**5. Conclusions**

This paper presents a numerical investigation of the forced heat transfer of a ionic liquid [C4mpyrr][NTf2] and ionanofluids with Al2O3 nanoparticles through a straight tube. Geometric and physical models were defined that adequately describe the problems, as well as the numerical grid on which the simulations were performed in the STAR-CCM + 2019.2 software. Numerical research was undertaken with the following approximations: the effects of natural convection were neglected, and fluid flow through a part of the tube

in the shape of a longitudinal wedge with angle of 5◦ was analyzed. Thermophysical properties of the analyzed ionic liquid and ionanofluids were assumed to be a function of temperature. The adequate equations for each thermophysical property were gained via curve-fitting the experimental values with square of correlation factors above 0.95 for density relations, 0.99 for viscosity relations, 0.98 for heat capacity relations and for a ratio of thermal conductivity above 0.8, regardless of weight percent. In order to obtain grid independent solutions, a grid sensitivity study was performed, and the model was validated with the results of Shah's equation. The presented analysis of the obtained results of the Nusselt number and the heat transfer coefficient of the ionic and nanoionic liquid shows the change in the considered parameters along the tube for different wt% and different Reynolds numbers. During the analysis of the numerical results of the Nusselt numbers and the heat transfer coefficient of both the ionic and nanoionic liquid, a decrease in the Nusselt numbers and the heat transfer coefficient with a decrease of a wt% was found. Moreover, it is concluded that an increase in the Reynolds number results in an increase in both heat transfer coefficient and Nusselt number. The Nusselt number and the heat transfer coefficient of the ionic and nanoionic liquid decrease exponentially along the tube for any case under consideration. The results were compared against the results related to the ionic liquid and ionanofluids with constant thermophysical properties. It is concluded that the assumption that thermophysical properties are constant has a significant influence on the heat transfer performance parameters of both ionic liquid and ionanofluids, and therefore such assumptions should not be made in research.

**Author Contributions:** Conceptualization, A.H. and J.H.A.; methodology, A.H.; software, A.H. and J.H.A.; validation, A.H., S.B. and E.D.; formal analysis, J.H.A.; investigation, A.H. and J.H.A.; resources, J.H.A.; writing—original draft preparation, A.H.; writing—review and editing, S.B. and E.D.; visualization, J.H.A.; supervision, E.D.; project administration, A.H.; funding acquisition, A.H.. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Canton Sarajevo, Ministry for Science, Higher Education and Youth, grant number 27-02-11-4375-6/21. The APC was funded with that grant.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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