Sie können Bookmarks mittels Listen verwalten, loggen Sie sich dafür bitte in Ihr SLUB Benutzerkonto ein.
Medientyp:
E-Book
Titel:
Mechanism of Heat Transfer Enhancement of Supercritical Pressure Co2 in a Horizontal Variable Cross-Section Circular Tube Under Cooling Conditions
Beschreibung:
This paper presents the results of computational investigations on cooling heat transfer of turbulent supercritical CO2 (scCO2) in three types of horizontal tubes (uniform cross-section tube, diverging tube and converging tube) using SST k- ω turbulence models at a pressure of P =8.0 MPa. The computational results demonstrate that the diverging tube effectively strengthens the heat transfer compared with the uniform cross-section tube, and the total heat transfer coefficient is increased by 32.3% and the pressure drop is decreased by 53.87% compared with that of the uniform cross-section tube. While the converging tube weakens the heat transfer, the heat transfer coefficient of the converging tube is 8.87% lower than that of the uniform cross-section tube, and the pressure drop increases by 150.2%. The volume-averaged field synergy angle θ of uniform cross-section tube, diverging tube and converging tube are 93.49o, 92.14o, 93.04o, respectively, indicating that field synergy is not the physical mechanism of enhanced heat transfer. The heat transfer mechanism of scCO2 in three types of horizontal tubes under cooling condition is revealed for the first time from the viewpoint of quasi-condensation. Similar to subcritical film-wise condensation, at supercritical pressure, when the wall temperature is below the Tpc, a liquid-like film is formed on the tube wall, the thickness of the liquid-like film increases with the cooling of the tube, and the increase in thickness will further deteriorate the heat transfer. The reason for the heat transfer coefficient of the bottom bus is less than that of the top bus is the thickness of liquid-like film at the top of the area is less than at the bottom, which is caused by the heavier liquid-like fluid in the near wall area flows downward under gravity effect, while the lighter gas-like fluid in the core area flows upward under buoyancy effect. Our work provides theoretical guidance for the design of supercritical coolers and enhances the understanding of supercritical heat transfer under cooling condition