In the present study, the thermal efficiency, convective heat transfer and friction factor analysis are investigated for a flat plate solar collector with thermosyphon (natural circulation) system using water and nanodiamond-cobalt oxide hybrid nanofluids as the working fluids. The nanodiamond-cobalt oxide hybrid nanoparticles were synthesized using in situ growth and chemical co-precipitation method and characterized using X-ray diffraction, transmission electron microscope and vibrating sample magnetometer. The investigations were performed at different volume flow rates 0.56-1.35 L min −1 and various mass concentrations of 0.05-0.15%. The thermal conductivity and viscosity of nanofluids were measured experimentally at various mass concentrations and temperatures. Due to the augmented thermo-physical properties of hybrid nanofluids, the collector reached higher coefficient of heat transfer as well as improved thermal efficiency than the water data. Maximum thermal conductivity and viscosity enhancements are found to be 15.71% and 45.83% at particle loadings of 0.15% mass concentration and at a temperature of 60 °C. Results show the Nusselt number enhancement of 0.15% mass concentration of hybrid nanofluid is 21.23% with maximum friction factor penalty of 1.13 times against water data. The collector thermal efficiency is found to 59% for the case of 0.15% mass concentration of hybrid nanofluid, where the thermal efficiency of water is 48%. The empirical correlations were developed for Nusselt number and friction factor for collector with hybrid nanofluids within the deviation of ± 3%.

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