## 2 Nanofluid Thermal Conductivity
### Effects of Nanoparticle Material on Nanofluid Thermal Conductivity  
Researchers have conducted experiments to explore the effects of nanoparticle material on the thermal conductivity of nanofluids, interestingly finding that nanoparticle material alone is not an influential parameter and it does not significantly influence nanofluid thermal conductivity [8; 26; 32; 36]. Several researchers have suggested that factors other than the intrinsic thermal conductivity of nanoparticles, such as nanoparticle size and clustering [26; 32; 36], have a more significant impact on nanofluid thermal conductivity. For example, [PERSON] et al. [36] studied the thermal conductivities of TiO\({}_{2}\)-water, Al\({}_{2}\)O\({}_{3}\)-water, Fe-Ethylene Glycol, and WO\({}_{3}\)-Ethylene Glycol nanofluids using the transient hot-wire method. It was observed that TiO\({}_{2}\)-water nanofluid had a higher thermal conductivity than Al\({}_{2}\)O\({}_{3}\)-water nanofluid, despite the higher intrinsic thermal conductivity of Al\({}_{2}\)O\({}_{3}\) in comparison to TiO\({}_{2}\). It was suggested that this was a result of the smaller size of the TiO\({}_{2}\) nanoparticles (25 nm) in comparison to the Al\({}_{2}\)O\({}_{3}\) nanoparticles (48 nm), resulting in a higher surface area to volume ratio in the TiO\({}_{2}\)-water nanofluid. It was also observed that Fe-Ethylene Glycol nanofluid had a higher thermal conductivity than WO\({}_{3}\)-Ethylene Glycol nanofluid. The size of the Fe nanoparticles (10 nm) was also smaller than that of the WO\({}_{3}\) nanoparticles (38 nm). It was suggested that the thermal conductivity of the suspended nanoparticles does not have a primary effect on the thermal conductivity of nanofluids, but the surface area to volume ratio does have a strong influence on the thermal conductivity of a nanofluid. It was also suggested that the thermal conductivities of nanofluids containing metallic nanoparticles are higher than that of those containing ceramic nanoparticles. Using the same method, [PERSON] et al. [32] measured the thermal conductivities of SiO\({}_{2}\)-methanol and Al\({}_{2}\)O\({}_{3}\)-methanol nanofluids. It was observed that SiO\({}_{2}\)-methanol nanofluid had a higher thermal conductivity despite alumina's higher intrinsic thermal conductivity. It was suggested that this was a result of the formation of larger nanoparticle clusters in the SiO\({}_{2}\)-methanol nanofluid. These results agree with the idea that the thermal conductivity of nanofluids is impacted more significantly by factors other than the thermal conductivity of nanoparticles. In the same way, [PERSON] et al. [26] measured the thermal conductivities of nanofluids composed of several oxide nanoparticles, includingMgO, TiO\({}_{2}\), ZnO, Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\), dispersed in Ethylene Glycol. Listed from highest to lowest, the intrinsic thermal conductivities of the nanoparticles were MgO, Al\({}_{2}\)O\({}_{3}\), ZnO, SiO\({}_{2}\), and TiO\({}_{2}\). The thermal conductivities of the nanofluids, listed from highest to lowest, were MgO-Ethylene Glycol, Al\({}_{2}\)O\({}_{3}\)-Ethylene Glycol, TiO\({}_{2}\)-Ethylene Glycol, ZnO-Ethylene Glycol, and finally SiO\({}_{2}\)-Ethylene Glycol nanofluid. These results also support the idea that the thermal conductivity of nanofluids is affected more significantly by factors other than the intrinsic thermal conductivity of nanoparticles. However, in some cases experimental data has shown controversial results, in which nanofluid thermal conductivity increased with nanoparticle thermal conductivity. For example, using the transient hot-wire method, [PERSON] et al. [37] measured the thermal conductivities of spherical Al\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), and ZrO\({}_{2}\) nanoparticles dispersed in water. Due to the high density of ZrO\({}_{2}\), yttrium oxide was used as a surfactant to aid in stability. The thermal conductivities of the nanoparticles listed from highest to lowest are Al\({}_{2}\)O\({}_{3}>ZrO>\) SiO\({}_{2}\).