Condensation is a phase change phenomenon often encountered in nature, as well as used in industry for applications including power generation, thermal management, desalination, and environmental control. For the past eight decades, researchers have focused on creating surfaces allowing condensed droplets to be easily removed by gravity for enhanced heat transfer performance. Recent advancements in nanofabrication have enabled increased control of surface structuring for the development of superhydrophobic surfaces with even higher droplet mobility and, in some cases, coalescence-induced droplet jumping. Here, we provide a review of new insights gained to tailor superhydrophobic surfaces for enhanced condensation heat transfer considering the role of surface structure, nucleation density, droplet morphology, and droplet dynamics. Furthermore, we identify challenges and new opportunities to advance these surfaces for broad implementation in thermofluidic systems.

/pdf/condensation-heat-transfer-on-superhydrophobic-surfaces-5b78jcldf3.pdf

Condensation Heat Transfer on Superhydrophobic Surfaces

Dropwise condensation (DWC) heat transfer depends strongly on the maximum diameter (Dmax) of condensate droplets departing from the condenser surface. This study presents a facile technique implemented to gain control of Dmax in DWC within vapor/air atmospheres. We demonstrate how this approach can enhance the corresponding heat transfer rate by harnessing the capillary forces in the removal of the condensate from the surface. We examine various hydrophilic-superhydrophilic patterns, which, respectively, sustain and combine DWC and filmwise condensation on the substrate. The material system uses laser-patterned masking and chemical etching to achieve the desired wettability contrast and does not employ any hydrophobizing agent. By applying alternating straight parallel strips of hydrophilic (contact angle ∼78°) mirror-finish aluminum and superhydrophilic regions (etched aluminum) on the condensing surface, we show that the average maximum droplet size on the less-wettable domains is nearly 42% of the width of the corresponding strips. An overall improvement in the condensate collection rate, up to 19% (as compared to the control case of DWC on mirror-finish aluminum) was achieved by using an interdigitated superhydrophilic track pattern (on the mirror-finish hydrophilic surface) inspired by the vein network of plant leaves. The bioinspired interdigitated pattern is found to outperform the straight hydrophilic-superhydrophilic pattern design, particularly under higher humidity conditions in the presence of noncondensable gases (NCG), a condition that is more challenging for maintaining sustained DWC.

Enhancing dropwise condensation through bioinspired wettability patterning.

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 nonjumping, 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 ( 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. Keywords: superhydrophobic condensation, jumping droplets, droplet coalescence, condensation optimization, environmental scanning electron microscopy; micro/nanoscale water condensation, condensation heat transfer.

http://dspace.mit.edu/bitstream/handle/1721.1/84986/Modeling%20and%20Optimization%20of%20Superhydrophobic%20Condensation.pdf;sequence=1

Modeling and Optimization of Superhydrophobic Condensation

When condensed droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump due to the conversion of excess surface energy into kinetic energy. This phenomenon has...

Electric-Field-Enhanced Condensation on Superhydrophobic Nanostructured Surfaces

Experiments show, for the first time, two water droplets coalescing and jumping on a superhydrophobic surface. Adhesion, contact angle hysteresis, and initial wetting behavior governed by the surface structure morphology and length scale, are all shown to play a role in droplet jumping.

Coalescence-induced nanodroplet jumping

The continually increasing heat generation rates in high performance electronics, radar systems and data centers require development of efficient heat exchangers that can transfer large heat loads. In this paper, we present the design of a new high-performance heat exchanger capable of transferring 1000 W while consuming less than 33 W of input electrical power and having an overall thermal resistance of 0.05 K/W. The low thermal resistance is achieved by using a loop heat pipe with a single evaporator and multiple condenser plates that constitute the array of fins. Impellers between the fins are driven by a custom permanent magnet synchronous motor in a compact volume of 0.1 × 0.1 × 0.1 m to maximize the heat transfer area and reduce the required airflow rate and electrical power. The design of the heat exchanger is developed using analytical and numerical methods to determine the important parameters of each component. The results form the basis for the fabrication and experimental characterization that is currently under development.

Design of an Integrated Loop Heat Pipe Air-Cooled Heat Exchanger for High Performance Electronics

Enhancement of convective heat transfer in an air-cooled heat exchanger using interdigitated impeller blades

We report the design and analysis of a high-power air-cooled heat exchanger capable of dissipating over 1000 W with 33 W of input electrical power and an overall thermal resistance of less than 0.05 K/W. The novelty of the design combines the blower and heat sink into an integrated compact unit (4″ × 4″ × 4″) to maximize the heat transfer area and reduce the required airflow rates and power. The device consists of multiple impeller blades interdigitated with parallel-plate condensers of a capillary-pumped loop heat pipe. The impellers are supported on a common shaft and powered with a low-profile permanent magnet synchronous motor, while a single flat-plate evaporator is connected to the heat load.

/pdf/design-and-analysis-of-high-performance-air-cooled-heat-20qdlvi6jr.pdf

Design and analysis of high-performance air-cooled heat exchanger with an integrated capillary-pumped loop heat pipe

We report the design, analysis, and preliminary testing of microstructured copper wicks for a novel capillary-pumped loop heat pipe. The heat pipe design is a key component in an integrated pump-heat sink that dissipates over 1000 W with an overall thermal resistance less than 0.05 °C/W. Microstructured wicks with sintered copper particle sizes of 5 - 150 µm were fabricated within copper tubing and permeabilities of 9.5x10 -14 - 1.5x10 -11 m 2 and maximum capillary pressures of 100 - 850 Pa have been achieved. Additionally, multi-layered microstructured wicks have been fabricated that simultaneously allow for high permeability and high capillary force. This work is a critical step towards developing new high-performance thermal management solutions for commericial, defense, and space applications.

A capillary-pumped loop heat pipe with multi-layer microstructured wicks

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Design of an Integrated Loop Heat Pipe Air-Cooled Heat Exchanger for High Performance Electronics

Enhancement of convective heat transfer in an air-cooled heat exchanger using interdigitated impeller blades

Design and analysis of high-performance air-cooled heat exchanger with an integrated capillary-pumped loop heat pipe

A capillary-pumped loop heat pipe with multi-layer microstructured wicks