The paper presents a 3D numerical simulation of turbulent heat transfer and flow characteristics in converging-diverging tubes (CDs) and converging-diverging tubes equipped with twin counter-swirling twisted tapes (CDTs). The effects of Reynolds number (Re = 10000-20000), pitch length (P = 11.25, 22.5 mm), rib height (e = 0. 5, 0.8, 1.1 mm), pitch ratio (delta= 1 : 8, 5 : 4, 8 : 1), gap distance between twin twisted tapes (b = 0.5, 4.5, 8.5 mm) and tape number (n = 2, 3, 4, 5, 6) on Nusselt number (Nu), friction factor (f) and thermal enhancement factor (eta) are investigated under uniform heat flux conditions, using water as working fluid. In order to illustrate the heat transfer and fluid flow mechanisms, flow structures in CDs and CDTs are presented. The obtained results reveal that all geometric parameters have important effects on the thermal performance of CD and CDT, and both CD and CDT show better thermal performance than plain tube at the constant pumping power. It is also found that the increases in the Nusselt number and friction factor for CDT are, respectively, up to 6.3%-35.7% and 1.75-5.3 times of the corresponding bare CD. All CDTs have good thermal performance with eta greater than 1 which indicates that the compound heat transfer technique of CDT is commendable for the maximum enhanced heat transfer rate.