Microlayer structure in nucleate boiling of water and ethanol at atmospheric pressure

This study investigates the structure of the microlayer that forms between growing bubbles and the heat transfer surface for nucleate pool boiling in the isolated bubble region. The microlayer thickness was measured for water and ethanol by a specially designed measurement system that employs the laser extinction method. A thin (diameter: 94 μm) optical fiber in a thin stainless-steel tube was inserted in a liquid pool. Gas flow was used to prevent the fiber, which was made of fluorinated glass, from becoming wet and to remove the pool liquid from the light path. A method for determining the initial microlayer thickness was developed based on numerical calculations. The microlayer thickness decreased over time due to evaporation. The thickness of the initially formed microlayer was found to be uniquely determined. For water, it was in the approximate range of 0–9 μm and it increased linearly with increasing distance between the incipient bubble site and the measurement position; this relation was independent of the heat flux. The initial microlayer structure for ethanol was similar to that for water, but its thickness was about twice that for water.