Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H12: Multiphase Flows: Bubbly Flow |
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Chair: Thomas Shepard, University of St. Thomas Room: C123 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H12.00001: Influence of bubble size on effervescent atomization. Part 1: bubble characterization and mean spray features Taylor Lewis, Thomas Shepard, David Forliti In the effervescent atomization process a gas-liquid bubbly mixture is ejected from a nozzle with the goal of enhancing liquid break-up. In this work, high speed images are taken of the bubbly flow inside of an effervescent atomizer as well as downstream of the atomizer exit. The use of varying porous plate media grades and channel inserts at the air injection site of the atomizer permitted independent control of mean bubble size. Digital image analyses were used for bubble characterization and measuring mean spray features. The roles of air injection geometry on bubble population parameters inside of the effervescent atomizer are detailed. The effect of bubble size is examined at multiple gas to liquid flow rate ratios for which the bubbly flow regime was maintained. Results are presented demonstrating the influence of bubble size on the average jet width, jet dark core length, and liquid break-up. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H12.00002: Influence of bubble size on effervescent atomization. Part 2: unsteady spatial and temporal features Thomas Shepard, David Forliti, Taylor Lewis In this work, high-speed images of the near-nozzle exit of an effervescent atomizer operating at low gas to liquid ratios are examined using proper orthogonal decomposition and digital image analyses. These techniques allow for the extraction of coherent spatial and temporal patterns present in the high-speed image sets. The effervescent atomizer was operated in the bubbly regime and the experimental facility allowed independent control over bubble size. The impact of varying the mean bubble size on the atomizer near exit field is presented at multiple gas to liquid flow rate ratios. The results demonstrate an influence of mean bubble diameter on peak instability frequency, instability amplitude, axial convection velocities and dominant mode structure. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H12.00003: Experimental Characterization of Interchannel Mixing of Multiphase Flow Through a Narrow Gap Simo A. M\"akiharju, James W. Gose, John R. Buchanan Jr., Alexander G. Mychkovsky, Kirk T. Lowe, Steven L. Ceccio Two-phase mass transfer through a gap connecting two adjacent channels was investigated as a function of gap geometry and flow conditions. An experiment with a simplified geometry was conducted to aid in the physical understanding and to provide data for validation of numerical computations. The flow loop consisted of two (127 mm)$^{\mathrm{2}}$ channels connected by a 1,219 mm ($L)$ x 229 mm ($W)$ gap, the height of which could be adjusted from 0 to 50 mm. The inlet Reynolds number in each channel could be independently varied from 4x10$^{\mathrm{4}}$ - 1x10$^{\mathrm{5}}$. During previous experiments, the single phase mixing was extensively investigated. The inlet void fraction was varied from 1 to 20{\%}. Gas was injected as nominally monodisperse bubbles with diameter $O$(5 mm). The mass transfer through the gap was determined from measurements of the flow rates of water and air, and tracer concentration taken at channel inlets/outlets. The void fraction, bubble diameter distribution and gas flux was determined at the inlets based on flow rate measurements prior to gas injection, optical probes and Wire Mesh Sensor (WMS) data. At the outlets the gas fluxes were based on WMS measurements and the liquid phase mixing was determined based on measurement of the tracer concentration and liquid flow rate after separation of gas. Imaging of fluorescent tracer dye was utilized for select conditions to examine the dynamics of the mixing. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H12.00004: Numerical Simulation of Bubbly Flows in an Aeration Tank with Biochemical Reactions Khateeb Noor Ul Huda, Kazuya Shimizu, Xiaobo Gong, Shu Takagi For bubbly flow with biochemical reactions, all the analyses including overall fluid flow, bubble motion, bubble dissolution at local level and bacterial reactions/consumption of substrates are important. The developed system is provided by mixed Eulerian-Lagrangian formulation in which liquid media is represented in Eulerian system and bubbles are tracked individually. Murai and Matsumoto [1] developed a model to track bubbles to predict plume structure in finely dispersed domain. Gong et al. [2] developed the model further to include mass transfer, gas dissolution and mixing phenomenon entrained in this model. In this research we are using the model to include simulation of bacterial biochemical reactions for the purification of water and make it resemble as the wastewater purification tank. The gas bubble dissolution and mass transfer from gas to liquid phase is linked with biochemical reactions for an overall comprehensive study. The main area associated with this research is to incorporate all biochemical reactions in this bubbly flow based on situation of water and demand. In this particular study, various kinds of biomass and substrates are considered. A detailed model for biological wastewater purification involving reactions using bacteria's is developed and primary validation has been carried out based on experimental study. Finally, we tried to achieve physical optimization for this biochemical reactions. [1] Murai, et al., ASME-Publications-FED, 236, (1996), pp. 67-74. [2] Gong, X., et al., 2009, Int J Multiphas Flow, 35, pp.155-162. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H12.00005: Bubble dynamics and bubble-induced turbulence of a single-bubble chain Joohyoung Lee, Hyungmin Park In the present study, the bubble dynamics and liquid-phase turbulence induced by a chain of bubbles injected from a single nozzle have been experimentally investigated. Using a high-speed two-phase particle image velociemtry, measurements on the bubbles and liquid-phase velocity field are conducted in a transparent tank filled with water, while varying the bubble release frequency from 0.1 to 35 Hz. The tested bubble size ranges between 2.0-3.2 mm, and the corresponding bubble Reynolds number is 590-1100, indicating that it belongs to the regime of path instability. As the release frequency increases, it is found that the global shape of bubble dispersion can be classified into two regimes: from asymmetric (regular) to axisymmetric (irregular). In particular, at higher frequency, the wake vortices of leading bubbles cause an irregular behaviour of the following bubble. For the liquid phase, it is found that a specific trend on the bubble-induced turbulence appears in a strong relation to the above bubble dynamics. Considering this, we try to provide a theoretical model to estimate the liquid-phase turbulence induced by a chain of bubbles. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H12.00006: The budget of turbulent kinetic energy in bubble plumes by acoustic Doppler velocimetry Chris LAI, Scott Socolofsky We present an experimental investigation on the TKE budget of a two-phase air-water bubble plume in an otherwise quiescent ambient. The required three-dimensional turbulent velocity field was measured by a profiling acoustic Doppler velocimeter. Experiments were carried out in a square water tank of 1m$^3$ and covered both adjustment phase (z/D $<$ 5) and asymptotic regime (z/D $\ge$ 5) of the plume in which the latter is characterized by a constant local $Fr_{p}$. The dynamic length scale $D$ has previously been derived from a two-fluid approach and delineates the two regimes. Data on the mean flow establish the existence of an asymptotic regime when $z/D > 8$ with an entrainment coefficient of 0.095 and a $Fr_{p}$ of 1.63. The data also corroborate well with previous measurements of large-scale bubble plumes. A budget of TKE was performed using curve-fits derived from the radial profiles of second- and third-order moments of turbulent velocities. From the budget, TKE production by bubbles was found to be larger than that by fluid shear. Approximately 55-60\% of the total work done by bubbles is used to create fluid turbulence. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H12.00007: DNS assisted modeling of bubbly flows in vertical channels Gretar Tryggvason, Ming Ma, Jiacai Lu The transient motion of bubbly flows, in vertical channels, is studied, using direct numerical simulations (DNS), where every continuum length- and time-scale are resolved. The results of several simulations, starting with laminar or turbulent liquid flows, including one with several hundred bubbles of different sizes and a friction Reynolds number of 500, are reviewed. At statistically steady state, nearly spherical bubbles in upflow form distinct wall-layers, but sufficiently deformable bubbles, as well as bubbles in downflow, do not. The transient evolution, particularly for nearly spherical bubbles in upflow is more complex. The bubbles first move toward the walls and then the liquid slowly slows down, eventually allowing some bubbles to return to the center of the channel. The use of the DNS results to help with modeling the average flow are discussed, including simple analytical models for laminar upflow and downflow and the use of statistical learning and neural networks to relate closure terms to resolved quantities for more complex flows. The prospects of using results from simulations of large system with bubbles of different sizes in turbulent flows for LES-like simulations are explored, including the simplification of the interface structure by filtering. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H12.00008: Numerical Simulation of Turbulent Bubbly Flow in a Vertical Square Duct Pratap Vanka, Purushotam Kumar, Kai Jin We numerically investigate the dynamics of a large number of gas bubbles in a turbulent liquid flow in a confined vertical square duct, a problem of interest to many industrial equipment. The fluid flow is simulated by Direct Numerical Simulations and the motions of the bubbles are resolved by an accurate Volume of Fluid (VOF) technique. The flow is considered periodic in the streamwise direction with an imposed pressure gradient. The surface tension force is incorporated through a Sharp Surface Force (SSF) method that is observed to generate only very small spurious velocities at the interface. The algorithm has been programmed on a multiple-GPU computer in a data parallel mode. The turbulence driven secondary flows are first ensured to agree with previous DNS/LES by other researchers. A very fine grid with 192 x 192 x 768 control volumes is used to resolve the liquid flow as well as 864 bubbles using 12 grid points across each bubble in all directions. The computations are carried out to 1.5 million time steps. It is seen that the bubbles preferentially migrate to walls, starting from a uniform layout. We present instantaneous and time mean velocities, turbulence statistics and compare them with unladen flow as well as with a bubbly flow in a planar channel. [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H12.00009: Direct Numerical Simulation of Insoluble Surfactant Effect on Turbulent Channel Bubbly Flows Jiacai Lu, Gretar Tryggvason Direct Numerical Simulations (DNS) have been successfully used to obtain detailed data for turbulent channel bubbly flows. However, most of DNS that have been done so far remain problematic in comparing to most experiments. One of the major reasons is that real bubbly flows contain surfactants. The surfactants adhere to the interface, and produce an uneven distribution of the surfactant concentration due to the moving of bubbles and result in uneven surface tension over bubble surfaces. In this project, the effect of surfactants on the flow of many bubbles in an upward turbulent channel flow is studied by using of Direct Numerical Simulation with 3D Front-tracking method. The surfactant mass and the interfacial area are directly tracked in the method, and the surfactant mass remains conserved during the evolution. By using of different elasticity numbers in the non-linear equation of state which relates the surface tension to the surfactant concentration, the simulations show that the evolution of the turbulent channel bubbly flow are much different among the cases with contaminated bubbles and clean bubbles. Profiles of many parameters, such as streamwise velocity, shear stress and etc., are also compared at the statistically steady state for these cases. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H12.00010: Bubbly flows around a two-dimensional circular cylinder Jubeom Lee, Hyungmin Park Two-phase cross flows around a bluff body occur in many thermal-fluid systems like steam generators, heat exchangers and nuclear reactors. However, our current knowledge on the interactions among bubbles, bubble-induced flows and the bluff body are limited. In the present study, the gas-liquid bubbly flows around a solid circular cylinder are experimentally investigated while varying the mean void fraction from 5 to 27 \%. The surrounding liquid (water) is initially static and the liquid flow is only induced by the air bubbles. For the measurements, we use the high-speed two-phase particle image velocimetry techniques. First, depending on the mean void fraction, two regimes are classified with different preferential concentration of bubbles in the cylinder wake, which are explained in terms of hydrodynamic force balances acting on rising bubbles. Second, the differences between the two-phase and single-phase flows (while matching their Reynolds numbers) around a circular cylinder will be discussed in relation to effects of bubble dynamics and the bubble-induced turbulence on the cylinder wake. [Preview Abstract] |
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