Table 1 shows some comparative results between optimized tube and smooth tube. Nu0 and f0 are Nu number and flow resistance coefficient of smooth tubes when Reynolds number is 200. The rate of radial velocity and axial velocity Ur/Ua is used to express the intensity of longitudinal swirl flow. The value of PEC ¼ (Nu/Nu0)/(f/f0)1/3 is used to evaluate the comprehensive performance. According to the field synergy principle, the performance of convective heat transfer will be improved if the angle between velocity and temperature gradient decreases. The longitudinal swirl flow improves the synergy of temperature and velocity field, and the performance is enhanced in comparison with general flow in tube. From Table 1, we can find that the radial velocity compared with axial velocity is not large, thus the flow resistance has small
increases. The radial velocity leads fluid flow along the direction of temperature gradient, thus the angle between velocity and temperature gradient will decrease and the heat transfer will be enhanced. Different conditions of fluid power consumption have different optimum field as shown in Figs. 2e5. Longitudinal swirl flow is the optimum flow pattern at different fluid power consumption, and the more vortexes, the better comprehensive performance