## 2 Nanofluid Thermal Conductivity
### Effects of Base Liquid on Nanofluid Thermal Conductivity  
Researchers have observed that the thermal conductivity of nanofluids increases with the thermal conductivity of the base liquid [3; 9; 10; 11; 12; 13; 14; 15]. For example, [PERSON] et al. [9] measured the thermal conductivities of CuO-water and CuO-Ethylene Glycol nanofluids. It was observed that for a given volume fraction and temperature, the thermal conductivity of CuO-water nanofluid was higher. Also, [PERSON] et al. [10] measured the thermal conductivities of CuO nanofluids, mixing CuO nanoparticles with water, Ethylene Glycol, and engine oil. The results are shown in Figure 12. The observations agreed with those of [PERSON] et al. [9], with the thermal conductivity of CuO-water nanofluid being higher than that of CuO-Ethylene Glycol nanofluid. The results showed that CuO-engine oil nanofluid had the lowest thermal conductivity. It was suggested that the variation in nanofluid thermal conductivity with different base fluids was related to the different thermal conductivities of the base fluids and the strength of interactions between the CuO nanoparticles and base fluids [10]. [PERSON] et al. [11] measured the thermal conductivities of CuO-water, CuO-Ethylene Glycol, Al\({}_{2}\)O\({}_{3}\)-water, Al\({}_{2}\)O\({}_{3}\)-Ethylene Glycol, Al\({}_{2}\)O\({}_{3}\)-pump fluid, and Al\({}_{2}\)O\({}_{3}\)-engine oil nanofluids. The observations also reinforced the findings of [PERSON] et al. [9] and [PERSON] et al. [10], with the thermal conductivities of water-based nanofluids being higher than those of Ethylene Glycol-based nanofluids for both CuO and Al\({}_{2}\)O\({}_{3}\) nanoparticles, at a given volume fraction. Al\({}_{2}\)O\({}_{3}\)-engine oil and Al\({}_{2}\)O\({}_{3}\)-Pump Fluid nanofluids had lower thermal conductivities than Al\({}_{2}\)O\({}_{3}\)-Ethylene Glycol nanofluid. The thermal conductivity of Al\({}_{2}\)O\({}_{3}\)-engine oil nanofluid was slightly higher than that of the pump fluid-based nanofluid. It was suggested that the thermal conductivity of a nanofluid increases with the thermal conductivity of the base liquid [11]. Additionally, [PERSON] et al. [12] observed that the thermal conductivity of Al\({}_{2}\)O\({}_{3}\)-water nanofluid was higher than that of Al\({}_{2}\)O\({}_{3}\)-Ethylene Glycol nanofluid, which agrees with the findings of [PERSON] et al. [11]. [PERSON] et al. [4] measured the thermal conductivities of Al\({}_{2}\)Cu and Ag\({}_{2}\)Al nanoparticles dispersed in Ethylene Glycol and water. It was again observed that water-based nanofluids had higher thermal conductivities than Ethylene  
Figure 11: Effect of nanoparticle shape on the thermal conductivity of ZnO–water nanofluid as a function of temperature. Glycol-based nanofluids [4]. [PERSON] et al. [13] measured the thermal conductivity of magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles dispersed in 20%:80%, 40%:60% and 60%:40% by weight Ethylene Glycol-water mixtures. The results, shown in Figure 13, indicated that the thermal conductivity decreased as the amount of Ethylene Glycol in the mixture increased. It was suggested that this was a result of the lower thermal conductivity of Ethylene Glycol in comparison to water. Additionally, [PERSON] et al. [14] measured the thermal conductivity of nanodiamond particles dispersed in 20%:80%, 40%:60% and 60%:40% by weight Ethylene Glycol-water mixtures. The results, shown in Figure 14, reinforce the results shown in Figure 13 for nanofluid containing Fe\({}_{3}\)O\({}_{4}\) particles. [PERSON] et al. [15] studied the variation of thermal conductivity as a function of temperature for aluminum nitride-Ethylene Glycol and aluminum nitride-propylene glycol nanofluids in comparison to that of the base liquids (Ethylene Glycol and propylene glycol). The thermal conductivity of aluminum nitride-Ethylene Glycol nanofluid was higher for a given temperature, which is consistent with the conclusion that the thermal conductivity of a nanofluid increases with the thermal conductivity of the base liquid.