## 3 Results and Discussion
For the base fluid (wt% = 0), the maximum decrease in viscosity is nearly 48% (from 0.89 at 25 \({}^{\circ}\)C to 0.46 at 60 \({}^{\circ}\)C), whereas for the highest mass fraction (wt% = 1), it is about 45% (from 1.81 at 25 \({}^{\circ}\)C to 1 at 60 \({}^{\circ}\)C). It to be mentioned that except at the beginning of the variations the increase slop of the viscosity is more important for higher mass fraction values especially for lower temperatures. This augmentation is due to the increase in molecular interaction between the nanoparticles and base liquid at higher mass fractions. The variation of the local coefficient of heat transfer with the axial position of the helically coiled tube with various mass fractions (wt% = 0.2% to wt% = 1%), for Re = 1590, is shown in Figure 7. Regardless of the axial position, this coefficient is markedly increased with the increase in the CuO-Gp nanoparticles mass fraction. At the beginning, this coefficient varies from 1500 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\) to 2692 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\), while at the exit it ranges from 234 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\) to 420 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\). For Re = 1590, the percentage of heat transfer coefficient enhancement versus nanoparticles mass fraction at the outlet of the helical coil (xi/D = 230) is plotted in Figure 8. As can be seen, the enhancement is nearly proportional to the mass fraction. In fact, it is around 23% for a mass fraction wt% = 0.2% while it reaches it maximum of 79.6% for wt% = 1%. Figure 5: Conductivity variation with hybrid nanoparticles mass fraction for different temperatures. Figure 6: Dynamic viscosity variation with hybrid nanoparticles mass fraction for various temperatures. Figure 7: Variation of the coefficient of heat transfer with the axial position at different CuO-Gp nanoparticles mass fractions for Re = 1590. Variation of the average coefficient of heat transfer, for Re = 1590, with hybrid nanoparticles' mass fraction is illustrated in Figure 9. Similar to the local heat transfer coefficient, there is a gradual augmentation with the rise of CuO-Gp nanoparticles loading. The average heat transfer coefficient reaches its maximum of 1012.14 W/m\({}^{2. }\)C for wt% = 1% and Re = 1590. The variation of the local coefficient of heat transfer with the axial position of the helically coiled tube with various Reynolds number values for wt% = 1% is illustrated in Figure 10. Disregarding the axial position, the increase in Reynolds number ameliorates  
Figure 8: Effect of nanoparticles mass fraction on the % of enhancement in heat transfer coefficient for Re = 1590 and xi/D = 230. Figure 4:0 Effect of CuO-Gp nanoparticles mass fraction on the average heat transfer coefficient for Re = 1590.  
the heat transfer coefficient. Furthermore, this amelioration is more pronounced at the coil entrance region where it is augmented from 1505 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\) to 4115 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\), while at the exit it was found to vary from 196 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\) to 536 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\). Figure 11 depicted the impact of Reynolds number on the average heat transfer coefficient, for wt% = 1%. Accordingly, a continuous increase is revealed with the rising of Reynolds number to attain its maximum of 1173 W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\) for Re = 1843. Figure 4:1 Variation of the average coefficient of heat transfer with Reynolds number for wt% = 1%. Figure 4:2 Variation of the local heat transfer coefficient with the axial position for various Reynolds number values and wt% = 1%.