*5.4. Hybrid Nanofluid with Different Compositions*

The comparison of heat transfer characteristics of water, 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) is presented in this section. The variation of heat transfer coefficient, Nusselt number, pressure drop, friction factor and performance evaluation criteria with various Reynolds number for above considered working fluids is presented in Figure 10. The heat transfer coefficient and Nusselt number of various working fluid have increased linearly with Reynolds number. This is because of increase in turbulence at higher Reynolds number has resulted into higher heat transfer rate. The hybrid nanofluids with all compositions show higher heat transfer coefficient and Nusselt number than single particle nanofluid and water. This is because of superior thermal conductivity and heat transfer performance of the single particle and hybrid nanofluids compared to water. The increase in the heat transfer coefficient and Nusselt number of hybrid nanofluid enhances with higher portion of Cu nanoparticle than Al2O<sup>3</sup> nanoparticle because the thermal conductivity of Cu nanoparticle is superior to Al2O<sup>3</sup> nanoparticle. This means 2.0% Al2O3/Cu (25/75%) nanofluid presents highest values of heat transfer coefficient and Nusselt number followed by 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%), 2.0% Al2O<sup>3</sup> nanofluids and water, respectively. The heat transfer coefficient enhances by 546.4% and Nusselt number increases from 9.14 to 59.09 for 2.0% Al2O3/Cu (25/75%) nanofluid with increase in the Reynolds number from 2000 to 12,000. The heat transfer coefficients of 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanofluids are higher by 10.2%, 14.8%, 13.3% and 16.3%, respectively compared with water. The water, 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanofluids show the Nusselt number of 50.83, 56.02, 58.29, 57.48 and 59.09, respectively at the Reynolds number of 12,000. The vari-

[3].

ation of pressure drop for various working fluid are not linear but increasing with increase in the Reynolds number. This is because the turbulence has increased with increase in the Reynolds number. The decreasing order of pressure drop is 2.0% Al2O3, 2.0% Al2O3/Cu (75/25%) 2.0% Al2O3/Cu (50/50%), water and 2.0% Al2O3/Cu (25/75%), respectively. The increase in Cu nanoparticle portion in hybrid nanofluid composition reduces the pressure drop. The 2.0% Al2O3/Cu (25/75%) nanofluid shows the lowest pressure drop among all working fluids. Compared to water, the pressure drops of 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%) and 2.0% Al2O3/Cu (75/25%) nanofluids are higher by 4.5%, 0.34% and 2.4%, respectively and pressure drop of 2.0% Al2O3/Cu (25/75%) nanofluid is lower by 1.6%. The variation in friction factor of various working fluids is parabolic with Reynolds number due to change of flow regime from laminar to transition and transition to turbulent with critical Reynolds number. The critical Reynolds number is 6000 where the friction factor is maximum for all working fluids. The 2.0% Al2O3/Cu (25/75%) nanofluid shows highest friction factor followed by 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%), 2.0% Al2O<sup>3</sup> nanofluids and water respectively in the decreasing order. This is due to higher viscosity as well as higher density of hybrid nanofluids, the density increases as the portion of Cu nanoparticle increases which results into lower velocity. The friction factors of 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanofluids are higher by 5.9%, 10.3%, 8.1% and 12.5%, respectively compared to water. The performance evaluation criteria of 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanofluids are affected very less by the variation in the Reynolds number due to significant effect of Reynolds number on both Nusselt number and friction factor. The increase of Cu nanoparticle portion in composition of hybrid nanofluid has shown significant enhancement in the performance evaluation criteria due to overall improvement in the thermophysical properties of hybrid nanofluid. Compared to 2.0% Al2O<sup>3</sup> nanofluid, the performance evaluation criteria of 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanofluids have improved by 2.8%, 2.1% and 3.5%, respectively. The 2.0% Al2O3/Cu (25/75%) nanofluid presents the highest value of performance evaluation criteria. *Symmetry* **2021**, *13*, x FOR PEER REVIEW 13 of 19 hybrid nanofluid shows higher friction factor than single particle nanofluid at the same volume fraction. Despite of lower pressure drop for hybrid nanofluid than single particle nanofluid, the higher density has caused higher friction factor. The friction factors of 0.5% Al2O3, 1.0% Al2O3, 2.0% Al2O3, 0.5% Al2O3/Cu, 1.0% Al2O3/Cu and 2.0% Al2O3/Cu nanofluids are higher by 1.5%, 2.9%, 5.9%, 2.6%, 5.2% and 10.3%, respectively compared to water. The 2.0% Al2O3/Cu nanofluid shows highest value of friction factor among all working fluids. The friction factor of 2.0% Al2O3/Cu nanofluid increases by 1.9% as the Reynolds number increases from 2000 to 12,000. Firoozi et al. have also presented the friction factor variation range from 0.02 to 0.14 for the maximum Reynolds number variation up to 5000

**Figure 7.** Effect of Reynolds number and volume fraction on friction factor of water, single particle and hybrid nanofluids. **Figure 7.** Effect of Reynolds number and volume fraction on friction factor of water, single particle and hybrid nanofluids.

The performance evaluation criteria present the combined effect of thermal and flow characteristics because it is calculated based on the Nusselt numbers and friction factors of water and single particle and hybrid nanofluids. The comparison of performance evaluation criteria (PEC) for single particle and hybrid nanofluids with various volume fractions and Reynolds number is depicted in Figure 8. The effect of Reynolds number on the performance evaluation criteria of single particle and hybrid nanofluids is very small. This is because both Nusselt number and friction factor are affected significantly due to variation in Reynolds number. Hence, the overall effect of Reynolds number on performance evaluation criteria is nullified. The Nusselt number and friction factor have improved with increase in volume fraction of single particle and hybrid nanofluids as a result of improvement in the thermophysical properties. Therefore, the performance evaluation criteria enhance for both single particle and hybrid nanofluids as volume fraction increases. Azmi et al. have proved that the variation in the performance evaluation criteria is very small and non-linear with Reynolds number. In addition, the performance evaluation criteria increase with increase in volume fraction of nanoparticles [24]. The performance evaluation criteria for hybrid nanofluid are superior compared to single particle

Al2O<sup>3</sup> nanofluid.

nanofluid for the same volume fraction because the increase in the Nusselt number is highly dominant compared to the increase in the friction factor for hybrid nanofluid. The 0.5% Al2O<sup>3</sup> nanofluid presents the lowest value of performance evaluation criteria and 2.0% Al2O3/Cu nanofluid presents the highest value of performance evaluation criteria among all nanofluids. Compared to the performance evaluation criteria of 0.5% Al2O<sup>3</sup> nanofluid, the performance evaluation criteria of 1.0% Al2O3, 2.0% Al2O3, 0.5% Al2O3/Cu, 1.0% Al2O3/Cu and 2.0% Al2O3/Cu nanofluids are higher by 1.9%, 5.9%, 0.7%, 3.3% and 8.9%, respectively. Overall, 2.0% Al2O3/Cu nanofluid with composition of 50/50% has presented the superior heat transfer characteristics among all working fluids. The 2.0% Al2O<sup>3</sup> nanofluid in single particle case and 2.0% Al2O3/Cu nanofluids in hybrid case present the superior performances in the respective groups. Therefore, the simulated contours of temperature and velocity for water, 2.0% Al2O<sup>3</sup> single particle nanofluid and 2.0% Al2O3/Cu hybrid nanofluid with composition of 50/50% are depicted in Figure 9. The most important section to observe the behavior of simulated results is the outlet of tube. Therefore, the temperature and velocity contours are presented for the working fluid domain at the tube outlet cross section considering the Reynolds numbers of 2000 and 12,000. As can be seen from Figure 9, the distribution of temperature and velocity contours at the outlet section of tube are symmetrical for water and nanofluids. The heat transfer characteristics of 2.0% Al2O3/Cu nanofluid is further investigated with additional two compositions of 75/25% and 25/75% and compared with composition of 50/50% as well as water and 2.0%

**Figure 8.** Comparison of performance evaluation criteria for single particle and hybrid nanofluids with various volume fractions and Reynolds number. **Figure 8.** Comparison of performance evaluation criteria for single particle and hybrid nanofluids with various volume fractions and Reynolds number. *Symmetry* **2021**, *13*, x FOR PEER REVIEW 15 of 19

**Figure 9.** Temperature and velocity contours for water, 2% Al2O3 single particle nanofluid and 2% Al2O3/Cu hybrid nanofluid with 50/50% composition at Re:2000 and Re:12,000. **Figure 9.** Temperature and velocity contours for water, 2% Al2O3 single particle nanofluid and 2% Al2O3/Cu hybrid nanofluid with 50/50% composition at Re:2000 and Re:12,000.

The comparison of heat transfer characteristics of water, 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) is presented in this section. The variation of heat transfer coefficient, Nusselt number, pressure drop, friction factor and performance evaluation criteria with various Reynolds number for above considered working fluids is presented in Figure 10. The heat transfer coefficient and Nusselt number of various working fluid have increased linearly with Reynolds number. This is because of increase in turbulence at higher Reynolds number has resulted into higher heat transfer rate. The hybrid nanofluids with all compositions show higher heat transfer coefficient and Nusselt number than single particle nanofluid and water. This is because of superior thermal conductivity and heat transfer performance of the single particle and hybrid nanofluids compared to water. The increase in the heat transfer coefficient and Nusselt number of hybrid nanofluid enhances with higher portion of Cu nanoparticle than Al2O<sup>3</sup> nanoparticle because the thermal conductivity of Cu nanoparticle is superior to Al2O<sup>3</sup> nanoparticle. This means 2.0% Al2O3/Cu (25/75%) nanofluid presents highest values of heat transfer coefficient and Nusselt number followed by 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%), 2.0% Al2O<sup>3</sup> nanofluids and water, respectively. The heat transfer coefficient enhances by 546.4% and Nusselt number increases from 9.14 to 59.09 for 2.0% Al2O3/Cu (25/75%) nanofluid with increase in the Reynolds number from 2000 to 12,000. The heat transfer coefficients of 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanofluids are higher by 10.2%, 14.8%, 13.3% and 16.3%, respectively compared with water. The water, 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanofluids show the Nusselt number of 50.83, 56.02, 58.29, 57.48 and 59.09, respectively at the Reynolds number of 12,000. The

variation of pressure drop for various working fluid are not linear but increasing with increase in the Reynolds number. This is because the turbulence has increased with increase in the Reynolds number. The decreasing order of pressure drop is 2.0% Al2O3, 2.0% Al2O3/Cu (75/25%) 2.0% Al2O3/Cu (50/50%), water and 2.0% Al2O3/Cu (25/75%), respectively. The increase in Cu nanoparticle portion in hybrid nanofluid composition reduces the pressure drop. The 2.0% Al2O3/Cu (25/75%) nanofluid shows the lowest pressure drop among all working fluids. Compared to water, the pressure drops of 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%) and 2.0% Al2O3/Cu (75/25%) nanofluids are higher by 4.5%, 0.34% and 2.4%, respectively and pressure drop of 2.0% Al2O3/Cu (25/75%) nanofluid is lower by 1.6%. The variation in friction factor of various working fluids is parabolic with Reynolds number due to change of flow regime from laminar to transition and transition to turbulent with critical Reynolds number. The critical Reynolds number is 6000 where the friction factor is maximum for all working fluids. The 2.0% Al2O3/Cu (25/75%) nanofluid shows highest friction factor followed by 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%), 2.0% Al2O<sup>3</sup> nanofluids and water respectively in the decreasing order. This is due to higher viscosity as well as higher density of hybrid nanofluids, the density increases as the portion of Cu nanoparticle increases which results into lower velocity. The friction factors of 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanofluids are higher by 5.9%, 10.3%, 8.1% and 12.5%, respectively compared to water. The performance evaluation criteria of 2.0% Al2O3, 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanofluids are affected very less by the variation in the Reynolds number due to significant effect of Reynolds number on both Nusselt number and friction factor. The increase of Cu nanoparticle portion in composition of hybrid nanofluid has shown significant enhancement in the performance evaluation criteria due to overall improvement in the thermophysical properties of hybrid nanofluid. Compared to 2.0% Al2O<sup>3</sup> nanofluid, the performance evaluation criteria of 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%) nanoflu-

**Figure 10.** Comparison of heat transfer characteristics of water, 2.0% Al2O3, 2.0% Al2O3/Cu **Figure 10.** Comparison of heat transfer characteristics of water, 2.0% Al2O<sup>3</sup> , 2.0% Al2O3/Cu (50/50%), 2.0% Al2O3/Cu (75/25%) and 2.0% Al2O3/Cu (25/75%).
