unsteady Gas-Phase CO2 in Loose Broken Coal: An Experimental Investigation

Understanding the seepage behavior of unsteady gas-phase CO2 within loose broken coal is crucial for evaluating the effectiveness of fire prevention and control strategies against coal spontaneous combustion in goafs. However, the underlying seepage mechanisms remain insufficiently understood, often leading to suboptimal control performance or local CO2 concentration exceedance, thereby limiting its practical engineering application. To address this issue, this study developed a seepage experimental system to explore the seepage behavior of unsteady gas-phase CO2 in loose broken coal under different working conditions, and analyzed the influence of flow rate and particle size on key seepage parameters. The results indicate that when unsteady gas-phase CO2 velocity exceeds 4 cm·s−1, seepage transitions from Darcy to non-Darcy flow. The permeability coefficient rises markedly with coal particle size, with a rapid increase near 2.81 mm. As particle size grows, transition velocity gradually decreases and stabilizes, while the critical Reynolds number increases monotonically. For a given particle size, both permeability coefficient and transition velocity follow the order (10 kPa, −20 °C) > (20 kPa, 40 °C) > (10 kPa, 40 °C), whereas the critical Reynolds number is independent of CO2 physical parameters. A seepage equation for unsteady gas-phase CO2 in loose broken coal is constructed and verified based on Darcy’s law and Forchheimer’s theory. This work provides a theoretical basis for predicting CO2 fire prevention performance and optimizing engineering parameters in goaf fire control.