**5. Conclusion**

The heat flux depends on the temperature difference between the local plate/tube and local air temperatures. In reality, these temperatures vary along the cross section of the air stream and along the fluid flow direction. All results are calculated considering the air flow and its streamline deviations caused by the plate and tube configuration and compared with the known correlation for circular fins of rectangular cross section. The model allows considering the heat transfer in three directions. This is an advantage, comparing to other optimization method, where the temperature profile is two-dimensional. The shape of the fin and tube is modified to calculate the heat transfer for different conditions, reduce the total mass that refers to the cost of the whole heat exchanger.

Described phenomena modify the conditions of the heat exchange between the plate and the fluid having the effect on the heat transfer. The rate of the heat transfer does not depend only on wall surface dimensions, heat transfer coefficient and the temperature difference between the fluid that surrounds the plate and the plate surface temperature. The air velocity and the fin shape are also essential because the fin profile influences the flow direction. For heat exchangers, built with many fins and designed for real industry, it is important to pay attention to and calculate the heat transfer considering the fluid flow and flow paths.

It should be also mentioned that if the fin is positioned into an air stream, the flow applies to a force from the fin tip surface in the direction of the oncoming flow (drag). The resistance of the body results in a pressure drop. The fin and tube surface orientation also modifies the resistance of the body that results in a pressure drop.
