Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session B24: Bubbles: Taylor Bubbles and Rising Bubbles II |
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Chair: Hangjie Ji, UCLA Room: 606 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B24.00001: Rising or Sinking --- An Elongated Bubble in a Vertical Capillary Tube Under External Flow Yingxian Estella Yu, Mirco Magnini, Suin Shim, Howard A. Stone When a confined bubble translates steadily in a vertical capillary tube under co-current fluid flow, the motion of the bubble depends on both the buoyancy and the mean fluid velocity. Although a wide variety of studies have been carried out analyzing the motion of the elongated confined bubble, most of them focus on the dynamics in one of two distinct limits --- either when buoyancy effects are negligible (Bo \textless \textless 1) or when Bo \textgreater \textgreater 1. In this work, we systematically investigate the motion of an elongated bubble and its film thickness profile with Bo \textasciitilde 1 under external downward flow. In a stagnant fluid, a bubble spontaneously rises only when Bo \textgreater 0.842 (Bretherton 1961), and intuitively, external downward flow can slow down the bubble motion or even reverse the translation direction. By varying the capillary number of the external downward flow, different film profile regimes are observed. A theoretical model based on the extended lubrication theory is developed in order to predict the bubble velocity and steady state solutions of the film profiles in different regimes. The critical external flow conditions are further characterized and validated by combining with the results from experiments and direct numerical simulations. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B24.00002: Effect of wake behind a rising bubble on the heat transfer of the vertical heated wall Hwiyoung Maeng, Hyungmin Park We conduct experiments with two kind of bubbles (linear and zigzagging bubble of which Reynolds number is 200 and 850) rising near the vertical heated (at a constant heat flux of 2000 W/m$^{\mathrm{2}})$ wall. While varying the distance between the bubble and the wall, we measure the gas- and liquid-phase flow fields with two-phase PIV and wall temperature distribution using an infrared camera simultaneously. We find out that the wake emanating behind the zigzag bubble convects laterally toward the wall, disturbing the natural convective boundary layer and enhances the local convective heat transfer, which is quantified as a Nusselt number (Nu) normalized by that of natural convection. As the distance between the bubble and the wall is smaller, the increasing slope and affected area of the heat transfer become larger. The maximum Nu augmentation is eight times larger than the reference value when the bubble-wall distance is short enough for the bubble to collide with the wall, but the affected area is maximum when the bubble does not collide but passes closely to the wall. In the case of linearly rising bubble, however, the heat transfer enhancement was not measured evidently, because the wake is stationary and does not convect to the vertical wall effectively. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B24.00003: Numerical study on the interaction of two bubbles rising in a power-law fluid Shu Takagi, Varun Jadon, Kazuyasu Sugiyama In this study, interactions of two bubble rising in a Non-Newtonian power-law fluid have been investigated through the direct numerical simulations. An interface capturing method with a continuous function is introduced and the three-dimensional model based on modified VOF method is used to study two-bubble interaction phenomena in a power-law Fluid. Here, we discuss the influence of shear-thinning and shear-thickening characteristics on the interaction of in-line or inclined configurations of two bubbles. The numerical results reveal that the shear-thinning fluid enhances the inviscid potential interaction and in-line motion becomes more unstable in shear-thinning fluid than that in Newtonian Fluid. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B24.00004: Effect of viscosity and density ratios on two bubbles rising side-by-side Sivanandan Kavuri, Mounika Balla, Manoj Kumar Tripathi, Kirti Sahu, Rama Govindarajan We study the dynamics of a pair of initially spherical `bubbles' of fluid rising side-by-side in a surrounding, denser, fluid. Interesting dynamics are reported, which cannot be extrapolated from previously known dynamics of gas-liquid systems. Similar to two air bubbles though, we find that two liquid bubbles move away from each other as they rise, in cases where a single bubble would rise vertically. A pair of light bubbles always remains in two-dimensional motion, and higher bubble viscosity increases the tendency of wobbling. This is in contrast with the dynamics of a single bubble that follows a highly three-dimensional trajectory at very low bubble viscosity, but is restricted to two dimensional motion at higher bubble viscosity. On the other hand, a pair of heavier bubbles displays three-dimensional behaviour at low bubble viscosity and two dimensional behaviour at high viscosity. We find that a pair of bubbles is far less sensitive to viscosity contrast than a single bubble is, in our parameter range. In contrast to gas-liquid systems, where shape change of the bubble was tied to nonlinear dynamics of the trajectory, we find in liquid-liquid systems that interesting bubble trajectories can occur without corresponding large shape changes. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B24.00005: On the Dynamics of an Oil Encapsulated Bubble Joel Karp, Ernesto Mancilla, Rigoberto Morales The dynamics of an oil encapsulated bubble was experimentally investigated. High-speed imaging was employed in the visualization of the motion of a coated bubble rising in quiescent tap water. The attachment of a bubble with diameter ranging from 300 to 3000 \textmu m to a 2.3 mm oil droplet resulted in two different regimes: oscillatory and non-oscillatory motion. The influence of the properties of the oil coating was investigated by employing three different oils, which were found to be more influential when no motion instabilities were observed and viscous forces dominate. Path oscillations decreased for the encapsulated bubble, being up to five times smaller in comparison with an isolated bubble with similar diameter. The oil coated bubble presented higher terminal velocity in comparison to a rising oil droplet, with increase factors from 100 up to 400{\%}. The shape deformation of the encapsulated bubble was less than 15{\%} within the diameter range evaluated, being essentially spherical in comparison to correspondent isolated bubbles. Transient motion assays indicated that developed regime was achieved in approximately half the rising distance than for isolated bubbles, after which a periodic oscillation of the velocity components was observed. [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B24.00006: Flow Boiling in a Horizontal Microchannel with high Aspect Ratio for Non-Uniform (One-Sided) Heating at Different Rotational Orientations Marius Vermaak, Jaco Dirker, Khellil Sefiane, Daniel Orejon, Prashant Valluri, Josua Meyer Flow boiling of FC72 in a semi-rectangular horizontal microchannel with one-sided heating, was investigated experimentally. Microchannel rotational orientations from bottom-heating (0\textdegree ) to top-heating (180\textdegree ), in 30\textdegree increments were considered. Mass fluxes of 10, 20 and 40~kg/m$^{\mathrm{2}}$s, paired with various heat fluxes which spanned from the onset of nucleate boiling to near dry out conditions, were tested in a borosilicate glass channel of 5x0.5mm internal cross-section. Ohmic resistive heating was obtained via a layer of tantalum oxide that was applied to only one of the outer surfaces of the microchannel. This optically transparent layer allowed for both transient surface temperature mapping and flow visualization. During quasi steady state average and local heat transfer coefficients as well as pressure drop were analyzed. Our results show that bottom-heating produced average heat transfer coefficients at least 17{\%} higher than top-heating. Flow boiling instabilities are promoted at greater wall super-heats, especially in rotations between top-heating and side-heating. The effect of inclination on flow boiling in such microchannels will also be presented. [Preview Abstract] |
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