**7. Discussion of the Obtained Results**

Nusselt number correlations for considered PFTHE (Figure 1) have been determined using the CFD simulation data. New individual correlations (Figures 7 and 8) for particular rows are significantly different from the average Nusselt number correlation for the entire PFTHE (Figure 6), also determined by the CFD simulation. Parameter *x2* in Equation (7) determined by the least-squares method is compatible only for the first row with parameters known in the literature [2], while in the second row, there is a 46% discrepancy compared to the parameter *x2* found by [2] for the two-row car radiator. The heat transfer correlations for the last two rows, the third, and fourth, are new and there are no existing correlations in the literature for comparison.

Additionally, Nusselt numbers fall between Reynolds numbers from 140 to 500, although they also increase from 500 to 1400. This increase may occur when the air stream does not separate away from the tube for low air velocity (0.3 m/s). The Nusselt number value is larger for higher air velocity (0.5 m/s–0.7 m/s) where the detachment of the air stream from the tube surface occurs.

The determined heat transfer correlations showed that the third row is the least efficient. It has been noticed that, for the entire air velocity range, the Nusselt number falls to the third row and then increases in the fourth row. This could mean that airflow in the penultimate row is less turbulent or airflow has formed only a narrow flow channel, leaving a wide heat transfer dead zone in front of and behind the tubes in the third row.

Experimental verification of derived CFD-based correlations has not been carried out in this paper. Experimental verification is planned as an extension of this research. The future of this research topic also considers a wider Reynolds number range, different tube and fin pitches, and tube diameters of PFTHE.
