## 2 Nanofluid Thermal Conductivity
### Effects of Concentration of Nanoparticles on Nanofluid Thermal Conductivity
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Nanoparticle** & **Base Liquid** & **Surfactant** & **Remarks** \\ \hline \multirow{8}{*}{[PERSON] et al. [17] (2016)} & \multirow{8}{*}{CuO Al\({}_{2}\)O\({}_{3}\)} & & & The negatively charged surfactant altered the surface charges of the positively charged nanoparticles, causing repulsion forces between the nanoparticles. These forces created a more stable nanofluid and led to increased thermal conductivity. \\  & & & & It was suggested that an ideal surface charge on the nanoparticles leads to ideal thermo-physical properties. \\  & & & & Above the surfactant concentrations of 0.1 \\  & & & & W\% for Al\({}_{2}\)O\({}_{3}\)-water nanofluid, and 0.15 W\% for CuO-water nanofluid the thermal conductivity decreased significantly. It was suggested that this decrease was caused by greater amounts of surfactant on the surfaces of the nanoparticles allowing a narrower area for heat transfer. \\ \hline \multirow{8}{*}{[PERSON] et al. [20] (2014)} & \multirow{8}{*}{Al\({}_{2}\)O\({}_{3}\)} & & & PVP surfactant produced a more stable nanofluid than SDS surfactant. \\  & & & & The surfactant decreased the thermal conductivity of the base liquid but improved the stability of the nanofluid by preventing agglomeration of the nanoparticles. \\ \hline \multirow{8}{*}{[PERSON] et al. [18] (2009)} & \multirow{8}{*}{Al\({}_{2}\)O\({}_{3}\)Cu} & & & It was suggested that obtaining an optimal pH value and surfactant concentration would lead to enhanced thermal conductivity by altering the surface charges of the nanoparticles in order to create repulsion forces and prevent agglomeration. \\  & & & & The thermal conductivity increased with surfactant concentration but decreased significantly at greater concentrations. It was suggested that this decrease was a result of greater amounts of surfactant on the surfaces of the nanoparticles allowing a narrower area for heat transfer. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._transient hot-wire method, [PERSON] et al. [23] measured the thermal conductivity of hydrophobic Ag-hexane and hydrophilic Ag-Ethylene Glycol nanofluids as a function of nanoparticle concentration. For both nanofluids, the thermal conductivity increased with the increase of particle concentration, however, the rate of increase in thermal conductivity slowed at higher concentrations. It was suggested that the decelerated rate of increase was related to agglomeration and settling of nanoparticles at higher concentrations. Using the same method, [PERSON] et al. [27] studied the thermal conductivity of ZnO-Ethylene Glycol nanofluid. A non-linear increase in thermal conductivity with nanoparticle concentration was observed. Two different linear relationships were found, one at lower concentrations, and one at higher concentrations. The linear relationship at lower concentrations had a higher slope than the linear relationship at higher concentrations. [PERSON] et al. [28] measured the thermal conductivities of CuO-transformer oil, Al\({}_{2}\)O\({}_{3}\)-water, Cu-Ethylene Glycol, and Al-water nanofluids as a function of nanoparticle concentration. For all nanofluids tested, the thermal conductivity increased with concentration. However, it was observed that the increase in thermal conductivity was relatively higher at low nanoparticle concentrations, which reinforces the observations of [PERSON] et al. [26], [PERSON] et al. [23], and [PERSON] et al. [27]. [PERSON] et al. [29] measured the thermal conductivity of nanodiamond-water nanofluid using the transient hot-wire method. It was observed that the thermal conductivity increased with concentration. However, contrary to the previously discussed observations, it was observed that the enhancement in thermal conductivity was greater at higher concentrations. It was suggested that this was caused by agglomeration of the nanodiamond particles. [PERSON] et al.