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 G11: Bubbles IV: Growth, Heat Transfer and Boiling |
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Chair: David Brutin, Polytech Marseille/Centre de la Recherche Room: 335 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G11.00001: Electrolytic Bubble Growth on Pillared Arrays Kenneth Lee, Omer Savas In current energy research, artificial photosynthetic (AP) devices are being designed to split water and harvest hydrogen gas using sunlight. In one such design, hydrogen gas bubbles evolve on catalytic surfaces of arrayed micropillars. If these bubbles are not promptly removed from the surface, they can adversely affect gas evolution rates, water flow rates, sunlight capture, and heat management of the system - all of which deteriorate device performance. Therefore, understanding how to remove evolved gas bubbles from the pillar surfaces is crucial. Flow visualization of electrolytic bubble nucleation and detachment from the catalytic pillar surfaces has been conducted. The bubble departure diameter and lift-off frequency are extracted and compared with known correlations from boiling heat transfer. Bubble tracking indicates that bubble detachment is enhanced by local interactions with neighboring bubbles. These observations suggest how hydrogen gas bubbles can be effectively removed from pillared surfaces to prolong AP device longevity. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G11.00002: How do bubbles grow in a weakly supersaturated solution? Oscar Enriquez, Chao Sun, Detlef Lohse, Andrea Prosperetti, Devaraj van der Meer Beer, champagne and soft-drinks are water-based solutions which owe their ``bubbliness'' to a moderate degree of carbon dioxide supersaturation. Bubbles grow sequentially from nucleation sites due to solute concentration gradients and detach due to buoyancy. The leading mass transfer mechanism is diffusion, but the advection caused by the moving surface also plays an important role. Now, what happens at the limit of very weak supersaturation? We take an experimental look at CO$_2$ bubbles growing in water under such a condition. Nucleation sites are provided by hydrophobic micro-cavities on a silicon chip, therefore controlling the number and position of bubbles. Although advection is negligible, measured growth rates for an isolated bubble differ noticeably from a purely diffusive theoretical solution. We can explain the differences as effects of the concentration boundary layer around the bubble. Initially, its interaction with the surface on which the bubble grows slows the process down. Later on, the growth rate is enhanced by buoyancy effects caused by the depletion of the solute in the surroundings of the bubble. When neighboring bubbles are brought into play they interact through their boundary layers, further slowing down their growth rates. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G11.00003: Mixing and drift by air bubbles crossing an interface of a stratified medium L. Diaz-Damacillo, A. Ruiz-Angulo, R. Zenit The dynamics of a single air bubble crossing the horizontal interface separating two different-density stagnant Newtonian miscible liquids are studied experimentally. Both liquids were water-glycerin mixtures. The bottom fluid was saturated with salt to make it denser the upper one. The size of the bubbles was widely varied to obtain a wide range of shapes from spherical to toroidal. The Planar Laser-Induced Flourescence (PLIF) technique was used to quantify the drift volume across the interface. When the bubble crosses the interface, it drags some amount of the heavy fluid into the upper lighter fluid. For small bubbles, the drift volume returns to the bottom liquid after sometime with negligible mixing. The dragged volume is inversely proportional to the bubble Reynolds number. For larger bubbles, the drift volume becomes unstable, which leads to mixing. Considering a balance of inertial, viscous and buoyant forces, we propose a dimensionless number to identify the onset of instability. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G11.00004: Effects of wakes and surface contamination on instantaneous mass transfer from a bubble to the surrounding liquid Yoshinori Nobata, Takayuki Saito The effects of wakes and surface contamination on instantaneous mass transfer from a zigzagging bubble to the surrounding liquid are discussed. A zigzagging CO2 bubble was captured with high spatial resolution by a pair of high-speed cameras, and the instantaneous changes in volume and surface area of the bubble were obtained from the images by our originally developed method. The instantaneous mass transfer from the bubble to surfactant-contaminated water was reduced, compared with that of the same-size bubble in pure water. In the surfactant-contaminated water, the surface tension of the bubble-liquid interface decreased. In association with the bubble ascent, the adsorbed surfactant was accumulated on the rear of the bubble. This non-uniform distribution of the surfactant on the interface caused the Marangoni flow on the bubble surface. As a result, the Marangoni effect attenuated the bubble motion and the vortex shedding. On the other hand, in pure water the shedding of hairpin vortexes from the bubble rear was very active. The hairpin vortexes transported the CO2-rich liquid away from the bubble. These indicate that the vortex shedding promotes convective transportation of CO2 and induced the enhancement of the instantaneous mass transfer. [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G11.00005: Using Improved Equation of State to Model Simultaneous Nucleation and Bubble Growth in Thermoplastic Foams Irfan Khan, Stephane Costeux, David Adrian, Diego Cristancho Due to environmental regulations carbon-dioxide (CO$_{2}$) is increasingly being used to replace traditional blowing agents in thermoplastic foams. CO$_{2}$ is dissolved in the polymer matrix under supercritical conditions. In order to predict the effect of process parameters on foam properties using numerical modeling, the P-V-T relationship of the blowing agents should accurately be represented at the supercritical state. Previous studies in the area of foam modeling have all used ideal gas equation of state to predict the behavior of the blowing agent. In this work the Peng-Robinson equation of state is being used to model the blowing agent during its diffusion into the growing bubble. The model is based on the popular ``Influence Volume Approach,'' which assumes a growing boundary layer with depleted blowing agent surrounds each bubble. Classical nucleation theory is used to predict the rate of nucleation of bubbles. By solving the mass balance, momentum balance and species conservation equations for each bubble, the model is capable of predicting average bubble size, bubble size distribution and bulk porosity. The effect of the improved model on the bubble growth and foam properties are discussed. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G11.00006: Asymmetric interface temperature during vapor bubble growth Antoine Diana, Martin Castillo, Ted Steinberg, David Brutin We investigate the nucleation, growth, and detachment of single vapor bubbles at the interface microscale. Shear flow is used to investigate pool and convective boiling situations using visible and infrared visualizations. We determine a threshold Reynolds number for the onset of asymmetric interfacial temperatures. Below this threshold, bubble growth is geometrically and thermally symmetric, while above, bubbles no longer grow thermally symmetrically. This is explained by the dominance of convective heat transfer removal over viscous effects at the bubble interface. We experimentally demonstrate asymmetric interfacial temperature profiles that should be taken into account for future bubble growth modeling. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G11.00007: Single Bubble Dynamics on Superhydrophilic Micropillar Arrays during Flow Boiling Jiansheng Feng, Siyu Chen, TieJun Zhang, Evelyn Wang Micro/nanoengineered surfaces have received recent interest for high heat flux thermal management solutions. In particular, micropillar arrays promise opportunities to enhance flow boiling performance, but an increasing understanding of the role of these structures are still needed. In this study, we used superhydrophilic micropillar arrays with well-defined geometries to investigate bubble growth and departure dynamics during boiling. These structures were individually tested in a closed-loop flow boiling setup. A combined side-view microscopy and high-speed videography technique was utilized to obtain images of bubble growth and departure. We demonstrated that by increasing the solid fraction of the microstructures, bubble departure can occur at smaller sizes and at higher frequencies comparing to that on a flat hydrophilic surface. Meanwhile, we observed that bubble sliding stage between departing from the nucleation site and detaching from the heated surface, which is present under a wide range of conditions during flow boiling on flat surfaces, was highly suppressed on some of the microstructured surfaces. In addition, we used a surface energy based model to explain the confinement effect of the liquid-vapor interface by the micropillar arrays, and to support our experimental findings that solid fraction is a key parameter dictating bubble dynamics. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G11.00008: Dual-Luminescent Imaging for Capturing Temperature Field around a Bubble Hirotaka Sakaue, Hideki Goya, Takeshi Miyazaki Dual-luminescent imaging uses two-luminescent outputs to extract the temperature information from an acquired image. This is applied to capture the temperature field around a bubble in boiling water. A laser sheet is used as an illumination source to obtain a temperature profile of the bubble. By using a fast frame-rate camera as an image acquisition unit, the time-resolved temperature information around the bubble can be captured. In the presentation, a current status of this measurement will be presented. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G11.00009: Heat transfer in turbulent bubbly flow in channels Sadegh Dabiri, Gretar Tryggvason In many applications convective heat transfer occurs in the presence of a multiphase turbulent flow such as in boilers and bubble column reactors. Turbulence in channel and pipe flows significantly increases the heat transfer rate. Here we examine the effect of turbulent bubbly flows on the heat transfer inside a vertical channel with uniform heat flux on the walls and compare it with the heat transfer in single phase flow. Both bubbles and the turbulence are fully resolved through Direct Numerical Simulation. The distribution of the bubbles in the channel is affected by the deformability of the bubbles. A wall-peaked distribution is observed for nearly spherical bubbles and a core-peaked distribution is observed for deformable bubbles. This change in distribution of the bubbles significantly affects the flow rate in the channel and the heat transfer rate as well. The results of heat transfer for different flow configurations are presented and compared to the heat transfer in a single phase channel flow. [Preview Abstract] |
Monday, November 25, 2013 9:57AM - 10:10AM |
G11.00010: Multi-Scale Acoustic Actuation of Vapor Bubbles for Pool Boiling Enhancement Thomas R. Boziuk, Marc K. Smith, Ari Glezer The effect of multi-scale acoustic actuation on heat transfer from a submerged structured surface in pool boiling is investigated experimentally. Actuation over a range of frequencies affects the growth, detachment, advection, and condensation of vapor bubbles and results in significant favorable changes to the boiling curve and critical heat flux. Heat transfer is also improved with a structured heated surface containing fixed but separate nucleation sites designed to limit the merger of vapor bubbles above the surface and to enable an efficient inflow of makeup liquid to the evaporation sites. However, the geometry of the surface between the evaporation sites can impede the effectiveness of the acoustic actuation within certain bandwidths related to the scale of the geometry. It is shown that a multi-scale approach combining low frequency (kHz-range) actuation, for bubble interface excitation and enhanced condensation, with high frequency (MHz-range) actuation, for induced interfacial forces near the contact line, yields effective control of the evolution of vapor bubbles over a broad range of scales and surface geometries and leads to a significant improvement in boiling heat transfer. [Preview Abstract] |
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