*5.1. Validation*

The numerical model based on CFD is validated with the experimental results presented by Hamid et al. for the same geometry (dimensions and structure) and boundary conditions [30]. The experimental and numerical results of Nusselt number and friction factor are compared as the heat transfer characteristics for various Reynolds number. The comparison of Nusselt number and friction factor for previous experimental study and present numerical model for various Reynolds number (Re) is depicted in Figure 3. The comparison is presented for water/EG flow in the considered tube geometry under the constant heat flux of 7957 W/m<sup>2</sup> . The trends of experimental and numerical results are sim-

ilar, Nusselt number increases, and friction factor decreases with increase in the Reynolds number. The average deviation between the experimental and numerical results of Nusselt number is 7.66% and that of friction factor is 8.83% for all considered Reynolds number variation. The presented numerical model based on CFD approach is validated within 10% error with previous experimental results for heat transfer characteristics. Therefore, the validated numerical model could be used for the detail comparison of heat transfer characteristics of single particle and hybrid nanofluids flow in uniformly heated plain straight tube. similar, Nusselt number increases, and friction factor decreases with increase in the Reynolds number. The average deviation between the experimental and numerical results of Nusselt number is 7.66% and that of friction factor is 8.83% for all considered Reynolds number variation. The presented numerical model based on CFD approach is validated within 10% error with previous experimental results for heat transfer characteristics. Therefore, the validated numerical model could be used for the detail comparison of heat transfer characteristics of single particle and hybrid nanofluids flow in uniformly heated plain straight tube.

. The trends of experimental and numerical results are

present numerical model for various Reynolds number (Re) is depicted in Figure 3. The comparison is presented for water/EG flow in the considered tube geometry under the

*Symmetry* **2021**, *13*, x FOR PEER REVIEW 8 of 19

constant heat flux of 7957 W/m<sup>2</sup>

**Figure 3.** Comparison of Nusselt number and friction factor for previous experimental study and present numerical model for various Reynolds numbers. **Figure 3.** Comparison of Nusselt number and friction factor for previous experimental study and present numerical model for various Reynolds numbers.

#### *5.2. Comparison of Nanofluid Properties 5.2. Comparison of Nanofluid Properties*

The thermophysical properties namely, density, specific heat, thermal conductivity and viscosity of single particle and hybrid nanofluids are calculated using equations presented in Section 3. The calculated thermophysical properties of single particle and hybrid nanofluids are depicted in Table 2. In case of hybrid nanofluid, the composition of Al2O<sup>3</sup> and Cu are mixed in the proposition of 50/50%. The density and thermal conductivity of hybrid nanofluid are higher than those of single particle nanofluid for all volume fractions. This is because the Cu nanoparticles have higher density and thermal conductivity compared to Al2O<sup>3</sup> nanoparticles. However, the specific heat of single particle nanofluid is superior to the specific heat of hybrid nanofluid for all volume fractions due to lower specific heat of Cu nanoparticles compared to specific heat of Al2O<sup>3</sup> nanoparticles. There is no difference for the viscosity of single particle and hybrid nanofluids for each volume fraction because the viscosity of base fluid is same for both single particle and hybrid nanofluids. However, the viscosity of nanofluids increases with increase in the volume fraction. The density and thermal conductivity of single particle and hybrid nanofluids have increased and the specific heat has decreased with increase in the volume fraction. This is because the density and thermal conductivity of nanoparticles enhances, and the specific heat of nanoparticles reduces with increase in volume fraction of nanoparticle in The thermophysical properties namely, density, specific heat, thermal conductivity and viscosity of single particle and hybrid nanofluids are calculated using equations presented in Section 3. The calculated thermophysical properties of single particle and hybrid nanofluids are depicted in Table 2. In case of hybrid nanofluid, the composition of Al2O<sup>3</sup> and Cu are mixed in the proposition of 50/50%. The density and thermal conductivity of hybrid nanofluid are higher than those of single particle nanofluid for all volume fractions. This is because the Cu nanoparticles have higher density and thermal conductivity compared to Al2O<sup>3</sup> nanoparticles. However, the specific heat of single particle nanofluid is superior to the specific heat of hybrid nanofluid for all volume fractions due to lower specific heat of Cu nanoparticles compared to specific heat of Al2O<sup>3</sup> nanoparticles. There is no difference for the viscosity of single particle and hybrid nanofluids for each volume fraction because the viscosity of base fluid is same for both single particle and hybrid nanofluids. However, the viscosity of nanofluids increases with increase in the volume fraction. The density and thermal conductivity of single particle and hybrid nanofluids have increased and the specific heat has decreased with increase in the volume fraction. This is because the density and thermal conductivity of nanoparticles enhances, and the specific heat of nanoparticles reduces with increase in volume fraction of nanoparticle in both single particle and hybrid nanofluids. The density and thermal conductivity of nanoparticles are higher than water which shows superior density and thermal conductivity of nanofluids than base fluid which further increases with increase in the volume fraction. Whereas the specific heat of nanoparticles is lower than the water which results into lower specific heat

of nanofluids than base fluid and further decreases with increase in the volume fraction. This is because the water has lower density, lower thermal conductivity and higher specific heat compared to Al2O<sup>3</sup> and Cu nanoparticles. The density enhances by 4.4% and 7.5% for Al2O<sup>3</sup> and Al2O3/Cu nanofluids, respectively with increase in volume fraction from 0.5% to 2.0%. The thermal conductivity improves by 4.3% and 6.1% for Al2O<sup>3</sup> and Al2O3/Cu nanofluids, respectively with increase in volume fraction from 0.5% to 2.0%. The specific heat of Al2O<sup>3</sup> and Al2O3/Cu nanofluids decease by 4.6% and 7.3%, respectively with increase in volume fraction from 0.5% to 2.0%. The viscosity increases by 3.9% for Al2O<sup>3</sup> and Al2O3/Cu nanofluids with increase in volume fraction from 0.5% to 2.0%. Addition of Cu nanoparticle along with Al2O<sup>3</sup> nanoparticle improves the thermophysical properties of hybrid nanofluid compared to single particle nanofluid.

**Table 2.** Properties of base fluid and nanoparticles.

