(e) Boundary conditions

Table 2 shows boundary condition types selected for the model. A commonly employed porous medium approach was used for the monolith, with a one-directional pressure gradient. Boundary conditions values are presented in Table 3.


**Table 2.** Boundary conditions included in the model.

**Table 3.** Measured temperatures, mass flow values and outlet pressure. These values have been employed as boundary conditions for the model.


A total amount of seven species are considered in gas composition (see Table 4). Oxidations of carbon monoxide and propylene, as explained in the previous section, are accounted for.


**Table 4.** Modeled species and volumetric fraction range in gas inlet composition within the test matrix.

(f) Boundary layer sensitivity analysis.

A mesh with a special refinement (three layers) near the walls and other without it were tested. Only a part of the domain was used (DOC and a section of the exhaust pipe) in order to reduce computational requirements. As can be seen in Table 5, refinement along the boundary layer adds a considerable number of extra cells and does not provide significant variations.


**Table 5.** Boundary layer sensitivity analysis results.

Boundary layer refinement results differed from the base case by less than 1% and required five times more cells. It can be stated that the model is not boundary layer-sensitive and, hence, boundary layer refinement is omitted.

(g) Wall treatment sensitivity analysis.

Three different wall approaches were tested to study their influence in the results of the model: standard, non-equilibrium and enhancement wall functions (Figure 8).

The standard wall functions proposed by Launder and Spalding [37] have been widely used, but the assumption of logarithmic velocity distribution treatment may not be adequate for complex non-equilibrium flows. To overcome this, non-equilibrium wall functions are based on pressure-gradient sensitized Launder and Spalding's [37] log-law for mean velocity.

Enhanced wall treatment is a near-wall modelling method that combines a two-layer model with enhanced wall functions. A one-equation relationship is used to evaluate the laminar sub-layer with fine mesh and transition to log-low function for the turbulent part of the boundary layer. The restriction that the near-wall mesh must be suitably fine might impose large computational requirements.

**Figure 8.** Results of test runs with different wall functions. No grea<sup>t</sup> disagreement between the three approaches was obtained.

More accurate wall functions such as non-equilibrium or enhanced showed no more than a 3% discrepancy with the standard approach, while requiring eight times more mesh elements. Consequently, the standard wall functions were selected.
