Scientists from different organizations throughout the world accomplished a benchmark research on the thermal conductivity of nanofluids, and the results indicated that the experimental data were in

good agreement when Nan’s model is used. According to Nan’s model, the thermal conductivity of the nanofluid can be calculated as follows: (2) where L ii and ϕ are the geometrical factor and the volume fraction of particles, respectively. β ii is defined as (3) where k p is the thermal conductivity of the particles. For GNPs, the aspect ratio is very high, so L 11 = 0 and L 33 = 1. It should be mentioned that the thermal conductivity determined here by Nan’s model has taken the matrix additive interface contact resistance into consideration. In Equation 2, the predicted thermal conductivity of composite is sensitive to the small change Ceritinib of the nanoparticles’ Apoptosis inhibitor thermal conductivity. Additionally, the theoretical calculation established that the thermal conductivity of graphene can be influenced by the dimensions, edge roughness, and defect density. Figure 11 shows the thermal conductivity enhancement of GNP nanofluids as a function of loading at a constant temperature of 30°C. From the results, it can be clearly seen that experimental results

can be validate using Nan’s model. Furthermore, the comparison between carbon-based nanofluids in most recent works is shown in Table 2. Figure 11 Thermal conductivity enhancement based on Nan’s model and experimental results at 30°C. Table 2 Thermal conductivity enhancement of recent nanofluids in literature Base fluid Concentration (wt.%) Dispersant + base fluid Maximum enhancement (%) Reference MWNTs 0.60 DW 34 [34] Graphite 0.5 DW + PVP 23 [35] GO 12 EG 61 [11] GNP 300 0.1 DW 14.8 Present study GNP 500 0.1 DW 25 Present study GNP 750

0.1 DW 27.6 Present study MWNTs, multiwall carbon nanotubes; GO, graphene oxide; DW, distilled water; EG, ethylene glycol; PVP, polyvinylpyrrolidone. Based on the results in Table 2, it is outstandingly evident that GNP nanofluids provide a significant thermal conductivity enhancement compared to those of other works when they have higher concentrations of nanoparticles. From these results, it can be seen that the use of low concentration of GNPs can achieve acceptable thermal conductivity enhancement for medium-temperature applications including solar collectors CYTH4 and heat exchanger systems. Electrical conductivity analysis Though important, the electrical conductivity of nanofluids has not yet been widely studied as compared to thermal conductivity. The electrical conductivity of a suspension can either increase or decrease depending on the background electrolyte, particle size, particle loading, and charge of the particle. The electrical conductivity (σ) of water is related to the temperature and increases by 2% to 3% for each 1°C increase (typical electrical conductivity of distilled water at 25°C is about 5.5 × 10−6 S/m).