Theoretical And Experimental Studies Of Heat Transfer Enhancement In Tube

The technology of convective heat transfer in tube is widely used in industry. Until now, the conventional technologies are mostly designed by empirical methods but not a theory on design, and this leads to a limit performance of the technologies. So the lack of a theory on design is a bottleneck problem of the heat transfer field, and many scholars in the world are searching for this theoretical breakthrough. This paper performs studies in convective heat transfer mechanism in tube. And the equations for optimizing the flow and temperature field are obtained by variational method. Through the solution results of these equations, the author proposes a flow-subdividing theory on designing the flow field, which is different from the conventional thinking of disturbing the whole flow field. Besides, this paper also performs studies on the methods of reducing the flow resistance in tube with inserts. And the the upwind area reduction theory is proposed.By the guidance of the flow-subdividing theory and the upwind area reduction theory, the author designed some new heat transfer enhancement devices such as the four-symmetrical vortex generator, the center-cleared twisted tapes, the single part twisted tape, the conical strips and the T-type strips. The numerical results show that, the four-symmetrical vortex generator has the best thermo-hydraulic performance by comparision with the single vortex generator, the twisted tape and the four-synclastic vortex generator. The results indicate that, the flow-subdividing theory has some applicability for the design of the heat transfer enhancement device in a tube. In the experimental range of Re from 5000 to 30000, the performance evaluation criteria (PEC) of the four-symmetrical vortex generator varies from 0.85 to 1.08, which is higher than that of the twisted tape. The heat transfer and the flow resistance are affected by the angle of attack and the spacing pitch. When the angle of attack is larger or the spacing pitch is smaller, the heat transfer and the flow resistance are both larger. A smaller angle of attack leads to more intensive vibrations which even could enhance the heat transfer in some cases.The numerical results indicate that, both of the center-cleared twisted tapes and the short-width twisted tape could reduce the flow resistance effectively. But the short-width twisted tape also reduces the heat transfer rate and PEC values. As compared with the conventional twisted tape, the PEC values of the center-cleared twisted tapes can be enhanced by up to 14%. And the PEC values of the single part twisted tape are higher than those of the center-cleared twisted tapes. The conical strips also have good thermo-hydraulic performances. When Re is 13000, the PEC value is 1. When Re is 17000, the PEC value is 0.9. But the PEC values decreases sharply with the increment of Re, which hinders the application of the conical strips in range of high Re. The T-type strips have better thermo-hydraulic performances than the conical strips in range of high Re, and they can be complementary.