## 3 Results and Discussions
### Characteristics of Nanoparticles
Since Fe\({}_{3}\)O\({}_{4}\) nanoparticles had the highest density among the single nanoparticles, the viscosities of Fe\({}_{3}\)O\({}_{4}\) nanofluids at 0.25, 0.05, 0.1, and 0.2 wt% were 1.81, 1.82, 1.85, and 1.87 mPa\(\cdot\)s, respectively. As the concentration increased, the viscosity of the nanofluids increased slightly. By adding MWCNT nanoparticles to Fe\({}_{3}\)O\({}_{4}\) nanofluids, the viscosities of the Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluids were 1.75, 1.77, 1.81, and 1.83 mPa\(\cdot\)s at concentrations of 0.25, 0.05, 0.1, and 0.2 wt%, respectively. According to the increase in the Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluids concentration, the increase in viscosity exhibited a similar tendency to that of Fe\({}_{3}\)O\({}_{4}\) nanofluid but had a lower viscosity than the Fe\({}_{3}\)O\({}_{4}\) nanofluid. The results confirmed that the viscosity of the Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluid was 2.6% lower than that of the Fe\({}_{3}\)O\({}_{4}\) nanofluid on average. Figure 8: XRD patterns of Fe\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)–MWCNT hybrid nanoparticles. Figure 10: Variation in viscosity of Fe\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)–MWCNT hybrid nanofluids according to the weight concentrations of nanoparticles. Figure 9: Variation in thermal conductivity of Fe\({}_{3}\)O\({}_{4}\), MWCNT, and Fe\({}_{3}\)O\({}_{4}\)–MWCNT nanofluids according to the weight concentrations. Convective Heat Transfer Performance of Fe\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)-MWCNT Nanofluids under a Magnetic Effect  
Figure 11 shows the enhancement in the convective heat transfer coefficient of the Fe\({}_{3}\)O\({}_{4}\) nanofluid compared with that of the base fluid. Generally, the convective heat transfer coefficient of Fe\({}_{3}\)O\({}_{4}\) nanofluids is higher than that of the base fluid. Under a constant Re, the convective heat transfer coefficient improvement was affected by the concentration and magnetic field intensity. Figure 11a shows the enhancement in the convective heat transfer coefficient of the Fe\({}_{3}\)O\({}_{4}\) nanofluid according to the concentration compared with the base fluid at Re = 1000. At Re = 1000, the convective heat transfer coefficient of the base fluid was 1232 W/m\({}^{2}\) K, and the convective heat transfer coefficient of Fe\({}_{3}\)O\({}_{4}\) nanofluid at a 0.2 wt%, the concentration increased by 21% to 1491 W/m\({}^{2}\) K. A similar trend was observed for other Re values other than 1000. As shown in Figure 11b-d, the increase in the convective heat transfer coefficient of the Fe\({}_{3}\)O\({}_{4}\) nanofluid was compared with that of the base fluid at different Re values of 1200, 1400, and 1600. When Re numbers were 1200, 1400, and 1600, the convective heat transfer coefficients of the base fluid were 1268, 1323, and 1347 W/m\({}^{2}\) K, respectively. In addition, when the magnetic field was not applied, the convective heat transfer coefficients of the 0.2 wt%, Fe\({}_{3}\)O\({}_{4}\) nanofluid were 1535, 1597, and 1616 W/m\({}^{2}\) K, respectively. Figure 11a shows that the average increase in the convective heat transfer coefficient was 20.6% for the 0.2 wt% Fe\({}_{3}\)O\({}_{4}\) nanofluid and Re = 1000. As the concentration of the nanofluid increased, Brownian motion intensified, causing the nanoparticles in the tube to move irregularly and actively, thereby improving the convective heat transfer of the Fe\({}_{3}\)O\({}_{4}\) nanofluid. The convective heat transfer coefficient was increased by the increase in both the concentration of nanofluids and magnetic field strength. Under the magnetic field effect, the Fe\({}_{3}\)O\({}_{4}\) nanoparticles formed a chain-like structure, thereby creating fine turbulence in the flow, resulting in improved convective heat transfer. Overall, as shown in Figure 11a-d, a significantly enhanced convective heat transfer coefficient at 750 G was observed, particularly the maximum convective heat transfer coefficient at 0.2 wt%.