Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L3: Multiphase Flows VI |
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Chair: Ivan Zadrazil, Imperial College London Room: 325 |
Monday, November 25, 2013 3:35PM - 3:48PM |
L3.00001: Numerical and experimental study of disturbance wave development in vertical two-phase annular flow Geoffrey Hewitt, Junfeng Yang, Yujie Zhao, Christos Markides, Omar Matar The annular flow regime is characterized by the presence of a thin, wavy liquid film driven along the wall by the shear stress exerted by the gas phase. Under certain liquid film Reynolds numbers, large disturbance waves are observed to traverse the interface, whose length is typically on the order of 20 mm and whose height is typically on the order of 5 times the thickness of the thin (substrate) layer between the waves. Experimental wok has been conducted to study the disturbance wave onset by probing the local film thickness for different Reynolds numbers. It is observed the disturbance waves grow gradually from wavy initiation and form the ring-like structure. To predict the wavy flow field observed in the experiment, 3D CFD simulations are performed using different low Reynolds number turbulence models and Large Eddy Simulation. Modeling results confirm that there is recirculation within the waves, and that they as a packet of turbulence traveling over a laminar substrate film. We also predict the coalescence and the break-up of waves leading to liquid droplet entrainment into the gas core. [Preview Abstract] |
Monday, November 25, 2013 3:48PM - 4:01PM |
L3.00002: The effect of surfactant on stratified and stratifying gas-liquid flows Baptiste Heiles, Ivan Zadrazil, Omar Matar We consider the dynamics of a stratified/stratifying gas-liquid flow in horizontal tubes. This flow regime is characterised by the thin liquid films that drain under gravity along the pipe interior, forming a pool at the bottom of the tube, and the formation of large-amplitude waves at the gas-liquid interface. This regime is also accompanied by the detachment of droplets from the interface and their entrainment into the gas phase. We carry out an experimental study involving axial- and radial-view photography of the flow, in the presence and absence of surfactant. We show that the effect of surfactant is to reduce significantly the average diameter of the entrained droplets, through a tip-streaming mechanism. We also highlight the influence of surfactant on the characteristics of the interfacial waves, and the pressure gradient that drives the flow. [Preview Abstract] |
Monday, November 25, 2013 4:01PM - 4:14PM |
L3.00003: Quantitative consideration of flow structures (bubble swarms and liquid motion) and dissolved CO2 concentration transportation, in a bubbly flow Daisuke Shinohara, Takayuki Saito The objective of the present study is to clarify the relationship between large scale flow structures (: bubble swarm and liquid motion) and dissolved CO2 concentration transportation, in a large-diameter bubble column. For this specific purpose, the time-series void fractions, dissolved CO2 concentration and liquid-phase-velocities were simultaneously measured by using a photoelectric optical fiber probe (POFP) and Laser Doppler Velocimetry. The POFP was newly developed in order to simultaneously measure bubble characteristics and dissolved CO2 concentration. We calculated the spatial scale of the bubble swarms and liquid motion based on the thinking of the integral length scale. The spatial scale of the bubble swarms and liquid motion was large in the bottom zone. Moreover, the size of this spatial scale changed with time; i.e. the flow structures changed with time in the bottom zone. The characteristics of the flow structures in the bottom zone faded out towards the upper zone of the column. The cross-correlation coefficients of dissolved CO2 concentration were calculated at several zones by height. As a result, the relationship between the flow structures and dissolved CO2 concentration transportation was found out. [Preview Abstract] |
Monday, November 25, 2013 4:14PM - 4:27PM |
L3.00004: The Annular Two-phase Flow on Rod Bundle: The Effects of Spacers Tomoaki Kunugi, Son Pham, Zensaku Kawara, Takehiko Yokomine The annular two-phase flow on rod bundle keeps an important role in many heat exchange systems but our knowledge about it, especially the interaction between the liquid film flowing on the rods' surfaces and the spacers is very limited. This study is aimed to the investigation of how the spacer affects the disturbance waves of the flow in a 3x3 simulating BWR fuel rod bundle test section. Firstly, the characteristics of the disturbance waves at both upstream and downstream locations of the spacer were obtained by using reflected light arrangement with a high speed camera Phantom V7.1 (Vision Research Inc.) and a Nikon macro lens 105mm f/2.8. The data showed that the parameters such as frequency and circumferential coherence of the disturbance waves are strongly modified when they go through the spacer. Then, the observations at the locations right before and after the spacer were performed by using the back light arrangement with the same high speed camera and a Cassegrain optical system (Seika Cooperation). The obtained images at micro-scale of time and space provided the descriptions of the wavy interface behaviors right before and after the spacer as well as different droplets creation processes caused by the presence of this spacer. [Preview Abstract] |
Monday, November 25, 2013 4:27PM - 4:40PM |
L3.00005: Two-Phase Lattice Boltzmann Modeling of Boiling Phenomena Mahmood Mohammadi Shad, Taehun Lee, Masahiro Kawaji Modern advanced technologies such as electronics cooling need large heat removal from surfaces. Nucleate boiling phenomena provides sufficient cooling for these purposes because of large value of latent heat stored in the liquid. A modified multiphase lattice Boltzmann equation model is developed for liquid-vapor phase change phenomena. The volumetric mass flow rate at the interface due to phase change is included in the non-zero value of divergence of velocity. The evolution equation for hydrodynamic pressure is used to force the incompressibility in the bulk regions and the compressibility in the interfacial region. The one-dimensional Stefan problem with analytical solution is used to validate the proposed model and the two-dimensional nucleate boiling on a flat surface is simulated as the main case study. [Preview Abstract] |
Monday, November 25, 2013 4:40PM - 4:53PM |
L3.00006: Slug front gas entrainment in gas-liquid two-phase horizontal flow using hi-speed slug-tracking Ivan Zadrazil, Omar Matar, Christos Markides A gas-liquid flow regime where liquid-continuous regions travel at high speeds (i.e. slugs) through a pipe separated by regions of stratified flow (i.e. elongated bubbles) is referred to as a ``slug flow.'' This regime is characterised by the turbulent entrainment of gas into the slug front body. We use a high-speed camera mounted on a moving robotic linear rail to track the formation of naturally occurring slugs over 150 pipe diameters. We show that the dynamics of the slugs become progressively more complex with increasing liquid and gas Reynolds numbers. Based on the slug- tracking visualization we present, over a range of conditions: (i) phenomenological observations of the formation and development of slugs, and (ii) statistical data on the slug velocity and gas entrainment rate into the slug body. [Preview Abstract] |
Monday, November 25, 2013 4:53PM - 5:06PM |
L3.00007: Using DNS Data for Modeling of Bubbly Flows Gretar Tryggvason, Jiacai Lu Direct numerical simulations (DNS) of bubbly flows in vertical channels have lead to significant insight into the structure and dynamics of the flow. However, for applications to industrial systems the range of scales is sufficiently large so that DNS will remain impractical for the foreseeable future. Furthermore, there are indications that the dynamics at the smallest scales is sufficiently universal so that it should not be necessary to recomputed those scales to accurately predict the large-scale motion. Thus, models where the large-scale motion is computed and the unresolved scales are modeled will continue to be of importance. Here, we report two efforts to generate data that can be used to help modeling. In the first study we have computed the lift and drag force on a single bubble in shear flow under a variety of conditions, focusing on both the effect of the deformability of the bubble as well as the changes in drag and lift as the bubbles are placed close to a wall. In the other study we examine the transient dynamics of a large number of bubbles of different sizes, initially placed in a turbulent upflow in a vertical channel, and use the fully resolved DNS date to compute the various quantities that are generally unresolved in a model of the large-scale flow. [Preview Abstract] |
Monday, November 25, 2013 5:06PM - 5:19PM |
L3.00008: Two-phase viscous flows in channels with chemically patterned walls Vladimir Ajaev, Elizaveta Gatapova, Oleg A. Kabov Recent experimental studies of two-phase channel flows past chemically patterned surface showed that the bubbles of gas phase in the liquid tend to accumulate in the regions of lower wettability. We investigate how the presence of such bubbles affects the flow, in particular the viscous resistance at a given imposed pressure gradient. Two regimes are considered. First, we study the limit of relatively low concentration of bubbles, obtaining expressions for effective slip past a hydrophobic stripe partially covered by bubbles. Then, we consider the regime when bubbles merge, leading to complete coverage of hydrophobic region by the gas phase. In the latter case, the viscous flow is affected by a competition between the slippage effect at the gas-liquid interface and the reduction of the channel flow cross-section. The shape of the gas-liquid interface is found from solving the coupled problems for the flows in two phases. [Preview Abstract] |
Monday, November 25, 2013 5:19PM - 5:32PM |
L3.00009: A computational model for large eddy simulation of dilute bubbly turbulent flows Mohammad Hajit, Fotis Sotiropoulos A mathematical formulation of filtered equations for two phase bubbly flows based on two-fluid method is presented. To remove high frequencies (noise), we extracted the filtered form of the equations in curvilinear coordinates, converting the microscopic governing equations to macroscopic equations via spatial averaging of solution variables. The set of equations describing the hydrodynamics in a gas-liquid system can be solved effectively if the solution procedure is decoupled so that an efficient iterative scheme can be employed. We propose a formulation for dilute bubbly flows in which the equations are converted to a loosely-coupled form. The resulting mathematical model is based on five distinct sets of equations, namely mixture momentum balance, pressure Poisson equation, Boyle's law and momentum and mass balances of gas phase. This mathematical formulation provides an efficient numerical procedure for two-way coupling of bubbly flows at low gas holdups. The subgrid-scale modeling is based on dynamic procedure of Germano for both phases. The formulation is validated for a fully turbulent bubble column test by comparing to available experimental results. [Preview Abstract] |
Monday, November 25, 2013 5:32PM - 5:45PM |
L3.00010: Numerical simulation of cavitating channel flows including non-condensable gases effects Michele Battistoni, Sibendu Som, Douglas E. Longman Fuel injectors often feature cavitation because of large pressure gradients which in some regions lead to extremely low pressure levels. Numerical results are assessed against quantitative high resolution experimental data collected at Argonne National Laboratory using synchrotron x-ray radiography on real-size fuel nozzles. Simulation are performed on structured embedded grids using finite volume method and second-order discretization schemes in space and time. A single fluid homogeneous mixture model is compared to a multi-fluid non-homogeneous model. Two mass transfer models for predicting cavitation are also studied. RANS and LES cases are presented. The presence of dissolved gases in the multi-phase flow is addressed and their effect has been accounted for by running compressible three-phase flow simulations. The study highlights the importance of accounting for dissolved gases in the liquid, since some void formations, which could be attributed to cavitation, are actually due to non-condensable gas expansion. A discussion about the effect of turbulent pressure fluctuations on cavitation inception is also presented. [Preview Abstract] |
Monday, November 25, 2013 5:45PM - 5:58PM |
L3.00011: Wall drag modification by large droplets in turbulent channel flow Luca Scarbolo, Alfredo Soldati Object of this work is to examine the influence of large deformable droplets on wall-bounded turbulence. To this aim we study the behavior of a swarm of droplets with the same density and viscosity of the surrounding fluid in turbulent channel flow. We use direct numerical simulations of turbulence coupled with a phase field model for the interface tracking. A wide range of Weber numbers (ratio between inertia and surface tension) is explored for shear Reynolds number $Re_{\tau}=150$. To quantify surface tension effects on the flow the wall shear stress, the average droplet deformability and the turbulent kinetic energy budgets will be analyzed. [Preview Abstract] |
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