## 2 Thermophysical Properties
### Thermal Conductivity
#### 2.3.2 Propylene-Glycol-Based Nanofluids  
Table 4 summarizes the main results regarding the thermal conductivity of EG- or PG-based nanofluids. It clearly shows, among other things, that the thermal conductivity of PG-based nanofluids has not been considered so far in the literature. [PERSON] et al. [5] studied experimentally the dispersion stability and the thermal conductivity of PG-based nanofluids containing \(Al_{2}O_{3}\) and \(TiO_{2}\) nanoparticles for different concentrations (1, 6, and 9 wt.%) within the temperature range [20 \({}^{\circ}\)C ; 80 \({}^{\circ}\)C]. They interestingly found that the average size of the nanoparticle suspensions was observed to decrease with increasing temperature in contrast to the common belief [65]. For example, at a 1 wt.% alumina concentration, the nanoparticle size decreased from 90-70 nm over the temperature range of interest. They found also that the thermal conductivity enhancement for both nanofluids was a nonlinear function of concentration and was temperature independent.  
[PERSON] et al. [6] carried out an experimental study to determine the thermal conductivity of five different nanofluids containing \(Al_{2}O_{3}\), \(CuO\), \(ZnO\), \(SiO_{2}\), and \(TiO_{2}\) nanoparticles dispersed in a base fluid composed of a 60:40 propylene-glycol and water mixture within the temperature range [\(-\)30; 90 \({}^{\circ}\)C]. Contrary to [5], they found that the thermal conductivity of nanofluids was strongly dependent on temperature, nanoparticle concentration, nanoparticle size, type of nanoparticles, and base fluid. The results showed an increase in the thermal conductivity of nanofluids by increasing either concentration or temperature. As the nanoparticle diameter increased, the thermal conductivity increased as well.  
[PERSON] and [PERSON] [66] considered the dispersion of \(TiO_{2}\) nanoparticles in a 70:30 (% by weight) water-propylene-glycol mixture for \(\varphi\) within the range [0.1-0.8%]. The results demonstrated that the thermal conductivity of the nanofluids increased with the volume concentration and temperature and could be correlated. Interestingly, these authors discussed also the potential heat transfer benefits of using these nanofluids under laminar and turbulent flow conditions.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Authors** & **Nanofluid** & \begin{tabular}{c} **Nanoparticle** \\ **Concentration \(\varphi\)** \\ \end{tabular} & \begin{tabular}{c} **Particle Size** \\ **(nm)** \\ \end{tabular} & \
\begin{tabular}{c} **Increase** \\ **in k (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Temperature** \\ **Range (\({}^{\circ}\)C)** \\ \end{tabular} \\ \hline [PERSON] et al. [54] & Cu/EG & 0.3 vol.\% & 10 & 40 & 17–37 \\ \hline [PERSON] et al. [47] & CuO/EG & 1–4 vol.\% & 24 & 4–20 & 17–37 \\ \hline  & \(Al_{2}O_{3}\)/EG & 1–4 vol.\% & 38 & 2–14 & 17–37 \\ \hline [PERSON] and [PERSON] [48] & \(Al_{2}O_{3}\)/EG & 6–10 vol.\% & 28 & 26–40 & 20 \\ \hline  & CuO/EG & 5–15 vol.\% & 23 & 20–55 & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Summary of the experimental data for the determination of the thermal conductivity of EG- and propylene-glycol (PG)-based nanofluids.