Experimental analysis of jet expansion geometry and the influence of pressure waves on the behavior of air discharge processes in subcooled stagnant water

Gas jet discharges into liquid media are extensively studied due to their relevance in several industrial processes. They are fundamental in nuclear reactor safety and venting systems, where they play roles in overpressure relief, fission product retention, and aerosol scavenging. The presence of pressure waves in these discharges significantly influences the behavior of the process, determining certain aspects of the jet dynamics. This study focuses on understanding the distribution and propagation of pressure waves within the jet, as they can generate shock waves that alter the geometry, trajectory, and conditions of the jet, introducing additional instabilities and complexities. To conduct this analysis, the JEBEA experimental facility was utilized, performing overexpanded and underexpanded discharges through nozzles with inner diameters ranging from 2 to 6 mm. Direct visualization techniques were employed using a high-speed camera to capture image sequences. Subsequently, image processing methods provided detailed information on the jet dynamics and the influence of pressure waves. Results indicate that shock waves generated near the nozzle significantly impact the intensity and frequency of instabilities in the discharges, varying notably between overexpanded and underexpanded conditions, with frequencies ranging from 5 to 40 Hz in overexpanded jets and 1-11 Hz in underexpanded jets.