INTERFACIAL RECONSTRUCTION OF TAYLOR BUBBLE AND RELATED FLUIDIC BEHAVIOUR IN VESSEL EMPTYING

Abstract

A combined experimental and numerical investigation has been performed to study the interfacial evolution of the slug bubble under different conditions. In this effort, four particular problems related to the interfacial evolution of the slug bubble have been targeted, namely, the transformation of the Taylor bubble to the annular bubble in presence of longitudinal insert, Taylor bubble pinch-off and its coalescence at macroscopic scales while bypassing a transverse insert, slug bubble evolution in a closed vessel and bubble slip phenomenon in a rectangular column. The transformation of a symmetric slug bubble to an asymmetric annular bubble takes place through six distinct stages, namely, plateau formation, doughnut shape bypass of obstruction followed by nucleation, preferential rise, retraction of the lagging lobe and subsequent thread formation, consumption of thread including bubble segment stage and finally the manifestation of the annular bubble before rising as steady annular sectorial wrap. A classical gravity-driven Rayleigh-Taylor instability causes the transformation to an annular bubble. The growth rate of the lobes corresponds to the growth rate of the bubble in the case of Rayleigh-Taylor instability. Some of the interfacial evolution stages are absent in the case of eccentric annuli and with viscous fluids. Various other physical feature like the thickness of the annular film, nose tip shift, bubble length ratio and bypass time has also been quantified. The modulation of the slug bubble size by using a perpendicular insert has been targeted in the viscous medium. A boundary has been demarcated between a unified bypass and fragmented bypass of the Taylor bubble on the basis of the diameter ratio of the insert in a glycerol solution. The stability of these slug trains has been established by computing the wake volume. An experimental effort has been reported for analysis of the fluidics during emptying of a glass bottle. Fluidic phenomena like formation and pinch-off of an encapsulated bubble, ejector jet, flooding, and stratification have been observed in a vertically upended bottle. The rise velocity, collapse dynamics, and growth rate of the bubble at the bottle mouth are affected by the angle of inclination and mainly viscosity of the emptying liquid Two distinct bottle emptying modes have been identified in one of which the discharge rate is increased due to high-frequency pinch-off of air bubbles inside the bottle and in another mode due to an increase in the volume of the pinched-off bubble at a comparatively lower frequency. Interaction of dominant forces during the emptying process has been established by quantifying Reynolds number, 𝑅𝑒, Weber number, 𝑊𝑒 and Bond number, 𝐵𝑜. For all emptying liquids, bottle emptying time reduces linearly up to a critical angle of inclination, 𝜃𝑐𝑟𝑖𝑡 ~ 20° and further follows an asymptote. We hypothesize that the transition between the linear regime and asymptotic regime is due to the saturation of the voidage of the air at the cylindrical section of the bottle mouth. Furthermore, the geometry of the bottle also facilitates the growth rate of the bubble inside the bottle at 𝜃𝑐𝑟𝑖𝑡 . Finally, from the studied collective fluidic physics from discharge dynamics, a pragmatic correlation for estimation of bottle emptying time, 𝑡𝑒 has been proposed as a function of bottle inclination, 𝜃, from the vertical and wide range of viscosity. A combined experimental and numerical study has been performed to investigate the interaction of the small bubble and the slug bubble in a rectangular column with viscous fluids. The interaction behavior of the small bubble depends upon its diameter, 𝑑𝑒𝑞 and thermo-physical properties of the fluid. The small bubble sprints away from the slug bubble at low Morton numbers, 𝑀𝑜= [(𝜌𝑙− 𝜌𝑔)𝑔𝜇4][𝜌𝑙2𝜎3]⁄ (sprint away regime). On the other hand, SB interacts with TB due to its lower terminal velocity at higher 𝑀𝑜 (bubble slip regime). The small bubble behaves independently ahead of the TB nose but accelerates linearly into its annular film. A regime map has been proposed to differentiate between bubble slip and sprint away regimes. The entrapped film between TB and SB is continuously fed from the annular film and avoids coalescence. An ad-hoc pressure jump model has been proposed to explain the repulsion of SB in the annular film. Furthermore, a modified lubrication theory-based model predicted the stability of the entrapped film due to interfacial velocities and curvature.