Abstract
The main topic of this paper is the influence of vertically aligned nucleation sites on each other in upward flow boiling. A setup was constructed to facilitate vertical up-flow of deminiralized water under saturation conditions. The main test section is a glass channel with a set of vertically aligned bubble generators. Each bubble generator is operated independently, where power and wall temperature are registered and the vapour bubbles are visualized by a high-speed camera. During the experiments, the downstream bubble generator (BG1) power is kept constant, while the power fed to the upstream bubble generator (BG2) is incrementally increased. Two main trends have been identified. The first trend is dominated by added convection from one site to the other. Both bubble frequency and detachment diameter on BG1 increase with increased power fed to upstream BG2. This effect decreases with increasing inter-site distance and becomes more significant with increasing liquid flow rate. When vapor bubbles start nucleating from BG2, these vapor bubbles inhibit bubble nucleation BG1 and can even lead to deactivation of this nucleation site. This second trend is only weakly dependent on inter-site distance, since the inhibition originates from bubbles flowing past BG1 in close proximity.
Highlights
Nucleate boiling is a very efficient mode of heat transfer, yet many aspects of nucleate boiling are not well understood even today
Many researchers studied nucleation site interactions in pool boiling, which provided better insight in the phenomenon. These interactions have never before been studied in forced convection, which is the main topic of this study
The increase in bubble frequency at the downstream bubble generator, BG1, with increasing PBG2 until significant bubble nucleation commences at BG2 (Fig. 8) is explained by the increasing convective heat added to the liquid flow by BG2
Summary
Nucleate boiling is a very efficient mode of heat transfer, yet many aspects of nucleate boiling are not well understood even today. Understanding of basic physical phenomena involved in nucleate boiling is essential to come to mechanistic models of nucleate boiling which yield realistic predictions. Interactions between vapour bubble nucleation sites form an important part of nucleate boiling heat transfer. Many researchers studied nucleation site interactions in pool boiling, which provided better insight in the phenomenon. These interactions have never before been studied in forced convection, which is the main topic of this study
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