Nucleation of Steam Bubbles from Large-Diameter Prepared Sites

摘要

The laboratory has been active for some time in the development of improved desalination equipment to provide potable water at advanced bases. A review of the current literature on the physics of boiling disclosed a consensus that (1) nucleation is necessary for boiling, (2) nucleation is accomplished by sites of certain sizes only, and (3) the amount of superheat above the boiling point that is necessary for bubble formation is a function of site orifice diameter. Because no supporting theory accompanied the latter two observations, a study was made to determine why these limiting factors should apply - if in fact they do. A simple theory derived by the author indicated, on the contrary, that for a given wetting angle at the solid-vapor-liquid interface, the superheat necessary should be a function of the effective radius of vapor bubbles as determined by the wetting angle and the angle formed by the site's walls. This hypothesis was tested using a small copper hot plate with several relatively large sites (on the order of 1 to 2 mm in diameter); nucleation proved possible with superheats of only a few degrees Fahrenheit. Although steam would form a bubble, the emergence and growth of the bubble was inhibited by the thickness of the boundary layer, because a large radius bubble would penetrate to the area in which the covering layer was at or below the boiling point. The vapor within the bubble would therefore be condensed by this relatively cooler water. (Author)