Understanding the effects of nozzle geometry on bubble generation is very important in nozzle design. Gas self-suction is discussed with different nozzle diameter by detecting the pressure at gas inlet. The effects of nozzle diameter on the bubble size distribution and spray morphology of bubble generators were investigated to assess the aeration capabilities. The increase in the geometric criterion K from 3.2 to 4.8 leads to an amazing transformation from non-self-suction to the maximum vacuum degree. The maximum vacuum degree with a geometric criterion K of 4.8, the maximum aeration capability with a K of 8.0, and the minimum bubble diameter with a K of 12 were achieved. The liquid handling capacity and spray angle increase together with increasing K. This work contributes to the foundation for the design of an industrial bubble generator through a pressure-swirl nozzle. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.