Any improvement in the efficiency of the process of boiling water would have a great impact on the total amount of energy used every day, since it is an activity that we carry out very often, whether it is to generate electricity in a power plant or to boil some eggs (or a cup of tea).
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). Normally they compensate each other: if one improves, the other worsens, and vice versa. Now, after years of research, it seems that science has found a way to improve both processes at once.
“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 of the Lawrence Berkeley National Laboratory in California and co-author of the paper published in the journal Advanced Materials . “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.”
This film prevents heat transfer from the hot surface to the water. “If we have steam between the surface and the water, it reduces the heat transfer efficiency and lowers the CHF value,” added the researcher.
Small scale lab tested
To get around this problem, they added a series of microscale cavities to a surface, thereby controlling how bubbles form on that surface. This kept the bubbles effectively attached to the dent locations and prevented them from spreading into a heat resistant film. The microcavities were then placed at the ideal length to optimize this process. In this work, the researchers created a series of 10-micrometer-wide dents spaced about 2 millimeters apart to prevent film formation.
While the various components of the new surface treatment he developed had been studied previously, the researchers say this work is the first to show that these methods could be combined to overcome the balance between the two competing parameters.
“These kinds of structures that we are making are not intended to be climbed in their current form,” the authors explained, but rather were used to show that such a system can work.
The team is now focused on finding additional ways to create these kinds of surface textures that can be used in practical dimensions.
“Showing that we can control the surface in this way to improve it is a first step,” he says. “So the next step is to think about more scalable approaches.” For example, although the pillars on the surface of these experiments were created using clean room methods commonly used to produce semiconductor chips, there are other, less demanding ways to create such structures, such as electrodeposition.
This same multiscale structuring technique could also be applied to different liquids, experts say, by adjusting the dimensions to account for different liquid properties.
Referencia: “Three-Tier Hierarchical Structures for Extreme Pool Boiling Heat Transfer Performance” by Youngsup Song, Carlos D. Díaz-Marín, Lenan Zhang, Hyeongyun Cha, Yajing Zhao and Evelyn N. Wang, 20 June 2022, Advanced Materials.