Thermo-hydraulic analysis of time-dependent anisotropic characteristics of crushed coal subjected to varied axial displacements

This study is aimed at investigating the transient anisotropic characteristics of coupled seepage and heat transfer in compressed crushed coal with complex void structures within abandoned mine reservoirs. To achieve this aim, multiple computed tomography (CT) scans were conducted first during the axial compression process to reconstruct and model the structural evolution of the crushed coal under varying compression displacements. Subsequently, the reconstructed models were incorporated into finite element simulations to compute transient dimensionless parameters related to fluid flow and heat transfer. The results show that as axial compression of the crushed coal intensifies, topological changes in the seepage channels lead to higher steady-state flow velocities and a more concentrated vortex distribution within the coal matrix. Moreover, the anisotropic differences in both thermal breakthrough time and the heat extraction rate at the breakthrough moment gradually decrease. The heat transfer coefficient during the compression process ranges from 667.51 W/(m 2 ·K) to 1062.05 W/(m 2 ·K). The anisotropy factor of Reynolds number ( Re) reaches a maximum of 0.19, while Nusselt number ( Nu ) varies between 1.17 and 1.71. The Nu-Re- Prandtl number ( Pr ) evolutionary surface during the coupled flow and heat transfer process follows a nonlinear trajectory. Overall, the findings provide a theoretical foundation for optimizing geothermal energy extraction from abandoned coal mines. • The compression of crushed coal will cause an increase in steady-state flow velocity and a more concentrated vortex distribution. • The difference in the moment of thermal breakthrough in each flow direction decreases with the increase of compression level. • The heat extraction rate at the moment of thermal breakthrough decreases with the increase of compression level. • The Nu-Re-Pr evolution surface during the flow and heat transfer process exhibits a nonlinear trajectory.