## 2 Nanofluid Thermal Conductivity
### Effects of Hybrid Nanoparticles on Nanofluid Thermal Conductivity
It was suggested that the increase with temperature was a result of increased Brownian motion, while that with concentration was a result of the increased area for heat transfer. It was also suggested that the Cu and MWCNTs prevented stacking of the graphene sheets, also contributing to increased thermal conductivity. In addition, although experimental data has shown that the thermal conductivity of nanoparticles themselves does not have a significant influence on the thermal conductivity of nanofluids [8; 26; 32; 36], it was suggested that the combination of multiple materials having high thermal conductivity contributed to the enhanced thermal conductivity of the nanofluid. [PERSON] et al. [55] measured the thermal conductivities of Cu-water, Au-water, Cu-CNT-water, and Au-CNT-water nanofluids using a TC-30 instrument which worked based on a modified hot-wire technique. The results showed a lower thermal conductivity for the hybrid nanofluids of Au-CNT-water and Cu-CNT-water nanofluids in comparison to the nanofluids containing a single type of nanoparticle. It was suggested that the decrease in thermal conductivity was a result of possible agglomeration, or a poor interaction between the Au and Cu with the CNT. Also, [PERSON] et al. [66] measured the thermal conductivity of a hybrid nanofluid composed of Cu and TiO\({}_{2}\) nanoparticles dispersed in a 50-50 Ethylene Glycol /water mixture. The data collected was compared with data for nanofluids composed of the same base liquid and each nanoparticle, individually. It was observed that the hybrid nanofluid containing PVP surfactant had a higher thermal conductivity in comparison to those containing Gum Arabic and SDS surfactants. It was observed that for a given concentration, temperature, and surfactant, the hybrid nanofluid containing PVP surfactant had a higher thermal conductivity than the nanofluids containing solely Cu or TiO\({}_{2}\) nanoparticles. However, for the nanofluid containing Gum Arabic, both the nanofluid containing solely Cu nanoparticles and that containing solely TiO\({}_{2}\) nanoparticles had higher thermal conductivities than the hybrid nanofluid,while for the SDS surfactant, the nanofluid containing solely TiO\({}_{2}\) nanoparticles had a higher thermal conductivity than the hybrid nanofluid. [PERSON] et al. [63] measured the thermal conductivity of ZnO-Ag-water hybrid nanofluid. An increase in thermal conductivity with both temperature and nanoparticle volume concentration was observed; however, it was observed that the increase in nanoparticle concentration had a greater influence on thermal conductivity. It was suggested that the increase with temperature was a result of increased Brownian motion at higher temperatures, while that with concentration was a result of nanoparticle clustering allowing for more rapid heat transfer between the liquid and solid. Using a KD2 Pro, [PERSON] et al. [64] studied the thermal conductivity of water-based and 50/50 water- Ethylene Glycol based hybrid nanofluid containing CuO and Al\({}_{2}\)O\({}_{3}\) nanoparticles. An increase in thermal conductivity with both temperature and nanoparticle volume concentration was observed. It was also observed that the Water-based nanofluid had a higher thermal conductivity than the water- Ethylene Glycol based nanofluid, likely due to the higher thermal conductivity of water in comparison to Ethylene Glycol. In the same way, [PERSON] et al. [67] measured the thermal conductivity of hybrid nanofluid composed of TiO\({}_{2}\) and SiO\({}_{2}\) nanoparticles dispersed in a 60%:40% by volume water- Ethylene Glycol mixture. The nanofluid was tested at the following various nanoparticle mixture ratios (TiO\({}_{2}\):SiO\({}_{2}\)): 20:80, 40:60, 50:50, 60:40, 80:20. It was observed that for each mixture ratio, the nanofluid thermal conductivity increased with temperature, likely due to increased Brownian motion and greater kinetic energy at higher temperatures.