Effect of Subcooling on Vapor Bubble Collapse and Microbubble Emission

In order to investigate the effect of subcooling on vapor bubble collapse and microbubble emission, the condensation process of vapor bubbles in a subcooled pool at different subcoolings was recorded with a high-speed video camera. The results show that the development of surface wave emerged on the bubble surface is slow and the vapor bubbles are condensed and split up gradually without sudden collapse at low subcooling (ΔT_sub=17 K). At high subcooling (40 K<ΔT_sub<75 K), the surface wave on the surface is violent and the vapor bubbles will collapse suddenly with the emission of a large number of microbubbles. Nevertheless, the process of tiny bubbles splitting off from large bubble is observed before sudden collapse of bubble at liquid subcooling of 30 K. The diameter and velocity of microbubbles after collapse of vapor bubble are within the same order of magnitude of those in microbubble emission boiling at 40 K<ΔT_sub<75 K. Furthermore, the diameter of microbubble decreases, while the velocity increases with the increase of subcooling. In addition, the wave number of the surface wave on bubble surfaces increases and the maximum amplitude of the surface wave increases firstly and then decreases with the increase of subcooling.