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Photo Source: Pixabay wants to boil the water, using a heat source to heat the water, but if the heat source is too hot, the water will no longer absorb heat. This is called the "Leighton Frost effect", and because of this effect, water is not a very good heat transfer medium when it is needed to cool high-temperature objects quickly. A recent study offers an alternative: instead of using water, use ice for heat transfer.

The heat doesn't boil? As we all know, water has three phases: solid, liquid and gaseous. At 1 standard atmospheric pressure, when the solid ice is heated to 0 ℃, it will melt into liquid water; if the water is further heated to 100 ℃, it will boil and vaporize into gaseous water vapor. This is not only the common sense of life, but also the most intuitive understanding of the physical phenomenon of "phase transition".

However, this common sense does not work in all cases. For example, what happens if you drop water onto a 150 ℃ aluminum plate? People will probably think that the temperature of the heat source is already higher than the boiling point of water, and the water droplets should vaporize quickly after touching the aluminum plate. However, if you do an experiment, such as cooking, heating the pan and then pouring water, you will find that the water droplets will float around on the bottom of the pan, keeping it liquid for a period of time rather than quickly evaporating into steam. This phenomenon was discovered a long time ago and was first described by the German doctor and theologian Johann Leidenfrost in 1751, so it was also named the "Leighton Frost effect".

Why does water keep liquid instead of boiling when it comes into contact with a heat source whose temperature is higher than its boiling point? The reason is actually very simple: an insulated water vapor layer will evaporate on the contact surface between the water droplets and the high-temperature heat source, which separates the water at the top from the heat source and continues to contact, thus blocking the whole boiling process of the water droplets. The Leighton Frost effect brings some trouble, which hinders the heat transfer from the heat source to the water if you want the water to boil to get steam as soon as possible. As early as in the steam era, many boiler designers have found that high-temperature boilers are less efficient to produce steam. In addition, liquid water is often used as a heat conduction medium, and the existence of Leighton Frost effect makes liquid water not so efficient in cooling some high temperature objects.

When liquid water comes into contact with a high temperature heat source, the lower layer will form a water vapor insulation layer, thus preventing the heat absorption of the upper water. (photo source: Wikipedia) in a paper published in January in physical Review fluids, researchers found a funny thing: if you want to transfer heat from the heat source quickly, you should not add water to the heat source, but ice.

At first, researchers from Virginia Tech wondered whether the Leighton Frost effect could also occur when ice came into contact with high-temperature heat sources, resulting in the co-existence of water vapor, water and ice. About five years ago, undergraduate Daniel Cusumano (Daniel Cusumano) observed in an experiment that even if the aluminum plate was heated to more than 150 ℃, the ice it came into contact with would not be suspended like water. Kusumano continued to raise the temperature of the aluminum plate, and he found that the critical temperature for the ice to levitate was much higher: about 550 ℃. When the critical temperature is not reached, the ice touches the heat source and the lower layer melts into a meltwater layer, but the meltwater layer remains liquid when it touches the heat source without vaporizing a steam insulation layer. This means that the meltwater layer can absorb heat continuously from the heat source.

What happens under the ice to keep liquid water in contact with high-temperature heat sources? Soon after, Mojtaba Edalatpour, a graduate student, restarted the study. He worked with associate professor Jonathan Boreyko to build a numerical model to simulate the heat conduction of ice on a high-temperature heat source. They found that the crux of the problem was the temperature difference in the meltwater layer under the ice: the temperature on the side of the meltwater layer in contact with the heat source was fixed at 100 ℃, while the temperature on the side in contact with the upper ice was fixed at 0 ℃. Most of the heat absorbed by the meltwater layer from the heat source is used to maintain this temperature difference, and only a small part of the energy can be used to generate steam, so that an insulated vapor layer will not be formed on the interface between the meltwater layer and the heat source.

The change process of 0.2 times slow-release ice in contact with a high temperature heat source. When the ice comes into contact with the heat source, the lower melting water layer continues to absorb heat. After all melting, the Leighton Frost effect occurs and the water is suspended. (video source: Mojtaba Edalatpour)

Borico explained that it is actually a good thing that ice is difficult to produce the Leighton Frost effect, in which case the heat transfer efficiency is higher. "once the water is suspended on the surface of the heat source, the heat transfer process is blocked. So, for heat transfer, the Leighton Frost effect is very bad."

The need for efficient heat transfer is so common in life-for example, we need to cool computer servers and car engines, so we need to find a substance or mechanism to remove energy from the hot surface and quickly redistribute the heat to reduce the heat loss of the parts. And water is a frequently used heat transfer medium, so it is necessary to avoid the Leighton Frost effect.

As a result of this discovery by Borico's team, we can expect to use ice instead of water for heat transfer in some practices. In nuclear power plants, for example, rapid cooling with ice heat conduction may be an emergency measure in the event of a power failure. There are also potential applications in metallurgy. In order to produce the alloy, the formed metal must be quenched in a short time to reduce its temperature rapidly, so that the alloy can have higher strength. If ice instead of water is used during quenching, heat can be released quickly by avoiding the Leighton Frost effect, thus cooling the metal more quickly.

Borico also foresees the potential of this heat conduction method in fire fighting. "you can imagine spraying ice shavings with a special hose instead of water, so you can put out the open fire more efficiently," he said. "this is not a novel plot. I visited an airline with ice pipes, and they already have this technology to spray ice particles instead of water droplets with nozzles to put out the fire."

Perhaps for ordinary people, if you want to experience the benefits of ice without the Leighton Frost effect, the next time you need to add water before cooking, you can try adding ice instead.

Reference source:

Https://vtx.vt.edu/articles/2022/01/eng-boreyko-boiling-ice-012022.html

Links to papers:

Https://journals.aps.org/prfluids/abstract/10.1103/PhysRevFluids.7.014004

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by Bai Defan, revision: Erqi

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