Study of enhanced heat transfer performance of subcooled and saturated flow boiling with R365mfc on sintered porous coating high heat flux tubes

Sintered porous coating tubes are high performance heat transfer components which are used to enhance boiling heat transfer. Sintering metal powder particles on the surfaces of plain tubes form porous coatings with numerous cavities which can promote nucleation of bubble generation in boiling processes and thus enhance boiling heat transfer enhancement. In the present study, experiments of the subcooled and saturated flow boiling heat transfer characteristics on the sintered porous coating tubes were conducted. The test tubes with porous coatings have an outer diameter of 25 mm, a length of 1 m and the coating thicknesses are 0.06 mm, 0.12 mm, 0.18 mm, and 0.25 mm, respectively. The heat transfer performance of high flux tubes is evaluated with a mass flow rate ranging from 128.3 to 252.03 kg/m²·s and the saturation temperature of the experimental section is controlled between 45 and 50℃. The influence of flow conditions, heat flux, and properties of the sintered layer on boiling heat transfer was discussed. The results indicate that sintered porous media can effectively reduce the degree of superheating required for boiling heat transfer, but they also inevitably increase in flow resistance. Remarkably, the heat transfer enhancement due to the porous media increases up to a certain point and then decreases, while the flow resistance increases as the sintered layers thicken. The maximum heat transfer coefficient of the sintered tube with a sintered thickness of 0.06 mm is 1.6 times greater than that of a smooth tube. However, increasing the thickness of the porous layer does not always enhance heat transfer. The effects of different particle sizes of the sintered grains and the thickness of the sintered layers under the conditions of subcooled boiling and the onset of nucleate boiling have been analyzed to understand the physical mechanisms. An empirical heat transfer correlation has been proposed according to the experimental results for the sake of design calculation in industry.