Water is boiled a lot – whether it is a cup of tea brewed in the kitchen or at the power plant. Any improvements in the efficiency of this process will have a significant impact on the total amount of energy used for it each day.
One such improvement could come with a newly developed treatment of surfaces used for water heating and evaporation. Processing improves two key parameters that define the boiling process: the heat transfer coefficient (HTC) and the critical heat flow (CHF).
Most of the time, there is a trade-off between the two – the better one, the worse the other. After years of searching, the search term behind this technique has found a way to enhance both.
“Both parameters are important, but optimizing both parameters together is kind of difficult because they have an intrinsic trade-off,” Bioinformatics scientist Yongsap Song says From Lawrence Berkeley National Laboratory in California.
“If we have a lot of bubbles on the boiling surface, then the boiling is very efficient, but if we have too many bubbles on the surface, they can fuse together, which could form a vapor layer above the boiling surface.”
Any vapor film between the hot surface and the water presents resistance, which reduces the heat transfer efficiency and CHF value. To get around this problem, the researchers devised three different types of surface modification.
First, a series of microtubules are added. This group of tubes of 10 µm width, spaced approximately 2 mm apart, controls bubble formation and keeps bubbles fixed in the cavities. This prevents a vapor film from forming.
At the same time, it reduces the concentration of bubbles on the surface, which reduces the boiling efficiency. To address this, the researchers introduced a smaller-scale treatment as a second modification, adding only nanometer-sized protrusions and edges within the surface of the hollow tubes. This increases the available surface area and enhances evaporation rates.
Finally, micro-cavities were placed in the center of a series of columns on the surface of the material. These plumes speed up the fluid withdrawal process by adding more surface area. Combined, the boiling efficiency increases significantly.
Above: A video slowed down by the researchers shows water boiling on a specially treated surface causing bubbles to form at specific separate points.
Since the nanostructures also promote evaporation under the bubbles, and the columns 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—which promotes maximum heat flow.
“Demonstrating our ability to manipulate the surface in this way to get the improvement is the first step,” says mechanical engineer Evelyn Wang from the Massachusetts Institute of Technology. “Then the next step is to think of more scalable approaches.”
“These types of structures that we make are not intended to scale into their current form.”
Moving work from a small-scale lab to something that can be used in commercial industries won’t be very easy, but the researchers are confident it can be done.
One challenge is finding ways to create the surface texture and the three “levels” of adjustments. The good news is that there are different methods that can be explored, and the procedure should work for different types of fluids as well.
“These kinds of details can be changed, and that could be our next step,” sung says.
The search was published in advanced materials.