**2. Nanofluids Preparation and Properties Characterization**

In this study, a two-step preparation method was followed to produce the Al2O<sup>3</sup> and TiO<sup>2</sup> NFs samples. The primary sizes (diameter) of both Al2O<sup>3</sup> and TiO<sup>2</sup> nanoparticle were <50 nm and 20 nm, respectively, and their purity was 99.5% (as provided by IoLiTec, Heilbronn, Germany). The nanoparticles were dispersed into the BF (DW) for the volumetric concentration of 0.01, 0.05, 0.1, 0.15, and 0.2 vol.% (equivalent to 0.04, 0.2, 0,4, 0.6, and 0.8% of the mass fractions). First, nanoparticles were precisely weighed using a KERN ABS 80-4N scale and then dispersed into the base fluid. The good dispersion of nanoparticles into the base fluid was obtained after employing a magnetic stirring process for 15 min in a first moment followed by an ultrasonication process for 25 min using a probe-type ultrasonicator (Hielscher UP200Ht) at an amplitude of 60%, power of 110 W and 40 kHz frequency, to improve the dispersion and stability of the nanoparticles into the fluids. Moreover, the viscosity and thermal conductivity (*k*) of the NFs were directly measured after the preparation to ensure very good stability during the measurements. The higher thermophysical properties of NFs, mainly the thermal conductivity and viscosity, are considered key factors that define the heat transfer characteristics when NFs are employed to flow through the heat exchangers channels/passages. Therefore, these thermophysical properties of the NF samples were carefully measured and evaluated in the following sections. On other hand, specific heat (*Cp*) and density of NFs do not change significantly at low particle concentrations, and mixture rules are widely used for the determination of these properties based on the volume fraction of particles (*ϕ*) into the BF. Thus, the mixture rules (e.g., [35,36]) which were applied to NFs are given by Equation (1) for density and Equation (2) for *Cp*, respectively,

$$
\rho\_{nf} = \varphi \rho\_p + (1 - \varphi)\rho\_{bf} \tag{1}
$$

$$\mathcal{L}\_{p,nf} = \frac{\varrho \left(\rho \mathbb{C}\_p\right)\_p + (1 - \varrho) \left(\rho \mathbb{C}\_p\right)\_{bf}}{\rho\_{nf}} \tag{2}$$

where the subscripts *n f* represent NF, *p* the particle, and *b f* the BF.
