Study On The Characteristics Of Flow Boiling And Condensation Heat Transfer In Annuli Outside Horizontal Smooth And Enhanced Tubes

Cooling facilities has been widely used in many fields.In order to develop the heatexchange equipment of low energy consumption,low emission,reduce the footprint of chillers and achieve a higher heat transfer coefficient,the application of enhanced surface tubes has gradually become one of the main ways to improve the efficiency of heat exchangers.In order to investigate the influence of the outer surface structures of the enhanced tubes on the two-phase forced convection heat transfer characteristics,this paper studies the single-phase,flow boiling and condensation heat transfer of refrigerant R410 A in annular space outside a smooth tube and three different enhanced tubes with saturation temperatures of 38°C,6°C and 45°C,respectively.The test tubes with the same outside diameter of 19 mm and the outer enhanced surface include micro-porous(No.1),integrated fin(No.2)and independent small bosses(No.3)structures.For flow boiling heat transfer,the heat transfer coefficient of the micro-porous tube is much higher than that of other tubes(about 2.52-2.64 times of smooth tube).Due to the deterioration of heat transfer,the performance of smooth tube is better than that of the small bosses tube.The corrections based on Gungor and winterton correlation can predict the experimental heat transfer coefficient within ±10% error range.In addition,the heat transfer characteristics of the micro-porous tube under different average qualities are also tested and analyzed.The introduction of efficiency index(?)indicates that the micro-porous tube is a good choice for the application of flow boiling in annular space because of its outstanding heat transfer performance.For condensation heat transfer,due to the influence of different surface structures,gravity and surface tension,the condensation heat transfer coefficient of refrigerant outside the three enhanced tubes varies with the refrigerant mass flux,and the heat transfer performance is better than that of the smooth tube.According to Nusselt theory,a new prediction correlation is established for the tested tubes,which can also be predicted all data points within ±10% of the error range.Finally,the efficiency index shows that the small bosses tube has some advantages in the condensation heat transfer outside the tubes.