## 4 Conclusions  
This study is focused on evaluating the effectiveness of the Mouromtseff number to predict the thermal performance of nanofluids by comparing _Mo_ ratios with experimental coefficients of convective heat transfer of new dispersions based on A-TiO\({}_{2}\) in a PG: W 30:70 vol% binary mixture. The A-TiO\({}_{2}\) nanoparticles were carefully designed and characterised by means of TEM and XRD, finding the pseudo-spherical shape, tetragonal crystalline structure (anatase), and a mean particle size of 21 nm. The effect of different surfactants, namely PEI, PSS, CTAB, and PVP, on the stability of A-TiO\({}_{2}\) nanofluids at different surfactant-nanoadditive mass ratios was analysed by DLS. These results clarified that the optimum surfactant for the proposed nanofluids is PSS at the 1:4 surfactant-nanoadditive mass ratio. The thermophysical and rheological profile of the A-TiO\({}_{2}\)/PG:W 30:70 + PSS 1:4 nanofluids were experimentally obtained. Slight enhancements in thermal conductivity of 1.5% were found for the 2.0 wt% A-TiO\({}_{2}\)/PG:W 30:70 + PSS 1:4 nanofluid. Density increases with nanoparticle loading, reaching 1.3% maximum average increments, whereas average decreases up to 12.9% of isobaric heat capacity values were observed for the highest nanoparticle mass concentration. Regarding the rheological profile, increases in dynamic viscosity with increasing nanoadditive content were noticed, assessing increments of 8.8% for the 2.0 wt% A-TiO\({}_{2}\)/PG:W 30:70 + PSS 1:4 nanofluid while average decreases of 69.9% were found for all analysed samples as the temperature increased. The thermal performance of the 0.25, 1.0, and 2.0 wt% A-TiO\({}_{2}\)/PG:W 30:70 + PSS 1:4 nanofluids was evaluated experimentally by determining the experimental \(h_{nf}/h_{bf}\) ratio through a tubular heat exchanger and through the \(Mo_{nf}/Mo_{bf}\) ratio obtained by using the Dittus-Boelter and Simons expression. Decreases up to 24% and 7% of the \(h_{nf}/h_{bf}\) and \(Mo_{nf}/Mo_{bf}\) ratios, respectively, were obtained for 2.0 wt% A-TiO\({}_{2}\)/PG:W 30:70 + PSS 1:4 nanofluid. Both ratios were in agreement, suggesting that the designed nanofluids do not improve the thermal performance of the base fluid. Nevertheless, the effect of the mixing phenomenon in the convective heat transfer performance of nanofluids in turbulent flow, along with the uncertainties derived from the experimental measurements, might cause deviations between the Figures-of-merit, the experimental convective heat transfer coefficients, and the pressure drop values. Hence, this work demonstrates that the Mouromtseff number ratio criteria can be a useful tool for discerning the potential advantages or disadvantages of using TiO\({}_{2}\) nanofluids. However, experimental tests like the one presented here are still needed to accurately determine the performance magnitude in tubular flow. U.C. : conceptualization, methodology, investigation, data curation, formal analysis, writing--original draft, writing--review and editing. J.I.P. : investigation, writing--original draft, writing--review and editing. J.P.V. : conceptualization, supervision, writing--review and editing. L.L. : conceptualization, project administration, validation, supervision, resources, writing--review and editing. All authors have read and agreed to the published version of the manuscript. This research was funded with Grant PID2020-112846 RB-C21 by MCIN/AEI/10.13039/ 501100011033 and Grant PDC2021-121225-C21 by MCIN/AEI/10.13039/501100011033 and by \"European Union NextGenerationEU/PRTR\". Not applicable. The authors declare no conflicts of interest. The thermal performance of the 0.25, 1.0, and 2.0 wt% A-TiO\({}_{2}\)/PG:W 30:70 + PSS 1:4 nanofluids was evaluated experimentally by determining the experimental \(h_{nf}/h_{bf}\) ratio through a tubular heat exchanger and through the \(Mo_{nf}/Mo_{bf}\) ratio obtained by using the Dittus-Boelter and Simons expression.