Biphilic surfaces reduce defrosting times in heat exchangers

Date:
July 29, 2020
Source:
University of Illinois Grainger College of Engineering
Summary:
Engineers have discovered a way to significantly improve the
defrosting of ice and frost on heat exchangers.



FULL STORY ==========================================================================
Ice formation and accumulation are challenging concerns for several
industrial applications including heating ventilation air conditioning
and refrigeration (HVAC&R) systems, aircraft, energy transmission,
and transportation platforms.

Frost formation on heat exchangers, for example, reduces heat transfer efficiency and results in significant economic losses. Moreover,
defrosting and de-icing techniques are energy-intensive, requiring
large masses of ice to be melted completely and surfaces to be cleaned
of leftover water during cyclic operation, making frosting-defrosting
a multi-billion-dollar problem in the U.S.


========================================================================== Nenad Miljkovic, along with researchers in his group, have discovered
a way to significantly improve the defrosting of ice and frost on heat exchangers. Their findings, "Dynamic Defrosting on Superhydrophobic and Biphilic Surfaces," have been published in Matter.

Defrosting of heat exchangers is a highly inefficient process. Common defrosting methods not only require significant energy to melt the frost
but additional energy to evaporate melted water from the wettable surface.

Researchers in the past have investigated the use of non-wettable
surfaces (hydrophobic or superhydrophobic) to delay frosting and reduce
ice adhesion, which does indeed improve defrosting performance. However,
water retention remains prevalent on such heat exchangers during frost, defrost, and re-frost cycles.

In an effort to eliminate water retention, Miljkovic and a team led by
graduate student Yashraj Gurumukhi and postdoctoral scholar Dr. Soumyadip
Sett studied dynamic defrosting on heterogeneous surfaces with spatially distinct domains of wettability, known as biphilic surfaces. These
biphilic surfaces have alternating superhydrophobic (water-repelling)
and hydrophilic (water-loving) regions. Through optical imaging,
the researchers showed that during defrosting, the frost layer on
a superhydrophobic region melts into a highly mobile slush, which is
pulled toward the hydrophilic regions by surface forces.

This mobility enables removal of slush from the superhydrophobic regions
prior to it completely melting, thereby cleaning the surface. Water is
then restricted to hydrophilic areas, where it evaporates quickly due
to the larger contact area.

Additionally, to optimize the design of their biphilic surfaces and
understand the effects of pattern heterogeneity, the team studied banana-leaf-inspired branched biphilic patterns to determine if it would
reduce cleaning time. They observed that binary biphilic designs were
simpler to manufacture when compared to branched designs and offered
better surface cleaning performance during defrosting.

"Defrosting cycles require systems to be shut down, frost completely
melted, and surfaces cleaned before restarting the system, consuming significant time and energy. Enhancing cleaning efficiency by utilizing wettability-patterned biphilic surfaces can reduce system downtime
and defrost energy input, thereby increasing overall efficiency,"
Miljkovic said.

In effect, when combined with suitable large-scale manufacturing methods, biphilic surfaces have the potential to outperform homogenous surfaces
in terms of heat transfer enhancements and energy requirements.

Their work not only provides fundamental design guidelines for fabricating biphilic surfaces, it illustrates the role of wettability gradients
on defrosting dynamics. Future work from the researchers will further
decrease defrost time by identifying critical bottlenecks in the process
and provide design methodologies to create effective defrost-enhancing
surfaces for industrial applications.

This project was funded by the Air Conditioning and Refrigeration Center
(ACRC) in MechSE.


========================================================================== Story Source: Materials provided by University_of_Illinois_Grainger_College_of_Engineering.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yashraj Gurumukhi, Shreyas Chavan, Soumyadip Sett, Kalyan Boyina,
Srivasupradha Ramesh, Peter Sokalski, Kirk Fortelka, Maury Lira,
Deokgeun Park, Juo-Yun Chen, Shreyas Hegde, Nenad Miljkovic. Dynamic
Defrosting on Superhydrophobic and Biphilic Surfaces. Matter,
2020; DOI: 10.1016/ j.matt.2020.06.029 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200729114748.htm

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