Nucleate pool boiling

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Nucleate Pool Boiling: Insights from Recent Research

Introduction to Nucleate Pool Boiling

Nucleate pool boiling is a heat transfer process where a heated surface is submerged in a liquid, causing the formation of vapor bubbles. This process is critical in various industrial applications, including power generation and cooling systems. Recent studies have explored different aspects of nucleate pool boiling, including surface structuring, fluid mixtures, and experimental conditions, to enhance heat transfer efficiency.

Surface Structuring and Heat Transfer Enhancement

Laser-Textured Surfaces

Research has shown that laser-textured surfaces significantly improve the heat transfer coefficient (HTC) in nucleate pool boiling. Experiments with saturated pure water and ethanol-water mixtures on laser-textured stainless-steel foils revealed that the HTC could be enhanced by up to 280% for pure water and 268% for pure ethanol, depending on the width of the laser patterns. The optimal enhancement for binary mixtures was achieved with 2.0-mm wide laser-textured regions, improving HTC by 235% and 279% for 0.4% and 4.2% ethanol-water mixtures, respectively.

Other Surface Structuring Methods

Other surface structuring methods, such as wire electrical discharge machining (WEDM) and etching, have also been investigated. These methods increase the nucleation site density (NSD) and modify the contact angle, leading to enhanced heat flux. For instance, surfaces structured by WEDM showed a 121% increase in HTC compared to plain surfaces, while etched surfaces showed a 37% increase. The highest performance was observed with a specific structured surface, which exhibited a 177% increase in HTC.

Copper-Diamond Textured Surfaces

Copper-diamond composite layers deposited by the Cold Spray technique have also been used to create textured surfaces. These surfaces support liquid wicking and exhibit mild hydrophobicity, resulting in a 300% increase in HTC and a 35% increase in critical heat flux (CHF). The capillary wicking effect at high heat fluxes plays a crucial role in these enhancements.

Fluid Mixtures and Boiling Behavior

Ethanol-Water Mixtures

The boiling behavior of ethanol-water mixtures has been extensively studied. The HTC enhancements for these mixtures depend on the concentration and the surface structuring. For example, laser-textured surfaces with specific pattern widths significantly improved the HTC for different ethanol-water mixtures.

Polymer Solutions

Aqueous solutions of hydroxyl ethyl cellulose (HEC) exhibit unique boiling characteristics due to their non-Newtonian, shear-thinning properties. At low heat fluxes, heat transfer is initially degraded but subsequently enhanced by up to 45% at higher heat fluxes. This behavior is influenced by changes in liquid-solid interface wetting, vapor-liquid interfacial tension, and the viscosity of the polymeric solutions.

Experimental Conditions and Boiling Dynamics

Pool Temperature Effects

Experiments with pure R113 under various pool conditions (subcooled, saturated, and superheated) have shown that bubble growth behavior and heat flow rates are significantly affected by the pool temperature. The bubble growth during different pool conditions can be characterized using dimensionless parameters and a modified Jakob number.

Microgravity Experiments

Nucleate pool boiling experiments conducted in microgravity on the International Space Station revealed that heat transfer coefficients and maximum heat fluxes are lower than those under normal gravity conditions. The presence of a large vapor bubble acting as a sink plays a crucial role in the boiling dynamics in microgravity.

Conclusion

Recent research on nucleate pool boiling has highlighted the importance of surface structuring and fluid properties in enhancing heat transfer efficiency. Laser-textured surfaces, WEDM, and etching methods have shown significant improvements in HTC and CHF. Additionally, the boiling behavior of fluid mixtures and the effects of experimental conditions, such as pool temperature and gravity, provide valuable insights for optimizing boiling processes in various applications.