New findings published in the Advanced Materials journal showed how to improve the two key parameters of the boiling process, heat transfer coefficient (HTC) and the critical heat flux (CHF), without compromising any of the parameters. In materials design, there's a trade-off between these two parameters: any improvement in one would make the other worse.
Trade-Off of Changing Both Heat Transfer Coefficient and Critical Heat Flux in Boiling Water
Youngsup Song, the study researcher, and a recent MIT graduate, explained that both HTC and CHF are important, but enhancing both parameters was tricky. "Because if we have lots of bubbles on the boiling surface, that means boiling is very efficient, but if we have too many bubbles on the surface, they can coalesce together, forming a vapor film over the boiling surface," he said. The vapor between the water and the surface could prevent the efficiency of heat transfer and lowers the CHF value.
All The Science explains that HTC describes how and how easily heat energy transfers from one medium to another. On the other hand, Springer defines the CHF condition by the sharp drop of the local heat transfer coefficient, which happens when liquid is replaced by vapor close to the heat transfer surface.
Song said that while the individual elements of the new surface treatment he created have been investigated before, this work is the first to demonstrate how these techniques may be combined to balance out the trade-off between the two competing criteria.
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Three Tier Modification to Improve Boiling Water Efficiency
Three surface modifications at different size scales were implemented to improve efficiency. One of the ways is to add a series of microscale cavities or dents to the surface. This method would control the way bubbles form on that surface. This would also keep them pinned to the dent's location, preventing them from spreading into a heat-resisting film.
To avoid film formation, the researchers in this study fabricated an array of 10 micrometers wide dents and spaced them by roughly 2 millimeters. However, that division also lessens the bubble concentration at the surface, which may decrease boiling efficiency. To make up for it, the scientists applied a surface treatment at a much smaller scale, producing tiny bumps and ridges at the nanometer scale. This would increase the surface area and accelerate the evaporation rate beneath the bubbles.
The cavities were created in the center of a series of pillars on the material's surface. The pillars were combined with nanostructures to promote the wicking of liquid from the base to the tops. The boiling process gets enhanced by providing more surface areas that are exposed to the water.
These form the three tiers, cavity separation, nanoscale texturing, and the posts, which helped enhance the boiling process.
The micro cavities define the position where the bubbles come up. Yet, the coalescence of the bubbles was minimized due to the 2-millimeter gap in the cavities. On the other hand, the nanostructures encourage evaporation under the bubbles, while the capillary action generated by the pillars provides liquid to the bubble base. This keeps a layer of liquid water between the boiling surface and the vapor bubbles, increasing the heat flux to its maximum.
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