**4. Mesh Parameters and Mesh Independent Study**

Mesh characteristics are presented in Table 1. The division of the modelled domain into finite elements considers mesh compaction at the junction of the air and fin wall surface and air and tube wall surface (Figure 2b).

**Table 1.** Specification of the finite element mesh for the model presented in Figure 2b.


All simulations for the entire range of air velocity were preceded by mesh independent studies, separately for all considered air velocities. The final mesh was selected only for the most favourable simulation. Temperature stabilisation for air velocity—4 m/s for the

fourth row of PFTHE—is shown in Figure 3. It was observed that, from air velocity in front of PFTHE, more than 2.5 m/s temperature stabilisation is not much different in comparison with temperature stabilisation in the fourth (last) row of PFTHE. Temperature stabilisation showed that, between chosen mesh element size and mesh with almost 30 million elements, relative differences were less than 1% for the fourth row of PFTHE (Figure 3). Moreover, for the first row, even for 10 m/s relative differences between chosen mesh and mesh with over 30 million, elements are less than 0.1% (Equation (1). This shows that, in the considered scenario, the greatest irregularity of flow and turbulence exists in the last row of PFTHE.

**Figure 3.** Temperature stabilisation for mesh independent study from the fourth row of PFTHE for 4 m/s air velocity in front of the heat exchanger.

A mesh with 6,790,572 elements was selected. Mesh elements' size belongs to the following CFX mesh sizes: mesh element size: 0.15 mm; mesh max size: 0.25 mm (Table 1). The whole mesh contains cuboidal elements only (Figure 1b).
