Study of bubbles entrapped into a gas-sheared liquid film

The surface of a thick liquid film under strong gas shear is covered by large-scale disturbance waves and small-scale ripples. Disruption of these ripples on top of disturbance waves by the gas stream leads to the creation of droplets that are entrained into the gas core and may deposit back onto the film surface. In addition, gas may be entrapped by the liquid film in the form of bubbles of various sizes. In this work, the study of gas bubble creation was performed in a horizontal rectangular duct using the brightness-based laser-induced fluorescence technique. With this technique, the instantaneous height of the liquid film was measured with a 40 µm spatial resolution over a 51 mm by 20 mm area at speeds of 10 kHz. The entrapped bubbles and entrained/depositing droplets are detectable in the data and can thus be studied simultaneously with the waves on the film surface. Several scenarios of bubble entrapment and collapse were identified and discussed. The dynamics of entrapped bubbles was studied quantitatively using an automatic processing algorithm, confirming and elucidating the results of qualitative observations. The effect of the flow parameters on the bubbles concentration, velocity and size distributions was studied separately for the bubbles inside the disturbance waves and inside the thin base film between the disturbance waves. It was shown that the bubbles are mostly created due to oblique impacts of droplets at the base film and are accumulated by the disturbance waves. A small number of bubbles of larger size are created in front of disturbance waves and remain inside the disturbance waves. The velocity of the bubbles is affected by the velocity of the surrounding liquid. Using the bubbles as tracers, a profile of longitudinal liquid velocity was constructed and a noticeable increase of wall shear under the rear slopes of disturbance waves was found.