Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R26: Multiphase Flows: General |
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Chair: Shankar Subramaniam, Iowa State University Room: 306 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R26.00001: Transport of temperature-velocity covariance in gas-solid flow and its relation to the axial dispersion coefficient Shankar Subramaniam, Bo Sun The presence of solid particles in a steady laminar flow generates velocity fluctuations with respect to the mean fluid velocity that are termed pseudo-turbulence. The level of these pseudo-turbulent velocity fluctuations has been characterized in statistically homogeneous fixed particle assemblies and freely evolving suspensions using particle-resolved direct numerical simulation (PR-DNS) by Mehrabadi {\it et al.}(JFM, 2015), and it is found to be a significant contribution to the total kinetic energy associated with the flow. The correlation of these velocity fluctuations with temperature (or a passive scalar) generates a flux term that appears in the transport equation for the average fluid temperature (or average scalar concentration). The magnitude of this transport of temperature-velocity covariance is quantified using PR-DNS of thermally fully developed flow past a statistically homogeneous fixed assembly of particles, and the budget of the average fluid temperature equation is presented. The relation of this transport term to the axial dispersion coefficient (Brenner, {\em Phil. Trans. Roy. Soc. A}, 1980) is established. The simulation results are then interpreted in the context of our understanding of axial dispersion in gas-solid flow. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R26.00002: Modeling Two-point Particle Dynamics of Homogeneous Gas-Solid Flows to describe Clustering and Stability Eric Murphy, Mohammad Mehrabadi, Sudheer Tenneti, Shankar Subramaniam The stability of a statistically homogeneous gas-solid flow is still incompletely understood in spite of several advances in our understanding of this fundamental problem. The stability of a homogeneous gas-solid flow is closely related to the formation of spatial patterns in the particle field, i.e. clusters. Although the precise definition of stability is lacking, much understanding has been gained through simulation of particle clustering. Previous modeling efforts have focused on the linear stability analysis of mean fields such as number density and mean particle velocity. Resolved simulations representing realizations of the particle field illustrates the importance of particle number fluctuations that occur in all gas-solid systems. These observations highlight the importance of two-particle statistics in analyzing clustering and stability of gas-solid flows. In this talk we present a framework, which may be utilized in modeling the evolution of two-point particle statistics. Using a stochastic particle-based approach to modeling, interaction laws between particles are modeled from DNS statistics. This modeling framework permits the analysis of clustering and stability of homogeneous gas-solid systems while accounting for naturally occurring particle number fluctuations. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R26.00003: Actuation of interfacial waves in oil-water flows Kyeong Park, Weheliye Weheliye, Maxime Chinaud, Panagiota Angeli Droplet detachment from interfacial waves in two-phase flows has pulled in noteworthy exploration interest. In order to examine this phenomenon experimentally and empower quantitative estimation, it is important to spatially confine the drop formation. In the present study, a cylinder, located close to the inlet of the test section and perpendicular to the direction of the flow, is placed in a two-phase stratified oil-water pipe flow. The introduction of this cylinder actuated interfacial waves and move from stratified to dispersed flow pattern. High speed visualisation and Particle Image Velocimetry (PIV) measurement are utilized to investigate the flow pattern maps of the two-phase flow and the velocity fields in the wake of the cylinder, respectively. These results will be compared with previous experimental studies. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R26.00004: Flow development investigation of concentrated unstable oil-water dispersions in turbulent pipe flows Victor Voulgaropoulos, Weheliye Weheliye, Maxime Chinaud, Panagiota Angeli This study explores the separation characteristics of unstable oil-water dispersed flows in pipes. The test section is a 7 m long acrylic pipe with a 37mm ID and the fluids used are tap water and an Exxsol oil (6.6cSt) An inlet system with more than a thousand capillary tubes of 1mm ID is implemented to actuate highly concentrated dispersions for a wider range of flow rates. High speed imaging combined with ring conductivity probes and pressure transducers are implemented in several axial positions along the pipe to study the flow development. Phase distribution and continuity are measured in the pipe cross-section and drop size information is acquired by high frequency dual impedance probes. The coalescence and sedimentation dynamics of the concentrated dispersions and the development of separate layers downstream the pipe are investigated. The experimental results are coupled with theoretical and semi-empirical models in an effort to predict the separation properties of the highly concentrated dispersed flows. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R26.00005: Droplet Size Distributions Resulting form Entrainment of Surface Oil Slick by Breaking Waves Cheng Li, Joseph Katz A spectrum of droplet sizes, ranging from submicron to several millimeters, is generated by breaking waves impinging on an oil slick. Their size distribution is crucial for modeling the fate of oil spill, and understanding the underlying flow physics. Digital holography microscopy (DHM) is used for measuring the droplet size distributions at high resolution (1.1 $\mu $m/pixel), and at varying temporal scale, from the initial plunging phase (seconds) to long term (hours). The time-resolved DHM data is acquired simultaneously with high speed visualizations of the breakup and large scale features of the entrainment process. Experimental conditions include: (i) plunging and spilling breakers with wave heights of 28.8, 24.9, 22.28 cm; (ii) crude oil (MC252 surrogate), and oil premixed with dispersants (Corexit-9500A) giving two order of magnitude range of water-oil interfacial tension; (iii) Crude, fish, and motor oils with viscosity of 9.4, 63.1 and 306.5 cst, respectively. Shortly after entrainment of crude oil, the droplet radius distribution is bimodal, with a primary peak in the 0-25 $\mu $m range, and a secondary peak at 200-250 $\mu $m. Adding dispersants reduces the latter to 150 $\mu $m. The drastic reduction in interfacial tension upon introduction of dispersants increases the primary peak, and causes short term micro threading. The Secondary peaks dampen within seconds, as the larger droplets rise, whereas the primary peaks are sustained for longer periods. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R26.00006: Modelling the Hydrodynamics and Transport in Multiphase Microreactors Lu Yang, Yanxiang Shi, Milad Abolhasani, Klavs Jensen Multiphase flow is prevalent in a variety of industrial applications, but the extent of these processes is often limited by the innate mass transfer resistance across phase boundaries. Microscale multiphase systems, owing to their reduced characteristic length scales, increase specific interfacial areas and unique hydrodynamic patterns, can significantly enhance the rate of mass transfer, thereby improving the efficiency of multiphase processes. However, many uncertainties still remain in the prediction of multiphase hydrodynamics and scalar transport on the microscale, primarily due to the complex nature of the multiphase flow. In this work, to elucidate the mechanism of mass transfer enhancement in microscale multiphase flows, a computational fluid dynamic (CFD) model using the volume-of-fluid (VOF) method is developed, and the method is validated with experiments. By introducing a scalar transport equation with sink/source terms using the one-fluid formulation, we enable the simultaneous capturing of multi-phase hydrodynamics, mass transfer and reactions. In tandem with the numerical simulations, we also perform mass transfer analysis of multiphase flows based on the penetration theory and a two-stage theory, which further examines the mechanism of mixing enhancement in multiphase flow, and reveals a two-fold increase in mass transfer coefficients in the microreactors compared to conventional multiphase contactors. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R26.00007: Linear stability analysis and direct numerical simulation of two layer channel flow Kirti Sahu, Rama Govindarajan, Manoj Tripathi We study the stability of two-fluid flow through a plane channel at Reynolds numbers of a hundred to a thousand. The two fluids have the same density but different viscosities. The fluids, when miscible, are separated from each other by a mixed layer of small but finite thickness, across which viscosity changes from that of one fluid to that of the other. When immiscible, the interface is sharp. Our study spans a range of Schmidt numbers, viscosity ratios and location and thickness of the mixed layer. Our two-dimensional linear stability results predict well the behaviour displayed by our three-dimensional direct numerical simulations at early times. In both linear and non-linear regimes, the miscible flow is more unstable than the corresponding immiscible one, and the miscible flow breaks spanwise symmetry more readily to go into three-dimensionality. We show that the miscible flow over our range of parameters is always significantly more unstable than the corresponding immiscible case. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R26.00008: Flow of two immiscible fluids in a periodically constricted tube: Transitions to stratified, segmented, churn, spray or segregated flow John Tsamopoulos, Dimitris Fraggedakis, Yiannis Dimakopoulos We study the flow of two immiscible, Newtonian fluids in a periodically constricted tube driven by a constant pressure gradient. Our Volume-of-Fluid algorithm is used to solve the governing equations. First the code is validated by comparing its predictions to previously reported results for stratified and pulsing flow. Then it is used to capture accurately all the significant topological changes that take place. Initially, the fluids have a core-annular arrangement, which is found to either remain the same or change to a different arrangement depending on the fluid properties, the pressure driving the flow or the flow geometry. The flow-patterns that appear are the core-annular, segmented, churn, spray and segregated flow. The predicted scalings near pinching of the core fluid concur with similarity predictions and earlier numerical results (Cohen et al. (1999)). Flow-pattern maps are constructed in terms of the Reynolds and Weber numbers. Our results provide deeper insights in the mechanism of the pattern transitions and are in agreement with previous studies on core-annular flow (Kouris {\&} Tsamopoulos (2001 {\&} 2002)), segmented flow (Lac {\&} Sherwood (2009)) and churn flow (Bai et al. (1992)). [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R26.00009: Reduced order modelling of counter-current two-layer flows Gianluca Lavalle, Mathieu Lucquiaud, Prashant Valluri The dynamics of two-layer flows has a great impact on absorption units of carbon-capture retrofits, since the wavy interface plays a crucial role on the transfer between the two fluids. Studying those flows by a direct numerical simulation (DNS) strategy results in a high computational cost requiring parallel computation. As an alternative approach, we present a reduced order model: the liquid film is computed with depth-integrated equations, and the coupling with the top phase is obtained by means of the Arbitrary Lagrangian-Eulerian (ALE) technique, according to which the grid follows the interface position. We study counter-current two-layer channel flows with a moderate density ratio, focusing on loading and flooding regimes, whose complete description is a central issue for many chemical applications. Also, we investigate the influence of flow rate and pressure gradient on the interface dynamics. Speed and growth rate of linear waves match with the Orr-Sommerfeld theory and our Level-Set DNS, and non-linear wave profiles agree with DNS. Finally, our model is tested with complex gas velocity profiles of cross-flow absorbers. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R26.00010: Two-Phase Flow Hydrodynamics in Superhydrophobic Channels Kimberly Stevens, Julie Crockett, Daniel Maynes, Brian Iverson Superhydrophobic surfaces promote drop-wise condensation and droplet removal leading to the potential for increased thermal transport. Accordingly, great interest exists in using superhydrophobic surfaces in flow condensing environments, such as power generation and desalination. Adiabatic air-water mixtures were used to gain insight into the effect of hydrophobicity on two-phase flows and the hydrodynamics present in flow condensation. Pressure drop and onset of various flow regimes in hydrophilic, hydrophobic, and superhydrophobic mini (0.5 x 10 mm) channels were explored. Data for air/water mixtures with superficial Reynolds numbers from 20-200 and 250-1800, respectively, were obtained. Agreement between experimentally obtained pressure drops and correlations in literature for the conventional smooth control surfaces was better than 20 percent. Transitions between flow regimes for the hydrophobic and hydrophilic channels were similar to commonly recognized flow types. However, the superhydrophobic channel demonstrated significantly different flow regime behavior from conventional surfaces including a different shape of the air slugs, as discussed in the presentation. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R26.00011: Experimental results and a self-consistent model of evaporation and high heat flux extraction by evaporating flow in a micro-grooved blade Reza Monazami, Mehdi Saadat, Jianzhong Zhu, Hossein Haj-Hariri The problem of evaporation from a vertical micro-grooved blade heated from above is investigated. The required superheat to handle the incoming flux is calculated using the results of the study by Monazami and Haj-Hariri (2012). The relation between the applied heat flux, dry-out length and the maximum equilibrium temperature for several geometries and working fluids are studied. Furthermore, a computational study of the evaporating meniscus is conducted to evaluate the evaporation rates and dissipated heat flux at the liquid-vapor interface. The computational study accounts for the flow and heat transfer in both liquid and vapor phases. The results of this study indicate that the micro-grooved structure can dissipate heat fluxes as high as 10MW/m2 for superheats as low as 5 degrees Kelvin. Experiments are conducted to verify the computational and analytical results. The findings of this work are applicable to the design of thermal management systems for high heat flux applications. {\em Ref:} Monazami, R. and Haj-Hariri, H. A mathematically-consistent formulation for evaporation of menisci in microchannels. American Physical Society, 65th Annual DFD Meeting, San Diego, CA, Nov 18–20, 2012. [Preview Abstract] |
Tuesday, November 24, 2015 3:13PM - 3:26PM |
R26.00012: The lifetime of evaporating dense sprays Alois de Rivas, Emmanuel Villermaux We study the processes by which a set of nearby liquid droplets (a spray) evaporates in a gas phase whose relative humidity (vapor concentration) is controlled at will. A dense spray of micron-sized water droplets is formed in air by a pneumatic atomizer and conveyed through a nozzle in a closed chamber whose vapor concentration has been pre-set to a controlled value. The resulting plume extension depends on the relative humidity of the diluting medium. When the spray plume is straight and laminar, droplets evaporate at its edge where the vapor is saturated, and diffuses through a boundary layer developing around the plume. We quantify the shape and length of the plume as a function of the injecting, vapor diffusion, thermodynamic and environment parameters. For higher injection Reynolds numbers, standard shear instabilities distort the plume into stretched lamellae, thus enhancing the diffusion of vapor from their boundary towards the diluting medium. These lamellae vanish in a finite time depending on the intensity of the stretching, and relative humidity of the environment, with a lifetime diverging close to the equilibrium limit, when the plume develops in an medium saturated in vapor. The dependences are described quantitatively. [Preview Abstract] |
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