New approach could make HVAC heat exchangers five times more efficient

New approach could make HVAC heat exchangers five times more efficient
Credit: Brown University

Researchers from Tsinghua University and Brown University have discovered a simple way to give a major boost to turbulent heat exchange, a method of heat transport widely used in heating, ventilation and air conditioning (HVAC) systems.

In a paper published in Nature Communications, the researchers show that adding a readily available organic solvent to common water-based turbulent heat exchange systems can boost their capacity to move heat by 500%. That's far better than other methods aimed at increasing heat transfer, the researchers say.

"Other methods for increasing —nanoparticle additives or other techniques—have achieved at best about 50% improvement," said Varghese Mathai, a postdoctoral researcher at Brown and co-first author of the study, who worked with Chao Sun, a professor at Tsinghua who conceived of the idea. "What we achieve here is 10 times more improvement than other methods, which is really quite exciting."

Turbulent heat exchangers are fairly simple devices that use the natural movements of liquid to move heat. They consist of a hot surface, a cold surface and tank of liquid in between. Near the hot surface, the liquid heats up, becomes less dense and forms warm plumes that rise toward the cold side. There, the liquid loses its heat, becomes denser and forms cold plumes that sink back down toward the hot side. The cycling of water serves to regulate the temperatures of each surface. This type of heat exchange is a staple of modern HVAC systems widely used in home heaters and units, the researchers say.

In 2015, Sun had the idea to use an organic component known as hydrofluoroether or HFE to speed the cycling of heat inside this kind of exchanger. HFE is sometimes used as the sole fluid in heat exchangers, but Sun suspected that it might have more interesting properties as an additive in water-based systems. Working with the study's co-first author Ziqi Wang, Mathai and Sun experimented with adding small amounts of HFE and, after three years of work, were able to maximize its effectiveness in speeding heat exchange. The team showed that concentrations of around 1% HFE created dramatic heat flux enhancements up to 500%.

Using high-speed imaging and laser diagnostic techniques, the researchers were able to show how the HFE enhancement works. When near the hot side of the exchanger, the globules of HFE quickly boil, forming biphasic bubbles of vapor and liquid that rise rapidly toward the cold plate above. At the cold plate, the bubbles lose their heat and descend as liquid. The bubbles affect the overall heat flux in two ways, the researchers showed. The bubbles themselves carry a significant amount of heat away from the hot side, but they also increase the speed of the surrounding water plumes rising and falling.

"This basically stirs up the system and makes the plumes move faster," Sun said. "Combined with the heat that the bubbles themselves carry, we get a dramatic improvement in ."

That stirring action could have other applications as well, the researchers say. It could be useful in systems designed to mix two or more liquids. The extra stir makes for faster and more complete mixing.

The researchers pointed out that the specific additive they used—HFE7000—is non-corrosive, non-flammable and ozone friendly. One limitation is that the approach only works on vertical heat exchange systems—ones that move heat from a lower plate to an upper one. It doesn't currently work on side-to-side systems, though the researchers are considering ways to adapt the technique. Still, vertical exchangers are widely used, and this study has shown a simple way to improve them dramatically.

"This biphasic approach generates a very large increase in heat flux with minimal modifications to existing heating and cooling systems," Mathai said. "We think this has great promise to revolutionize exchange in HVAC and other large-scale applications."


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More information: Ziqi Wang et al, Self-sustained biphasic catalytic particle turbulence, Nature Communications (2019). DOI: 10.1038/s41467-019-11221-w
Journal information: Nature Communications

Provided by Brown University
Citation: New approach could make HVAC heat exchangers five times more efficient (2019, August 1) retrieved 23 August 2019 from https://phys.org/news/2019-08-approach-hvac-exchangers-efficient.html
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Aug 01, 2019
Go to
Forcedphysics.com for another approach that uses no fluids.

Aug 02, 2019
This could significantly cut down on electric heating thus cut down on fossil fuels. How long before I can have one installed on my furnace?

Aug 02, 2019
Interesting. I don't think this could make much difference beyond the condensing furnace that's already available in home applications; but in industrial applications it could make a lot.

Aug 02, 2019
I don't get it. Please help me to understand.

HVAC system employ two heat exchangers.

One is a gas (air) > gas (refrigerant) exchanger (the evaporator coil inside)

The other is a liquid (refrigerant) > gas (air) exchanger (the condensing coil outside)

This new development is for liquid heat exchange between two surfaces.

I don't see any area in "traditional" Carnot Cycle HVAC system that this has advantage. The size of the outdoor condensing coil is dictated by the air rate transfer to the coil surface (not tranfer internally in the liquid refrigerant.

And for other non-HVAC systems (industrial) that employ traditional heat exchangers, allowing a liquid to phase change is a very dangerous game. The moment bubbles form around the surface, conduction drops dramatically and surface temperatures can rise out of control (depending on the process), destroying the system. That is called thermal runaway.

What am I missing here?

Aug 02, 2019
This is not about the refrigerant. It's about the actual heat exchanger. In an industrial system this isn't solid copper or aluminum or whatever like in your home HVAC plant; it's a liquid heat exchanger, because it has to handle more heat than a solid exchanger can (for instance for a whole office building) at least economically. Copper is expensive, especially in the massive quantities that would be needed for such a heat exchanger. Such a thing would be prohibitively expensive.

Aug 03, 2019
The moment bubbles form around the surface, conduction drops dramatically and surface temperatures can rise out of control (depending on the process), destroying the system. That is called thermal runaway.
What am I missing here?
This would only apply to very hot surfaces and strictly water. These fluoro-ethers have BP's well below 100° C and are only ~1% of the aqueous mix. The high heat capacity of water allows the system to rapidly squelch any positive thermal feedback loop before it could possibly happen. It's about as dangerous as trying to light wet tissue-paper.

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