## 2 Nanofluid Thermal Conductivity
### Effects of Nanoparticle Material on Nanofluid Thermal Conductivity
[29] also observed relatively low values for the thermal conductivity of nanodiamond-water nanofluids. Within the temperature range of 20-60 \({}^{\circ}\)C, for \(\varphi=1\%\), the thermal conductivity of nanodiamond-water nanofluid ranged from 0.67-0.80 \(W/m.k.\) The thermal conductivities of nanofluids composed of various nanoparticles dispersed in water at room temperature are shown in Figure 2 as a function of nanoparticle volume concentration, \(\varphi\), for concentrations below \(\varphi=0.01\%\). It is observed that for a given nanoparticle concentration, 5000 nm oxidized graphene nanofluid has higher thermal conductivity than the nanofluid composed of unoxidized graphene nanoparticles having the same size, despite the nanoparticles having the same thermal conductivity. This may be a result of the greater stability of the nanofluid containing oxidized graphene nanoparticles. It is also observed that the thermal conductivity of 5000 nm graphene-water nanofluid is higher than that of 15,000 nm graphene-water nanofluid, which is likely caused by the smaller size of the 5000 nm particles. The thermal conductivities of nanofluids composed of various nanoparticles dispersed in water at room temperature are shown in Figure 3 as a function of nanoparticle volume concentration, \(\varphi\), for concentrations below \(\varphi=1\%\). It is observed that the thermal conductivity of TiO\({}_{2}\)-water nanofluid is higher than that of Al\({}_{2}\)O\({}_{3}\)-water nanofluid. This may be a result of the smaller size of the TiO\({}_{2}\) nanoparticles. The thermal conductivities of nanofluids composed of various nanoparticles dispersed in water at room temperature are shown in Figure 4 as a function of nanoparticle volume concentration, \(\varphi\), for concentrations below \(\varphi=5\%\). Despite the low thermal conductivity of Fe\({}_{3}\)O\({}_{4}\) in comparison to other oxide nanoparticles, it is observed that the thermal conductivity of Fe\({}_{3}\)O\({}_{4}\)-water nanofluid is unexpectedly high. This may be a result of more significant clustering and alignment of the Fe\({}_{3}\)O\({}_{4}\) nanoparticles in comparison to other oxide nanoparticles dispersed in water [1]. It is also observed that the thermal conductivity of Al-water nanofluid is higher than the thermal conductivities of nanofluids containing oxide nanoparticles, other than Fe\({}_{3}\)O\({}_{4}\). This may be a result of the higher intrinsic thermal conductivity of metals in comparison to metal oxides. The thermal conductivities of water-based nanofluids containing various nanoparticles are shown in Figure 5 as a function of temperature, at a nanoparticle volume concentration of \(\varphi=1\%\). It is observed that for a given temperature, the thermal conductivity of Fe\({}_{3}\)O\({}_{4}\)-water nanofluid is higher than that of the CuO-water nanofluid. This observation is inconsistent with Figure 1, in which it is observed that the thermal conductivity of CuO-water nanofluid is slightly higher than that of Fe\({}_{3}\)O\({}_{4}\)-water nanofluid for a given temperature, at \(\varphi=2\%\). The thermal conductivities of nanofluids composed of various nanoparticles dispersed in Ethylene Glycol at room temperature are shown in Figure 5 as a function of nanoparticle volume concentration, \(\varphi\), for concentrations below \(\varphi=5\%\). It is observed that nanodiamond-Ethylene Glycol nanofluid has a higher thermal conductivity than other nanofluids, besides MgO-Ethylene Glycol nanofluid, for a given nanoparticle volume concentration. The higher thermal conductivity of nanodiamond-Ethylene Glycol nanofluid may be a result of the combination of the high thermal conductivity of nanodiamond, and the strong compatibility between nanodiamond particles and Ethylene Glycol base fluid, which is caused by the carboxyl and hydroxyl groups on the surfaces of the nanodiamond particles having strong interactions with polar Ethylene Glycol in an alkaline environment (having a PH value higher than 7.0) [45].