## 2 Thermophysical Properties
### Rheological Behaviour of Nanofluids  
The dynamic viscosities \(\mu\) of pure ethylene- and propylene-glycol are 21- and 57.6-times higher than that of liquid water at 20 \({}^{\circ}\)C, respectively (Table 3). It is now a well-known fact the addition of nanoparticles in these base fluids will accentuate that. For industrial applications where the HTF needs to be transported, it is a primary concern as the dynamic viscosity of the HTF will directly impact the pressure drop, and so the required pumping power to transport it. The dynamic viscosity is then certainly as critical as the thermal conductivity when choosing the appropriate nanofluid for a given engineering system. However, most of the published works to date have mainly focused their effort on the heat transfer part, and as shown in Table 5, fewer provided reliable data on the dynamic viscosity of EG- or PG-based nanofluids. Most authors still use general correlations to predict the rheological behaviour of PG-W- or EG-W-based nanofluids like the Brinkman equation:  
\[\mu_{nf}=\mu_{bf}\frac{1}{(1-\varphi)^{2.5}} \tag{9}\] \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] and [PERSON] [55] & Fe/EG & 10 vol.\% & 0.55 & 18 & 20 \\ \hline [PERSON] et al. [60] & CNTs/EG & 1 vol.\% & \(15\times 310^{-4}\) & 12.6 & 20 \\ \hline [PERSON] et al. [58] & SiC/EG & 4.2 vol.\% & 26 & 15.8 & 4 \\ \hline \multicolumn{6}{c}{(spherical shape)} \\ \hline  & SiC/EG & 4 vol.\% & 600 & 22.9 & 4 \\ \hline \multicolumn{6}{c}{(cylindrical shape)} \\ \hline [PERSON] et al. [52] & TiO\({}_{2}\)/EG & \(\leq\)18 vol.\% & 25 & \(\leq\)14.5 & 20, 40 \\ \hline [PERSON] et al. [67] & Cu/EG & 0.04–2 vol.\% & 200 & 2–12.6 & 25 \\ \hline [PERSON] et al. [57] & MgO/EG & 5 vol.\% & 20 & 40.6 & 10–60 \\ \hline  & \(Al_{2}\)O\({}_{3}\)/EG & 5 vol.\% & 20 & 28.2 & 10-60 \\ \hline  & TiO\({}_{2}\)/EG & 5 vol.\% & 20 & 27.2 & 10-60 \\ \hline  & ZnO/EG & 5 vol.\% & 20 & 26.8 & 10–60 \\ \hline  & SiO\({}_{2}\)/EG & 5 vol.\% & 20 & 25.3 & 10–60 \\ \hline [PERSON] et al. [68] & \(Al_{2}\)Ca/EG & 0.2–2.5 vol.\% & 15 & 6–220 & 20 \\ \hline  & \(A_{\rm 2}Al/\)EG & 0.2–2.5 vol.\% & 18 & 8–250 & 20 \\ \hline [PERSON] et al. [5] & \(Al_{2}\)O\({}_{3}\)/PG & 1-9 wt.\% & 70–100 & 3.6–11 & 20–80 \\ \hline  & TiO\({}_{2}\)/PG & 1-9 wt.\% & 100–140 & 1.4–9 & 20–80 \\ \hline [PERSON] et al. [62] & SWCNT/EG & 0.2 vol.\% & \(1.6\times 150\) & 14.8 & 27–57 \\ \hline [PERSON] et al. [53] & Diamond/EG-W(45:55) & 0.5–2 vol.\% & 10 & 6–18 & 10–60 \\ \hline [PERSON] et al. [59] & SiC/EG-W(50:50) & 4 vol.\% & 16, 29, 66 and 90 & 11.5–17 & 15-85 \\ \hline [PERSON] and [PERSON] [45] & \(Al_{2}\)O\({}_{3}\)/EG-W(60:40) & 1–10 vol.\% & 53 & 8–69 & 25–90 \\ \hline  & CuO/EG-W(60:40) & 1–6 vol.\% & 29 & 14–60 & 25–90 \\ \hline  & ZnO/EG-W(60:40) & 1–7 vol.\% & 29 and 77 & 9.8–48.5 & 25–90 \\ \hline [PERSON] et al. [50] & \(Al_{2}\)O\({}_{3}\)/EG-W(50:50) & 0.2–0.8 vol.\% & 36.5 & 9.8–17.89 & 15-50 \\ \hline  & CuO/EG-W(50:50) & 0.2–0.8 vol.\% & 27 & 15.6–24.56 & 15-50 \\ \hline [PERSON] and [PERSON] [49] & TiO\({}_{2}\)/EG-W(40:60) & 0.2–1 vol.\% & 21 & 1.94–4.3 & 30–70 \\ \hline  & TiO\({}_{2}\)/EG-W(50:50) & 0.2–1 vol.\% & 21 & 10.64–14.2 & 30–70 \\ \hline [PERSON] et al. [6] & \(Al_{2}\)O\({}_{3}\)/PG-W(60:40) & 0.5–6 vol.\% & 15–45 & 2–12.2 & –30–90 \\ \hline  & ZnO/FG-W(60:40) & 0.5–6 vol.\% & 36–76 & 1.6–21.36 & –30–90 \\ \hline  & CuO/PG-W(60:40) & 0.5–6 vol.\% & 30 & 2.3–15.6 & –30–90 \\ \hline  & SiO\({}_{2}\)/PG-W(60:40) & 6 vol.\% & 30 & 4.7–7.2 & –30–90 \\ \hline  & TiO\({}_{2}\)/PG-W(60:40) & 0.5–1.5 vol.\% & 15 & 1–7.3 & –30–90 \\ \hline \hline \end{tabular}
\end{table}