Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session L09: Bubbly Flows |
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Chair: Brian Elbing, Oklahoma State University Room: Georgia World Congress Center B214 |
Monday, November 19, 2018 4:05PM - 4:18PM |
L09.00001: Energy cascade in a homogeneous swarm of bubbles rising in a vertical channel Bruño Fraga, Chris C.K. Lai, Ronald Chan, Michael Dodd The cascade of kinetic energy is a defining characteristic of fluid turbulence. For single-phase flows, the Richardson-Kolmogorov phenomenology provides a satisfactory first approximation of the energy cascade with which many existing turbulence models/theories are based. However, the phenomenology has not been demonstrated in two-phase flows where the production by the dispersed phase is an additional source of liquid turbulent kinetic energy. We use bubble-resolved, direct numerical simulations to investigate the energy cascade in homogeneous swarms of air bubbles rising a vertical channel. We consider millimeter-sized bubbles with a bubble Reynolds number of 500. The von Karman-Howarth-Monin (K-H-M) equation is adapted for the two-phase flow and used to quantify the interscale energy transfer and to compute the scale-by-scale energy budget. |
Monday, November 19, 2018 4:18PM - 4:31PM |
L09.00002: Bubble induced mixing in a vertically vibrating bubble column Shahrouz Mohagheghian, Brian R. Elbing A homogeneous bubble swarm rising in quiescent liquid generates liquid velocity agitations different from that of grid generated isotropic turbulence. Dispersion of a passive scalar using the mixing effect of bubble-induced turbulence was studied. In addition, the effect of vibration on the mixing performance of a bubble swarm was investigated. An instrumented vibrating bubble column test facility was used to study the mixing time of a non-reactive dye. Mixing time was measured by tracking the temporal evolution of a batch of dye using optical photography. In the present work gas superficial velocity, vibration frequency, and amplitude are the independent variables. In addition to the mixing time and independent parameters, measurements of the bulk void fraction, bubble size, and rise velocity were acquired. Effect of vibration frequency and amplitude on mixing time was investigated independently. In a bubble swarm, mixing was observed to occur via capture and transport in the bubble wake and via direct interaction of bubble induced liquid agitations with the dye. The mixing performance of the bubble swarm will be presented and compared with and without vibration while matching the specific input power. |
Monday, November 19, 2018 4:31PM - 4:44PM |
L09.00003: Bubble plumes beneath an inclined wall Frederik Brasz, Dayoung Kim, Mark Menesses, Jesse L. Belden, James Bird Bubbles injected beneath a submerged inclined wall can form a two-dimensional plume, entraining surrounding liquid and spreading out laterally as they rise along the wall. These plumes are being considered as a technique for preventing biofouling in marine environments, for example on ship hulls. We use high-speed imaging experiments to obtain ensemble statistics and bubble trajectories, allowing us to investigate how the bubble plume structure and velocity depend on flow rate, nozzle size, and inclination angle. The results are compared to integral plume theory and previous studies of axisymmetric bubble plumes. |
Monday, November 19, 2018 4:44PM - 4:57PM |
L09.00004: Bubble trapping in a stirred vessel Steven Wang, Lyes Kahouadji, Susan Frey, Erich J Windhab, Yuchi Murai, Chao Yang, Howard A Stone, Richard V Craster, Omar K. Matar We study a highly unexpected focusing phenomenon encountered in high-viscosity stirred fluids. In this project, we aim to uncover the fundamental focusing mechanism theoretically and experimentally. A fully three-dimensional direct numerical simulation is performed where the interface solver is based on a hybrid Front Tracking/Level Set method and designed to handle highly deforming interfaces with complex topology changes including pinch-off and coalescence. The form of the six-bladed Rushton turbine is constructed by means of a module for the definition of immersed objects via a distance function that takes into account the object’s interaction with the flow and treated as a fictitious fluid in the Navier-Stokes solution and its velocity is corrected in order to satisfy rigid body motion constraint. Bubbles are introduced into the system at various locations, and we demonstrate the conditions under which the motion of the bubbles is focused near the turbine impeller. The mechanisms underlying the bubble ‘trapping’ are also elucidated |
Monday, November 19, 2018 4:57PM - 5:10PM |
L09.00005: Complex vortex formation and aeration in air-water mixing flows in stirred tanks Damir Juric, Lyes Kahouadji, Assen Batchvarov, Ricardo Constante-Amores, Jalel Chergui, Seungwon Shin, Richard Craster, Omar K. Matar Three-dimensional (3D) direct numerical simulations are carried out of air-water mixing flows using pitched blade turbine in an open vessel. We useahybrid front-tracking/level-set solver for 3D parallel simulations of multiphase flows. The turbine is constructed through a module that defines immersed objects using a distance function that accounts for the objects interaction with the flow and with appropriate boundary conditions in the Navier-Stokes solution the velocity is corrected in order to satisfy a rigid body motion constraint. In addition to ordinary primary vortices occurring in any kind of rotating flow, our configuration also features several secondary vortices, vortex breakdown, blade-tip, and end-wall corner vortices. For certain turbine rotational speeds, aeration takes places; this corresponds to the breakup of at the air-water interface leading to the entrainment of bubbles into the water phase. The mechanisms underlying the entrainment, and bubble formation processes, are elucidated. |
Monday, November 19, 2018 5:10PM - 5:23PM |
L09.00006: Direct numerical simulation of a gas-liquid two-phase bubble plume Hyunduk Seo, Sivaramakrishnan Balachandar, Kyung Chun Kim Complex nature of the gas-liquid two-phase plume comes from the turbulent mixing between different phases and the mechanical shear converting mean kinetic energy into turbulent kinetic energy. These turbulent characteristics of bubbly plume have significant role on the rising process of the bubble plume accompanying with entrainment and lifting of ambient liquid phase. To understand the effect of bubble on mixing and entrainment mechanism of the bubble plume, turbulence resolving simulation with high fidelity is necessity. In this study, two-phase fluids governing equations are solved by DNS using the spectral element method code Nek5000 with Euler-Euler approach. From the numerical data, turbulence of the bubble induced flow is quantified with high order turbulence statistics with different bubble conditions which are reflected in slip velocity of the gas phase and inlet conditions. |
Monday, November 19, 2018 5:23PM - 5:36PM |
L09.00007: Bubble dynamics in two-phase flows in an upward 90-degree elbow pipe Hongseok Choi, Hyungmin Park For a flow through a bent pipe, it is known that a centrifugal force and the boundary layer on the wall induce secondary vortical structures, which affects the interfacial structure of the flow. Since the transfer characteristics of two-phase flows are greatly influenced by this interfacial structure, the effect of pipe orientation on the two-phase flow should be studied in detail. We investigate bubble dynamics in gas-liquid bubbly flows in an upward 90-degree bent pipe, with different Reynolds numbers (Re = 0, 400 and 6000) of background flow and mean void fraction of 0-3%. The cross-section of the pipe is a square (20 mm x 20 mm), and we use high-speed shadowgraphy to track the three-dimensional path (velocity and size distributions) of the bubbles. When the flow is laminar, bubbles separate at about 15 degrees from the bent entrance, while more bubbles reside at center of the pipe as mean void fraction increases. For turbulent flow (Re = 6000), on the other hand, most of the bubbles separate from the inner wall at about half way of the bent and more bubbles migrate to outer wall as the mean void fraction increases. More detailed bubble dynamics and its mechanism will be discussed. |
Monday, November 19, 2018 5:36PM - 5:49PM |
L09.00008: Liquid-Gas Flow-Induced Vibration in Flexible Pipe Narakorn Srinil, Hossein Zanganeh, Konstantinos Bakis, Bowen Ma, Omar K Matar, Chris Pain Liquid-gas flows propagating through a pipe may evolve into a variety of flow patterns governed by the flow-pipe parameters and interfacial characteristics. Mechanisms of liquid-gas flow-induced vibrations in a long flexible pipe with a high length-to-diameter ratio are poorly understood owing to the lack of studies dealing with such complex flow phenomena inside the bendable geometries. Previous studies have mostly focused on a rigid inflexible pipe with a short span, small diameter or fixed inclination. Here, the most problematic slug flows with fluctuating liquid-gas volumetric fractions, velocities and pressure are studied through a hierarchy of mathematical, phenomenological and computational fluid dynamics models. We predict liquid-gas flow features and effects on flexible pipes with variable inclinations, multiple bends and multi-directional flows, and assess the pipe dynamic deformations, stresses and frequencies. The effects of slug properties (length, velocity, frequency, intermittency) are investigated. Numerical results reveal several key fundamental aspects of flexible pipe vibrations caused by travelling liquid-gas slug flows. |
Monday, November 19, 2018 5:49PM - 6:02PM |
L09.00009: Vapour bubble drag reduction in boiling Taylor-Couette turbulence Rodrigo Ezeta, Sander G. Huisman, Dennis Bakhuis, Sander Bonestroo, Chao Sun, Detlef Lohse |
Monday, November 19, 2018 6:02PM - 6:15PM |
L09.00010: Abstract Withdrawn An experimental study on the steam-water direct contact condensation in an unstable bubbling condensation oscillation regime was conducted to investigate the effects of non-condensable gas on the heat transfer rate. As a working fluid, purified DI water though the reverse osmosis method was utilized. The experiments were performed at pool temperatures of 70 to 95°C ± 1°C, with different steam/air mixture mass fluxes between 5 and 50 kg/m^{2}-s with the inner diameter of 10 mm gas injection nozzle. The steam and air mixing ratio was varied from 1.0:0.0 to 0.25:0.75. The pool temperature was measured using thermocouples, and the surface area of the bubble and the volume flow rate were measured with high-speed camera image processing to obtain the heat transfer coefficient. As a result of the presence of the non-condensable gas in the steam, decrease of heat transfer coefficient and change in the pattern of condensation were observed. The correlations between the heat transfer coefficient, the steam mass flux, and non-condensable gas mixing ratio were analyzed. |
Monday, November 19, 2018 6:15PM - 6:28PM |
L09.00011: The effects of non-condensable gas on cavitating flow over a cylinder Filipe Leite Brandao, Mrugank P Bhatt, Krishnan Mahesh The effects of non-condensable gas (NCG) in cavitating flow over a circular cylinder are investigated using a homogeneous mixture model. We consider Reynolds number = 200 at different cavitation numbers (σ = 0.7 to σ = 1.0). The numerical method developed by Gananaskadan and Mahesh (2015) for the mixture of water and vapor is extended to include the effect of non-condensable gas. It is observed that the non-condensable gas has a stabilizing effect on the cavity being formed. The decrease in sound speed when the gas is added to the flow creates a damping effect, effectively decreasing the magnitudes of shock-waves generated during the cavity collapse. Thus, weaker pressure waves impinge on the body. In the bubbly shock regime (σ = 0.7), it is shown that the NCG substantially decreases the strength of the condensation shock. A shock speed equation is derived from the Rankine-Hugoniot jump conditions to explain the phenomenon. We show that this decrease in condensation shock speed in the presence of NCG is due to the decrease in the pressure ratio across the shock as it moves. |
Monday, November 19, 2018 6:28PM - 6:41PM |
L09.00012: Influence of thermal sensitivity on Venturi cavitation characteristics in a blow-down type tunnel with controlled dissolved gas content Zhigang Zuo, Haochen Zhang, Knud Aage Mørch, Shuhong Liu nfluence of dissolved gas content (measured by dissolved oxygen, DO) on Venturi cavitation (10mm-scale test section) in water at different temperature, i.e., different degree of thermal sensitivity, is investigated in a blowdown-type cavitation tunnel. Investigations on the cavitation length and the unsteady cavitation behavior are presented. It is shown that the cavitation length decreases with increasing temperature (or the degree of the thermodynamic effect). In addition, the DO content has little influence on the mean cavitation length at thermally sensitive conditions. |
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