Vapor condensation on metallic surfaces is a phase-change phenomenon that has widespread application in many processes. Jumping-droplet-enhanced condensation is an effective mode of dropwise condensation due to its higher droplet removal rate, enabling more efficient heat transfer. However, maintaining stable jumping-droplet condensation, requires surface structures to be suitably designed to prevent droplet pinning and surface flooding. In recent years, using metal additive manufacturing (AM) process to create heat exchanger surfaces has received significant attention due to its design freedom and versatility in fabricating highly complex functional parts. Here, we present a highly scalable method of fabricating superhydrophobic (SHP) AM surfaces from aluminum alloy, AlSi10Mg, and experimentally investigate their thermal performance during steam condensation. The test samples are fabricated by Selective Laser Melting (SLM), an AM technique for producing metallic parts. This AM technique utilizes a laser source to melt the base metallic powder, where the molten pool then rapidly solidifies to form a solid layer. Layer-by-layer repetition of this process allows fabrication of three-dimensional solid parts. Through detailed materials characterizations, we found that it is possible to achieve superior superhydrophobicity on AM surfaces, with unique cellular-like nanoscale surface features, by simple chemical etching and functionalization processes. Furthermore, tuning of the nanostructure feature size can also be realized by controlling the surface treatment time.&nbsp;To understand the droplet dynamics and obtain insights on the effects of AM nanostructures on the condensate droplet morphology and jumping, we carried out condensation experiments with an environmental scanning electron microscope (ESEM) at low supersaturation of ~1.05. The important relations between the fabricated AM nanostructure morphology and droplet dynamics are established by characterizing the droplet distribution density, droplet departure diameter and droplet jumping frequency. To determine the anti-flooding and condensation heat transfer performances of the AM SHP surfaces, pure vapor condensation experiments under higher supersaturation conditions are carried out in a well-controlled environmental chamber. Together with the aid of high-resolution imaging and heat transfer measurements, we demonstrate significant reduction in droplet pinning sites due to the implementation of the AM cellular-like structure. This reduces the thermal barrier between the condensing surface and surrounding vapor, and hence, increases the condensation heat transfer. Our results show that excellent droplet jumping performance and better droplet mobility can be achieved by using AM SHP surfaces as compared to conventional SHP aluminum extruded tubing. These results underscore the potential of advancing AM structured surfaces for jumping-droplet-enhanced condensation under high heat flux conditions.

Nanostructuring of Metallic Additively Manufactured Surfaces for Enhanced Jumping Droplet Condensation

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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