## 5 Conclusions  
This study experimentally investigated the forced convective heat transfer of TiO\({}_{2}\)-W NFs in a CFI. The effects of the NP concentrations and the flow Reynolds numbers on the heat transfer performance of NPs were evaluated. The following conclusions were obtained:  
* Over particular ranges of NP volume concentrations and Reynolds numbers, the heat transfer was considerably higher in the proposed system than in the base fluid (i.e., water) flowing through the CFI and potentially superior than in water flowing in a helical coiled tube and TiO\({}_{2}\)-W NFs in a straight tube. * NP volume concentrations of 0.2, 0.5, 1.0, and 1.5 \(v\)/\(v\)% were studied at Reynolds numbers ranging from 1400 to 9500. The heat transfer was enhanced in the \(6000\leq N_{Re}\leq 9500\) and at all NP concentrations, except for 1.5 \(v\)/\(v\)%. At the latter concentration, it is theorized that the higher viscosity of the NF than that of the base fluid outweighed the thermal conductivity advantage of the NF. * The most significant \(N_{Nu}\) increase of the NF in the CFI was 41-52% relative to water, obtained for the 1.0 \(v\)/\(v\)% NF in the higher Reynolds number range (i.e., \(6000\leq N_{Re}\leq 9500\)). At lower Reynolds numbers (i.e., \(1400\leq N_{Re}<6000\)), the CFI geometry failed to enhance the heat transfer of the NFs (relative to water) at any concentration. * From the experimental results of the TiO\({}_{2}\)-W NFs in the CFI, two new empirical equations for \(N_{Nu}\) prediction were proposed. The correlations are valid in the \(1400\leq N_{Re}\leq 9500\) range of Reynolds numbers, the \(4.5\leq N_{Pr}\leq 5.2\) range of Prandtl numbers, and NP volume fractions of \(0.002\leq\phi\leq 0.010\) and \(0.002\leq\phi\leq 0.015\). The latter was built using previous theoretical findings for CFI, and the enhancement was added considering the limiting effect of having excess NPs. * The physical and transport properties of the TiO\({}_{2}\)-W NFs were measured experimentally in the concentration domain of 0.2-1.5 \(v\)/\(v\)% and with temperature varied from 25 to 45 \({}^{\circ}\)C. The experimental results indicated that the classical theoretical models to estimate the thermophysical properties of NFs do not accurately predict the data for the TiO\({}_{2}\)-W NFs used in this study. This work should assist the determination of the flow ranges that will benefit from the CFI geometry and impose practical limits on the NP volume concentrations when enhancing the heat transfer is the objective. It also extends the state-of-the-art experimental methods for enhancing heat transfer in the CFI geometry to TiO\({}_{2}\)-W NFs as the working fluid, which is of great current interest. Finally, this work highlights the potential importance of chaotic advection in thermal mixing applications of TiO\({}_{2}\)-W NFs. Conceptualization, methodology, and supervision: J.L.L.-S. and A.J.G.-C. Investigation, visualization, and writing (original draft): B.A.-T. Data curation, formal analysis, resources, software, and writing (review and editing): B.A.-T., J.L.L-S., and A.J.G.-C. Validation: B.A.-T. and J.L.L-S. Funding acquisition and project administration: A.J.G.-C. All authors have read and agree to the published version of the manuscript. This research was funded by Conacyt and SENER through the Sustainable Energy Fund, grant number CEMIE-Sol P09. **Acknowledgments:** This work was supported by Tecnologico de Monterrey through the Energy and Climate Change Research Group and NOVUS initiative, and Conacyt and SENER through the Sustainable Energy Fund and CEMIE-Sol Project P09. **Conflicts of Interest:** The authors declare no conflict of interest. **Nomenclature**