Visualization study on the bubble behavior on a downward facing hemispherical surface during saturated pool boiling

Saturated pool boiling experiments were conducted in a scale-down three-dimensional downward facing hemispherical vessel. In this study, a high-speed digital video camera was employed to capture the bubble behavior along the overheated convex surface. The general morphology of coalescent bubble at the bottom center was analyzed based on the observed pictures and videos. The visualization results showed that the bubble behavior along the hemispherical curved surface was quite different from that on the plane and upward surfaces. It was found that the coalescent bubble was cyclical repeatedly forming a stratified vapor layer, and then randomly sliding upward along the convex surface. The downward facing boiling heat flux was directly related to the duration of the boiling cycles, which were obtained based on the visualization by high speed camera and the image processing technique. A Matlab program was developed to recognize and analyze the images, as well as to calculate the characteristic parameters of the coalescent bubble. The boiling cycle was almost constant during the nucleate boiling regime, which was independent with the heat flux level. This study also provided an in-depth physical understanding of the 3-D downward facing boiling process during ERVC that could be even useful for hydrodynamic modeling of the CHF phenomenon. (C) 2019 Elsevier Ltd. All rights reserved.