Superhydrophobic Metal and Heat Transfer

Question: Could a superhydrophobic metal surface still be useful in a heat exchanger?

I would think it would be a very poor heat exchanger.

I'll stick with these

Those are great, but there are plenty of instances that require water as the coolant, whether it is a shell and tube exchanger or some other device (maybe the back side of a PV panel), and I am just curious at this point.

In the power generation industry, there really not much telling how much energy is wasted (not converted to electrical power) due to improper maintenance of condenser cooling efficiency, especially due to fouling by particulates in the water, or outright scale formation, or corrosion leading to deposition on the tubes in a condenser that is a special class of shell and tube cooling technology.

If the contact of the water is such that metal ions are not subject to finding a path for the corrosion reaction to take place in the first place, nor are minerals in the water able to make contact (say) with the higher pH metal of a corrosion cathode (condenser tube), then the fouling factor of such a system might be kept as near "factory" condition as possible. The result is generator performance that stays at the "top of the curve" for as long as is physically possible. The implications would be immense, not only for cycle efficiency, but for the bottom line of these businesses that rely on the generation heat rate being as low as physically possible. Then also comes into the equation water conservation issues that are now becoming more and more of a driving force in new plants and in re-fit of older ones.

K would be nearly zilch.

At least the hypothesis for or against should not be hard to test. I intend to contact the lab at Rochester U, and see if they are even remotely interested in conducting the test with their materials.

I have installed a few heat exchangers with 'high-flux tubes' in petrochem plants. These were where clean, light hydrocarbons were being vaporized. The modified surface enhances the nucleate boiling process. I didn't read your referenced article, but the discussion sounds like it is along these lines.

You can Google 'high-flux heat exchange' or 'enhanced surface heat exchanger' for more background.

I'm sure if it would make a difference or make it worse. If the water does actually boil I would expect it to be a poorer heat exchanger because the steam bubbles would become an insulating bubble on the surface. If the water does not boil would the presumed lower friction coefficient allow for a more fluid motion to happen or would the ease in motion induce easier stratification.

I agree this might not work well in a boiler as DNB might be way easier to take place.

I am not entirely convinced that there is a really poor K factor, or just a mediocre one. After all, there still would be minimal surface interaction because we have forced a tube full of water that does not want to wet the metal at all.

Question is still - does the boundary layer exist, and does that have any influence on heat exchange at all? Why?

I read that the other day too. cool stuff wonder about cost for commercial uses?

Just about everything that involves high enough power laser to change the surface probably costs more. I don't know that economy of scale will improve or "phase in" on this one, but it might. Certainly would be nice not to have to clean a toilet at all in the future, but surely there might be other more pertinent uses for this technology, maybe even in the anti-icing, rain proofing etc.

I was thinking windshields

Maybe so, but if this results in a "grating" on the surface of the glass, would it not be like looking through a transparent CD disc? One might think it evoked vision similar to an LSD trip? Not that I know what psychedelic vision is like.

Having read the linked article , I question the description of the material as superhydrophobic. I would argue that the "repellency" is entirely due to physics and not to any innate properties that repel water and other highly polar liquids. According to the way it is described in the article, it should also be oleophobic, at least to a degree depending on the surface tension of any liquid splashed on it. Immersion and splashing are two very different things.

I would suggest that this treatment could be very useful for a heat exchanger. Why? Because the process of nano scale etchings on the metal surface must increase the area significantly. Also, in a flooded (and especially pressurized) environment as found in a heat exchanger, I would imagine the gaseous "cushion" described would soon be absorbed into the process fluids, causing intimate contact and removing the hydrophobic effect until dried. It could be a real pain for holding scale though!

If this material allows scale to form, then it is game over. That would be the No.1 reason for using something as advanced as this -- the quest to eradicate fouling/scaling of heat exchangers. I could see that this treatment of the metal on the condensing side might be a real advantage in a surface condenser due to likelihood of quickly shedding or eliminating condensate surface film loading on the metal that might inhibit condensation heat transfer at high throughput. On that side replellancy is precisely what is useful.

On the cold side (coolant circulation), the surface interaction will simply need to be studied, but first we have to learn if the cost of this material supercedes any useful effect.

That seems to me to be a perfect test of whether or not this surface treatment yields true hydrophobicity- how it works as a condenser surface, where the vapour would be contacting the entire surface and not just the "points". With surface treatment being limited in size by the wavelength of the laser as an absolute maximum for control, the intervening areas must still have lots of room on a molecular scale.

As these images show, water repulsion will reduce the contact area if no other force (in this case just gravity) forces contact with the superhydrophobic material. A smaller contact area will reduce heat transfer.

Surface area may have increased by etching but the gaps created by repellency means your thermal conduction is influenced by the thermal conductivity of whatever can wet out or fill those gaps.

I'm picturing air (less conductive than water and metal) filling gaps water can't reach. The increased surface area is now made up of two K values interacting with water instead of one.

As posted, my question is whether surface tension is sufficient to make the repellency last in this particular environment? Short of using a fluid that is pre-saturated with air or a suitable noncondensing gas, it would appear likely that the void space would gradually become filled as the original gas dissolves and is replaced by water vapour, which could condense.

I see where your question arises, and it makes sense that water vapor should replace at least some of the "air". But suppose that this "air" were to be replaced with essentially a vacuum (the former air, all having been lost to solubility), oh but wait, as the air dissolves, its pressure decreases until it cannot reach a level of solution that results in loss of additional air, so then water vapor will take over until the air is displaced. Water vapor is in equilibrium (supposedly) with the liquid, and over infinitesimally small regions, the transfer of vapor to liquid and back should be considerably rapid.

The only way to really resolve the "issue" is to test it, and I have attempted to contact the principle researcher on this to ask for a test.

Exactly!

Results will be interesting. My earlier answer is not much more than a guess. It would be neat to be proven wrong, in this case.

treat the hull, especially the nose