Thermohydraulics of laminar flow of viscous oil through a circular tube having integral axial rib roughness and fitted with center-cleared twisted-tape

► Laminar flow in axially ribbed channel with center-cleared twisted-tape is studied. ► Friction factor and Nusselt number correlations are developed. ► The thermohydraulic performance is evaluated. ► This finned geometry performs significantly better within limits. ► Results are useful for solar thermal heating and heat exchangers.