## 3 Results and Discussions  
### Characteristics of Nanoparticles  
Figure 7 shows the transmission electron microscopy (TEM) images of the Fe\({}_{3}\)O\({}_{4}\) nanoparticles and Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanoparticles used in this experiment. The Fe\({}_{3}\)O\({}_{4}\) nanoparticles were as spherical as most conventional nanoparticles and had an average size of 20 nm. In addition, as shown in Figure 7b, the Fe\({}_{3}\)O\({}_{4}\) nanoparticles were attached to the surface of the MWCNT by cohesion, thus forming Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanoparticles. The thickness of the MWCNT nanoparticles with Fe\({}_{3}\)O\({}_{4}\) nanoparticles was 10-30 nm. The structures of the Fe\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)-MWCNT nanoparticles were investigated using X-ray diffraction (XRD). The XRD results for Fe\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)-MWCNT nanoparticles are shown in Figure 8. The XRD measurement results of the Fe\({}_{3}\)O\({}_{4}\) nanoparticles indicated the characteristics of a cubic spinel structure at (220), (311), (400), (422), (511), and (440). The strongest reflection was observed at (311) [51]. The XRD measurements of the Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanoparticles indicated the characteristic of the cubic spinel structures of both the Fe\({}_{3}\)O\({}_{4}\) and MWCNT nanoparticles at (002) and (100). This indicated that the structure of the MWCNTs was not modified during the synthesis of composite nanoparticles with Fe\({}_{3}\)O\({}_{4}\) nanoparticles [52]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Uncertainty (\%)** \\ \hline T-Type thermocouple (K) & 0.75 \\ Power meter (V) & 3.0 \\ Pressure transmitter (PSID) & 0.2 \\ Mass flow meter (kg/s) & 1.0 \\ Magnetic strength meter (G) & 5.0 \\ Convective heat transfer coefficient (W/m\({}^{2}\) K) & 0.3 \\ Nusselt number & 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Uncertainties of measuring devices. Figure 7: TEM images Fe\({}_{3}\)O\({}_{4}\) nanoparticles and Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanoparticles. Thermal Conductivity and Viscosity of Fe\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)-MWCNT Hybrid Nanofluids  
Figure 9 shows the thermal conductivity of the MWCNT and Fe\({}_{3}\)O\({}_{4}\) single nanofluids and Fe\({}_{3}\)O\({}_{4}\)-MWNCT hybrid nanofluids of various concentrations at a temperature of 25 \({}^{\circ}\)C. The thermal conductivity of MWCNT nanofluids is used here for comparison from a previous study [45]. For Fe\({}_{3}\)O\({}_{4}\) nanofluids, the thermal conductivity was 0.515, 0.518, 0.521, and 0.521 W/m\(\cdot\)K at concentrations of 0.025, 0.05, 0.1, and 0.2 wt%, respectively. We observed that the thermal conductivity of Fe\({}_{3}\)O\({}_{4}\) nanofluids was lower than that of MWCNT nanofluids at the corresponding concentrations. Adding a small amount of MWCNT nanoparticles to the Fe\({}_{3}\)O\({}_{4}\) nanofluid can synergistically enhance its convective heat transfer due to the high thermal conductivity of the MWCNT nanoparticles. The thermal conductivity of the Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluid was 0.516, 0.519, 0.524, and 0.531 W/m\(\cdot\)K at concentrations of 0.025, 0.05, 0.1, and 0.2 wt%, respectively. Compared with the Fe\({}_{3}\)O\({}_{4}\) single nanofluid, the thermal conductivity of the Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluid increased by approximately 2%, which confirmed the low thermal conductivity of the Fe\({}_{3}\)O\({}_{4}\) nanofluid through the addition of MWCNT nanoparticles. Figure 10 shows the change in viscosity according to the various concentrations of the MWCNT and Fe\({}_{3}\)O\({}_{4}\) single nanofluids and Fe\({}_{3}\)O\({}_{4}\)-MWNCT hybrid nanofluid at 25 \({}^{\circ}\)C. The viscosity values of MWCNT nanofluids were used for comparison from a previous study [45].