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The water here refused to boil, so the scientists decided to add some ice

Global science

By jsyeem shekelsPublished 3 years ago 5 min read

If you want to boil the water, use 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 high temperature does not 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, if you drop water on an aluminum plate at 150 ℃, 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".

When water comes into contact with a heat source whose temperature is higher than its boiling point, it does not boil, but remains liquid. The reason is actually very simple. An insulated water vapor layer is evaporated on the contact surface between the water droplet and the high-temperature heat source. It separates the top water 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.

Ice can't be suspended.

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 above 150C, the ice in contact with it would not be suspended like water. Kusumano continued to raise the temperature of the aluminum plate, and he found that the critical temperature for ice to levitate was much higher: about 550 degrees Celsius. 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.

Mojtaba Edalatpour, a graduate student, restarted the study shortly after what happened under the ice to keep liquid water exposed to hot sources of heat. 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 lies in the temperature difference of the meltwater layer under the ice: the temperature on one side of the meltwater layer in contact with the heat source is fixed at 100 ℃, while the temperature on the side in contact with the upper ice is 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.

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

Efficient heat transfer

The need to transfer heat 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 that removes energy from the hot surface and redistributes heat quickly 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 spraying water, which 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 the 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.

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jsyeem shekels

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