## 2 Nanofluid Thermal Conductivity
### Effects of Hybrid Nanoparticles on Nanofluid Thermal Conductivity  
Recently, researchers have conducted experiments on nanofluids containing multiple types of nanoparticles. These nanofluids are called hybrid nanofluids. Experimental data has shown that similarly to nanofluids containing one type of nanoparticle, the thermal conductivity of hybrid nanofluids generally increases with temperature and nanoparticle volume concentration [59; 60; 61; 62; 63; 64]. In many cases, researchers combined metal or metal-oxide nanoparticles with various types of carbon nanotubes [55; 59; 60; 62; 65]. For example, using a KD2 Pro Thermal Properties Analyzer, [PERSON] et al. [65] tested the thermal conductivity of CuO-MWCNT-water nanofluid. It was observed that the thermal conductivity of the nanofluid increased with both temperature and nanoparticle volume concentration, having a more significant increase with temperature at higher  
Figure 25: Effect of surfactant on the thermal conductivity of MWCNT–water nanofluid as a function of nanoparticle volume concentration, \(\varphi\), at room temperature. nanoparticle concentrations. It was suggested that the increase with temperature was a result of increased Brownian motion, while the increase with concentration was a result of the greater area for heat transfer and possible clustering of the nanoparticles. In the same way, [PERSON] et al. [60] measured the thermal conductivity of SWCNT-Al\({}_{2}\)O\({}_{3}\)-water nanofluid. A non-linear increase with both temperature and concentration was observed, with a stronger dependence on temperature shown at higher nanoparticle concentrations. It was suggested that this was a result of increased collisions of the nanoparticles due to Brownian motion. Additionally, using a KD2 Pro Thermal Property Analyzer, [PERSON] et al. [62] measured the thermal conductivity of ZnO-MWCNT-50/50 Ethylene Glycol-water nanofluid. The results are shown in Figure 26. A non-linear increase with both temperature and concentration was observed. It was suggested that the increase with temperature was a result of Brownian motion, whereas the increase with nanoparticle volume concentration was a result of the increased surface area to volume ratio allowing for more effective heat transfer, along with a greater number of particles, resulting in a greater number of collisions. Through comparing the collected data for the hybrid nanofluid with previously collected data for MWCNT-water and ZnO-water nanofluids, it was observed that for a given concentration, the hybrid nanofluid had a greater thermal conductivity. Additionally, it was suggested that the hybrid nanofluid was a more efficient choice, due to its lower cost in comparison to a nanofluid containing solely MWCNTs in a higher concentration. Using the same method, [PERSON] et al. [61] conducted experiments to explore the effects of concentration and temperature on MgO-functionalized MWCNT-Ethylene Glycol nanofluid. A non-linear increase with concentration was observed. It was suggested that this increase was a result of clustering of the nanoparticles creating paths with low thermal resistance, thus leading to accelerated heat transfer, and therefore increased thermal conductivity. However, while the thermal conductivity continually increased, the percent enhancement was lower at concentrations above \(\varphi=0.2\%\). It was suggested that this was a result of increased van der Waals forces resulting in increased size of the clusters. It was suggested that the increased thermal conductivity with temperature was a result of increased Brownian motion. Using the thermal hot plate method, [PERSON] et al. [59] measured the thermal conductivity of Ethylene Glycol -based nanofluid containing nanoparticles composed of Cu bonded to the surfaces of MWCNTs and graphene sheets. A non-linear increase was observed with both temperature and nanoparticle concentration.