New Method Could Greatly Enhance Performance of HVAC Heat Exchangers

Scientists from Tsinghua University and Brown University have found an easy way to deliver a significant boost to turbulent heat exchange, a technique of heat transport extensively used in heating, ventilation and air conditioning (HVAC) systems.

Turbulent heat exchangers use fluid dynamics to move heat. Plumes of warmer water rise from a hot bottom surface to a cold upper surface, where they lose their heat and sink back to the bottom. A new study shows that adding a small amount of a liquid additive to a water-based heat exchanger can speed the motion of the plumes and increase heat exchange capacity. (Image credit: Chao Sun)

In a paper published in Nature Communications, the scientists illustrate that incorporating a readily available organic solvent to typical water-based turbulent heat exchange systems can improve their capacity to move heat by 500%. That is much better than other techniques aimed at boosting heat transfer, the scientists say.

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

Turbulent heat exchangers are moderately simple devices that use the natural movements of liquid to shift heat. They comprise a hot surface, a cold surface, and tank of liquid in between. Close to the hot surface, the liquid heats up, becomes less dense and forms warm plumes that float toward the cold side. There, the liquid loses its heat, turns denser and forms cold plumes that return back down to the hot side. The cycling of water serves to control the temperatures of each surface.

This type of heat exchange is a core aspect of contemporary HVAC systems extensively used in home heaters and air conditioning units, the scientists say.

In 2015, Sun had the notion to use an organic component called hydrofluoroether (HFE) to accelerate the cycling of heat within this type of exchanger. HFE is occasionally used as the sole fluid in heat exchangers, but Sun supposed that it might have more stimulating properties as an additive in water-based systems.

Collaborating with the study’s co-first author Ziqi Wang, Mathai, and Sun tested with adding small quantities of HFE and, after three years of research, were successful in maximizing its effectiveness in accelerating heat exchange. The team demonstrated that concentrations of about 1% HFE formed significant heat flux improvements up to 500%.

Applying high-speed imaging and laser diagnostic methods, the scientists were able to demonstrate how the HFE enhancement happens. When near the hot side of the exchanger, the globules of HFE rapidly boil, developing biphasic bubbles of vapor and liquid that rise quickly toward the cold plate above.

At the cold plate, the heat from the bubbles is lost and they descend as liquid. The bubbles influence the overall heat flux in two ways, the scientists demonstrated. The bubbles themselves transport a substantial amount of heat away from the hot side, but they also boost the speed of the neighboring water plumes rising and falling.

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

Chao Sun, Professor, Tsinghua University

The scientists say this stirring action may have other applications as well. It could be beneficial in systems engineered to mix two or more liquids. The additional stir makes for faster and more thorough mixing.

The scientists highlighted that the particular additive they used—HFE7000—is non-corrosive, ozone friendly, and non-flammable. One drawback is that the method only functional on vertical heat exchange systems—ones that move heat from a lower plate to an upper one. It does not presently work on side-to-side systems, though the scientists are looking for ways to adapt the method. Still, vertical exchangers are extensively used, and this research has revealed a basic way to enhance them considerably.

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

Varghese Mathai, Study Co-First Author and Postdoctoral Researcher, Brown University


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