*2.8. Model Implementation*

The multi-scale reactor models were implemented in the FLUENT software. The computational domains were discretized using rectangular meshes refined in the inlet regions and adjacent to the walls. Spatial discretization was conducted by the second-order upwind differencing scheme. The pressure–velocity coupling was performed with the SIMPLEC algorithm. User defined function (UDF) of FLUENT was used to couple the heat and mass transport models and chemical kinetics to the reactor-level models. The inlet boundary was set as uniform velocity. The outlet boundary was set as constant pressure (operating pressure) and zero normal gradients of velocity, temperature and species mass fractions. A no-slip, adiabatic or thermally coupled boundary was imposed on the reactor walls depending on the reactor type. Axisymmetric boundaries were imposed on the centerline of the reactor. The convergence criteria were set as all residuals below 10−3. Simulations were performed on PC with Intel i9-9900X processors (Intel Corporation, Santa Clara, CA, USA) and 64 GB RAM (G.skill International Enterprise Co., Ltd., Taipei, China). The typical computation time under each operating condition is between 0.5 and 2 h.
