Stability of periodic bubble departures at a low frequency

• Dynamics of bubble departures from glass nozzle has been studied.
• Stability loss of periodic bubble departures has been investigated.
• Stability of periodic bubble departures depends on the volume of air supply system.
• Non-linear gas compression is responsible for chaotic bubble departures.

The dynamics of bubble departures (at a frequency of f=3 Hz) from a glass nozzle submerged in a tank filled with distilled water has been experimentally and theoretically studied. The volume of the system that supplies air to the nozzle (plenum chamber volume) and the air volume flow rate were changed in the experiment. The air pressure, bubble paths and liquid flow inside the nozzle were simultaneously recorded using a data acquisition system and a high-speed camera. It was shown that an increase in the plenum chamber volume leads to an increase in the intensity of the occurrences of chaotic changes in the subsequent waiting times. The analysis of the mechanism of the stability loss of the periodic bubble departures was based on changes in the time of the air pressure, the depth of the liquid penetration into the nozzle, the time of the bubble growth, the waiting time, and the bubble paths and their sizes, which is presented in this paper. The results of the analysis are compared with simulations that are based on the models of bubble growth and liquid flow inside the nozzle during the waiting time. It was shown that the air pressure rise, Δpl, during the waiting time is a non-linear function of the gas pressure after the bubble departure and the liquid velocity around the nozzle outlet. The nonlinearity of Δpl increases when the plenum chamber volume increases, and it decreases when the air volume flow rate increases.