## 1 Introduction
[9] presented the Mouromtseff number ratios for nanofluids of quasi-spherical zirconium oxide (ZnO) nanoparticles suspended in an aqueous ethylene glycol mixture at 50 vol% at three nanoparticle concentrations up to 5 wt%. The sample at 1 wt% showed the highest _Mo_ number ratio, indicating that it should exhibit the best thermal performance. However, they found no improvement in the heat transfer coefficient at 1 wt% with regard to the base fluid. Moreover, [PERSON] et al. [10] found differentiated conclusions depending on the flow regime. They compared the ratio _Mo\({}_{nf}\)_/_Mo\({}_{bf}\)_ to the thermal efficiency of water-based nanofluids with different suspended nanomaterials (Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), SiO\({}_{2}\), polystyrene, graphene nanoplatelets, and single-wall carbon nanotubes). For laminar flow, results from the _Mo_ ratio totally agree with the experimental thermal efficiencies, whilst for turbulent flow, they did not fully correspond. They concluded that the _Mo_ number criterion is more representative of FOM than the thermal efficiency for flat plate solar collectors. It is indeed clear that the use of _Mo_ numbers to evaluate nanofluids for heat transfer applications is widespread in the literature. Nevertheless, no further discussions on the causes for the controversy between the analysis based on _Mo_ numbers and based on heat transfer coefficients and _Nu_ numbers can be found. Several authors have reported better heat transfer performance of common base fluids than the derived nanofluids in turbulent forced convection. [PERSON] and [PERSON] [11] analysed the pressure drop ratio, h/\(\Delta\)P, and convective heat transfer coefficients of various nanofluids (PCM, Al\({}_{2}\)O\({}_{3}\), CNT, Al\({}_{2}\)O\({}_{3}\)+CNT, and Al\({}_{2}\)O\({}_{3}\)+PCM) at two concentrations (0.01 and 0.1 vol%) in a shell-and-tube heat exchanger, comparing them to distilled water. At low flow rates, 0.01 vol.% Al\({}_{2}\)O\({}_{3}\), CNT, Al\({}_{2}\)O\({}_{3}\)/PCM, and Al\({}_{2}\)O\({}_{3}\)/CNT exhibited lower _h_/\(\Delta\)P values than distilled water, suggesting that the pressure drop increase precedes the coefficient of convective heat transfer. [PERSON] et al. [12] evaluated the effect of various mass loadings (0.002, 0.005, 0.01, and 0.015 wt%) of multiwall carbon nanotubes acetone nanofluids on a flat plate heat pipe thermal performance. It was evidenced that an optimal mass concentration of 0.005 wt% decreased the convective thermal resistance and improved the heat transfer performance, whereas 0.01 and 0.015 wt% multiwall carbon nanotube nanofluidsevidenced lower heat transfer performance than the base fluid. [PERSON] et al. [13] investigated the convective efficiency of SiO\({}_{2}\)/water nanofluids at several volume concentrations (from 1 to 4 vol%) in a cylindrical tube exposed to a constant heat flux. It was observed that convective heat transfer coefficients increased with volume concentration up to 3 vol% and then decreased in comparison to the water. [PERSON] et al. [14] analysed Al\({}_{2}\)O\({}_{3}\)/water nanodispersions heat transfer characteristics with volume fractions of 2, 3, and 4 vol%) as coolants in counter-flow corrugated plate heat exchangers at various flows. Their findings indicated a decrease in the heat transfer rate with increasing particle concentration above the optimal volume fraction of 2 vol%. [PERSON] et al. [15] evaluated the Al\({}_{2}\)O\({}_{3}\)/water nanofluids heat transfer performance in a confined and submerged impinging jet on a flat, horizontal, and circular heated surface. They considered three distinct nozzle-heated surface distances (from 2 to 10 mm) as well as various volume concentrations (0, 2.8, and 6%). For the case of 2 mm, nanofluids showed no enhancement in coefficients of convective heat transfer with regard to water. However, at a distance of 5 mm, the 2.8% Al\({}_{2}\)O\({}_{3}\)/water nanosized dispersion showed superior thermal performance.