The Leidenfrost Effect by Dan Hendricks Peter Ashworth

The Leidenfrost effect is a phenomenon in which a liquid, in near contact with a mass significantly hotter than the liquid's boiling point, produces an insulating vapor layer which suspends that liquid above the surface. Leidenfrost Point

 Observe the Leidenfrost effect.  Compare and contrast the forms of boiling which occur at temperature differences less than and greater than the Leidenfrost point.  Determine which form of boiling would transfer heat away from the surface at a faster rate.

 Materials ◦ 1 cup water ◦ 1 tsp ◦ 1 dropper ◦ 1 pencil ◦ 1 pan ◦ Gas Stove ◦ Towel

Time Pan Heated (s) Water Drop Size ¼ tsp Nucleate boiling, droplets splattered on surface Nucleate/transition boiling, droplets splattered on surface Film boiling, droplets split, but remained spherical Dropper Nucleate boiling, droplets splattered on surface Transition/film boiling, droplets remained spherical Film boiling, droplets remained spherical Pencil Tip Nucleate boiling, droplets splattered on surface Transition/film boiling, droplets remained spherical Film boiling, droplets remained spherical

 At temperatures less than the Leidenfrost point, the water “splatters” over the surface and heat is transferred to the water through contact with the surface, thus quickly boiling all the water.  At temperatures greater than the Leidenfrsot point, a layer of vapor forms between the water and the surface and heat is transferred to the water by convection, which increases the amount of time needed to boil the water. Random side note: It was discovered why many pans actually have ridged bottoms. These ridges act as fins to heat the pan faster over an open flame. So if you are heating your pan on an electric stove, its better to have a flat bottomed pan (more surface contact with the coils), but on a gas stove, its better to have a ridged bottom. Water boils faster when the temperature difference is Less than the Leidenfrost point. (This is why quenching is more effective in oil than in water. Since the leidenfrost point in oil is much higher than water, the oil can conduct heat away from the object being quenched faster than the water vapor could through convection. ) Thus boiling a liquid at a temperature less than the Leidenfrost point will transfer heat away from the surface at a higher rate.

 Calculated Values of convection coefficient Nucleate Boiling: Film Boiling: h=52,317 W/m^2*k h=42.6 W/m^2*k Matlab Code:

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