Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session H11: Bubbles III |
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Chair: Marc Smith, Georgia Institute of Technology Room: 26A |
Monday, November 19, 2012 10:30AM - 10:43AM |
H11.00001: Bubble contraction in free-boundary Hele--Shaw flow with surface tension and kinetic undercooling regularisation Michael Dallaston, Scott McCue When an inviscid bubble expands into a viscous fluid in a Hele-Shaw cell, the bubble boundary is unstable, in general forming long fingers (the Saffman--Taylor instability). In order to make the problem well-posed, a regularising boundary effect must be included. The most widely studied of these are surface tension, which penalises high curvatures, and kinetic undercooling, which penalises high velocities. Both these effects act as a stabilising influence on the free boundary. Less attention has been paid to the case of contracting bubbles, which shrink to a single point (or points) in finite time. In this case, the two effects are in competition, as surface tension stabilises the boundary, while kinetic undercooling destabilises it. This leads to bifurcation behaviour in the asymptotic (near-extinction) shape of the bubble as the relative strengths of the two effects are varied. In particular, there is a critical range of parameter values for which both circular and slit-type bubbles are stable, with a third (unstable) oval-type shape also present. We discuss some numerical and analytic techniques for solving the full free boundary problem and for exploring this interesting extinction behaviour. [Preview Abstract] |
Monday, November 19, 2012 10:43AM - 10:56AM |
H11.00002: Cavitation propagation in water under tension Xavier Noblin, Yann Yip Cheung Sang, Mathieu Pellegrin Cavitation appears when pressure decreases below vapor pressure, generating vapor bubbles. It can be obtain in dynamical ways (acoustic, hydraulic) but also in quasi-static conditions. This later case is often observed in nature, in trees, or during the ejection of ferns spores [1]. We study the cavitation bubbles nucleation dynamics and its propagation in a confined microfabricated media. This later is an ordered array of microcavities made in hydrogel filled with water [2]. When the system is put into dry air, it dehydrates, water leaves the cavities and tension (negative pressure) builds in the cavities. This can be sustained up to a critical pressure (of order -20 MPa), then cavitation bubbles appear. We follow the dynamics using ultra high speed imaging. Events with several bubbles cavitating in a few microseconds could be observed along neighboring cells, showing a propagation phenomenon that we discuss.\\[4pt] [1] X. Noblin, N. O. Rojas, J. Westbrook, C. Llorens, M. Argentina, J. Dumais. The Fern Sporangium: A Unique Catapult. Science, 335, 1322, 2012.\\[0pt] [2] Tobias D. Wheeler and Abraham D. Stroock. The transpiration of water at negative pressures in a synthetic tree. Nature, 455, 208-212, 2008. [Preview Abstract] |
Monday, November 19, 2012 10:56AM - 11:09AM |
H11.00003: Vortex Cavitation on Delta-Wings Harish Ganesh, Steven Ceccio Vortex cavitation experiments were carried out on delta wing with 70 degrees sweep back at various attack angles in a re-circulating water tunnel. The flow speed was maintained at a constant value of 4 m/s and the free stream pressure was varied to observe cavitation events ranging from inception to fully attached bubbles. By changing the free stream cavitation number two types of bubbles were observed; stationery bubbles that occupied an axial position for a sufficient time and travelling bubbles that convected with the flow. Variation of the vortex properties on a delta wing facilitates the occurrence of these two bubble types. Stationery bubbles, by the virtue of their equilibrium position and shape provide information about the axial and radial gradients of the flow respectively. A simple analysis based on equilibrium of a stationery bubble is used to illustrate the relation between the vortex properties and the stationery bubble properties. Acoustic signature of these bubbles were also measured and compared. Dye injection was also used to determine the breakdown location and it was found that vortex cavitation had no influence on the breakdown location. Finally, a relationship between stationery bubbles and the vortex properties is proposed based on the measurements and analysis. [Preview Abstract] |
Monday, November 19, 2012 11:09AM - 11:22AM |
H11.00004: Enhancing cavitation with micromachined surfaces David Fernandez Rivas, Laura Stricker, Aaldert G. Zijlstra, Han Gardeniers, Detlef Lohse, Andrea Prosperetti When a silicon surface with micromachined pits submerged in a liquid is exposed to continuous ultrasound at 200 kHz, bubbles are ejected from the air filled cavities. Depending on the pressure amplitude different scenarios are observed, as the bubbles ejected from the micropits interact in complex ways with each other, and with the silicon surface. We have determined the size distribution of bubbles ejected from one, two and three pits for three different electrical power settings, and correlated them with sonochemical OH* radical production. Numerical simulations of the sonochemical conversion reaction rates were obtained using the empirical bubble size distributions and are compared with experimental results. Experimental evidence of shock wave emission from the microbubble clusters, deformed microbubble shapes, jetting and surface erosion are also presented. [Preview Abstract] |
Monday, November 19, 2012 11:22AM - 11:35AM |
H11.00005: Instability arisen on condensing vapor bubble Ichiro Ueno, Ryota Hosoya, Chungpyo Hong In the present study a special attention is paid to the growing and collapsing processes of vapor bubble injected into a subcooled pool; the authors try to extract the vapor-liquid interaction by employing a vapor generator that supplies vapor at designated flow rate to the subcooled pool instead of using a immersed heated surface to realize a vapor bubble by boiling phenomenon. This system enables ones to detect a spatio-temporal behavior of a single bubble of superheated vapor exposed to a subcooled liquid. The authors indicate the condensation rates as functions of the injection velocity of the vapor and the degree of subcooling of the pool. The authors indicate that an abrupt condensation of the injected vapor results in a fine disturbance over the vapor bubble surface before the collapse stage of the bubble. The wave number is sharply dependent on the degree of subcooling of the pool. The threshold of such a fine disturbance formation over the bubble corresponds with that the occurring condition of the maximum volume reduction rate of the vapor bubble. [Preview Abstract] |
Monday, November 19, 2012 11:35AM - 11:48AM |
H11.00006: The behavior of vapor bubbles during boiling enhanced with acoustics and open microchannels Thomas Boziuk, Marc K. Smith, Ari Glezer Boiling heat transfer on a submerged heated surface is enhanced by combining a grid of surface micromachined open channels and ultrasonic acoustic actuation to control the formation and evolution of vapor bubbles and to inhibit the instability that leads to film boiling at the critical heat flux (CHF). The microchannels provide nucleation sites for vapor bubble formation and enable the entrainment of bulk subcooled fluid to these sites for sustained evaporation. Acoustic actuation excites interfacial oscillations of the detached bubbles and leads to accelerated condensation in the bulk fluid, thereby limiting the formation of vapor columns that precede the CHF instability. The combined effects of microchannels and acoustic actuation are investigated experimentally with emphasis on bubble nucleation, growth, detachment, and condensation. It is shown that this hybrid approach leads to a significant increase in the critical heat flux, a reduction of the vapor mass above the surface, and the breakup of low-frequency vapor slug formation. A large-scale model of the microchannel grid reveals details of the flow near the nucleation site and shows that the presence of the microchannels decreases the surface superheat at a given heat flux. [Preview Abstract] |
Monday, November 19, 2012 11:48AM - 12:01PM |
H11.00007: Development of a GPU and multi-CPU accelerated non-isothermal, multiphase, incompressible Navier-Stokes solver with phase-change Christopher J. Forster, Ari Glezer, Marc K. Smith Accurate 3D boiling simulations often use excessive computational resources -- in many cases taking several weeks or months to solve. To alleviate this problem, a parallelized, multiphase fluid solver using a particle level-set (PLS) method was implemented. The PLS method offers increased accuracy in interface location tracking, the ability to capture sharp interfacial features with minimal numerical diffusion, and significantly improved mass conservation. The independent nature of the particles is amenable to parallelization using graphics processing unit (GPU) and multi-CPU implementations, since each particle can be updated simultaneously. The present work will explore the speedup provided by GPU and multi-CPU implementations and determine the effectiveness of PLS for accurately capturing sharp interfacial features. The numerical model will be validated by comparison to experimental data for vibration-induced droplet atomization. Further development will add the physics of boiling in the presence of acoustic fields. It is hoped that the resultant boiling simulations will be sufficiently improved to allow for optimization studies of various boiling configurations to be performed in a timely manner. [Preview Abstract] |
Monday, November 19, 2012 12:01PM - 12:14PM |
H11.00008: Heat transfer enhancement in turbulent thermal convection close to the boiling point: Numerical simulations R. Lakkaraju, R.J.A.M. Stevens, P. Oresta, F. Toschi, C. Sun, R. Verzicco, A. Prosperetti, D. Lohse We perform numerical simulation of turbulent Rayleigh-B\'{e}nard convection close to the boiling point of water, i.e., at 100$^{\circ}$ C and ambient pressure, in a cylinder of aspect ratio 1 for the Rayleigh number range $2\times 10^6\le Ra \le 5\times 10^9$, modeling the vapor bubbles as two-way coupled point particles. We quantified the heat transfer enhancement as function of the number of bubbles, the degree of superheating (i.e., temperature excess of the plate as compared to the temperature of the bubble) and $Ra$. Heat transport is enhanced up to 6 times for low $Ra$ and by to 2 times for high $Ra$ through the presence of bubbles. Our results are consistent with the recent experimental findings of Zhong et al. (Phys. Rev. Lett. \textbf{102}, 124501, 2009), if one considers that the vapor bubble nucleation rate increases with the super heating. [Preview Abstract] |
Monday, November 19, 2012 12:14PM - 12:27PM |
H11.00009: Numerical Investigation of Boiling Michael Sagan, Sebastien Tanguy, Catherine Colin In this work, boiling is numerically investigated, using two phase flow direct numerical simulation based on a level set / Ghost Fluid method. Nucleate boiling implies both thermal issue and multiphase dynamics issues at different scales and at different stages of bubble growth. As a result, the different phenomena are investigated separately, considering their nature and the scale at which they occur. First, boiling of a static bubble immersed in an overheated liquid is analysed. Numerical simulations have been performed at different Jakob numbers in the case of strong density discontinuity through the interface. The results show a good agreement on bubble radius evolution between the theoretical evolution and numerical simulation. After the validation of the code for the Scriven test case, interaction of a bubble with a wall is studied. A numerical method taking into account contact angle is evaluated by comparing simulations of the spreading of a liquid droplet impacting on a plate, with experimental data. Then the heat transfer near the contact line is investigated, and simulations of nucleate boiling are performed considering different contact angles values. Finally, the relevance of including a model to take into account the evaporation of the micro layer is discussed. [Preview Abstract] |
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