Numerical Analysis of Bubble Formation Mechanism in Nucleate Pool Boiling

Abstract

This study comprehensively investigates nucleate pool boiling by focusing on bubble formation, growth, and detachment mechanisms. A numerical analysis of saturated nucleate pool boiling of water on a heated surface, specifically emphasizing a single cavity, was conducted and compared with experimental results documented in the literature. Accurate modeling of boiling phenomena is crucial, particularly in effectively capturing the mass transfer and phase change processes between the liquid and vapor phases. The dynamic separation of these phases through a moving interface presents a significant challenge when simultaneously applying the Navier–Stokes equations to both phases, as it complicates the continuity conditions at the interface. Various numerical methods, incorporating implicit and explicit schemes, have been developed to address these challenges for two-phase flow simulations. Interface tracking techniques such as the Volume of Fluid (VOF), Level-Set, and Lattice Boltzmann methods are commonly employed. This study used Ansys Fluent software to perform a detailed boiling model analysis. Based on the findings from detailed literature reviews, the Volume of Fluid (VOF) method is considered the most suitable simulation approach for modeling pool boiling. After establishing an appropriate computational domain, a two-dimensional simulation of single bubble formation on a microscale heated surface was carried out using a custom-developed User Defined Function (UDF). The objective was to analyze the bubble's geometric characteristics and diameter evolution throughout the boiling process. The accuracy of the numerical model was evaluated by comparing simulation results with experimental observations reported in the literature, showing a high degree of agreement.CFD analyses were conducted for both a flat copper surface and a surface with a single cavity. The results showed that, due to nucleate boiling, the copper surface's superheat values were higher than those on the surface with a cavity. This indicates improved heat transfer performance on the structured surface. These findings suggest that in processes where boiling-induced heat transfer is applied, surfaces that are roughened either through etching or coating methods may yield enhanced thermal performance compared to smooth surfaces, in line with observations reported in the literature.

Keywords

• Pool boiling
• VOF
• UDF
• saturated nucleate pool boiling
• CFD.

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