Pore-Scale Simulation of Hydrogen–Air Premixed Combustion Process in Randomly Packed Beds

Real geometric structures of randomly packed beds are modeled using discrete element software LIGGGHTS. The pore-scale computational region is screened and chosen with nonextreme local distortion, porosity, and other structural parameters, and in this region the distribution of spherical walls of pellets are representative and typical. The wall-adapting local eddy-viscosity (WALE) model and EBU-Arrhenius combustion model are used to simulate the ignition process in a closed cavity, and the calculated results are compared with the direct simulation results. The results show that the turbulence model and the combustion model used in this work are basically reasonable. Then the propagation of the hydrogen-air premixed flame and the interaction of flame with turbulence in the porous and nonporous structures during the ignition process are simulated and analyzed. Results show that the temperature propagating velocity in the porous structure is higher than that in the nonporous one, and the porous structure can make the temperature field more uniform. In the porous structure, at the initial ignition stage the temperature gradient is relatively high, and at the following time points, the top temperature gradients gradually decline and remain stable. The distances between adjacent temperature gradient tops have no obvious changes with the passage of time, and the flame propagation is faster in the porous structure. In the region near the ignition point, the vorticity in the porous structure is lower than that in the nonporous structure, in the region away from the ignition point, the vorticity in the porous structure is higher. In addition, the interaction of the flame and turbulence is quantitatively described by the Karlovitz number, and the flame regimes at different times are identified. Results show that the ignition process in the porous structure experiences two turbulent flame regimes: corrugated flamelets regime and thin reaction regime, which occur mainly in the thin reaction zone. In the nonporous structure two turbulent flame regimes are experienced too, thin reaction regime and corrugated flamelets regime, which occur mainly in the corrugated flamelets zone.