Wetting Mode Evolution of Steam Dropwise Condensation on Superhydrophobic Surface in the Presence of Noncondensable Gas

Superhydrophobic micro/nanostructured surfaces for dropwise condensation have recently received significant attention due to their potential to enhance heat transfer performance by shedding water droplets via coalescence-induced droplet jumping at length scales below the capillary length. However, achieving optimal surface designs for such behavior requires capturing the details of transport processes that is currently lacking. While comprehensive models have been developed for flat hydrophobic surfaces, they cannot be directly applied for condensation on micro/nanostructured surfaces due to the dynamic droplet-structure interactions. In this work, we developed a unified model for dropwise condensation on superhydrophobic structured surfaces by incorporating individual droplet heat transfer, size distribution, and wetting morphology. Two droplet size distributions were developed, which are valid for droplets undergoing coalescence-induced droplet jumping, and exhibiting either a constant or variable contact angle droplet growth. Distinct emergent droplet wetting morphologies, Cassie jumping, Cassie non-jumping, or Wenzel, were determined by coupling of the structure geometry with the nucleation density and considering local energy barriers to wetting. The model results suggest a specific range of geometries (0.5-2 μm) allowing for the formation of coalescence-induced jumping droplets with a 190% overall surface heat flux enhancement over conventional flat dropwise condensing surfaces. Subsequently, the effects of four typical self-assembled monolayer promoter coatings on overall heat flux were investigated. Surfaces exhibiting coalescence-induced droplet jumping were not sensitive (< 5%) to the coating wetting characteristics (contact angle hysteresis), which was in contrast to surfaces relying on gravitational droplet removal. Furthermore, flat surfaces with low promoter coating contact angle hysteresis (< 2°) outperformed structured superhydrophobic surfaces when the length scale of the structures was above a certain size (> 2 μm). This work provides a unified model for dropwise condensation on micro/nanostructured superhydrophobic surfaces and offers guidelines for the design of structured surfaces to maximize heat transfer. 2

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“…6), surface inclination (Sec. 7), and contact angle hysteresis with different promoter coatings (Sec. 8) on overall surface heat transfer.…”

Section: Introductionmentioning

confidence: 99%

Modeling and Optimization of Superhydrophobic Condensation

Miljkovic

Enright

Wang

2013

Journal of Heat Transfer

243

241

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“…[33][34][35][36] In view of this situation, a system with high-speed cameras is designed as shown in Figure 2. However, it's difficult to capture the detailed coalescence process as droplet coalescence in a few milliseconds and the visibility under condensation is poor.…”

Section: Methodsmentioning

confidence: 99%

“…However, it's difficult to capture the detailed coalescence process as droplet coalescence in a few milliseconds and the visibility under condensation is poor. [33][34][35][36] In view of this situation, a system with high-speed cameras is designed as shown in Figure 2.…”

Section: Methodsmentioning

confidence: 99%

Morphology evolution and dynamics of droplet coalescence on superhydrophobic surfaces

et al. 2018

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In this work, the coalescence of two equal‐sized water droplets on superhydrophobic surfaces (SHSs) is experimentally investigated. The morphologies of droplet coalescence are observed from side‐view and bottom‐view using high‐speed camera system. The related morphology evolution and dynamics of droplet coalescence are explored. The dynamic behaviors of droplet coalescence on SHSs can be decomposed into liquid bridge growth, contact line evolution, and droplet jumping. The liquid bridge radius is proportional to the square root of time, whereas the dimensionless prefactor is decreased from 1.18 to 0.83 due to the transition of interface curvature. The retraction velocity of the contact line shows limited dependence on initial droplet radii as the retraction dynamics considered here are governed by the capillary–inertial effect. The coalesced droplet finally departs the substrate with a dimensionless jumping velocity of around 0.2. A heuristic argument is made to account for the nearly constant dimensionless jumping velocity. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2913–2921, 2018

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“…Instead of using the jacket cooling method, Oh and Revankar [11] conducted steam/air condensation experiments with secondary pool boiling. Recently, Ma et al [12] experimentally studied the heat transfer characteristics of dropwise condensation on a vertical superhydrophobic plate for a variety of noncondensable gas concentration.…”

Section: Introductionmentioning

confidence: 99%