A visualized study of micro-bubble emission boiling

Experiments were designed, and experimental equipment was built, to study the characteristics of micro-bubble emission boiling (MEB) in water contacting a copper heating surface 10 mm in diameter. The behavior of bubbles on the heating surface was captured by a high-speed video camera. The results of these experiments indicated that after subcooling exceeded 25 K, MEB occurred and was accompanied by the emission of numerous extremely small bubbles. During the initial stage of MEB, two different bubble behaviors were observed: a film of vapor on the heating surface expanded and shrank periodically, emitting micro-bubbles, and the film of vapor expanded unevenly before condensing or collapsing into many micro-bubbles. During fully developed MEB, the film of vapor exhibited irregular changes at its surface and partially collapsed in several milliseconds. Nearly simultaneously, a new vapor film layer formed on the heating surface. MEB never occurred during water subcooling when the heating surface was embedded 0.5 mm within a ceramic thimble.