In order to investigate the effect of vapor injection rate on the direct contact condensation and evolution of vapor bubble surface

a high-speed video camera was employed to record the process of vapor condensation in the cold bulk at a certain liquid subcooling and different vapor injection rates. Experimental results showed that the bubble surface was relatively smooth throughout the whole slow process of bubble condensation when the vapor injection rate was less than 0.36m3/h. As the vapor injection rate increased to 0.74m3/h

surface wave appeared on the bubble surface and the bubble was split up into some tiny bubbles finally. When the vapor injection rate was larger than 1.13m3/h

surface wave on the bubble was very violent. The bubble instability was enhanced and the bubble would collapse into many microbubbles. Furthermore

with the increase of the vapor injection rate

the diameter of these microbubbles decreased and the average velocity increased. When the vapor injection rate exceeded 1.13m3/h

the bubble relative radius decreased and the change rate of bubble radius increased rapidly. The strong surface wave on the bubble could increase the bubble effective area of heat transfer significantly

resulting in a substantial enhancement of heat and mass transfer between vapor and cold bulk.