## 2 Nanofluid Thermal Conductivity
### Effects of Concentration of Nanoparticles on Nanofluid Thermal Conductivity
[12] measured the thermal conductivities of Al\({}_{2}\)O\({}_{3}\)-water and Al\({}_{2}\)O\({}_{3}\)-Ethylene Glycol nanofluids using KD2 Pro equipment which worked based on the transient line heat source method. For both nanofluids, the thermal conductivity increased with nanoparticle concentration. The Ethylene Glycol-based nanofluid showed a more rapid increase. At higher concentrations, the results showed a more significant increase in thermal conductivity with nanoparticle concentration, which agrees with the observations of [PERSON] et al. [29]. It was suggested that Ethylene Glycol-based nanofluid would be a more appropriate choice for applications involving rapid changes in particle concentration. [PERSON] et al. [51] measured the thermal conductivity of CuO-oleic acid nanofluid using the LFA 447 NanoFlash Analyzer. It was observed that at low concentrations, the thermal conductivity increased linearly with concentration. At higher concentrations, the thermal conductivity increased non-linearly with concentration. It was suggested that this relationship was a result of nanoparticle agglomeration at high concentrations. [PERSON] et al. [16] measured the thermal conductivity of TiO\({}_{2}\)-water nanofluid containing CTAB surfactant. The results showed a non-linear increase in thermal conductivity with the increase of particle concentration at concentrations below \(\varphi=2\%\), and a linear increase at higher concentrations. It was suggested that the non-linear relationship at lower particle concentrations was caused by the surfactant. Using the transient line heat source method, [PERSON] at al [31] measured the thermal conductivity of nanodiamond-water nanofluid. It was observed that the thermal conductivity increased non-linearly with nanoparticle volume fraction. At higher volume concentrations, a stronger non-linear relationship was observed. It was suggested that the thermal conductivity could be improved by increasing the dispersion quality of the nanofluid. In the same way, [PERSON] et al. [1] measured the thermal conductivity of Fe\({}_{3}\)O\({}_{4}\)-water nanofluid. A non-linear increase in thermal conductivity with nanoparticle volume fraction was observed. [PERSON] et al. [32] conducted experiments to determine the thermal conductivities of SiO\({}_{2}\)-methanol and Al\({}_{2}\)O\({}_{3}\)-methanol nanofluids. A non-linear increase in thermal conductivity with the increase of volume fraction was observed. [PERSON] et al. [8] measured the thermal conductivities of CuO-water and Al\({}_{2}\)O\({}_{3}\)-water nanofluids as a function of volume fraction. It was observed that the thermal conductivity of both nanofluids increased non-linearly with nanoparticle volume concentration. Using a KD2 Pro Thermal Property Analyzer, [PERSON] et al. [33] measured the thermal conductivity of TiO\({}_{2}\)-water nanofluid. The results are shown in Figure 18. It was observed that for a given temperature, the thermal conductivity increased non-linearly with nanoparticle concentration. Using the same method, [PERSON] et al. [34] measured the thermal conductivity of SiC-water nanofluid as a function of nanoparticle volume concentration. The results showed a non-linear increase in thermal conductivity with increased nanoparticle volume fraction. Using the transient hot-wire method, [PERSON] et al. [13] measured the thermal conductivity of nanofluids composed of magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles dispersed in 20%:80%, 40%:60% and 60%:40% by weight Ethylene Glycol-water. The results indicated a non-linear increase in thermal conductivity with the increase of nanoparticle concentration. Additionally, [PERSON] et al. [30] measured the thermal conductivity of magnetic Fe\({}_{3}\)O\({}_{4}\)-water nanofluid containing CTAB surfactant. The results showed a non-linear increase in thermal conductivity with nanoparticle concentration. [PERSON] et al. [35] studied the thermal conductivity of Ag-water nanofluid. The results showed a non-linear increase in the thermal conductivity with nanoparticle concentration.