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

This study investigates the performance of enhanced surfaces in flow boiling, focusing on both microstructures (laser-textured surfaces) and macrostructures (reticular-thread). Four tube configurations were tested: a smooth tube, a laser-textured tube (LT), a reticular-thread tube (RT), and a composite laser-textured/reticular-thread tube (LT/RT). All tubes were made of stainless steel with an outer diameter of 9.52 mm and a wall thickness of 0.6 mm, using Refrigerant 32 as the working fluid at a saturation temperature of 279 K. The experiments were conducted for various mass fluxes (100, 200, and 300 kg·m−2·s−1) and vapor qualities ranging from 0.2 to 0.8. The results showed that both the heat transfer coefficient (HTC) and the frictional pressure drop increased with mass flux. The HTCs for the RT and LT/RT tubes were 1.24–1.79 times and 1.49–2.63 times higher, respectively, compared to the smooth tube. The reticular-thread structure significantly influenced the frictional pressure drop, while the laser-textured surface had a smaller effect. The RT structure increased the heat transfer area and vaporization core, promoting bubble growth and detachment, which enhanced fluid turbulence and reduced the liquid film thickness. The HTC of the LT tube was slightly lower than that of the smooth tube, as the laser-textured surface increased nucleation sites but sometimes inhibited bubble detachment. However, when the LT surface was combined with the reticular-thread structure (LT/RT tube), the HTC improved due to the increased bubble growth and detachment facilitated by the reticular structure. The results also indicated that under variable average vapor quality conditions, both the HTC and frictional pressure drop increased with vapor quality. When the average vapor quality was below 0.6, the HTC of the LT tube was smaller than that of the smooth tube, but above 0.6, the HTC of the LT tube became higher. This is due to a higher vapor quality reducing the average fluid density and increasing fluid velocity, which aids in faster bubble detachment and reduces the likelihood of localized dry outs.