Leidenfrost effect

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The Leidenfrost effect is a physical phenomenon in which the liquid, in close contact with the mass much hotter than the boiling point of liquid, produces an insulating vapor layer keeping the liquid from boiling rapidly. Because of this 'repulsive force', droplets float above the surface rather than making physical contact with it. This is most often seen when cooking: one drop of water in a pan to measure its temperature: if the pan temperature is at or above the Leidenfrost point, the water skitters cross the pot and take longer to evaporate than in the pan below the Leidenfrost point temperature (but still above boiling temperature).

The effect is also responsible for the ability of liquid nitrogen to slide on the floor.

It has also been used in several potentially dangerous demonstrations, such as dipping a wet finger in liquid tin or blowing a mouth full of liquid nitrogen, both of which apply unscathed on the demonstrators. The latter is potentially lethal, especially if accidentally ingesting liquid nitrogen.

It was named after Johann Gottlob Leidenfrost, who discussed it in 1756.

Video Leidenfrost effect

Detail

The effect can be seen as the water droplets sprinkled into the pot at various times as it heats up. Initially, because the pan temperature is just under 100 ° C (212 ° F), the water flattens and slowly evaporates, or if the pan temperature is well below 100 ° C (212 ° F), the water remains liquid. Since the pan temperature exceeds 100 ° C (212 ° F), the water droplets hiss when touching the pan and the droplet evaporates quickly. Then, as the temperature exceeded the Leidenfrost point, the Leidenfrost effect came into play. When in contact with the pan, water droplets rise into small balls of water and circle around, lasting longer than when the pan temperature is lower. This effect works to a much higher temperature causing more water droplets to evaporate too quickly to cause this effect.

This is because at temperatures above the Leidenfrost point, the bottom of the water droplet evaporates immediately upon contact with the hot pot. The resulting gas delayed the remaining water droplets just above it, preventing further direct contact between the liquid water and the hot pot. Since the steam has a thermal conductivity that is worse than a metal pot, further heat transfer between the pan and the droplet slows dramatically. This also leads to a decrease that can slip around the pot on the gas layer just below it.

The temperature at which the Leidenfrost effect starts to happen is not easy to predict. Even if the volume of liquid droplets remains the same, the Leidenfrost point may be very different, with a complicated dependence on the surface properties, as well as any impurities in the liquid. Some research has been done into the theoretical model of the system, but it is quite complicated. As a very rough estimate, the Leidenfrost point for a drop of water on a fryer may occur at 193 ° C (379 ° F).

The effect is also explained by the renowned Victoria-style boiler designer Sir William Fairbairn, which refers to his massive influence in reducing heat transfer from the surface of the hot iron to water, as in the boiler. In a pair of lectures on boiler design, he cites the work of Pierre Hippolyte Boutigny (1798-1884) and Professor Bowman of King's College, London in studying this. A drop of water that immediately evaporates at 168 ° C (334 ° F) lasts for 152 seconds at 202 ° C (396 ° F). Lower temperatures in the fire box of the boiler can evaporate faster as a result; compare the Mpemba effect. An alternative approach is to increase the temperature beyond the Leidenfrost point. Fairbairn considers this as well, and may have thought of a flash steam boiler, but considers the technical aspects not insurmountable for the time being.

The Leidenfrost point can also be regarded as the temperature at which the drifting droplets last for the longest.

It has been shown that it is possible to stabilize the Leidenfrost water vapor layer by exploiting the superhydrophobic surface. In this case, once the vapor layer is formed, the cooling never shrinks the layer, and no nucleation nucleation occurs; the layer slowly relaxes until the surface is cooled.

The Leidenfrost effect has been used for the development of high sensitivity ambient mass spectrometry. Under the influence of Leidenfrost conditions, Levitating droplets do not release enriched molecules and molecules inside the droplets. At the last moment of droplet evaporation, all enriched molecules are released in a short time domain and thus increase the sensitivity.

A heat engine based on the effects of Leidenfrost has been made prototype. It has the advantage of very low friction.

Maps Leidenfrost effect

Leidenfrost Point

Leidenfrost point marks the beginning of a steamy stable film. This is the point on the boiling curve where the heat flux is at a minimum and the surface is completely covered by a vapor blanket. Heat transfer from surface to liquid occurs through conduction and radiation through the vapor. In 1756, Leidenfrost observed that the water droplets supported by the steam film slowly evaporate as they move on the hot surface. As surface temperatures increase, the radiation through the steam film becomes more significant and the heat flux increases with increasing excess temperature.

Minimum heat flux for large horizontal plates can be derived from the Zuber equation,

where the properties are evaluated at the saturation temperature. The Zuber constant, C is about 0.09 for most liquids at medium pressure.

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heat transfer correlation

The heat transfer coefficient can be approximated using the Bromley equation,

Di mana, ${\ displaystyle {{D} _ {o}}} Â$ adalah diameter luar tabung. Korelasi constant C adalah 0.62 untuk silinder horizontal dan pelat vertikal dan 0.67 untuk ball. Sifat uap dievaluasi pada suhu film.

For a stable film that boils on a horizontal surface, Berenson has modified the Bromley equation to produce,

For vertical tubes, Hsu and Westwater have correlated with the following equation,

Di mana, m adalah laju aliran massa dalam ${\ displaystyle l {{b} m}}/hr} Â Â$ di ujung atas tabung

At the above excess temperatures that at minimum heat flux, the radiation contribution becomes considerable and becomes dominant at excessive high temperatures. The total heat transfer coefficient can thus be a combination of both. Bromley has suggested the following equation for boiling the boiling film from the outer surface of the horizontal tube.

${\ displaystyle {{h} ^ {{} ^ 4 \! \! \ diagup \! \! {} _3} \;}} = {{h} _ {conv}} ^ {{4} \! \! \ diagup \! \! {} _3} \;} {{h} rad}} {{h} ^ {{}} 1! \! \ Diagup \! \! {} _3} \;}}} Â Â$

Jika ${\ displaystyle {{h} rad}} & lt; {{h} _ {conv}}} Â Â$ ,

${\ displaystyle h = {{h} _ {conv}} {\ frac {3} {4}} {{h} rad}} Â Â$

Koefisien radiasi yang efektif, ${\ displaystyle {{h} _ {rad}}} Â$ dapat dinyatakan sebagai,

${\ displaystyle {{h} rad {}} = {\ frac {\ varepsilon \ sigma \ kiri (T_ {s} 4 -T_ {sat} 4} \ right)} {\ left ({{T} _ {s}} - {{T} _ sat}} \ right)}}} Â Â$

Di mana, ${\ displaystyle \ varepsilon}  ÂÂ$ adalah emisivitas solid dan ${\ displaystyle \ sigma}  ÂÂ$ adalah konstanta Stefan-Boltzmann.

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Medan tekanan dalam tetesan Leidenfrost

The equation for the pressure plane in the vapor region between the droplet and the solid surface can be solved because it uses standard momentum and continuity equations. For the sake of simplicity in solving, linear temperature profiles and parabolic speed profiles are assumed in the vapor phase. The heat transfer in the vapor phase is assumed through conduction. With this approach, the Navier-Stokes equation can be solved to obtain a pressure field.

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Temperature and Leidenfrost surface tension effects

Leidenfrost temperature is the property of a given pair of solid-liquid. The solid surface temperature outside the liquid undergoes a Leidenfrost phenomenon called the Leidenfrost temperature. Leidenfrost temperature calculation involves calculating the temperature of a minimum boiling liquid film. Berenson acquires a connection for minimum boiling film temperature of a minimum heat flux argument. While the equations for the minimum film boiling temperature, which can be found in the above reference, are quite complex, its features can be understood from a physical perspective. One important parameter to consider is surface tension. The proportional relationship between the minimum temperature of the boiling film and the surface tension is expected because the liquid with higher surface tension requires a higher amount of heat flux for the nucleation nucleation onset. Since the boiling film takes place after boiling nucleation, the minimum temperature for boiling film should have a proportional dependence on the surface tension.

Henry developed a model for the Leidenfrost phenomenon that includes temporary evaporation and microlayer evaporation. Since the Leidenfrost phenomenon is a special case of boiling film, the temperature of Leidenfrost is related to the temperature of the boiling film to a minimum through the relationship of a factor in the properties of the solids used. While Leidenfrost's temperature is not directly related to the surface tension of the liquid, it indirectly depends on it via the boiling temperature of the film. For liquids with the same thermophysical properties, having a higher surface tension usually have a higher Leidenfrost temperature.

For example, for a saturated water-copper interface, the temperature of Leidenfrost is 257 Â ° C (495 Â ° F). Leidenfrost temperatures for glycerol and common alcohols are significantly smaller due to lower surface tension values ​​(density and viscosity differences are also a contributing factor.)

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Leidenfrost Reactive Effects

Non-volatile material is found in 2015 to also exhibit a 'reactive Leidenfrost effect', in which solid particles are observed floating on a hot surface and circling around irregularly. Detailed characterization of the reactive Leidenfrost effect is solved for small particles of cellulose (~ 0.5 mm) on high temperature polished surfaces by high speed photography. Cellulose is shown to decompose into short-chain melt oligomers and smooth wet surfaces with increased heat transfer associated with increased surface temperatures. Above 675 ° C (1.247 ° F), cellulose is observed to indicate a boiling transition with a loud bubbling and associated reduction in heat transfer. The removal of cellulose droplets (pictured to the right) observed over 750 ° C (1.380 ° F) is associated with a dramatic reduction in heat transfer.

High-speed photography of the cellulosic reactive Leidenfrost effect on porous surfaces (porous alumina) is also shown to suppress the reactive Leidenfrost effect and increase the overall heat transfer rate to the particles from the surface. The new phenomenon of 'Reidenfrost (RL) reactive effect' is characterized by a dimensionless quantity (? _RL =? _conv /? _rxn ), constant heat transfer of solid particles to the time of particle reaction constants, with reactive Leidenfrost effect occurring for 10 ^-1 & lt; ? _RL & lt; 10 ¹ . The reactive Leidenfrost effect with cellulose will occur in a variety of high temperature applications with carbohydrate polymers including conversion of biomass into biofuel, preparation and cooking of food, and tobacco use.

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See also

• Calefaction
• Critical heat flux
• Effects mpemba
• Nucleic boiling
• the paradox of the beta-region

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References

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External links

• Essay on effects and demonstrations by Jearl Walker (PDF)
• Sites with high-speed video, image and movie-boiling annotations by Heiner Linke at University of Oregon, USA
• "Scientists make water run uphill" by BBC News on the use of the Leidenfrost effect to cool computer chips.
• "Uphill Water" - Story of ABC Catalyst
• "Leidenfrost Maze" - University of Bath students, Carmen Cheng and Matthew Guy
• "When Water Flows Uphill" - Science Friday with Univ of Bath Professor Kei Takashina
• Jeffrey, Colin (March 10, 2015). "Machines running on frozen carbon dioxide can drive missions to Mars". Gizmag . Retrieved March 10 2015 .
• "Leidenfrost effect on BBC QI"

Source of the article : Wikipedia