Local phase and thermal behaviors in pool boiling on different wettability surfaces

Abstract

In this work, we study the effect of surface wettability on boiling heat transfer and critical heat flux through experimental and numerical methods. In the experiments, four surfaces with different wettability, i.e., bare, SiO2-coated, TiO2-coated, and PTFE-coated on smooth sapphire surfaces, were examined. The visualization results of liquid–vapor phase distributions on the boiling surfaces indicated that the behaviors of dry spots, bulk liquid, and microlayers on the surfaces are significantly deviated from each other surface with varying wettability. Particularly, the rewetting behaviors of the relatively large dry spots occurred under the high heat flux condition showed that the surrounding liquid to wet the dry spots suffers a stronger resistance from the evaporative force at the triple contact line for the less wettable surface. In the numerical analysis, the local surface temperature and heat flux data on the boiling surfaces were obtained and systemically analyzed. The numerical results showed that the nucleate boiling regime can be maintained under the high heat flux condition by the repetitive process of nucleation of the dry spots and their destructions by the wetting of the surrounding liquid, i.e., quenching. However, for a particular dry spot appeared at a high surface heat flux, the quenching action was seriously limited, and the spot expanded irreversibly on the surface, indicating the CHF trigger. From the numerical results, we found that such a dry spot could be initiated by overheating the spot beyond a critical temperature limit which increases with the surface wettability and the heat flux focusing effect around the spot. Finally, the local and transient boiling behaviors were analyzed using the boiling curves separated into wet and dry regions.