## Materials and Methods  
### Nanoparticles and Nanofluids  
The Fe\({}_{3}\)O\({}_{4}\) nanoparticles used in this study were prepared using a co-precipitation method. Commercially available MWCNTs (US Research Nanomaterials Inc., Houston, TX, USA) were used for hybrid nanofluid preparation. Table 1 shows the thermal properties of the Fe\({}_{3}\)O\({}_{4}\) and MWCNT nanoparticles used in the experiments. Measured thermal properties are provided by nanoparticle manufacturers (AVENTION, Incheon, Korea). MWCNT nanoparticles have a high thermal conductivity of 3000 W/m\(\cdot\)K, and Fe\({}_{3}\)O\({}_{4}\) nanoparticles have a high density of 4950 kg/m\({}^{3}\). Both nanoparticles have a similar specific heat of 650-670 J/kg\(\cdot\)K. Therefore, for preparing Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluid in which the two nanoparticles were dispersed in the base fluid at an appropriate ratio, synergy is expected to be generated in the direction with excellent thermal properties through thermal conductivity and the magnetic effect. The nanofluid used in this study was prepared using a two-step process. Initially, the base fluid was prepared by mixing water and EG at a ratio of 80:20. EG is known for its low freezing temperature and is widely used as a working fluid in heat transfer systems, particularly in cold winters. An 80:20 ratio water-EG mixture is commonly used in industries to prevent freezing and minimize viscosity while increasing conductivity [45]. Fe\({}_{3}\)O\({}_{4}\) nanoparticles, which were prepared using the co-precipitation method, were added to the base fluid, stirred for 1 h with a stirrer, and sonicated for 3 h using an ultrasonic disperser. To ensure reliable dispersion stability of the Fe\({}_{3}\)O\({}_{4}\) nanofluid, a small amount of NaOH was added to adjust the pH to 11. This is because when the pH value of the Fe\({}_{3}\)O\({}_{4}\) nanofluid is 11, magnetite is formed, and the dispersion stability can be ensured [46]. To prepare the Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluid, Fe\({}_{3}\)O\({}_{4}\), and MWCNT nanoparticles were added to the base fluid at a 1:1 ratio, and the mixture was stirred for 1 h. After stirring, the mixture was sonicated for 2 h using an ultrasonic disperser. In addition, a small amount of Arabic gum was added as a surfactant to disperse the MWCNT nanoparticles in the base fluid evenly [47]. In this study, Fe\({}_{3}\)O\({}_{4}\) nanofluids were prepared at different concentrations of 0.025, 0.05, 0.1, and 0.2 wt%, and Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluids were prepared by adding Fe\({}_{3}\)O\({}_{4}\) and MWCNT nanoparticles at a 1:1 ratio. The prepared Fe\({}_{3}\)O\({}_{4}\) nanofluids and Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluids are shown in Figure 1. The Fe\({}_{3}\)O\({}_{4}\) nanofluid was dark brown, and as the concentration increased, the nanofluid became darker. The Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluid was black compared with the base fluid owing to the effect of MWCNT nanoparticles. To investigate the dispersion stability of the manufactured nanofluids, observation and measurement of zeta potential methods were conducted simultaneously. A one-month visual inspection of the nanofluids manufactured in this study revealed no significant precipitation. In addition, the zeta positional values of the Fe\({}_{3}\)O\({}_{4}\) nanofluid and Fe\({}_{3}\)O\({}_{4}\)-MWCNT hybrid nanofluid were measured and observed to be stable at \(-34\) to \(-47\) mV and \(-32\) to \(-56\) mV, respectively. Generally, when the measured Zeta potential is more than the \(\pm 30\) mV, it is judged that the dispersion stability of nanofluids is secured [48]. \begin{table}
\begin{tabular}{c c c} \hline
**Item** & **MWCNT** & **Fe\({}_{3}\)O\({}_{4}\)** \\ \hline Purity & \textgreater{}95\% & 99.9\% \\ Thermal conductivity (W/m\(\cdot\)K) & 3000 & 80 \\ True density (kg/m\({}^{3}\)) & 1350 & 4950 \\ Specific heat (J/kg\(\cdot\)K) & 650 & 670 \\ \hline \end{tabular}
\end{table}