Microstructured uniformly and non-uniformly wettable surfaces were created on 25-m-thin stainless steel foils by laser texturing using a marking nanosecond Nd:YAG laser ( = 1064 nm) and utilizing various laser fluences and scan line separations. High-speed photography and high-speed IR thermography were used to investigate nucleate boiling heat transfer on the microstructured surfaces. The most pronounced results were obtained on a surface with nonuniform microstructure and non-uniform wettability. The obtained results show up to a 110 % higher heat transfer coefficients and 20-40 times higher nucleation site densities compared to the untextured surface. We show that the number of active nucleation sites is significantly increased in the vicinity of microcavities that appeared in areas with the smallest (10 m) scan line separation. Furthermore, this confirms the predictions of nucleation criteria and proves that straightforward, cost-effective nanosecond laser texturing allows the production of cavities with diameters of up to a few micrometers and surfaces with non-uniform wettability. Additionally, this opens up important possibilities for a more deterministic control over the complex boiling process.
Functionalized
interfaces enhancing phase-change processes have immense applicability
in thermal management. Here, a methodology for fabrication of surfaces
enabling extreme boiling heat transfer performance is demonstrated,
combining direct nanosecond laser texturing and chemical vapor deposition
of a hydrophobic fluorinated silane. Multiple strategies of laser
texturing are explored on aluminum with subsequent nanoscale hydrophobization.
Both superhydrophilic and superhydrophobic surfaces with laser-engineered
microcavities exhibit significant enhancement of the pool boiling
heat transfer. Surfaces with superhydrophobic microcavities allow
for enhancements of a heat transfer coefficient of over 500%. Larger
microcavities with a mean diameter of 4.2 μm, achieved using
equidistant laser scanning separation, induce an early transition
into the favorable nucleate boiling regime, while smaller microcavities
with a mean diameter of 2.8 μm, achieved using variable separation,
provide superior performance at high heat fluxes. The enhanced boiling
performance confirms that the Wenzel wetting regime is possible during
boiling on apparently superhydrophobic surfaces. A notable critical
heat flux enhancement is demonstrated on superhydrophobic surfaces
with an engineered microstructure showing definitively the importance
and concomitant effect of both the surface wettability and topography
for enhanced boiling. The fast, low-cost, and repeatable fabrication
process has great potential for advanced thermal management applications.
A vast majority of heat exchangers suffer from unwanted deposition of material on the surface, which severely inhibits their performance and thus marks one of the biggest challenges in heat transfer. Despite numerous scientific investigations, prediction and prevention of fouling remain unresolved issues in process engineering and are responsible for large economic losses and environmental damage. This review article focuses specifically on crystallization fouling, providing a comprehensive overview of the state-of-the-art of fouling in heat exchangers. The fundamentals of the topic are discussed, as the term fouling resistance is introduced along with distinct fouling behaviour, observed in laboratory and industrial environments. Insight into subsequent phases of the fouling process is provided, along with the accompanying microscale events. Furthermore, the effects of fluid composition, temperature, flow velocity, surface condition, nucleate boiling and composite fouling are comprehensively discussed. Fouling modelling is systematically reviewed, from the early work of Kern and Seaton to recently used artificial neural networks and computational fluid dynamics. Finally, the most common fouling mitigation approaches are presented, including design considerations and various on-line strategies, as well as off-line cleaning. According to our review, several topics require further study, such as the initial stage of crystal formation, the effects of ageing, the interplay of two or more fouling mechanisms and the underlying phenomena of several mitigation strategies.
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