Water is boiled a lot – whether it’s in the kitchen making a cup of tea or in a power plant generating electricity. Any improvement in the efficiency of this process will have a major impact on the total amount of energy used for it each day.
One such improvement could be through a newly developed treatment for surfaces that deal with water heating and evaporation. The treatment improves two key parameters that determine the boiling process: heat transfer coefficient (HTC) and critical heat flux (CHF).
Often, there is a trade-off between the two – as one gets better, the other gets worse. After years of research, the research period behind the technique found a way to improve both.
“Both parameters are important, but increasing both parameters together is kind of difficult because they have internal trade-offs” says bioinformatics scientist Youngsup Song from Lawrence Berkeley National Laboratory in California.
“If we have a lot of bubbles on the boiling surface, that means the boiling is very efficient, but if there are a lot of bubbles on the surface, they can coalesce and that creates a vapor film on the boiling surface.”
Any vapor film between the hot surface and the water creates resistance, reducing the heat transfer efficiency and CHF value. To solve the problem, the researchers developed three different surface modifications.
First, a series of microscale tubes is attached. This array of 10-micrometer-wide tubes, spaced about 2 millimeters apart, controls bubble formation and keeps the bubbles trapped in the voids. This prevents the formation of a vapor film.
At the same time, it reduces the concentration of bubbles on the surface, reducing boiling efficiency. To overcome this, the researchers introduced a smaller-scale treatment as a second modification, simply adding nanometer-sized bumps and ridges to the surface of the hollow tubes. This increases the available surface area and increases the rate of evaporation.
Finally, micro-scale voids were placed in the center of a series of pillars on the surface of the material. These columns add more surface area, speeding up the liquid removal process. Together, the boiling efficiency increases significantly.
Above: A slow-motion video of the researchers’ setup shows water boiling on a specially treated surface and bubbles forming at certain points.
Because the nanostructures also aid evaporation below the bubbles and the columns maintain a continuous supply of liquid to that bubble base, a layer of water can be maintained between the boiling surface and the bubbles, maximizing heat flow.
“Showing that we can manipulate the surface in this way to get improvement is the first step” says mechanical engineer Evelyn Wang from the Massachusetts Institute of Technology. “Then the next step is to think about more scalable approaches.”
“These kinds of structures that we’re developing are not designed to scale in their current form.”
Taking the work from a small-scale laboratory setting to one that can be used in commercial industry will not be straightforward, but the researchers are confident that it can be done.
One of the challenges will be finding ways to create three “levels” of surface textures and modifications. The good news is that there are different approaches that can be explored, and the procedure should work for different types of fluids.
“Those kinds of details are subject to change and that could be our next step.” sings a song.
The study was published Advanced Materials.
