The heat transfer, friction factor, and collector efficiency are estimated experimentally for
multi-walled carbon nanotubes+Fe3O4 hybrid nanofluid flows in a solar flat plate collector under
thermosyphon circulation. The combined technique of in-situ growth and chemical coprecipitation
was utilized to synthesize the multi-walled carbon nanotubes+Fe3O4 hybrid nanoparticles. The
experiments were carried out at volume flow rates from 0.1 to 0.75 L/min and various concentrations
from 0.05% to 0.3%. The viscosity and thermal conductivity of the hybrid nanofluids were experimentally
measured at different temperatures and concentrations. Due to the improved thermophysical
properties of the hybrid nanofluids, the collector achieved better thermal efficiency. Results show that
the maximum thermal conductivity and viscosity enhancements are 28.46% and 50.4% at 0.3% volume
concentration and 60 C compared to water data. The Nusselt number, heat transfer coefficient,
and friction factor are augmented by 18.68%, 39.22%, and 18.91% at 0.3% volume concentration and
60 C over water data at the maximum solar radiation. The collector thermal efficiency improved by
28.09% at 0.3 vol. % at 13:00 h daytime and a Reynolds number of 1413 over water data. Empirical
correlations were developed for friction factor and Nusselt number