Tsinghua and Brown University scientists have made use of a simple modification to boost the capability of HVAC systems.
The new process is said to improve turbulent heat exchange, a method of heat transport widely used in heating, ventilation and air conditioning (HVAC) systems.
According to the scientists, adding a readily available organic solvent to common water-based turbulent heat exchange systems can boost their capacity to move heat by 500%.
Turbulent heat exchangers 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.
In 2015, Tsinghua professor Chao Sun – the original creator of the process – had the idea of using an organic component known as hydrofluoroether (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 and Brown University post-doc Varghese Mathai, 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%. They demonstrated this with high-speed imaging and laser diagnostic techniques.
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.
According to the scientists, these bubbles not only 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 – thereby improving heat transfer.
Image and content: ShutterStock Image/Brown University