## 1 Introduction  
Nanofluids have been broadly studied for the last decades, especially concerning their colloidal stability, thermophysical characteristics, flow behaviour, and their efficiency of heat transfer [1]. However, the establishment of appropriate and reliable comparative analyses becomes critical to evaluate these nanodispersions' improvement compared to the corresponding base fluid. This allows the design of the optimal working fluid for each specific application. Different thermal transfer approaches have been proposed in the literature based on theoretical studies or experimental results. In the case of laminar flow that is fully developed both hydrodynamically and thermally, thermal conductivity is widely understood to be proportional to the coefficient of convective heat transfer [2]. Instead, for turbulent flow, there are numerous correlations depending on the flow characteristics, geometry, or thermophysical properties, the classical Dittus-Boelter correlation [3] being one of the most widely spread and popular. Figures-of-merit (FOMs) and dimensionless numbers are commonly employed to assess the thermal transfer performance of different nanofluids. Despite the Nusselt number (_Nu_) allowing the analysis of heat transfer mechanisms by comparing convective to conductive heat transfer, it is inappropriate to compare fluids (or nanofluids) with different thermal conductivities [4]. In fact, the use of Mouromtseff numbers (_Mo_), first introduced by [PERSON] in 1942 [5], is widely extended in nanofluids literature since they allow to  
evaluate the influence of the combined thermophysical profile of the fluid on the convective heat transfer coefficient. Indeed, it is common to evaluate the thermal transfer potential of a nanofluid in terms of its _Mo_ number (_Mo\({}_{nf}\)_) in comparison to the base fluid's _Mo_ number (_Mo\({}_{nf}\)_). In this sense, the ratio _Mo\({}_{nf}\)_/_Mo\({}_{bf}\)_ is identified as a performance index for the relative nanofluid thermal transfer performance. Several examples of the utilisation of _Mo_ number for the assessment of the potential convective heat transfer capability of nanosized dispersions can be found in the literature. [PERSON] and [PERSON] [2] analysed how changes in thermophysical properties with temperature and nanoadditive concentration impact the thermal transfer capability of three sets of nanofluids based on aluminum oxide (Al\({}_{2}\)O\({}_{3}\)), silicon dioxide (SiO\({}_{2}\)), and copper oxide (CuO) suspended in ethylene glycol:water mixture 60:40 wt% (EG:W 60:40). They found _Mo\({}_{nf}\)_/_Mo\({}_{bf}\)_ greater than 1 for all the studied dispersions, indicating superior performance of the nanofluids compared to the EG:W 60:40 base fluid. [PERSON] et al. [6] compared the base fluid Mouromtseff numbers to those of the nanofluids obtained by means of the Dittus-Boelter [3] and Pak and [PERSON] [7] equations for functionalized graphene nanoplatelets suspended in an aqueous propylene glycol mixture at 30:70 wt%. They reported improvements of _Mo_ ranging from 16% to 36%, observing that these enhancements increase with the lower temperatures and lower concentration of dispersed nanoadditive. Despite the number of studies on how variables such as temperature and nanoparticle concentration affect Mo numbers, there is no clarity on the suitability of the Mo criterion for evaluating the nanofluid's convective heat transfer efficacy. Contradictory results can be found in the available literature. A strong correlation between the experimental heat transfer coefficients and those predicted from the Mouromtseff number ratio was reported by [PERSON] et al. [8], who investigated Therminol 59-based nanofluids containing suspended copper (Cu) nanoparticles of 50-100 nm (mass concentrations of 0.50 wt%, 0.75 wt%, and 2 wt%) under turbulent conditions. Conversely, [PERSON] et al.