Flow Boiling Heat Transfer of R32 in Laser-Textured and Reticular-Thread Enhanced Tubes

Abstract Improving heat transfer performance in heating, ventilation, and air conditioning systems is crucial for enhancing energy efficiency and reducing operating costs in buildings. This study investigates the flow boiling performance of enhanced surfaces, focusing on both microstructures (laser-textured (LT) surfaces) and macrostructures (reticular-thread (RT)): a smooth tube, an LT tube, an RT tube, and a composite LT/RT tube. Results show that both the heat transfer coefficient (HTC) and the frictional pressure drop increase with mass flux. The HTCs of the RT and LT/RT tubes were 1.24–1.79 times and 1.49–2.63 times higher, respectively, than that of the smooth tube. The RT structure significantly affected the pressure drop, while the LT surface had a relatively smaller impact. The RT grooves enlarged the heat transfer area and increased vaporization cores, promoting bubble growth and detachment, enhancing turbulence, and reducing the liquid film thickness. The LT surface increased nucleation sites but sometimes hindered bubble departure, resulting in slightly lower HTC than the smooth tube. However, when combined with RT, the LT/RT tube exhibited enhanced HTC due to improved bubble dynamics facilitated by the reticular structure. Under varying average vapor quality, both HTC and pressure drop increased with vapor quality. Specifically, when the average vapor quality was below 0.6, the HTC of the LT tube was lower than that of the smooth tube. Once the vapor quality exceeded 0.6, the HTC of the LT tube surpassed that of the smooth tube.