Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session T23: Multiphase Flows: Bubbly Flows |
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Chair: Simo Makiharju, UC Berkeley Room: North 224 A |
Tuesday, November 23, 2021 12:40PM - 12:53PM |
T23.00001: Effects of split power-law entrainment on bubble fragmentation cascades Declan B Gaylo, Kelli L Hendrickson, Dick K Yue Understanding the size distribution of bubbles due to entrainment of gas at a free surface is important to a variety of natural and engineering applications. The bubble-size distribution is the result of multiple processes, and in this work, we focus on the interaction between steady spectral entertainment and turbulence-driven fragmentation at large Weber numbers. We recently demonstrated that a simplified population balance model (S-PBM) with entertainment modeled using a cutoff power-law predicts the shape of observed bubble-size distributions for large bubbles. Analytic analysis shows that, within a weak-entrainment regime, the commonly observed -10/3 power-law size distribution of large-radius bubbles is independent of both the entrainment power-law slope and the fragmentation daughter distribution. Previous work has suggested that entrainment can contain two separate regimes, such as the surface tension-dominant and gravity-dominant regimes of turbulent entertainment, separated by the capillary scale. This work extends our cutoff power-law entertainment model to a split cutoff power-law characterized by two power-law regimes separated at a critical radius. Using S-PBM and Monte Carlo simulations, we show that the shape of the resulting bubble-size distribution is still independent of the entrainment power-law slopes when both are in the weak-entertainment regime. |
Tuesday, November 23, 2021 12:53PM - 1:06PM |
T23.00002: Bubble Trapping and Clustering Dynamics in an Upward Liquid-Gas Flow around a Cylinder Dohwan Kim, Matthew J Rau Two-phase wakes from bluff bodies in liquid-gas flows can have non-uniform distributions of void fraction due to bubble trapping and clustering. We have conducted an experiment to characterize these bubble dynamics using a water-air mixture in an adiabatic upward flow channel. In this experiment, the air superficial velocity of the two-phase mixture was 0.061 m/s and the liquid Reynolds number (Re), based on a cylinder diameter of 9.5 mm, was 3,000. Additionally, the equivalent bubble diameters ranged from 1 mm to 4 mm. We measured the bubble diameters and calculated bubble trajectories using shadowgraphs of the flow obtained at 2,000 fps and Lagrangian Particle Tracking Velocimetry (PTV). In addition to the bubbles, we injected liquid flow tracers and used Particle Shadow Image Velocimetry (PSIV) to analyze the liquid flow fields. The two-phase tracking results allowed us to evaluate the effect of bubble size on buoyancy, inertial, drag, lift, and pressure gradient forces. The force balance analysis showed inertial, lift, and pressure gradient forces contributed to the formation of the bubble trapping wakes. While the pressure gradient forces were the strongest, the results suggested inertial and lift forces are comparably important. |
Tuesday, November 23, 2021 1:06PM - 1:19PM |
T23.00003: Effect of Superhydrophobic Internals on Flow in a Bubble Column Angel F Rodriguez, Simo A Makiharju This study investigates the influence on bubble dynamics of superhydrophobic coatings on internal components within a bubble column. Multiphase flows similar to those considered are relevant to heat exchangers and rod bundles, and we hypothesize that superhydrophobic-coated internal components will result in a change of flow regime, which can be beneficial for maximizing heat transfer performance or avoiding an undesirable operating regime.The presence of a superhydrophobic surface can enable gas to escape faster to the free surface along solid surfaces. This can result in reduced gas volume fractions compared to conditions without superhydrophobic coated internals and allow the flow to remain dispersed at higher gas fluxes. Advanced measurement techniques can measure the phase fraction in these optically opaque systems. We employ multi-plane wire mesh sensors and X-ray computed tomography to study flow patterns in a bubble column with and without internals. We observe for low gas fluxes a minimal effect from the coating, but for a gas flux that in our column leads to a void fraction exceeding 0.2 we observe a plateau in phase fraction as gas layer indeed forms an 'escape route' to the free surface along the superhydrophobic surfaces. The overall flow pattern is significantly modified, and flow remains in a disperse regime in most of the column at higher gas fractions. |
Tuesday, November 23, 2021 1:19PM - 1:32PM |
T23.00004: Experimental Investigation of Temperature Effects on Breakup Mechanisms of Supercritical-Assisted Atomization Shadi Shariatnia, Amir Asadi, Dorrin Jarrahbashi Supercritical CO2 is proved as an excellent choice in supercritical-assisted atomization (SAA) systems. The sensitivity of SAA to operational conditions and properties of injected mixture enables practical customization. Since the rheological properties of supercritical fluids strongly depend on temperature, the breakup mechanism is significantly affected by crossing the critical temperature. Here, we experimentally investigate the breakup mechanism of CO2-assisted atomization (CO2-A) at subcritical, critical, and supercritical states and compare it with cases where N2 is utilized as the assisting fluid at the same conditions. High-speed imaging and laser diffraction are used to analyze the evolution of the liquid jet during the primary and secondary atomization processes. The primary breakup of CO2-A is found to be the emergence, expansion, and burst of CO2 bubbles and formation of ligaments that break up into small droplets, due to the high solubility of CO2 in water and low interfacial tension of the CO2-water mixture. |
Tuesday, November 23, 2021 1:32PM - 1:45PM |
T23.00005: Vortex Shedding in a Dispersed Multiphase Flow Eric W Thacher, Per-Olof Persson, Simo A Makiharju Vortex-induced vibration from cross flow over a cylinder is an important design consideration in devices from flow meters to nuclear reactors. While past researchers have shown that introducing bubbles to the flow decreases vibration amplitude while increasing shedding frequency, the mechanisms causing these changes are not fully understood. Considering individual bubble transport may provide insight, as previous studies demonstrated that the frequency shift depends nonlinearly on bubble size. Using NSF funding, we begin by experimentally and numerically investigating the flow of individual monodispersed bubbles over a cylinder in cross flow, to assess the size-dependent location and time needed for bubbles to be captured in the shed vortices. These statistics are initially predicted using a one-way coupled point-particle tracking model with the flow field computed using high-order LES. The numerical results are then verified experimentally using high-speed camera visualization of a stream of monodisperse bubbles (range of 40-400 microns) introduced upstream of the cylinder. The bubble capture statistics provide insight into the mechanism for frequency shift, which will be explored further in higher phase fraction flows that are no longer one-way coupled. |
Tuesday, November 23, 2021 1:45PM - 1:58PM |
T23.00006: Bubble dynamics and cavitation inception mechanism characterization in aviation fuel liquids via computer vision tools Igal Gluzman, Flint O Thomas Aviation fuel cavitation inception mechanisms triggered by a single injected bubble are characterized in the diverging part of the converging-diverging nozzle using advanced computer vision (CV) algorithms. The gained CV blob statistics provided unprecedented quantitative data from non-intrusive imaging techniques, revealing valuable insights into the bubble spatial-temporal evolution, breakup dynamics, and cavitation inception mechanisms in fuel liquids. Two distinct constant velocities of the bubble before its breakup and the resulting voids cloud after the breakup are observed consistently, despite the rapid fluid velocity decrease in that range. We have characterized the initial bubble size role in the resulting void fraction variation, bubble breakup site, and its terminal velocity before the breakup. Additionally, We have defined a unique dimensionless number, distinguishing between the breakup dynamic parameters accounting for different fuel liquids and flow regimes. The obtained results shed some light on the dynamics of a group of nonspherical cavities and complex fuel cavitation mechanisms. |
Tuesday, November 23, 2021 1:58PM - 2:11PM |
T23.00007: Modeling Gas Oscillations Following a Confined Underwater Release Idan Eizenberg, Dan Liberzon, Ian Jacobi The sudden, underwater rupture of a finite canister of air was studied by time-resolved pressure and interfacial geometry measurements. Two distinct dynamical regimes for the air remaining in the canister were identified: an inertial regime, in which the air oscillated with increasing frequency, and a capillary regime, in which emerging air bubbles pinched off from the air remaining in the canister. The temporal scales for these regimes were identified, and the inertial regime was shown to follow Rayleigh-Plesset dynamics with an empirically defined characteristic length scale associated with the air volume. The spectral dynamics of the two regimes were reproduced by modifying the Rayleigh-Plesset equation to include mass/energy losses, viscous damping, and a time-dependent capillary forcing. |
Tuesday, November 23, 2021 2:11PM - 2:24PM |
T23.00008: Direct numerical simulations of surfactants in bubbly tri-periodic turbulent flows Ianto Cannon, Giovanni Soligo, Marco Edoardo Rosti Turbulent bubbly flows are common in nature, industry and even our daily life; e.g. breaking waves, water treatment facilities and laundromats. A common element to these applications is the presence of surface-active agents, also known as surfactants. Surfactants have profound effects on these flows: by lowering the surface tension, they modify deformability of the interface and introduce Marangoni stresses. |
Tuesday, November 23, 2021 2:24PM - 2:37PM |
T23.00009: Numerical simulations of hydrogen production in alkaline water electrolysers Morgan Kerhouant, Thomas Abadie, Andre Nicolle, Omar K Matar Alkaline water electrolysers are commonly used for the industrial-scale production of hydrogen, and the gas-liquid flow within the electrochemical cell has an influence on the electrolyser performance. Operating at higher current densities often leads to enhanced hydrogen production but lower efficiency, due to blockage of the electrode surface and reduced electrical conductivity of the electrolyte arising from an increase in void fraction. We use a multifluid Eulerian model to perform three-dimensional numerical simulations of the bubbly flow in a small electrochemical cell immersed in a sodium sulphate solution. Hydrogen and oxygen are modelled as dispersed phases, and flow through the cell is driven from buoyancy of the generated gas bubbles. We explore the impact of bubble size, distribution and inter-phase coupling terms on the gas-liquid flow in the cell, including the study of bubble induced turbulence and the unsteady behaviour of the bubble layer along the electrodes. Gas volume fractions, velocity profiles and turbulent intensity are compared with experimental work at current densities of 500, 1000 and 2000 Am-2. Improved modelling of flow within electrolysers paves the way for optimisation of cell design and operation from a fluid mechanics perspective. |
Tuesday, November 23, 2021 2:37PM - 2:50PM |
T23.00010: Volume of Fluid simulations of surfactant-laden interfaces Giovanni Soligo, Marco Edoardo Rosti Flowing systems characterized by the presence of fluid-fluid interfaces are commonly found in nature and in a large number of industrial processes. These flows are often characterized by the presence of surface-active agents, also known as surfactants, which influence the formation, growth, breakage, coalescence and dynamics of the interface separating the different fluid phases. The term surfactant encompasses molecules and particles that naturally collect at the interface and locally reduce surface tension according to their concentration. Local modifications of the surface tension value introduce complex interfacial dynamics: (i) the deformability of the interface changes according to the local surfactant concentration, (ii) Marangoni stresses are generated, proportional to the surface tension gradients and acting along the interface. We propose a numerical method to describe the dynamics of surfactant over moving and deforming interfaces, which can also undergo breakage and coalescence; the dynamics of the interface is simulated using an algebraic volume of fluid method. Several test cases and benchmarks will also be presented to show the performances of the proposed method. |
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