In recent years, the theory of heat transfer enhancement was perfected step by step and some new theories were proposed. Guo et al. [13] proposed the field synergy principle and it reveals the relationship between local behaviour and the comprehensive performance of convective heat transfer in two-dimensional laminar flow. They point out that the synergy between temperature and velocity field is one of factors which have an effect on the performance of convective heat transfer. Liu et al. [14e17] explained physical quantity synergy principle from field synergy principle by reflecting the physical mechanism of convective heat transfer in the laminar and turbulence flows. They explained physical essentials
on enhancing heat transfer and reducing flow resistance by revealing how heat transfer performance and flow resistance were influenced by physical quantity synergy relation associated with temperature, velocity and pressure. According to the field synergy principle, the synergetic degree between the temperature and velocity fields has an important effect on the performance of heat transfer. Therefore, changing the velocity field is a direct method to enhance the heat transfer. And we need to know what kind of velocity field has the best performance, then heat transfer enhancement unit can be invented to form the close velocity field