▎ 摘　　要

This research attempts to investigate the benefits of utilizing two heat transfer enhancement methods simultaneously. These techniques include employing biological graphene-based nanofluid and using rotating coaxial double-twisted tapes (RCDTT) to enhance heat transfer inside tubes. Using the nanofluid is desirable such that at twisted ratio of 2.5 and rotational speed of 0 rpm, the heat transfer coefficient improves about 16.3% by increase of the concentration from 0 to 0.1%. Furthermore, higher rotational speed is suggested such that its increase from 0 to 900 rpm, at twisted ratio of 3.5 and concentration of 0%, causes a 77% increment in the heat transfer coefficient. Lower twisted ratios are also more effective for enhancing heat transfer rate. The rotational speed increment has the great impact on the pressure drop, such that a 34% increment in the pressure drop occurs with increasing the rotational speed from 300 rpm to 900 rpm at weight fraction of 0.075% and twisted ratio of 3.5. Owing to the more intense disturbances generated by greater rotational speeds, the particle residence time becomes greater leading to pumping power increment. The figure of merit for all the cases is higher than 1, which is an excellent outcome from energy efficiency viewpoint.