| Heat transfer flat tube is the most important component in heat exchanger, it is the carrier of heat transfer. The heat transfer and friction factors performance of heat transfer flat tube is the main factor to affect the efficiency of heat exchanger. Pits structure has been developed as a new heat transfer enhancement structure recently, a large quantity of experiments proved pits structure has a better integrated heat transfer performance than smooth channel. In additional, to be compared with other heat exchange enhancement technologies, it can enhance the heat exchange performance without creating big friction factors, as a result it is widely used in industries.This thesis introduced both physical and mathematic models for pits flat tube, basing on the characterize and fluid status of the research object, RNG κ-ε turbulence model and rectangle model are applied to analyze the numerical simulation of different types of heat transfer flat tubes in different manufacturing condition and carrier gas was chosen as air.First of all, pits flat tube with different kinds of geometry structure factors were simulated with high Reynolds numbers, focusing on how did the depth, width, distance of pits and height of channel affect to the flat tube heat transfer, friction factors and integrated heat exchange performance. The result shows the best structure factors that match flat tube heat transfer with friction factors in different manufacturing condition, afterward, the principle of flat tube heat transfer enhancement was discussed. It was assumed that the existence of pits increased the heat exchange area which achieved the heat exchange enhancement. On the other hand, pits destroyed the boundary layers of fluid and created vortex which cause increasing of certain fluid turbulent fluctuation, the closer to pits the stronger enhancement occurred. Compared to smooth flat tube, pits flat tube has an integrated heat exchange factor η between 1.5 and 2.2 which is obviously higher than smooth flat tube. It was followed by the analysis of low Reynolds numbers narrow pits flat tube which discussed about in rectangle fluctuation condition, how did the depth, horizontal distance, vertical distance and width of pits affect the sense of integrated heat exchange performance, and obtained the best scheme.This thesis analyzed the heat exchange performance in different manufacturing and geometry condition, discussed the fluid and heat exchange principle which provided the evidence for further design and works. |
