Thermal efficiency, friction factor, heat transfer, and cost analysis of a flat plate collector operates with water-based nanodiamond
nanofluids under thermosyphon (natural circulation) conditions are investigated experimentally at 0.2%, 0.4%,
0.6%, 0.8%, and 1.0% particle volume concentrations. The thermophysical properties of the working fluids are analyzed
as well. Results show that the maximum thermal conductivity and viscosity enhancements are obtained by using 1.0 vol%
concentration of nanofluid and found to be 22.86% and 79.16%, respectively, compared to water at a temperature of 60 °C.
The maximum increases in Nusselt number are 19.53% and 36.17% using 1.0 vol% concentration of nanofluid compared to
water at Reynolds number of 140 and 345, respectively. The maximum increases of friction factor are attained by using 1.0
vol% concentration of nanofluid and found to be 1.14 times and 1.25 times of water friction factor at Reynolds number of
143 and 345, respectively. The collector thermal efficiency increases from 57.15% using water to 69.85% using nanofluid
with a concentration of 1.0%. The collector cost decreases approximately by 18.18% for 1.0 vol% nanofluid compared to
water. The relative deviations of the equations developed to evaluate Nusselt number and friction factor are within ± 2.5%.