Flow regimes of the reactive submerged gas jet from the Laval nozzle into liquid

The reactive submerged gas jet into a liquid bath is used in numerous industrial processes. The jet is featured with violent gas–liquid interface fluctuations. Previous studies showed that the most prominent feature is the cyclic formation and disappearance of the “gas cavity” near the nozzle. This near-nozzle interface fluctuation causes the “back attack” which is usually hazardous in actual applications. In the current study, another flow regime was observed: the near-nozzle interface is stable, and the most violent interface oscillation and the main reaction zone are away from the nozzle. In this regime, the “gas cavity” fluctuation is detached to the nozzle, the gas phase break-off position is pushed downstream (L/d₀ from <3.0 to >6.0), and the “back attack” vanishes. This regime is termed as the “Detached Reaction Regime” (if the previous regime is the “Attached Reaction Regime”). The transition from the Attached Reaction Regime to the Detached Reaction Regime is usually caused by an increase in gas momentum. Laval nozzles with different divergent angles (10°, 15° and 20°) were tested. For each Laval nozzle, the effects of the gas total pressure and liquid temperature were investigated. The length and width of the jet were analyzed. Results show that the length and width of the gas jet suddenly decrease because of an enhancement of the gas–liquid mixing rate at the regime transition. In addition, the length-to-width ratio is approximately 3.0 for both regimes and all nozzles. Finally, a criterion for predicting the flow regime transition was proposed.