**8. Conclusions**

New heat transfer correlations for the air-side Nusselt number have been proposed for four-row PFTHE. Consideration of individual correlations on individual rows of heat exchanger tubes can improve designing of the PFTHEs. In the analysed range of air velocities in front of the heat exchanger from 0.3 m/s to 2.5 m/s, the average HTC for the first tube row has the highest value. In the subsequent tube rows, the HTC is lower compared to the HTC on the first tube row. This paper also presents a simple method for the determination of Nusselt number correlation for individual rows, which can reduce or even eliminate experimental research. The adoption of a constant external surface temperature for the ribs and tubes simplifies the determination of HTC on individual tube rows. It is shown that, for constant fin and tube surface temperatures of 60 ◦C, 70 ◦C, and 80 ◦C, the resulting HTC values for the same airflow velocity differ slightly. However, CFD simulation must be precisely conducted with a proper finite volume mesh and turbulence model for turbulent flows. CFD modelling is an effective tool for determining individual correlations for calculating the air-side Nusselt number on the individual tube rows of the exchanger.

**Author Contributions:** Conceptualisation, D.T., J.T.; methodology, software, validation, formal analysis, investigation, writing—original draft preparation, M.M.; resources, data curation, M.M., K.W.; writing—review and editing; supervision, D.T., J.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
