Abstract
We report on Leidenfrost patterns and boiling with compressed sulfur hexafluoride (hbox {SF}_6). The experiments were carried out in a large aspect ratio Rayleigh–Bénard convection cell, where the distance between the horizontal plates is comparable with the capillary length of the working fluid. Pressures and temperatures were chosen such that the bottom plate was above and the top plate was below the liquid–vapor transition temperature of hbox {SF}_6. As a result, hbox {SF}_6 vapor condenses at the top plate and forms drops that grow in size. Leidenfrost patterns are formed as the drops do not fall but levitate by the vapor released in the gap between the hot bottom plate and the colder drops. When the size of these drops became too large, one or more vapor bubbles—chimneys—form inside them. We determine the critical size for the formation of a chimney as a function of the capillary length. For even larger drops and extended puddles many disconnected chimneys occur that can grow to sizes large enough for the formation of new drops inside them. By varying the temperatures and the pressure in the system, we observe various such patterns. When the area covered by a puddle becomes large it touches the hot bottom plate locally and boils off rapidly. This can be attributed to a local reduction of the bottom plate surface temperature below the Leidenfrost temperature.
Highlights
We dedicate this paper to Pierre Hohenberg, a great scientist, scholar and friend
The Leidenfrost effect plays an important role in heat transfer applications involving boiling, as the presence of a vapor layer between a liquid and a hot substrate severely restricts the heat transfer from the substrate to the liquid
We report experimental results on Leidenfrost drops and puddles in a Rayleigh–Bénard setup, where a fluid is confined between a hot bottom and a cold top horizontal plate
Summary
We dedicate this paper to Pierre Hohenberg, a great scientist, scholar and friend. We were always impressed by his openness to new ideas, creativity and scientific rigor. A liquid drop placed on a surface, sufficiently hotter than its boiling temperature, may levitate when the pressure from evaporation of the drop overcomes its weight. This phenomena was described in detail by Johann Gotlob Leidenfrost in 1756 [1], and is called the Leidenfrost effect. The Leidenfrost effect plays an important role in heat transfer applications involving boiling, as the presence of a vapor layer between a liquid and a hot substrate severely restricts the heat transfer from the substrate to the liquid. The sudden reduction of the heat transport due to the formation of an insulating vapor layer for larger liquid volumes, is termed boiling crisis [4]. For a detailed account on the dynamics associated with the Leidenfrost effect see, e.g., [2,5]
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