Pore-scale lattice Boltzmann simulation of capillary-driven phase-change transpiration cooling in heterogeneous porous media

The capillary force plays a crucial role in phase-change transpiration cooling within the porous media; however, its role at the pore scale is still less studied. In this study, the capillary-driven phase-change transpiration cooling in heterogeneous porous media is initially investigated at the pore scale using a pseudo-potential phase-change lattice Boltzmann method with a modified wetting boundary scheme. The influences of heat flux, surface wettability, and porosity on the liquid fraction, coolant mass flow rate, and dimensionless temperature rise are analyzed. In terms of the dynamic characteristics of phase-change transpiration cooling, the porous layer reaches thermal equilibrium through the combined effect of the evaporation and the capillary-driven self-replenishment of the coolant. For the heat flux, the coolant mass flow rate increases as the heat flux increases below a critical value. Once the heat flux exceeds this critical value, the capillary-driven pumping capability is suppressed, leading to a reduced mass flow rate and a significant decrease in the liquid fraction. For the surface wettability, hydrophilic surfaces increase the coolant mass flow rate, resulting in a higher liquid fraction and a lower temperature rise compared to less wetting surfaces. For the porosity, the lowest equilibrium surface temperature rise is achieved at ε = 0.5, while deviations from this value lead to higher surface temperature rises. At higher porosities, the surface temperature rise increases gradually and rises sharply at ε = 0.8. These findings provide mechanistic insight into the pore-scale capillary effects within the phase-change transpiration cooling systems.