Water gets boiled a lot, whether it’s a cup of tea that’s brewed in the kitchen or a power plant that generates electricity. Any improvement in the efficiency of this process will have a huge impact on the total amount of energy used every day.
One of those improvements could come with a newly developed treatment for surfaces involved in heating and evaporating water. The treatment improves two key parameters that determine the boiling process: the heat transfer coefficient (HTC) and the critical heat flux (CHF).
Most of the time, there is a trade-off between the two: as one gets better, the other gets worse. After years of research, the research term behind the technique has found a way to improve both.
“Both parameters are important, but improving both parameters together is a bit tricky because they have an intrinsic trade-off,” says bioinformatic scientist Youngsup Song from the Lawrence Berkeley National Laboratory in California.
“If we have a lot 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, which can form a vapor film on the boiling surface.”
Any vapor film between the hot surface and the water introduces resistance, which reduces the heat transfer efficiency and the CHF value. To get around the problem, the researchers came up with three different types of surface modification.
First, a series of microscale tubes are added. This array of 10-micrometer-wide tubes spaced about 2 millimeters apart controls bubble formation and keeps the bubbles attached to the cavities. This prevents a vapor film from forming.
At the same time, it reduces the concentration of bubbles on the surface, reducing the boiling efficiency. To address that, the researchers introduced a smaller-scale treatment as a second modification, adding bumps and ridges just nanometers in size within the surface of the hollow tubes. That increases the available surface area and promotes evaporation rates.
Ultimately, microscale cavities were lodged in the center of a series of pillars on the surface of the material. These pillars speed up the liquid extraction process by adding more surface area. In combination, the boiling efficiency is increased significantly.
Above: A slowed-down video of the researchers’ setup shows water boiling on a specially treated surface that causes bubbles to form at specific separate points.
As the nanostructures also promote evaporation below the bubbles, and the pillars maintain a constant supply of liquid to the base of the bubble, a layer of water can be maintained between the boiling surface and the bubbles, enhancing maximum heat flux. .
“Showing that we can control the surface in this way for improvements is a first step,” says mechanical engineer Evelyn Wang from the Massachusetts Institute of Technology. “So the next step is to think about more scalable approaches.”
“These kinds of structures that we’re making are not meant to be climbed in their current form.”
Taking the work from a small-scale laboratory setting to something that can be used in commercial industries won’t be that simple, but the researchers are confident it can be done.
One challenge will be finding ways to create the surface textures and the three “tiers” of mods. The good news is that there are different approaches that can be explored, and the procedure should also work for different types of liquids.
“Those kinds of details can be changed, and that may be our next step.” says song.
The research has been published in advanced materials.