Turbulent heat exchange is a method of heat transport widely used in heating, ventilation, and air conditioning (HVAC) systems. Adding a readily available organic solvent to common water-based turbulent heat exchange systems can boost their capacity to move heat by 500%. Other methods for increasing heat flux — nanoparticle additives or other techniques — have achieved about 50% improvement.

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 regulates the temperatures of each surface. This type of heat exchange is commonly used in modern HVAC systems in home heaters and air conditioning units.

The organic component known as hydrofluoroether (HFE) was used to speed the cycling of heat inside this kind of exchanger. HFE is sometimes used as the sole fluid in heat exchangers; in this case, it is used as an additive in water-based systems. Concentrations of around 1% HFE created dramatic heat flux enhancements up to 500%.

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, where the bubbles lose their heat and descend as liquid. The bubbles affect the overall heat flux in two ways: they carry a significant amount of heat away from the hot side and also increase the speed of the surrounding water plumes rising and falling. This stirs up the system and makes the plumes move faster. Combined with the heat that the bubbles carry, dramatic improvement is achieved in heat transfer. That stirring action could have applications in systems designed to mix two or more liquids. The extra stir makes for faster and more complete mixing.

The specific additive used — HFE7000 — is non-corrosive, non-flammable, and ozone friendly. One limitation is that the approach only works on vertical heat exchange systems — those that move heat from a lower plate to an upper one. It does not currently work on side-to-side systems.

For more information, contact Kevin Stacey at This email address is being protected from spambots. You need JavaScript enabled to view it.; 401-863-3766.

Tech Briefs Magazine

This article first appeared in the December, 2019 issue of Tech Briefs Magazine.

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