Abstract: “Visualization of bubble's dynamics in vertical upward subcooled flow boiling was carried out to investigate the mechanisms causing the onset of significant void (OSV). It was found that formation of large bubbles is a key phenomenon since they were large in size as well as their lifetime was remarkably longer than those of the smaller bubbles. Most bubbles were detached from the heated surface to disappear immediately after nucleation or during rise along the vertical heated surface. Therefore, for the large bubbles to be formed, it was necessary to coalesce with other bubbles during the growth period at the nucleation site and the rising period along the heated surface. However, when the bubble size exceeded the critical value, the bubbles rose along the heated surface stably. Thus, the bubble lifetime was elongated substantially to contribute to the rapid increase of the void fraction.”

Keywords: subcooled flow boiling, void fraction, onset of significant void (OSV), net vapor generation (NVG), bubble coalescence