Experimental investigation of bubble size evolution released from a fine bubble ejector

The effects of gas flow rate, liquid flow rate, and liquid properties on the bubble size evolution at the outlet zone of an ejector have been investigated systemically. The liquid properties, including surface tension and viscosity, were changed by adding glycerol, ethanol, or Carbopol 2020 into tap water. It was found that increasing the liquid flow rate is beneficial for producing smaller bubbles with a narrower size distribution in the ejector, while the gas flow rate investigated here shows less influence on the generated fine bubble diameter. An intense bubble coalescence phenomenon, which has a negative effect on mass transfer, was first observed at the small outlet zone of the ejector by exploring the bubble size evolution along the axial height. It was found that the mean diameter of the fine bubbles could increase by 48% similar to 90% when they rose only 25 cm from the ejector outlet, and the bubble coalescence could be effectively suppressed by adding a small amount of glycerol to water. The bubble diameter could be reduced by 32% similar to 43% by using a 2 wt.% glycerol solution under the experimental conditions. A comparative study with tap water, glycerol solution, Carbopol solution, and ethanol solution was taken, and the underlying mechanism of reducing the generated bubble size by changing the liquid properties was revealed.