## 2 Thermophysical Properties
### Thermal Conductivity
#### 2.3.1 Ethylene-Glycol-Based Nanofluids
The measurements by the KD2 pro for the thermal conductivity were performed for \(d_{np}\) = 13 nm, \(T\) = [30-70 \({}^{\circ}\)C] and \(0.5\leq\varphi\leq 2\)%. All their results can be correlated within a 5% confidence interval by:  
\[k_{nf}=0.634(1+\varphi)^{0.1045}\left(\frac{T}{70}\right)^{0.1094}(1+BR)^{-1. 159} \tag{6}\]  
where \(BR\) is the base ratio, equal to \(BR\) = 0.4 for a 40:60 W:EG mixture. Figure 2 summarizes the experimental data obtained by [PERSON] et al. [52] for \(TiO_{2}\) nanoparticles dispersed in pure ethylene-glycol at two temperatures, 20 and 40 \({}^{\circ}\)C. The addition of these nanoparticles enhanced the thermal conductivity of the mixture, and this enhancement was slightly higher if \(T\) increased from 20-40 \({}^{\circ}\)C. However, the differences remained within the error bars. More interestingly, their results showed firstly that a better thermal enhancement could be achieved by choosing nanoparticles (here, 25 nm) if one compared their data to the Maxwell theory obtained  
for micro-sized particles. Secondly, these authors proposed an interesting correlation for the thermal conductivity, which is a modified version of the Maxwell correlation accounting for the thermal conductivity of aggregates. This rather complex correlation works particularly well in their case for \(\varphi\geq 0.4\) vol.%. The thermal performance enhancement of nanofluid containing diamond nanoparticles dispersed into a mixture of EG and water (45:55) was carried out by [PERSON] et al. [53]. They observed that the fluid thermal conductivity increased linearly with the nanoparticle volume fraction. They noted that loading 2 vol.% diamond nanoparticles enhanced the EG-water thermal conductivity by more than 18%. The authors claimed that the optimal nanofluid thermal performance was found to be at \(\varphi=0.5\) vol.%. Significantly larger improvements in terms of thermal conductivity were reported by [PERSON] et al. [54] by adding copper nanoparticles to EG. Only 0.3 vol.% of \(Cu\) nanoparticles increased the EG thermal conductivity by 40%. [PERSON] and [PERSON] [55] reported an 18% enhancement by dispersing 0.55 vol.% \(Fe\) nanocrystalline powder. Loading 0.1 vol.% of \(Cu\) nanoparticles in EG led to enhancement of its thermal conductivity by almost 9%, after [56]. [PERSON] et al. [57] studied the thermal conductivity of EG-based nanofluids containing oxides including \(MgO\), \(TiO_{2}\), \(ZnO\), \(Al_{2}\)O\({}_{3}\), and \(SiO_{2}\) nanoparticles. The results demonstrated that, at the same volume fraction, the \(MgO\)-EG nanofluid exhibited the highest improvement in terms of thermal conductivity compared to the other nanofluids. They noted that the maximum enhancement was up to 40.6% at \(\varphi=4\) vol.% of \(MgO\) nanoparticles. [PERSON] and [PERSON] [45] provided a comprehensive set of measured thermal conductivity data for three nanofluids: \(Al_{2}O_{3}\), \(CuO\), and \(ZnO\) dispersed in a 60:40 ethylene-glycol-water mixture. The results revealed that the thermal conductivity was a linear function of \(\varphi\) for all the studied nanofluids. For instance, at a given temperature, the thermal conductivity increased over the base fluid one by 42%, 46%, and 60% by suspending 6 vol.% of \(ZnO\), \(Al_{2}\)O\({}_{3}\), and \(CuO\) nanoparticles, respectively. The authors also examined the influence of the nanoparticle size on EG-W thermal conductivity. Two sets of measurements were conducted for the \(ZnO\)-based nanofluid with particle sizes 29 and 77 nm. They found that the thermal conductivity was higher for the smaller nanoparticle diameter since the former provided more surface area for the transfer of thermal energy. As an example, at \(\varphi\) = 4 vol.%, the thermal conductivity was 3.3% higher for 29-nm particles compared to that of 77-nm particles.