抄録
Many photographic studies have been conducted so far to measure the number density of active nucleation sites in subcooled flow boiling. In these studies, however, overlapping of bubbles is a serious problem in counting the number of nucleation sites accurately particularly under high void fraction condition. In the present work, we used a transparent glass heater to mitigate the influence of bubbles' overlapping in the video images and to observe bubble nucleation process in water subcooled flow boiling in detail. Using the movie data obtained in this work, important parameters in subcooled flow boiling such as the nucleation site density, bubble departure diameter, and bubble release frequency were measured. It is emphasized that the bubble departure diameter and bubble release frequency could be measured individually for all the active nucleation sites. It was found that the bubble release frequency tended to decrease with an increase in the bubble departure diameter but the bubble release frequency was exceptionally low at some nucleation sites; the number of these sites increased rapidly when the wall superheat exceeded the critical value. Based on these experimental results, active nucleation sites were divided into the two categories to investigate the appropriate functional form of correlations. The experimental data accumulated in this work were also used to evaluate the predictive performance of existing correlations.
