We experimentally realized and elucidated kinetically limited evaporation where the molecular gas dynamics close to the liquid–vapour interface dominates the overall transport. This process fundamentally dictates the performance of various evaporative systems and has received significant theoretical interest. However, experimental studies have been limited due to the difficulty of isolating the interfacial thermal resistance. Here, we overcome this challenge using an ultrathin nanoporous membrane in a pure vapour ambient. We demonstrate a fundamental relationship between the evaporation flux and driving potential in a dimensionless form, which unifies kinetically limited evaporation under different working conditions. We model the nonequilibrium gas kinetics and show good agreement between experiments and theory. Our work provides a general figure of merit for evaporative heat transfer as well as design guidelines for achieving efficient evaporation in applications such as water purification, steam generation, and thermal management.
Evaporation plays a critical role in a range of technologies that power and sustain our society. Wicks are widely used as passive, capillary-fed evaporators, attracting much interest since these devices are highly efficient, compact, and thermally stable. While wick-based evaporators can be further improved with advanced materials and fabrication techniques, modeling of heat and mass transport at the device level is vital for guiding these innovations. In this perspective, we present the design and optimization of capillary-fed, thin film evaporation devices through a heat and mass transfer lens. This modeling framework can guide future research into materials innovations, fabrication of novel architectures, and systems design/optimization for next generation, high-performance wick-based evaporators. Furthermore, we describe specific challenges and opportunities for the fundamental understanding of evaporation physics. Finally, we apply our modeling framework to the analysis of two important applications—solar vapor generation and electronics cooling devices.
Boiling is an essential process in
numerous applications including
power plants, thermal management, water purification, and steam generation.
Previous studies have shown that surfaces with microcavities or biphilic
wettability can enhance the efficiency of boiling heat transfer, that
is, the heat transfer coefficient (HTC). Surfaces with permeable structures
such as micropillar arrays, in contrast, have shown significant enhancement
of the critical heat flux (CHF). In this work, we investigated microtube
structures, where a cavity is defined at the center of a pillar, as
structural building blocks to enhance HTC and CHF simultaneously in
a controllable manner. We demonstrated simultaneous CHF and HTC enhancements
of up to 62 and 244%, respectively, compared to those of a smooth
surface. The experimental data along with high-speed images elucidate
the mechanism for simultaneous enhancement where bubble nucleation
occurs in the microtube cavities for increased HTC and microlayer
evaporation occurs around microtube sidewalls for increased CHF. Furthermore,
we combined micropillars and microtubes to create surfaces that further
increased CHF by achieving a path to separate nucleating bubbles and
rewetting liquids. This work provides guidelines for the systematic
surface design for boiling heat transfer enhancement and has important
implications for understanding boiling heat transfer mechanisms.
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