A lot of water boils – be it 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 energy used each day.
One such improvement could come with a newly developed treatment of surfaces used for water heating and evaporation. Processing optimizes two key parameters that define the boiling process: heat transfer coefficient (HTC) and critical heat flux (CHF).
In most cases, there is a trade-off between the two – one is better, the other is worse. After years of searching, the search term behind this technique has found a way to optimize both.
“Both parameters are important, but optimizing both parameters together is difficult because they have an inherent trade-off.” says bioinformatics scientist Yongchap Chang From Lawrence Berkeley National Laboratory in California.
“If there are a lot of bubbles on the boiling surface, boiling is more efficient, but if there are too many bubbles on the surface, they can stick together, which can create a vapor layer on the boiling surface.”
Any vapor film between the heated surface and the water provides resistance, which reduces the heat transfer efficiency and CHF value. To overcome this problem, the researchers developed three different types of surface modifiers.
First, a series of capillaries is added. This set of 10 micron wide tubes, spaced about 2 mm apart, controls bubble formation and stabilizes the bubbles within the cavities. This prevents the formation of a vapor film.
At the same time, the concentration of bubbles on the surface decreases, which reduces the boiling capacity. To address this, the researchers introduced a small-scale treatment as a second modification, adding only nanometer-sized protrusions and edges to the surface of the hollow tubes. This increases the available surface area and increases the rate of evaporation.
Finally, micro-cavities were placed in the center of the array of columns on the surface of the material. These worms accelerate the process of fluid withdrawal by adding more surface area. In combination, the boiling capacity increases significantly.
Above: A slow-motion video by the researchers shows water boiling on a specially treated surface, causing bubbles to form at specific discrete points.
Because the nanostructures promote evaporation under the bubbles, and the columns maintain a steady supply of liquid to the bottom of the bubble, a layer of water can be maintained between the boiling surface and the bubbles—promoting maximum heat flow.
“The first step is to demonstrate the ability to manipulate the surface in this way to achieve improvement,” Mechanical engineer Evelyn Wang says: from the Massachusetts Institute of Technology. “The next step is to think about more scalable approaches.”
“These kinds of structures we’re building are not meant to scale in their current form.”
Transferring the work from a small-scale lab to commercial applications isn’t easy, but researchers believe it can be done.
A challenge is finding ways to create surface textures and three “levels” of adjustments. The good news is that different methods can be explored, and the process should work for different types of liquids.
“These kinds of details can be changed, and that could be our next step.” He says he sang.
Search was published in Advanced Materials.
“Professional coffee fan. Total beer nerd. Hardcore reader. Alcohol fanatic. Evil twitter buff. Friendly tv scholar.”