Water is boiled a lot – whether it’s a cup of tea brewed in the kitchen or at a power station. Any increase 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 newly developed surface treatments used for heating and evaporating water. Processing improves the two main parameters that determine the boiling process: heat transfer coefficient (HTC) and critical heat flow (CHF).
Most of the time, there is a trade-off between the two – the better, the worse the other. After years of searching, the search terms behind this technique have found a way to improve both.
“Both parameters are important, but optimizing the two parameters together is a bit difficult because they have intrinsic trade-offs,” Bioinformatics scientist Yongsap Song said From Lawrence Berkeley National Laboratory in California.
“If we have a lot of bubbles on the boiling surface, then boiling is very efficient, but if we have too many bubbles on the surface, they can melt together, which can form a vapor layer over the boiling surface.”
Any vapor layer between the hot surface and the water creates resistance, which reduces the heat transfer efficiency and the CHF value. To solve this problem, the researchers designed three different types of surface modification.
First, a series of microtubules is added. This group of tubes 10 m wide, spaced about 2 mm apart, controls bubble formation and keeps bubbles in the cavity. This prevents the formation of a vapor film.
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 small-scale treatment as a second modification, adding only nanometer-sized protrusions and inner edges of the hollow tube surface. This increases the available surface area and increases the evaporation rate.
Finally, a micro cavity is placed in the center of a series of columns on the surface of the material. These clumps speed up the liquid withdrawal process by adding more surface area. Combined, boiling efficiency increases significantly.
Above: A video slowed down by researchers showing boiling water on a specially treated surface causing bubbles to form at specific, separate points.
Because the nanostructure also promotes evaporation below the bubble, and the column maintains a constant supply of liquid to the bottom of the bubble, a layer of water can be maintained between the boiling surface and the bubble—which promotes maximum heat flow.
“Demonstrating our ability to manipulate the surface in this way to get optimization is the first step,” Mechanical Engineer Evelyn Wang said: from the Massachusetts Institute of Technology. “Then the next step is to think of a more scalable approach.”
“This type of structure we created was not intended to scale to its current form.”
Moving work from a small-scale laboratory to something usable in commercial industry won’t be easy, but the researchers believe it can be done.
One of the challenges was finding ways to create surface textures and “three tiers” of adjustments. The good news is that there are various methods that can be explored, and the procedure should also work for different types of fluids.
“These kinds of details can be changed, and that could be our next step.” sung says.
Search published in advanced materials.
–
