Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session L4: Bubbles: Nucleation, Growth, Heat Transfer and Boiling |
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Chair: Michel Versluis, University of Twente Room: 3006 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L4.00001: A high-fidelity approach towards heat transfer prediction of pool boiling Miad Yazdani, Abbas Alahyari, Thomas Radcliff A novel numerical approach is developed to simulate the multiscale problem of pool-boiling phase change with an unprecedented fidelity and cost. The particular focus is to predict the heat transfer coefficient of pool-boiling regime and its transition to critical heat flux on surfaces of arbitrary shape and roughness distribution. The large-scale of the phase change and bubble dynamics is addressed through employing off-the-shelf methods for interface tracking and interphase mass and energy transfer. The small-scale of the microlayer which forms at early stage of bubble nucleation is resolved through asymptotic approximation of the thin-film theory which provides a closed-form solution for the distribution of the micro-layer and its influence on the evaporation process. In addition, the surface roughness and its role in bubble nucleation and growth is represented based on thermodynamics of nucleation process which allows the simulation of pool boiling on any surface with known roughness and enhancement characteristics. The numerical model is validated for dynamics and hydrothermal characteristics of a single nucleated bubble on a flat surface against available literature data. In addition, the model's prediction of pool-boiling heat transfer coefficient is verified against reputable correlations for various roughness distributions and different surface alignment. Finally, the model is employed to demonstrate pool-boiling phenomenon on enhanced structures with reentrance cavities and to explore the effect of enhancement features on thermal and hydrodynamic characteristics of these surfaces. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L4.00002: Effect of surface micro-texture on bubble dynamics and boiling critical heat flux Navdeep Dhillon, Jacopo Buongiorno, Kripa Varanasi We present results of an experimental study on the effect of surface texture on the dynamics of bubble growth and departure in pool boiling of water and correlate them to the measured values of critical heat flux (CHF) on these surfaces. Although it is well known that surface roughness or micro-texture has a significant impact on macroscale boiling parameters such as boiling heat transfer coefficient (HTC) and CHF, the physics underlying these processes is not well understood. Using high speed optical and infrared (IR) imaging, we explored the mechanism of single bubble growth and departure on micro-textured surfaces fabricated using photolithography techniques. Interestingly, we observed that the introduction of the micro-texture not only completely changed bubble dynamics and boiling surface thermal characteristics but there was a clear correlation between the micro-texture parameters and the salient bubble characteristics such as the departure diameter and frequency. To explain these results, we propose a physical model based on micro-texture-induced surface microflows supplementing the conventional bubble growth and departure theory based on buoyancy and capillary pinning forces, and verify it using CHF measurements. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L4.00003: Kinetics-based phase change approach for VOF method applied to boiling flow Paolo Cifani, Bernard Geurts, Hans Kuerten Direct numerical simulations of boiling flows are performed to better understand the interaction of boiling phenomena with turbulence. The multiphase flow is simulated by solving a single set of equations for the whole flow field according to the one-fluid formulation, using a VOF interface capturing method. Interface terms, related to surface tension, interphase mass transfer and latent heat, are added at the phase boundary. The mass transfer rate across the interface is derived from kinetic theory and subsequently coupled with the continuum representation of the flow field. The numerical model was implemented in OpenFOAM and validated against 3 cases: evaporation of a spherical uniformly heated droplet, growth of a spherical bubble in a superheated liquid and two dimensional film boiling. The computational model will be used to investigate the change in turbulence intensity in a fully developed channel flow due to interaction with boiling heat and mass transfer. In particular, we will focus on the influence of the vapor bubble volume fraction on enhancing heat and mass transfer. Furthermore, we will investigate kinetic energy spectra in order to identify the dynamics associated with the wakes of vapor bubbles. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L4.00004: Growth and collapse of a single nucleated bubble in a subcooled flow Maryam Medghalchi, Nasser Ashgriz Subcooled flow on heated surfaces may result in the condensation and collapse of nucleated bubbles. A numerical study accounting for the heat and mass transfer at the gas-liquid interfaces of a nucleated bubble is performed. The model considers (i) the microlayer between the bubble and the heated surfaces, where the liquid is trapped and its temperature rises above the saturation temperature; and (ii) bubble evaporation and condensation using Hertz-Knudsen equation. The results show that the thermal boundary layer inside the bubble grows faster than that inside the liquid. This is mainly due to the buoyancy induced circulating flows inside the bubble, and different interface heat and mass transfer rates at the top and the bottom of the bubble. The calculated microlayer thicknesses are found to be less than those provided by the existing correlations. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L4.00005: Asymptotic approach in the limit of small contact angles to sessile vapor bubble growth in a superheated environment Alexey Rednikov, Nicolas Hollander, Marta Hernando Revilla, Pierre Colinet A model of nucleate pool boiling is considered, and more concretely the growth dynamics of a single spherical-cap vapor bubble on a flat superheated substrate in a large volume of an equally superheated liquid. An asymptotic scheme is developed valid in the limit of small contact angles. These are basically supposed to be the evaporation-induced ones and hence finite even in the case of a perfectly wetting liquid implied here. The consideration generally involves four regions: i) microregion, where the contact line singularities are resolved and the evaporation-induced contact angles are established, ii) Cox-Voinov region, iii) foot of the bubble, and iv) macroregion. It is only in the latter region, which remarkably appears to leading order in the form of the exterior of a sphere touching a planar surface in one point (hence a fixed geometry even for variable contact angles), that the full Navier-Stokes and heat equations are to be (numerically) resolved. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L4.00006: Visualization of bubble formation induced by femtosecond laser pulses in water/acetone on a time scale from sub-picosecond to microseconds Yuki Mizushima, Takayuki Saito Laser induced bubble formation is usually understood as a trigger pulled by a plasma formation in a bulk media. During the plasma growth, normally, bright light emission due to excitation of the energy state of the electrons in the molecules can be observed. However, femtosecond laser pulses (fs pulses) generate bubbles through a process without bright light emission. The fs pulse leads extraordinary phenomena due to their extremely higher energy density than usual laser pulses (nano- or pico-second). We think the bubble formation by fs pulses must be different from the ordinary laser-induced cavitation. In this study, a single fs pulse was focused on water and acetone in a glass cell through several types of lens. We visualized bubble formation processes from sub-picosecond to microsecond order through time-resolved visualization. We found out a strange time-series process of refraction index changes of the media irradiated by the fs pulse: the bubble nucleation, rapid growth of bubble nucleation and interesting bubble properties. Based on these results, we will discuss a relationship between those and fs pulse peak intensity, and differences in bubble formation in water and acetone. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L4.00007: Nucleation and ultrafast vaporization dynamics of laser-activated polymeric microcapsules Guilaume Lajoinie, Erik Gelderblom, Ceciel Chlon, Marcel Boehmer, Wiendelt Steenbergen, Nico de Jong, Srirang Manohar, Michel Versluis Precision control of vaporization, both in space and time, has many potential applications; however, the physical mechanisms underlying controlled boiling are not well understood. The reason is the combined microscopic length scales and ultra-short timescales associated with the initiation and subsequent dynamical behavior of the vapor bubbles formed. Here we study the nanoseconds vapor bubble dynamics of laser-heated single oil-filled microcapsules using coupled optical and acoustic detection. Pulsed laser excitation leads to vapor formation and collapse, and a simple physical model captures the observed radial dynamics and resulting acoustic pressures. Continuous wave laser excitation leads to a sequence of vaporization and condensation cycles, the result of absorbing microcapsule fragments moving in and out of the laser beam. A model incorporating thermal diffusion from the capsule shell into the oil core and surrounding water reveals the mechanisms behind the onset of vaporization. Excellent agreement is observed between the modeled dynamics and experiment. [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L4.00008: Surface wetting and bubble dynamics of dielectric fluids boiling in high electric fields Navdeep Dhillon, Christopher Love, Seyed Reza Mahmoudi, Kripa Varanasi We present results of an experimental study on the effect of high electric fields on the nature of bubble formation and departure in nucleate pool boiling of dielectric fluids. Despite some past studies looking at the application of electric fields to enhance boiling performance, the exact mechanism of interaction of these fields with the fluid/surface is not well understood. In this study, we employed high-speed optical and infrared (IR) imaging to observe changes in wetting behavior of the fluid on the solid surface and the mode of bubble formation and departure under applied electric fields. The experimental results point towards a liquid film stabilization effect of the applied electric field on the boiling surface. Both the bubble departure size and surface dry spot dynamics is visibly altered under the effect of the electric field. These effects can be attributed to the development of surface charges on the bubble microlayer adjacent to the liquid-vapor contact line, which affect the liquid receding and surface rewetting mechanisms. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L4.00009: Numerical study of heat transfer in bubbly flows in channels Saul Piedra, Jiacai Lu, Eduardo Ramos, Gretar Tryggvason The effects of bubbles on the heat transfer in channel flows is examined by direct numerical simulations (DNS), where every continuum length and time scale is resolved. Earlier simulations of bubbles in turbulent flow in vertical channels have shown that the presence of bubbles increases the Nusselt number, compared to flow without bubbles. This is the case for both nearly spherical as well as deformable bubbles, even though the flow structure is very different. Here we examine how bubbles modify the heat transfer in horizontal and sloping channels, for both laminar and turbulent flows. The results show that the bubbles generally increase the heat transfer, but the exact amount depends of the degree that the bubbles modify the structure of the flow. Preliminary efforts to use the results to aid in the development of models for the average flow are discussed and early results for more complex transient flows with bubbles of different sizes are shown. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L4.00010: GPU-based multi-resolution direct numerical simulation of multiphase flows with phase change Christopher J. Forster, Marc K. Smith Nucleate pool boiling heat transfer can be enhanced in several ways to increase the critical heat flux (CHF) and delay the transition to film boiling. Changes to the heated surface geometry using open microchannels and direct forcing of the vapor bubbles using acoustic interfacial excitation are being investigated for their effects on the CHF. The numerical simulation of boiling with these effects lends itself to multi-resolution techniques due to the multiple length and time scales present during evolution of the bubbles from initial nucleation in the microchannels to forming a bubble cloud above the heated surface. To this end, a wavelet multi-resolution boiling simulation based on a parallel GPU architecture is being developed to solve the compressible Navier-Stokes equations using a dual time stepping method with preconditioning to alleviate the stiffness problems associated with the liquid phase. Interface tracking is handled by the level-set method with a prescribed interface thickness based on the maximum amount of local grid refinement desired, which can approach the physical interface thickness. Initial cases to validate the simulation will be demonstrated, including the rising bubble test problem. [Preview Abstract] |
Monday, November 24, 2014 5:45PM - 5:58PM |
L4.00011: Vapor explosions during the impact of molten tin droplets into a liquid pool Nadia Kouraytem, Er Qiang Li, Sigurdur Thoroddsen High-speed video imaging is used to study the impact of a molten tin droplet into a liquid pool. Three different regimes have been identified as nucleation boiling, film boiling or vapor explosion. The latter generally comprises two stages; during the first stage, vapor gets entrapped into the molten tin drop and then, at a second stage, the vapor is superheated by the tin material, creating a violent expansion (explosion). It was observed that the addition of surfactant to the fluid pool could promote the explosion and make it occur at a lower temperature. Furthermore, other parameters such as the pool liquid surface tension, boiling temperature, viscosity and molten tin temperature have been varied to examine the explosion dynamics. [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L4.00012: A generation method of single bubbles of various sizes using a slitting elastic tube and acoustic pressure wave Toshiyuki Sanada, Kimihiko Abe A bubble generation method (Sanada and Abe, Rev. Sci. Instrum. 2013) using a slitting elastic tube and acoustic pressure wave in the gas phase can produce single bubbles of various sizes. In this study, we experimentally investigated the bubble generation mechanism in a slitting elastic tube. We used high-speed photography to observe the bubble generation process and slit motion in different liquids with different surface tensions. The results indicated that there was no significant difference in the slit opening time even if the amplitude of the acoustic pressure wave was changed, and the generated bubble radius was determined by the opening displacement of the slit, which was governed by the surface tension. In addition, the shape oscillation of a bubble due to surface tension promoted its detachment from an elastic tube with poor wettability. [Preview Abstract] |
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