###### Abstract  
The thermal performance enhancement of a vertical helical coil heat exchanger using distilled water-based copper oxide-graphene hybrid nanofluid has been analyzed experimentally. Accordingly, the focus of this study is the preparation of CuO-Gp (80-20%) hybrid nanoparticles-based suspensions with various mass fractions (0% \(\leq\) wt \(\leq\) 1%). The volume flow rate is ranged from 0.5 L-min\({}^{-1}\) to 1.5 L-min\({}^{-1}\) to keep the laminar flow regime (768 \(\leq\) Re \(\leq\) 1843) and the supplied hot fluid's temperature was chosen to equal 50 \({}^{\circ}\)C. To ensure the dispersion and avoid agglomeration an ultrasound sonicator is used and the thermal conductivity is evaluated via KD2 Pro Thermal Properties Analyzer. It has been found that the increment in nanoparticles mass fraction enhances considerably the thermal conductivity and the thermal energy exchange rate. In fact, an enhancement of 23.65% in the heat transfer coefficient is obtained with wt = 0.2%, while it is as high as 79.68% for wt = 1%. Moreover, increasing Reynolds number results in a considerable augmentation of the heat transfer coefficient. copper oxide-graphene hybrid nanofluid; experimental study; heat transfer coefficient; helical coil heat exchanger +
Footnote †: journal: Applied  
12% and 20% was recorded for nanoparticles concentration of 0.1%, 0.4%, and 0.8%, consecutively, and they have concluded that Al2O3/water nanofluids can be used as an efficient coolant in a helically coiled tube with a concentration up to 0.4%. Furthermore, compared to parallel flow, the counter flow configuration leads to an improvement of 4% to 8% in the thermal performance with nanoparticles volume percentage of 0.4%. [PERSON] et al. [22] studied the thermal and hydraulic effectiveness of aqueous MWCNT nanofluid in a helically coiled heat exchanger. The obtained viscosity increased significantly more than the thermal conductivity. Moreover, there is no noticeable improvement in the thermal energy exchange for a given pumping power and constant flow speed. [PERSON] et al. [23] examined experimentally thermal energy exchange enhancement adopting the combination of multi-walled carbon nanotube nanofluids with helical coil tubes. Compared to the straight tube type, a considerable enhancement of the thermal energy transport was achieved. [PERSON] et al. [24] made a comparison between 25\({}^{\circ}\), and 50\({}^{\circ}\) helical baffles, and segmental type STHEs in terms of heat exchange and entropy generation. They indicated that the maximum thermal energy exchange rate is achieved using 25\({}^{\circ}\) helical baffles heat exchanger whereas the minimum generated entropy occurred with the 50\({}^{\circ}\) arrangement. [PERSON] et al. [25] studied the effectiveness of a helical coil heat exchanger operated with different nanofluids. The increment in nanoparticles concentration was accompanied by an important augmentation of thermal transport. In comparison with the pure water, an amelioration of 26.8%, 30.37%, and 32.7% in thermal efficiency was recorded utilizing TiO2, Al2O3, and CuO/water-based nanofluids, respectively. [PERSON] et al. [26] conducted both numerical and experimental studies of the turbulent forced convection flow inside helically coiled tubes operating with CuO-water nanofluids. It was revealed that the coefficient of heat exchange and the pressure decline were raised with the increase in curvature ratio and Reynolds number value. Larger increases were recorded with experimental results that exhibit 16% of rising in pressure reduction and 17% of enhancement in the heat exchange coefficient. [PERSON] et al. [27] investigated numerically the combination of nanofluids and chaotic advection in coiled heat exchangers. Al2O3 and CuO nanoparticles were used to prepare nanofluids with concentrations up to 3%. Higher thermal energy exchange was achieved with chaotic coils and pure water when compared to the normal coils with nanofluids.