Engineers invent better way to boil water

Engineers at the Massachusetts Institute of Technology (MIT) have developed a new surface treatment for heat-conducting materials that makes them more efficient at boiling water.

The research team published its findings in a paper in the Advanced Materials journal.

They explained they had created a texture consisting of three modifications at different size scales to radically improve a material’s boiling efficiency.

The treatment simultaneously increases two important parameters of the conducting material — the heat transfer coefficient (HTC) and the critical heat flux (CHF).

Existing attempts to improve the HTC or CHF typically result in one of these improving to the detriment of the other.

One of the team’s members, MIT graduate Youngsup Song, explained how it worked.

“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, which can form a vapour film over the boiling surface.”

Song said that film introduced resistance to the heat transfer from the hot surface to the water.

“If we have vapour in between the surface and water, that prevents the heat transfer efficiency and lowers the CHF value,” he said.

To control the formation of bubbles on the surface, the researchers investigated adding different microscale cavities or dents.

With the right combination, the engineers could effectively keep the bubbles pinned to the locations of the dents and prevent them from spreading out into a heat-resisting film.

The combination consisted of an array of 10-micrometre-wide dents separated by about two millimetres to prevent film formation.

To compensate for the reduction in the concentration of bubbles at the surface, they added a much smaller-scale surface treatment, creating tiny bumps and ridges at the nanometer scale, which increased the surface area and promoted the evaporation rate under the bubbles.

“In these experiments, the cavities were made in the centres of a series of pillars on the material’s surface,” MIT explained.

“These pillars, combined with nanostructures, promote wicking of liquid from the base to their tops, and this enhances the boiling process by providing more surface area exposed to the water.”

While the work has confirmed the combination of surface treatments could enhance both HTC and CHF and improve boiling efficiency, it was done in small-scale laboratory conditions. It did not yet offer a way to scale up practically.

Aside from potential use in the thermal management of electronic devices with semiconductors, it could pave the way for increasing the output capability of many of the world’s power plants, which mostly rely on steam to turn turbines and produce electricity.

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Engineers invent better way to boil water