Visualization of capillary boiling

Abstract

Capillary boiling has been experimentally analyzed using an optical technique, based on the extraction of gray levels at specific locations from video frames. Boiling is achieved by placing a small capillary in a large container filled with water at boiling temperature. A heating wire located inside the capillary provides the additional heat that triggers the evaporation. The phenomenon is videotaped and digitized. A small region of the filmed field is chosen and the gray level of all the digitized frames is stored in a file which is then analyzed. This constitutes in essence the possibility of using a large number of non-intrusive, fixed (Eulerian) virtual sensors. The information extracted concerns the global properties of the phenomenon such as the time between the departure of two subsequent bubbles, and more detailed properties such as the shape and volume of the departing bubbles or even the motion of liquid packets inside the capillary. Moreover, this technique permits the study of time and space correlations. It is observed that the bubbles depart from the capillary tip with a variable frequency that can be approximately grouped around two values. Clear evidence that this is due to two different mechanisms is given. The time between the departure of two bubbles is correlated with the volume of the departing bubbles.