**6. Conclusions**

(50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%).

The heat transfer characteristics of water, single particle and hybrid nanofluids in uniformly tube under constant heat flux condition are compared. The heat transfer coefficient, Nusselt number, pressure drop, friction factor and performance evaluation criteria are investigated for various Reynolds number, volume fractions and nanoparticle compositions of hybrid nanofluid. The valuable findings of the present numerical study are summarized as follows.


**Author Contributions:** Conceptualization, K.S.G.; and M.-Y.L.; methodology, K.S.G.; and M.-Y.L.; formal analysis, K.S.G.; and M.-Y.L.; investigation, K.S.G.; and M.-Y.L.; resources, K.S.G.; and M.-Y.L.; data curation, K.S.G.; validation, K.S.G.; software, K.S.G.; writing—original draft preparation, K.S.G.; and M.-Y.L.; writing—review and editing, K.S.G.; and M.-Y.L.; visualization, K.S.G.; and M.-Y.L.; supervision, M.-Y.L.; project administration, M.-Y.L.; funding acquisition, M.-Y.L. 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 will be available on request to the corresponding author.

**Acknowledgments:** This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1A2C1011555).

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