The principle of heat transfer enhancement in the core flow of tube has been proposed to improve the temperature uniformity and reduce flow resistance, which is different from that of heat transfer enhancement in the boundary flow of tube. Helical screw-tape inserts with four different widths (w = 7.5 mm, 12 mm, 15 mm and 20 mm) have been investigated for different inlet volume-flow rates ranging from 200 L/h to 500 L/h. A three-dimensional turbulence analysis of heat transfer and fluid flow is performed by numerical simulation. The simulation results show that the average overall heat transfer coefficients in circular plain tubes are enhanced with helical screw-tape of different widths by as much as 212 similar to 351% at a constant tube-side temperature and the friction factor are enhanced by as much as 33% to 1020%. The PEC value of the helical screw-tape inserts of different width varies between 1.58 and 2.35. Physical quantity synergy analysis is performed to investigate the mechanism of heat transfer enhancement. The synergy angles alpha, beta, gamma, theta and eta are calculated, and the numerical results verify the synergy regulation among physical quantities of fluid particle in the flow field of convective heaet transfer, which can guide the optimum design for better heat transfer units and high-efficiency heat exchangers. Entropy generation analysis is also performed to explain how to get the optimum helical screw-tape. (C) 2013 Elsevier Ltd. All rights reserved.