The collapse behavior of vapor bubbles containing non-condensable gas by direct contact condensation

Abstract

Direct contact condensation (DCC) is a phenomenon that occurs when vapor is injected into a subcooled water pool and is essential in many industrial devices. However, the actual phenomenon involves non-condensable gases in the vapor, and there has been insufficient research on how non-condensable gases affect direct contact condensation. In this study, through the detailed observation of the bubble collapse behavior using a high-speed video camera, we aimed to clarify the bubble collapse behavior and heat transfer coefficient of vapor bubbles containing non-condensable gas by DCC. Experiments were compared between DCC under pure vapor conditions and DCC under conditions containing non-condensable gas. Characteristic quantities for each condition were calculated from image analysis of the captured images. The temperature distribution inside the plume was also measured using thermocouples. From these data, the relationship between contraction rate and microbubbles diameter and the average heat transfer coefficient (HTC) were estimated. As a result, the construction rate of the plume containing non-condensable gas was reduced to 4 ~ 20 % of that of pure vapor, and large number of microbubbles were observed under lower air mass fraction with the log-normal distribution in diameter. The average HTC decreased to about 5 ~ 13 % when the non-condensable gas was mixed with about 8 % of air mass fraction. These results were proposed to be because vapor concentration decreases as the plume shrinks and vapor diffusion is suppressed by the mass conservation of non-condensable gas as the vapor condenses.