Abstract
The boiling performance of functionalized hybrid aluminum surfaces was experimentally investigated for water and self-rewetting mixtures of water and 1-butanol. Firstly, microstructured surfaces were produced via chemical etching in hydrochloric acid and the effect of the etching time on the surface morphology was evaluated. An etching time of 5 min was found to result in pitting corrosion and produced weakly hydrophilic microstructured surfaces with many microcavities. Observed cavity-mouth diameters between 3.6 and 32 μm are optimal for efficient nucleation and provided a superior boiling performance. Longer etching times of 10 and 15 min resulted in uniform corrosion and produced superhydrophilic surfaces with a micropeak structure, which lacked microcavities for efficient nucleation. In the second stage, hybrid surfaces combining lower surface energy and a modified surface microstructure were created by hydrophobization of etched aluminum surfaces using a silane agent. Hydrophobized surfaces were found to improve boiling heat transfer and their boiling curves exhibited a significantly lower superheat. Significant heat transfer enhancement was observed for hybrid microcavity surfaces with a low surface energy. These surfaces provided an early transition into nucleate boiling and promoted bubble nucleation. For a hydrophobized microcavity surface, heat transfer coefficients of up to 305 kW m−2 K−1 were recorded and an enhancement of 488% relative to the untreated reference surface was observed. The boiling of self-rewetting fluids on functionalized surfaces was also investigated, but a synergistic effect of developed surfaces and a self-rewetting working fluid was not observed. An improved critical heat flux was only obtained for the untreated surface, while a lower critical heat flux and lower heat transfer coefficients were measured on functionalized surfaces, whose properties were already tailored to promote nucleate boiling.
Highlights
Pool boiling is utilized in diverse industrial applications, such as the cooling of electronic devices [1], heat exchangers [2], distillation [3], refrigeration [4] and others
It is evident that long etching times, which completely change the morphology of the boiling surface, impact the heat transfer coefficients negatively since the boiling curves for surfaces A10 and A15 are shifted to the right relative to the untreated reference surface
The results indicate that the micropeak morphology, which is not tailored to promote nucleation and nucleate boiling through the presence of microcavities, does not enhance the boiling heat transfer in a superhydrophilic state
Summary
Pool boiling is utilized in diverse industrial applications, such as the cooling of electronic devices [1], heat exchangers [2], distillation [3], refrigeration [4] and others. Aluminum has a high thermal conductivity and is relatively cheap (approximately six-times cheaper compared with a similar volume of copper), lightweight and machinable, which makes it suitable for the mass production of functionalized boiling surfaces. It is soft and can be roughened [7]. A stepwise approach to boiling heat transfer enhancement on hybrid functionalized surfaces is investigated by first changing the microstructure and morphology of flat aluminum surfaces, reducing the surface energy of prospective surfaces through hydrophobization and, by evaluating a possible further enhancement by using self-rewetting mixtures as the working fluid. The main factors and techniques used for the development of functionalized surfaces are briefly discussed in the following subsections, together with the employment of self-rewetting fluids for an enhanced pool boiling performance
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