5.3.1. Heat Transfer Coefficient

The heat transfer coefficient (h) of water, single particle and hybrid nanofluids flow inside tube for various volume fractions and Reynolds number is presented in Figure 4. The heat transfer coefficients of all working fluids have increased with increase in the Reynolds number and the variation trends are linear. Saedodin et al. have also presented the increasing linear trend of heat transfer coefficient for various working fluids flow inside tube with Reynolds number [7]. The turbulence increases with increase in the Reynolds number for all working fluids. The heat transfer rate increases as the turbulence increases. Higher turbulence at higher Reynolds number of working fluids absorbs higher heat and presents higher heat transfer rate compared with lower turbulence at lower Reynolds number of working fluids. Therefore, the heat transfer coefficient of working fluids is higher at the higher Reynolds number and lower at lower Reynolds number. The heat transfer coefficient of single particle and hybrid nanofluids for all volume fractions are higher than water because of improved thermophysical properties of single particle and hybrid nanofluids compared to water. The heat transfer coefficient improves further with increase in the volume fractions. In addition, for the same volume fraction, the heat transfer coefficient of hybrid nanofluid is better than the single particle nanofluid due to addition of Cu nanoparticles with higher thermal conductivity in hybrid nanofluid. The heat transfer coefficients of Al2O<sup>3</sup> nanofluid are higher by 2.5%, 4.9% and 10.2% and those of Al2O3/Cu nanofluids are higher by 3.6%, 7.2% and 14.8% for volume fractions of 0.5%, 1.0% and 2.0%, respectively compared with water. The Al2O3/Cu nanofluid with 2.0% volume fraction shows highest heat transfer coefficient among all working fluids. The heat transfer coefficient increases by 546.1% for Al2O3/Cu nanofluid with 2.0% volume fraction when the Reynolds number increases from 2000 to 12,000.

heat transfer coefficients of Al2O<sup>3</sup> nanofluid are higher by 2.5%, 4.9% and 10.2% and those of Al2O3/Cu nanofluids are higher by 3.6%, 7.2% and 14.8% for volume fractions of 0.5%, 1.0% and 2.0%, respectively compared with water. The Al2O3/Cu nanofluid with 2.0% volume fraction shows highest heat transfer coefficient among all working fluids. The heat transfer coefficient increases by 546.1% for Al2O3/Cu nanofluid with 2.0% volume fraction

when the Reynolds number increases from 2000 to 12,000.

**Figure 4.** Heat transfer coefficient of water, single particle and hybrid nanofluids flow inside tube for various volume fractions and Reynolds number. **Figure 4.** Heat transfer coefficient of water, single particle and hybrid nanofluids flow inside tube for various volume fractions and Reynolds number.

#### 5.3.2. Nusselt Number 5.3.2. Nusselt Number

The comparison of Nusselt number of various working fluids with Reynolds number is presented in Figure 5. The Nusselt number depends on the heat transfer coefficient hence, the variation trend of Nusselt number is similar as of heat transfer coefficient for each working fluid. The Nusselt number increases linearly with Reynolds number for all working fluids because of increase in the turbulence with increase in the Reynolds number. The Nusselt number of hybrid nanofluid is better than the single particle nanofluid for the same volume fraction due to higher heat transfer coefficient of hybrid nanofluid compared to single particle nanofluid. In addition, due to the improved thermophysical properties of nanofluids by dispersion of nanoparticles in water, the Nusselt number of single particle and hybrid nanofluids are superior compared to water. Addition of solid nanoparticles into water has enhanced the thermal conductivity which improves continuously as larger amounts of nanoparticles are dispersed, which results in increase in heat transfer coefficient. Therefore, the Nusselt number increases with increase in the volume fraction for both single particle and hybrid nanofluids. The Nusselt numbers of 50.83, 52.13, 53.36, 56.02, 52.65, 54.49 and 58.29 are evaluated for water, 0.5% Al2O3, 1.0% Al2O3, 2.0% Al2O3, 0.5% Al2O3/Cu, 1.0% Al2O3/Cu and 2.0% Al2O3/Cu, respectively at Reynolds number of 12000. The 2.0% Al2O3/Cu nanofluid presents superior value of Nusselt number for all Reynolds number compared to other working fluids. The Nusselt number of 2.0% Al2O3/Cu nanofluid increases from 9.02 to 58.29 as the Reynolds number increases from 2000 to 12000. Firoozi et al. have shown the Nusselt number ranging from 10 to 40 for the variation of Reynolds number up to 5000 [3]. Torii and Hajime have reported the Nusselt The comparison of Nusselt number of various working fluids with Reynolds number is presented in Figure 5. The Nusselt number depends on the heat transfer coefficient hence, the variation trend of Nusselt number is similar as of heat transfer coefficient for each working fluid. The Nusselt number increases linearly with Reynolds number for all working fluids because of increase in the turbulence with increase in the Reynolds number. The Nusselt number of hybrid nanofluid is better than the single particle nanofluid for the same volume fraction due to higher heat transfer coefficient of hybrid nanofluid compared to single particle nanofluid. In addition, due to the improved thermophysical properties of nanofluids by dispersion of nanoparticles in water, the Nusselt number of single particle and hybrid nanofluids are superior compared to water. Addition of solid nanoparticles into water has enhanced the thermal conductivity which improves continuously as larger amounts of nanoparticles are dispersed, which results in increase in heat transfer coefficient. Therefore, the Nusselt number increases with increase in the volume fraction for both single particle and hybrid nanofluids. The Nusselt numbers of 50.83, 52.13, 53.36, 56.02, 52.65, 54.49 and 58.29 are evaluated for water, 0.5% Al2O3, 1.0% Al2O3, 2.0% Al2O3, 0.5% Al2O3/Cu, 1.0% Al2O3/Cu and 2.0% Al2O3/Cu, respectively at Reynolds number of 12000. The 2.0% Al2O3/Cu nanofluid presents superior value of Nusselt number for all Reynolds number compared to other working fluids. The Nusselt number of 2.0% Al2O3/Cu nanofluid increases from 9.02 to 58.29 as the Reynolds number increases from 2000 to 12000. Firoozi et al. have shown the Nusselt number ranging from 10 to 40 for the variation of Reynolds number up to 5000 [3]. Torii and Hajime have reported the Nusselt number enhancement for graphene-oxide nanofluid as twice than water for horizontal circular tube under constant heat flux [45].
