Experimental Imaging of Supercritical Discharge of Bubbles in a Liquid Jet

Abstract

Primary breakup in twin-fluid internal mixing atomisers is governed by the intermittent passage of gas–liquid structures through the discharge orifice; yet, the instability mechanisms at the orifice remain insufficiently characterised at the relevant temporal and spatial scales. This study presents a high-speed imaging investigation of orifice flow instability in a transparent nozzle with a refractive-index-matched working fluid, enabling direct imaging of bubble transit events at 120 000 fps across a range of discharge pressures and bubble sizes. Vena-contracta separation was found to limit optical access, restricting observations to bubbles large enough to cause wall attachment. In the wall-attached regime, the dominant jet oscillation frequency appears to scale near-linearly with the discharge Weber number, suggesting that surface tension may be a governing parameter. During bubble transit, periodic gas-jet pinch-off consistent with Kelvin–Helmholtz instability is observed; the measured instability wavelength is systematically underestimated by analytical predictions for both Rayleigh and Rayleigh–Taylor instability modes.