## 2 Thermophysical Properties
### Thermal Conductivity
#### 2.3.1 Ethylene-Glycol-Based Nanofluids  
Among the first studies on the thermal enhancement due to EG-based nanofluids are those done at the Argonne National Laboratory. [PERSON] et al. [47] investigated experimentally the effect of nanoparticles and base fluid type on the thermal conductivity of the solution. Two oxide nanoparticles, \(Al_{2}O_{3}\) and \(CuO\), were dispersed separately in water or in EG, resulting in four different nanofluids. The results showed that the mixture thermal conductivity increased linearly with the nanoparticle concentration. The authors stated that the \(CuO\)-ethylene-glycol nanofluid exhibited a better thermal conductivity enhancement compared to \(CuO\)-water, \(Al_{2}O_{3}\)-EG, and \(Al_{2}O_{3}\)-water nanofluids. For instance, for the same nanoparticle volume fraction \(\varphi\) = 4%, the EG thermal conductivity increased by about 20% and 14% by adding \(CuO\) and \(Al_{2}O_{3}\) nanoparticles, respectively. [PERSON] and [PERSON] [48] considered nanofluids composed of \(Al_{2}O_{3}\) and \(CuO\) nanoparticles dispersed in water, a vacuum pump fluid, engine oil, or ethylene-glycol. They noted that adding a small nanoparticle volume fraction to one of these base fluids increased noticeably the mixture thermal conductivity. The latter increased as \(\varphi\) increased. For instance, the effective EG thermal conductivity was enhanced by 26% and 40% for \(\varphi\) = 5% and 8% of \(Al_{2}O_{3}\) nanoparticles, respectively. The enhancement rates for EG and engine oil were the highest, whereas that of the pump fluid was the lowest. Dispersing \(\varphi\) = 5% of \(Al_{2}O_{3}\) and \(CuO\) powders into EG increased the effective thermal conductivity by about 20% and 26% compared to pure EG, respectively. They stated that the nanofluid-based EG was very stable up to a volume fraction of 16%. However, the water-, engine oil- and pump fluid-based nanofluids started flocculating from \(\varphi\) = 10%. [PERSON] and [PERSON] [49] performed experimental measurements of the effective thermal conductivity of nanofluids containing \(TiO_{2}\) nanoparticles (\(d_{np}\) = 21 nm) dispersed separately in water, 40:60 and 50:50 EG-water mixtures for \(30<T<70\)\({}^{\circ}\)C and \(0.2<\varphi<1\)%. They noted that as the titanium oxide concentration increased, the nanofluid thermal conductivity was enhanced. The 50:50 EG-water-based nanofluid showed a higher \(k\) enhancement compared to pure water and 40:60 EG-water-based fluids. For example, at \(T\) = 30 \({}^{\circ}\)C, the thermal conductivity enhancements were measured to be 10.64% and 14.2% by adding 0.2 vol.% and 1 vol.% of \(TiO_{2}\) nanoparticles to a 50:50 EG-water mixture, respectively. However, under the same conditions, the enhancement in the thermal conductivity was much lower, namely between 0.649% and 5.01% and between 1.94% and 4.38% for water and 40:60 EG-water-based nanofluids, respectively. Their results can be correlated by:  
\[k_{nf}=k_{bf}[a+b\varphi] \tag{5}\]  
with \(a\) and \(b\) adjusting parameters, which depend on \(\varphi\). Experimental thermal conductivity measurements of \(Al_{2}O_{3}\) and \(CuO\) nanoparticles dispersed in a 50:50 ethylene-glycol-water mixture were conducted by [PERSON] et al. [50]. They reported that the thermal conductivities of both nanofluids increased by increasing both the nanoparticle volume fraction and temperature. The \(CuO\)-based nanofluid exhibited a better thermal conductivity enhancement compared to the \(Al_{2}O_{3}\)-based nanofluid under the same operating conditions. They noted that the thermal conductivity enhancement of the \(Al_{2}O_{3}\) nanofluid varied from 9.8-17.89%, and for \(CuO\) nanofluid, it varied from 15.6-24.56% within the temperature range [15 \({}^{\circ}\)C; 50 \({}^{\circ}\)C] at \(\varphi\) = 0.8 vol.% compared to the base fluid. [PERSON] et al. [51] investigated the thermal conductivity of \(Al_{2}O_{3}\)/EG-W nanofluids in different water:EG proportions: 40:60, 50:50, and 60:40.