Scalably-manufactured etched surface structures for enhanced flow boiling heat transfer of water

Flow boiling is essential in moving heat in refrigeration and air conditioning systems, nuclear reactors, and chemical manufacturing plants due to its efficient utilization of the latent heat of liquid to vapor phase change during vaporization. However, the flow boiling heat transfer performance of conventional bare metallic surfaces can be limited by the low number of active nucleation sites, which are a function of the surface roughness among other factors. Wet chemical etching is a cost-effective, scalable, surface structure fabrication technique that has been shown to significantly influence boiling heat transfer performance. In this study, two distinct copper etching recipes are developed specifically for flow boiling performance enhancement of water. The heat transfer coefficient and pressure drop of the two etch recipes are experimentally investigated in 0.25″ round copper tubes and compared with a reference bare tube of the same size. Experiments are carried out at atmospheric pressure using deionized water as a working fluid over a range of heat fluxes (10 kW/m² < q < 70 kW/m²), mass fluxes (140 kg/(m²·s) < G < 255 kg/(m²·s)), and vapor qualities (0 < x < 0.11). The results demonstrate that both etching recipes achieve heat transfer coefficient improvements over the plain tube, with the rougher surface providing the highest enhancement (up to 36%). The enhanced thermal performance of the etched tubes is attributed to the increased active nucleation site density and improved surface wetting characteristics. Despite the heat transfer coefficient enhancement, the pressure drop of the etched tubes is found to be similar to that of the bare tube. By carefully selecting the etching parameters, it is possible to fabricate a wide range of cavity sizes for boiling heat transfer enhancement optimized for any working fluid. This work provides insights into how chemical etching can be utilized as an effective technique to impact passive heat transfer enhancements for flow boiling applications.