CHF enhancement for a finned surface in flow boiling with explanations based on vapor observations

摘要

In this study, effects of a finned structure on downward-facing flow boiling were investigated. Compared to a bare surface, a heated surface with a finned structure has a higher CHF and heat transfer coefficient. Two highspeed cameras were utilized to observe boiling behaviors from the downward view and side view. Under low heat flux conditions, there existed a front zone and wake zone due to the existence of the extended heating surface, which was confirmed by a numerical simulation. The bubbles tended to escape from two sides of the extended surface. Under high heat flux conditions, two vapor films formed due to the existence of the finned structure, allowing more water to be supplied to the surface. Meanwhile, the existence of the fin enhanced the turbulence and made the flow field velocity distribution non-uniform, thus enhancing heat transfer and CHF. The maximum vapor film thickness of finned surface was larger than that of the bare surface. A larger maximum volume of vapor generating on the surface suggested a larger amount of latent heat consumption and thus, leading to a bigger CHF. These results demonstrated the potential of CHF enhancement using the finned structure design.