The present study numerically examines heat transfer and flow in circular tubes fitted with dimpled twisted tape inserts, and Al2O3-water nanofluid is employed. Considering the effects of dimples, protrusions, nanofluid volume fraction a and nanoparticle" diameter tip, the discussion and analysis on heat transfer, flow characteristics, turbulence kinetic energy TICE, thermal property, entropy generation and maximum local wall temperature T-max are detailedly provided. The results show that dimple side and protrusion side both realize great heat transfer enhancement, and dimple side behaves better compared with protrusion side. Utilization of dimples leads to an increase by 25.53% in convective heat transfer coefficient It at most compared with smooth tape. Heat transfer performance is greatly improved on both tape wall and tube wall owing to disturbance to flow structures. The overall TILE level significantly rises especially close to the core flow region when dimples are adopted, and overall swirl flow intensity greatly increase, further resulting in enhancement of turbulent mixing and heat transfer. Remarkable decline in average heat transfer entropy generation rate S-ah is identified, accompanied by slight rise in average friction entropy generation rate S-af, where average total entropy generation rate S-a inclines by 29.10% at most in comparison with smooth tape. Besides, the employment of nanofluids results in great improvement of heat transfer, with further enhanced effect when alpha increases, accompanied by slight growth in resistance. A maximum increase by 58.96% in h is identified compared with basefluid case, and it is quite helpful that in this case the maximum rise in friction factor is only 5.05%. The wall temperature distribution is greatly improved using nanofluid, and nanofluid case provides markedly improved thermal conductivity distribution in spite of slight growth in dynamic viscosity. The heat transfer in recirculation-caused low heat transfer regions is further promoted through nanofluids when dimple technique is adopted. Utilization of nanofluid brings about a significant reduction in S-ah, the effect of which is further promoted by a rising cc, with a slight increase in S-af where S-a is reduced by 28.89% at most. The T-max greatly declines using nanofluids with further enhanced effect when alpha ascends. Furthermore, utilization of smaller d(p) gives rise to significant heat transfer enhancement compared with the larger one, with extremely mild resistance increase. Lower S-a and lower T-max are both realized when smaller d(p) is used, and the effect of d(p) reaches an extremely low level when it exceeds 40 nm. (C) 2017 Elsevier Ltd. All rights reserved.