The Influence of Expansion Tube Diameter on Microbubble Production in a Confined Cavitating Jet

The influence of expansion tube diameter (dₑ) on microbubble production via rapid depressurization of supersaturated water through an orifice and expansion tube is explored in the context of artificial nuclei seeding in hydrodynamic facilities. Microbubble size distribution and production rate are investigated experimentally using high-magnification shadowgraphy. To eliminate the need for a depth-of-field correction, the bubbly plume is discharged into a 0.5 mm thick Hele-Shaw cell. The experiments were performed for a device with fixed orifice diameter, D = 0.25 mm, with the expansion tube diameter in range 0.25 mm ≤ dₑ ≤ 0.75 mm and fixed length of 100 mm. Microbubble production was characterized for a range of generator cavitation numbers (σin j), controlled by varying the device inlet pressure with the outlet open to the atmosphere. Two distinct microbubble populations were generated based on the flow Reynolds (Re) and Weber (We) numbers. For the large dₑ (high Re and We), a polydisperse bubble population with a power law like size distribution is observed, with the bubble production frequency increasing as the diameter approaches the lower resolution limit of the used optical system (≈ 7µm). For the small dₑ (low Re and We), a change in the bubble production regime results with a peak at a value above the lower optical resolution limit, which may be attributed to a decrease in production of sub-Hinze scale products during bubble break-up process. σ_inj is found to be the main parameter controlling the total production rate, with the measured production frequencies of the order 10³ to 10⁷ Hz. Through variation of the expansion tube geometry, control of the resulting microbubble population is demonstrated, contributing to enhanced capability for modelling of nucleation effects on cavitating flow in hydrodynamic facilities.