## 2 Nanofluid Thermal Conductivity
### Effects of Nanoparticle Material on Nanofluid Thermal Conductivity
It was observed that Al\({}_{2}\)O\({}_{3}\)-water nanofluid had the highest thermal conductivity, and SiO\({}_{2}\)-water nanofluid had the lowest. Also, [PERSON] et al. [4] measured the thermal conductivities of nanofluids composed of Ag\({}_{2}\)Al and Al\({}_{2}\)Cu dispersed in both water and Ethylene Glycol. They obtained measurements using a thermal comparator, which worked based on the principle that the total heat transferred by a hot probe was a function of both the thermal conductivity of the fluid and the thermal conductivity of the solid nanoparticles. When an electrically heated source was brought into contact with the fluid, a metallic copper probe sensed the temperature change through an attached thermocouple. The temperature was then measured and recorded through a circuit covering the probe assembly and nanofluid. Thermal conductivity values recorded were very high, with a thermal conductivity of 1.44 \(W/mk\) for Ag\({}_{2}\)Al-water nanofluid at nanoparticle volume concentration, \(\varphi=1.8\%\), and a thermal conductivity of 1.35 \(W/mk\) for Al\({}_{2}\)Cu nanofluid at the same nanoparticle volume concentration. Although no direct evidence was found, it was suggested that the formation of loose chain-like structures between nanoparticles may have caused the high thermal conductivity values by occupying more volume within the nanofluid than individual nanoparticles, thus allowing a greater area for heat transfer. It was observed that the nanofluids containing Ag\({}_{2}\)Al nanoparticles had higher thermal conductivities than those containing Al\({}_{2}\)Cu nanoparticles. [PERSON] et al. [28] performed experiments to determine the thermal conductivities of Al\({}_{2}\)O\({}_{3}\)-water, Al\({}_{2}\)O\({}_{3}\)-Ethylene Glycol, CuO-transformer oil, Cu-water, Cu-Ethylene Glycol, Al-water, and Al-transformer oil nanofluids. The transient hot-wire method was used to obtain measurements for the oxide nanofluids, and temperature oscillation equipment was used for the metallic nanofluids. It was observed that nanofluids containing metallic nanoparticles had higher thermal conductivities than those containing oxide nanoparticles, which supports the observation made by [PERSON] et al. [36]. Additionally, using the transient hot-wire method, [PERSON] et al. [13] measured the thermal conductivity of magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles dispersed in a mixture of Ethylene Glycol and water. It was stated that an advantage to using magnetic nanoparticles is the possibility of separating the particles from the base fluid after the particles are fully dispersed. From the observations discussed above, it can be suggested that the intrinsic thermal conductivity of nanoparticles does not always have a significant influence on the thermal conductivity of nanofluids. However, further experimentation should be completed in order to determine the parameters which overpower the impact of nanoparticle material on nanofluid thermal conductivity. The thermal conductivities of water-based nanofluids containing various nanoparticles, are shown in Figure 1 as a function of temperature, at a nanoparticle volume concentration of \(\varphi=2\%\). It is observed that for a given temperature, the thermal conductivity of CuO-water nanofluid is higher than the thermal conductivity of Cu-water nanofluid. These results contradict -the observations made by [PERSON] et al. [44], who observed a thermal conductivity of 0.706 \(W/mk\) for 50 nm Cu nanoparticles dispersed in water, and a thermal conductivity of 0.67 \(W/mk\) for 30 nm CuO nanoparticles dispersed in water, at a nanoparticle volume concentration \(\varphi=3\%\) and a temperature of 30 \({}^{\circ}\)C. It is also observed that the thermal conductivity of nanodiamond-water nanofluid is very low despite the high intrinsic thermal conductivity of nanodiamond 1000 \(W/mk\) within the temperature range of 27-52 \({}^{\circ}\)C, for \(\varphi=2\%\), the thermal conductivity of nanodiamond-water nanofluid ranged from 0.66-0.69 \(W/m.k.\) [PERSON] et al.