Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session G13: Multiphase Flow III: Experiments |
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Chair: Alberto Aliseda, University of Washington Room: 316 |
Monday, November 21, 2011 8:00AM - 8:13AM |
G13.00001: The Breakup Mechanism and the Spray Pulsation Behavior of a Three-Stream Atomizer Chin Hei Ng, Anne Dord, Alberto Aliseda In many processes of industrial importance, such as gasification, the liquid to gas mass ratio injected at the atomizer exceeds the limit of conventional two-fluid coaxial atomizers. To maximize the shear rate between the atomization gas and the liquid while maintaining a large contact area, a secondary gas stream is added at the centerline of the spray, interior to the liquid flow, which is annular in this configuration. This cylindrical gas jet has low momentum and does not contribute to the breakup process, which is still dominated by the high shear between the concentric annular liquid flow and the high momentum gas stream. The presence of two independently controlled gas streams leads to the appearance of a hydrodynamic instability that manifests itself in pulsating liquid flow rates and droplet sizes. We study the dependency of the atomization process on the relative flow rates of the three streams. We measure the size distribution, droplet number density and total liquid volumetric flow rate as a function of time, for realistic Weber and Ohnesorge numbers. Analysis of the temporal evolution of these physical variables reveals the dominant frequency of the instability and its effect on the breakup and dispersion of droplets in the spray. We present flow visualization and Phase Doppler Particle Analyzer results that provide insight into the behavior of this complex coaxial shear flow. [Preview Abstract] |
Monday, November 21, 2011 8:13AM - 8:26AM |
G13.00002: Flash atomization in splash plate nozzles Reza Karami, Nasser Ashgriz, Honghi Tran An experimental study was conducted to determine the effect of mass flow rate and temperature on characteristics of liquid sheets formed by splash plate nozzles. Two different splash plate nozzle sizes were used at five flow rates and 6 temperatures. The splash plate angel of the nozzles was 55 degrees and the working liquid was water. Direct visualization via photography was performed to investigate the sheet breakup process. Reynolds number and Jacob number were used to characterize the sheet formation and breakup regime. Four different sheet instabilities and breakup mechanisms can occur depending on the temperature and mass flow rate of the nozzle. The results were compared to the results obtained from other experiments using different liquid and nozzle size. The characterization can help to predict the sheet breakup mechanism at any condition. [Preview Abstract] |
Monday, November 21, 2011 8:26AM - 8:39AM |
G13.00003: Classification of secondary atomization mechanisms Nicolas Rimbert In the 20$^{\rm th}$ century both drop towers and shock tubes have been used to study the breakup of an accelerated drop in an air strream. This led Faeth and colleagues to synthesize the different breakup modes observed in a chart involving the Weber \textit{We} and the Ohnesorge \textit{Oh} number of the droplet (\textit{We} is the ratio between kinetic energy and surface energy of the droplet whereas \textit{Oh} governs the damping rate of its surface oscillations). The most important drop breakup mechanisms are for increasing values of \textit{We}: the bag breakup, the shear breakup and the so-called catastrophic breakup. Rayleigh-Taylor instability (RTI) is one of the main explanations and it has recently elucidated some experimental results with success, first thanks to accurate measures of the drop acceleration and then by estimating it through use of drag coefficients. The point is that the droplet firstly deforms into a spheroid which increases its drag coefficient and eventually accelerates the growth of RTI. Unlike most linearized theory, The Droplet Deformation and Breakup (DDB) model makes use of a variational technique to obtain the non linear evolution equation of the semi-axis of the spheroid. It will be shown how to combine both the DDB theory and either the classical RTI or a viscous extension. We will firstly show how the different domain can be theoretically derived. Then the classical hypothesis that the deformation mechanism is much quicker than the growth of the RTI will be proved invalid for high \textit{We}. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G13.00004: PTV implementation on two-phase flow in a forced impinging jet Rahul Mulinti, Kenneth Kiger Two-phase flow experiments have been conducted to predict particle suspension and sedimentation within coupled particle-laden flows relevant to rotorcraft brownout conditions. A hybrid PIV/PTV technique has been implemented to improve the performance in high concentration regions, while still retaining the flexibility inherent to PTV to resolve multi-valued velocity displacements within a given interrogation region. These processing tools have been optimized and their reliability has been validated using synthetic particle images in a prescribed Taylor-Green vortex flow model. The parametric space of investigation included particle image density, Stokes number and image delay times. Experiments have been conducted to study the interaction of a mobile sediment bed with characteristic flow structures similar to those within a rotor wake. The mobilization conditions and wall-normal flux of particulates by the vortex-wall interaction will be reported for different particle size classes, and are correlated to the local vortex conditions such as vortex decay and its subsequent three dimensionalization. The effect of turbulent coupling between the particle and fluid momentum, as based on a point-particle drag law valid for dilute concentrations of particles has been examined. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G13.00005: Cavitation and bubble cloud dynamics in a high-intensity focused ultrasonic field Yuan Lu, Joseph Katz, Andrea Prosperetti We focus a high power (200 W max, 500 kHz) ultrasonic beam to generate cavitation in quiescent water, and observe the process using high-speed holography. The intense pressure fluctuations cause corresponding variations in water density and refractive index, generating diffraction patterns that are evident in the holograms. These enable us to map the instantaneous spatial structures of the pressure field, and resolve its mean features, such as pressure nodes in a partial standing wave. At low powers, we observe slow growth of bubbles and their migration to the nodes due to the primary Bjerknes force. At high powers, this process persists in the periphery of the focal zone, but bubble clouds grow explosively near the center of the beam. These clouds travel in the sound propagation direction at a velocity of up to 5 m/s, but slow down briefly at the pressure nodes, while growing and shrinking. The bubbles contain mostly vapor, as the clouds vanishes in $<$100 $\mu$ s when the sound is turned off. Likely causes for these phenomena include acoustic streaming, Bjerknes forces, and attenuation of sound in the clouds. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G13.00006: Wind tunnel experiments on the interactions between turbulence and small inertial droplets Colin Bateson, Alberto Aliseda Understanding the dynamics of particles in turbulent flows is important to many engineering and environmental problems including spray atomization as well as cloud- droplet growth and precipitation. Specifically, we have studied the effect of turbulence on droplet collision-coalescence in an effort to clarify its role in the process of warm rain formation. We are exploring the hypothesis that turbulence-induced-collisions can explain the size gap between the limit of condensational growth and the onset of gravitational collisions and sedimentation. We use wind tunnel experiments to study the evolution of water droplets in homogeneous, isotropic, slowly decaying grid turbulence. We analyze the process of preferential concentration and the enhanced relative velocity of droplets in the 1-100 micron range due to their inertial interactions with the underlying turbulence. We collect droplet size and velocity data from a Phase Doppler Particle Analyzer (PDPA) to understand the influence of turbulence on the droplet collision kernel, and to quantitatively model it in terms of the Radial Distribution Function. We use high-speed visualizations to obtain two-dimensional droplet velocity fields to validate the PDPA point measurements, and to model the relative velocity distribution as a function of droplet pair spacing and Stokes number. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G13.00007: Turbulent boundary layer on a large freely moving particle suspended in high-Reynolds number isotropic turbulence Margaret Byron, Colin Meyer, Gabriele Bellani, Evan Variano We experimentally measure the flow surrounding neutrally buoyant particles freely moving in homogeneous, isotropic turbulence. Particle size is within the inertial subrange of the ambient turbulence. We measure the flow using stereoscopic particle image velocimetry which resolves 3 velocity components in a nearly 2-dimensional planar volume. We calculate ensemble average velocity statistics, conditional on the particle kinematics. Two-point statistics indicate the effect of the moving particle on the surrounding turbulent flow. We use these results to consider the impact of ambient turbulence on aquatic microorganisms, specifically those which are larger than the Kolmogorov timescale and small enough that swimming does not dominate transport. [Preview Abstract] |
Monday, November 21, 2011 9:31AM - 9:44AM |
G13.00008: Time Resolved 2D X-Ray Densitometry of a Ventilated Partial Cavity Closure Simo Makiharju, Steven Ceccio A time resolved x-ray densitometry system was developed to measure the spatial distribution of void fraction for nominally two-dimensional flows. The system can image a region of (15 cm)$^{2}$ at a frame rate of up to 4000 fps. The source was a rotating anode type normally used for cineradiography and angiography. Supplied by a 65 kW high frequency generator with a high speed starter, it could be operated at up to 433 mA at 150 kV. The imager subsystem comprised of a high speed camera coupled with a high resolution image intensifier. The range of measured void fraction can be changed to span a desired range yielding an uncertainty on the order of 1{\%} of the measurement range. The system is used to examine the void fraction field in the closure region of a ventilated partial cavity behind a backward facing step. The cavity has Reynolds number of $O$(10$^{5})$ based on the cavity length, and a non-dimensional gas flux of $Q$* = 0.0048. The bubbly flow created in the cavity wake is examined using the x-ray densitometry system, duel fiber optical probes, and high speed cinematography. The local void fraction and bubble size distributions in the cavity wake are determined, and the measurements methods are compared. [Preview Abstract] |
Monday, November 21, 2011 9:44AM - 9:57AM |
G13.00009: Dynamics of single rising bubbles in a liquid-solid system Nasim Hooshyar, Robbert F. Mudde, Peter J. Hamersma, Sankaran Sundaresan, J. Ruud van Ommen While the dynamics of single rising gas bubbles in
clear liquids has been studied extensively, the dynamics of
bubbles in liquid-solid slurries containing small particles (
$d_{s} <$ 100$\mu$m ) is not yet well understood.
\noindent We have investigated the rise characteristics of
single gas bubbles ($ |
Monday, November 21, 2011 9:57AM - 10:10AM |
G13.00010: Experimental Validation of Dam-Break Problem for Two-Phase Flow Numerical Simulation Keisuke Hara, Zensaku Kawara, Tomoaki Kunugi, Taku Nagatake In order to validate the numerical method it is important to compare the results of the numerical simulation to the experimental ones. The dam-break problem is one of the well-known validation tests for the multiphase flow computations. However, a few experimental data are available until now regarding the problem. In the validation test, it is necessary to consider the differences between the experiment and the numerical simulation. In this study, we conducted the dam-break experiments and measured the position of the water-front and the gravity center of the water region. These experimental results were compared with the numerical results obtained by the MARS (Kunugi, 2001). The water-front position has been used as a reference to validate the numerical method. When the size of the liquid column changed to double without changing the aspect ratio, the non-dimensional water-front positions were changed. On the other hand, the gravity center positions of the water region obtained by the experiments for both cases were in good agreement with each other and also with the numerical results. As a conclusion, the gravity center position must be used as the validation measure instead of the water-front position. Kunugi,T. CFD Journal, 9, (2001) pp.563-571 [Preview Abstract] |
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