Bubble coalescence plays a fundamental role in nucleate boiling, as it redistributes interfacial energy and modifies vapor organization locally. This study investigates the post coalescence dynamics of wall-attached bubbles in subcooled mini-channel boiling using deionized water with high-speed imaging at 10,000 Hz. Advances in computer vision enable the extraction of time-resolved quantitative bubble behavior using CNN segmentation with Kalman tracking. Thanks to the implemented tracking processing, the focus is on the short-term deformation and oscillatory relaxation of merged bubbles and their relation to coalescence symmetry and detachment. Directly after coalescence, bubbles undergo a strong shape deformation characterized by high ellipticity, followed by damped oscillations. Under the experimental conditions, an empirical transition near Λ≈0.11, based on the parent-bubble diameters, was observed between post-coalescence detachment and sliding events. The deformation intensity and oscillation damping then depend mainly on parent bubbles symmetry. Indeed, nearly symmetric coalescence produces larger deformation amplitudes and more sustained oscillatory motion, favoring detachment, whereas asymmetric interactions result in weaker and more rapidly damped oscillations, favoring sliding. The dominant oscillation frequencies range from 500 to 700 Hz and are consistent with early-time dynamics mechanisms despite significant deformation and wall attachment, with a mean dimensionless frequency K∗≈ 0.49, close to the theoretical Rayleigh–Lamb for quadrupolar oscillations. These results are consistent with coalescence-induced detachment mechanisms and the work extends previous studies on bubble detachment by quantifying post-coalescence oscillation frequency, damping at ONB.
This study investigates pulsation phenomena for dual gas-assisted annular sheet nozzles, employing air and water as working fluids. Three nozzle configurations with varying liquid sheet thickness were examined experimentally utilizing high-speed imaging of the primary sheet breakup, complemented by CFD analyses to clarify the underlying physical mechanism for the observed pulsation. Numerical results revealed that the pulsation originates from liquid-sheet pinching induced by the outer gas stream, causing pressure buildup within the central gas flow and subsequent radial expansion of the liquid sheet.Experimental results obtained over a broad range of operating conditions show that increasing either inner or outer gas velocity, or liquid velocity, generally increases the pulsation frequency, whereas increasing liquid sheet thickness leads to a decrease. Based on the experimental data, an empirical correlation for calculation of the pulsation frequency as a function of the major operating parameters was proposed. However, the pulsation frequency is primarily governed by liquid sheet thickness and liquid velocity.
