3.2.2. Assessment of the Shell-Side Flow Behavior

In order to verify whether the assumption of uniform velocity distribution over the flue gas duct cross-section was appropriate in the FEA-based computation discussed in the next section, a simplified 3-D CFD model of the duct was created in ANSYS Fluent [46]. Parameters of the model are listed in Table 5. To keep computational demand at a reasonable level, the bundles were replaced by porous zones. Additionally, the entire duct was split into several parts, and hexahedral cells were used whenever possible to further lower the cell count while maintaining acceptable mesh quality. After the necessary mesh adaptation, the final cell count was ca. 3.3 M (see the *y*<sup>+</sup> histogram in Figure 10). As for cell equiangle skewness, only 1231 cells (ca. 0.04% of the total number of cells) featured skewness greater than 0.6, of which 1227 fell between 0.6 and 0.7. The obtained contour plot of velocity magnitude just below the bottom bundle is then shown in Figure 11. This indicates that, although the velocity distribution was not entirely uniform (see also the pathlines in Figure 12), the non-uniformity was still at a reasonable level, which should not lead to significant inaccuracy in the calculated overall heat transfer rate.

**Table 5.** Parameters of the simplified 3-D CFD model of the flue gas duct.

**Figure 10.** Histogram of *y*<sup>+</sup> values of wall-adjacent cells in the model from Table 5. There were 275 cells with *<sup>y</sup>*<sup>+</sup> <sup>≤</sup> 30 (all of these but 12 were between 25 and 30, ca. 0.04% of the total number of wall-adjacent cells) and no cells with *y*<sup>+</sup> > 300.

**Figure 11.** Contour plot of velocity magnitude just below the bottom bundle, which was obtained using the CFD model from Table 5. The velocity distribution was not uniform (due to the flue gas duct being bent) and fluctuated in time.

**Figure 12.** Pathlines in the flue gas duct, which were colored by velocity magnitude. One can see that the flow behind the bend is pushed to one side as indicated in Figure 11 and that there are relatively large recirculation zones present.
