Enhancing critical heat flux (CHF) during boiling with structured surfaces has received much attention because of its important implications for two-phase flow. The role of surface structures on bubble evolution and CHF enhancement remains unclear because of the lack of direct visualization of the liquid- and solid-vapor interfaces. Here, we use high-magnification in-liquid endoscopy to directly probe bubble behavior during boiling. We report the previously unidentified coexistence of two distinct three-phase contact lines underneath growing bubbles on structured surfaces, resulting in retention of a thin liquid film within the structures between the two contact lines due to their disparate advancing velocities. This finding sheds light on a previously unidentified mechanism governing bubble evolution on structured surfaces, which has notable implications for a variety of real systems using bubble formation, such as thermal management, microfluidics, and electrochemical reactors.

Liquid film–induced critical heat flux enhancement on structured surfaces

Boiling is an effective energy‐transfer process with substantial utility in energy applications. Boiling performance is described mainly by the heat‐transfer coefficient (HTC) and critical heat flux (CHF). Recent efforts for the simultaneous enhancement of HTC and CHF have been limited by an intrinsic trade‐off between them—HTC enhancement requires high nucleation‐site density, which can increase bubble coalescence resulting in limited CHF enhancement. In this work, this trade‐off is overcome by designing three‐tier hierarchical structures. The bubble coalescence is minimized to enhance the CHF by defining nucleation sites with microcavities interspersed within hemi‐wicking structures. Meanwhile, the reduced nucleation‐site density is compensated for by incorporating nanostructures that promote evaporation for HTC enhancement. The hierarchical structures demonstrate the simultaneous enhancement of HTC and CHF up to 389% and 138%, respectively, compared to a smooth surface. This extreme boiling performance can lead to significant energy savings in a variety of boiling applications.

/pdf/three-tier-hierarchical-structures-for-extreme-pool-boiling-6jjiik3k.pdf

Three‐Tier Hierarchical Structures for Extreme Pool Boiling Heat Transfer Performance

Superior Antidegeneration Hierarchical Nanoengineered Wicking Surfaces for Boiling Enhancement

Creating thin (<100 nm) hydrophobic coatings that are durable in wet conditions remains challenging. Although the dropwise condensation of steam on thin hydrophobic coatings can enhance condensation heat transfer by 1000%, these coatings easily delaminate. Designing interfaces with high adhesion while maintaining a nanoscale coating thickness is key to overcoming this challenge. In nature, cell membranes face this same challenge where nanometer-thick lipid bilayers achieve high adhesion in wet environments to maintain integrity. Nature ensures this adhesion by forming a lipid interface having two nonpolar surfaces, demonstrating high physicochemical resistance to biofluids attempting to open the interface. Here, developing an artificial lipid-like interface that utilizes fluorine-carbon molecular chains can achieve durable nanometric hydrophobic coatings. The application of our approach to create a superhydrophobic material shows high stability during jumping-droplet-enhanced condensation as quantified from a continual one-year steam condensation experiment. The jumping-droplet condensation enhanced condensation heat transfer coefficient up to 400% on tube samples when compared to filmwise condensation on bare copper. Our bioinspired materials design principle can be followed to develop many durable hydrophobic surfaces using alternate substrate-coating pairs, providing stable hydrophobicity or superhydrophobicity to a plethora of applications.

A Lipid-Inspired Highly Adhesive Interface for Durable Superhydrophobicity in Wet Environments and Stable Jumping Droplet Condensation.

Two-phase internal flow is ubiquitous to many systems due to its ability to transfer large amounts of heat effectively. Recent advances motivated by sustainability have pushed towards augmenting the heat transfer between the tube and the two-phase working fluid. Augmenting heat transfer in two-phase flows enables process intensification, and compactness leading to reduced cost and material consumption. This review investigates the effect of heat transfer enhancement techniques on different fluids including R134a, R407C, R22, etc … The techniques are categorized into passive and active and focus on durable enhancement methods which do not include surface structuring or coating. Passive techniques utilize geometrical and surface modifications to induce better mixing. Several passive techniques are analyzed including fins, inserts, and more. In analyzing these techniques, the effect of changing geometric parameters is studied, and the limitations are highlighted. Further, active techniques that enable heat transfer enhancement through applying power are also investigated. Electrohydrodynamic approaches are analyzed and compared to passive techniques. Additionally, the implications of heat exchange enhancement in the context of global energy are discussed. An energy analysis discretized by the use sector is conducted, with an emphasis on predicting the future potential for renewable and sustainable development. Moreover, although active approaches provide the highest heat transfer enhancement, due to implementation difficulties, passive techniques are more frequently adopted. The review ends with discussion of next-generation heat exchangers which will rely more on additive manufacturing due to its flexibility and reduced cost per part as the technology matures. 

http://manuscript.elsevier.com/S1364032121011631/pdf/S1364032121011631.pdf

Review of heat transfer enhancement techniques in two-phase flows for highly efficient and sustainable cooling

Endoscopic Visualization of Contact Line Dynamics during Pool Boiling on Capillary-Activated Copper Microchannels

The dislocation and thermodynamic properties of L1 2 -Al 3 X(X Sc, Ti, V) intermetallics were studied based on first-principles calculations. The computed stacking fault energies indicate the energy barrier for dislocation formation in Al 3 Sc is higher than that of Al 3 Ti and Al 3 V. Also, the negative anti-phase boundary(APB) energy for Al 3 V suggests the formation of APB is energetically favored, and this finding is further evidenced by electron localization function(ELF) and densities of states(DOS). Moreover, the computed Peierls energies and Peierls stress suggest the movement of edge dislocation is easier than screw dislocation in Al 3 Sc, Al 3 Ti, and Al 3 V. Further, the < 110 > (111) screw dislocation of Al 3 Sc and the < 110 > (111) edge dislocation of Al 3 V is the most difficult and easiest to move in all the studied systems, respectively. It is consistent with the computed ideal shear stress(τ iss ) for Al 3 Sc, Al 3 Ti, and Al 3 V. Moreover, we speculated the Al 3 Sc and Al 3 Ti in aluminum alloy may possess a better strengthening effect in comparison with Al 3 V. Finally, the thermodynamic properties of Al 3 Ti was found less dependent on temperatureare than that of Al 3 Sc and Al 3 V, the Al 3 Ti can serve as a better candidate for improving the heat resistance of aluminum alloys. ● The dislocation properties of L1 2 -Al 3 Sc, Al 3 Ti, and Al 3 V were studied via PN model. ● The dislocation formation in Al 3 Sc is harder than that in Al 3 Ti and Al 3 V. ● The formation of anti-phase boundary(APB) in Al 3 V is energetically favored. ● The Al 3 Sc and Al 3 Ti in aluminum possess a better strengthening effect than Al 3 V. 

Insight into the stacking fault energy, dislocation, and thermodynamic properties of L12-Al3X(X Sc, Ti, V) intermetallics from first-principles calculations

Yongyi Zhao

Papers

Endoscopic Visualization of Contact Line Dynamics during Pool Boiling on Capillary-Activated Copper Microchannels

Superior Antidegeneration Hierarchical Nanoengineered Wicking Surfaces for Boiling Enhancement

Insight into the stacking fault energy, dislocation, and thermodynamic properties of L12-Al3X(X Sc, Ti, V) intermetallics from first-principles calculations