An experimental study of relationship between pool boiling CHF and periodically modified surfaces by LASER processing

Abstract

Boiling heat transfer is one of the effective heat transfer methods because phase change from liquid to vapor is involved. Critical heat flux (CHF) is considered as an upper limit of boiling heat transfer because when CHF is reached, the heating surface is covered with a vapor layer and then temperature of the heating surface increases drastically. For this reason, there have been various researches to enhance CHF by modifying heating surfaces. Liter et al.(2001) produced modulated porous coated surfaces and their CHF are up to 3 times as high as that of the bare surface. Also, they obtain CHF model by modifying Zuber’s CHF model and its predictions showed good agreement with experimental results. This implies that flow modulation (periodic arrangement of vapor and liquid columns near CHF condition) is induced by periodically modified surface and periodicity of induced flow modulation is exactly identical to that of the surface. In this sense, periodic surface modification can be an effective approach to enhance CHF by changing flow modulation near the heating surface.Accordingly, the objective of this research is defined as to analyze the effects of parameters of periodic surface modification on CHF. Various periodically modified copper surfaces were produced by Nd:YAG laser and corresponding CHFs were obtained by pool boiling experiments for saturated water at atmospheric pressure. As a result, the surface with larger S (size of the modified region) and smaller G (gap between the modified regions) exhibits more enhanced CHF. In other words, virtual surface with high CHF is considered that thin grid of large periodicity is removed from fully modified surface. CHF enhancement by periodic surface modification can be explained that this thin grid disturbs coalescence between vapor columns and it delays complete vapor coverage on the heating surface

CHF. Also, it is implied that there exist optimum value of parameter G and this means that there is the limit of CHF enhancement by decreasing G for given S.