Turbulent heat exchangers are widely used in HVAC systems around the world, and a new study demonstrates a simple modification that can improve their capability by 500%.
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 heat flux — 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 air conditioning units, the researchers say. 1
- 1. Source: Brown University
Ziqi Wang et al, Self-sustained biphasic catalytic particle turbulence, Nature Communications (2019).
Turbulence is known for its ability to vigorously mix fluid and transport heat. Despite over a century of research for enhancing heat transport, few have exceeded the inherent limits posed by turbulent-mixing. Here we have conceptualized a kind of “active particle” turbulence, which far exceeds the limits of classical thermal turbulence. By adding a minute concentration (ϕv ∼ 1%) of a heavy liquid (hydrofluoroether) to a water-based turbulent convection system, a remarkably efficient biphasic dynamics is born, which supersedes turbulent heat transport by up to 500%. The system operates on a self-sustained dynamically equilibrated cycle of a “catalyst-like” species, and exploits several heat-carrier agents including pseudo-turbulence, latent heat and bidirectional wake capture. We find that the heat transfer enhancement is dominated by the kinematics of the active elements and their induced-agitation. The present finding opens the door towards the establishment of tunable, ultra-high efficiency heat transfer/mixing systems.