Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H24: Bubbles: Growth, Heat Transfer and Boiling I |
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Chair: Prashant Valluri, University of Edinburgh Room: 606 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H24.00001: Viscous growth and rebound of confined bubbles Sebastien Michelin, Giacomo Gallino, Francois Gallaire, Eric Lauga In many applications (e.g. bubble-powered micro-rockets), microscopic gas bubbles nucleate and grow in close proximity with a rigid bounding surface. This has profound hydrodynamic consequences, as the bubble is geometrically constrained and must translate away from the wall as its radius increases, in order to maintain mechanical equilibrium with the non uniform hydrodynamic stresses applied on its surface. \\ This dynamic process is analyzed theoretically for a spherical inflating gas bubble in a viscous fluid, focusing specifically on the drainage dynamics of the thin lubricating film separating the bubble from the wall and for bubble surfaces of different physical nature, ranging from ``clean'' (i.e. stress-free) to ``polluted'' or rigid (i.e. slip-free) interfaces. Different bubble surface conditions lead to fundamentally different behaviors: bubbles may thus drain the lubrication film monotonically or bounce off the surface before eventually draining the film. A final universal regime (i.e. for all bubble surface conditions) is finally identified. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H24.00002: Bubble Coalescence at the Free Surface Daniel Shaw, Luc Deike Bubble coalescence at a free surface occurs in our daily lives at the surface of drinks as well as at the surface of the ocean. While coalescence inside water has been largely investigated, the studies at the free surface remain scarce. In this talk, we study experimentally the coalescence of two bubbles at a free surface. Three primary regimes are identified. During an `attraction' phase, the bubbles accelerate together due to the capillary distortion of the free-surface. As the bubbles draw near, the resistance provided by the fluid between them increases and slows the advancing bubbles. This `drainage' regime begins when the relative velocity of the bubbles start decreasing and ends when the film is ruptured. `Confluence,' the third regime, is dominated by the rapid expansion of the neck separating the newly united air pockets. A balance of capillary, inertial, and viscous forces determine the dynamics of the newly-formed interface. Unlike coalescence in a bulk outer-fluid, the asymmetry created by the presence of the free surface alters previous models and presents new challenges for both measuring and modeling coalescence. [Preview Abstract] |
(Author Not Attending)
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H24.00003: Effect of Non-Uniform Circumferential Heat Fluxes and Orientation on Microchannel Flow Boiling -- A Numerical Investigation Marius Vermaak, Mohammad Moghimi, Josua Meyer, Khellil Sefiane, Prashant Valluri Flow boiling in microchannels exhibits incredibly high heat transfer characteristics, which could be revolutionary for multiple industries. However, the underlying physical phenomena that cause these characteristics are not well understood, especially the 3D effects of flow boiling in non-circular channels and the influence of confinement on bubbles. In this study, single bubble growth is considered in a high aspect ratio microchannel, hydraulic diameter 909 $\mu $m and aspect ratio 10, at varying gravitational orientations with only 1 face heated. While common microchannel theory neglects gravitational effects, at large aspect ratios gravity affects the hydrothermal characteristics of the flow. In particular, if microchannels are rotated along the axial direction. Bubble behavior after nucleation and its effect on heat transfer is investigated until slug formation. This investigation found that the heat transfer characteristics of the bottom heated case are the highest. In addition, the thermofluidic interaction between two sequential slugs moving down a microchannel is also presented. The results of this study have been validated against past numerical and current experimental work, some of which is being performed at the University of Edinburgh. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H24.00004: Nucleation of Plasmonic Bubbles in Binary Liquids Marvin Detert, Binglin Zeng, Yuliang Wang, Harold J. W. Zandvliet, Detlef Lohse When a noble metal, plasmonic nanoparticle is immersed in a liquid and irradiated with a laser of resonant frequency, it can heat rapidly and a vapor bubble can be nucleated. Bubbles generated this way are called plasmonic bubbles and have garnered a lot of attention due to their variety of applications. We want to disentangle the effect of various control parameters, such as the boiling temperature, the heat conductivity, the latent heat of vaporization etc., by measurements via ultra-highspeed imaging. A perfect candidate for these measurements are binary liquids, because their parameters can be tuned by their composition. We show both experimentally and theoretically that the time of bubble nucleation is determined by the total amount of dissolved gas. In contrast, the maximum volume of the bubble is governed by the energy needed for vaporization. Consequently, the bubble's nucleation time and its maximal size can be tuned by varying the corresponding liquid parameters. We envisage that our findings will not only have important consequences for current applications, but might also result in new applications. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H24.00005: Nucleation and Growth of a Nanobubble on Rough Surfaces Shantanu Maheshwari, Cor Van Kruijsdijk, Suchismita Sanyal, Albert Harvey We study the nucleation and growth of a nanobubble on rough surfaces by using molecular dynamics simulations. A nanobubble nucleates and grows by the virtue of a heterogenous surface reaction which results in the production of gas molecules near the surface. We study the role of surface roughness in the nucleation and growth behaviour of a nanobubble. We perform simulations at various reaction rates and surface morphology, and quantified the growth dynamics of a nanobubble. Our simulations show that after the onset of nucleation, nanobubble grows rapidly with radius following $t^{1/3}$ behaviour followed by diffusive growth regime which is marked by $t^{1/2}$ growth behaviour. This growth behaviour remains independent of surface roughness and reaction rates over the range considered in this study. We also quantified the oversaturation of gas required for the nucleation of a nanobubble and demonstrated its dependence on the surface morphology. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H24.00006: Near-wall bubble expansion and jetting collapse in generalized Newtonian fluids Jonathan Freund, Ratnesh Shukla The jetting dynamics of a gas bubble near a wall in a non-Newtonian fluid are investigated using axisymmetric simulations. The bubble gas is assumed homogeneous, with density and pressure related through a polytropic equation of state. An incompressible, Eulerian-frame, Navier-Stokes solver for generalized Newtonian fluids is used, with discretization modified to sharply represent the shear-free bubble-liquid interface. Simulations show both stabilization and destabilization due to non-Newtonian effects. In general, for fixed zero- and infinite-shear-rate viscosities, shear-thinning promotes and shear-thickening suppresses jet formation and impact. For a shear-thinning fluid, a threshold Carreau time scale $\lambda$ is found that suppresses both jetting and impact. Likewise, for shear-thickening, a minimum is found that suppresses both. The bubble-wall speed increases sharply with shear thinning and decreases for shear thickening. However, the bubble volume is far less sensitive, changing less than 50\% for $0 < \lambda < \infty$. The general trends, and particularly the high sensitivity of the jet speed to $\lambda$, suggest a criterion that could potentially protect tissue in biomedical application and be used for high-strain-rate, large-deformation rheology. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H24.00007: Bubbles and droplets with mobile surfaces bounce stronger but coalesce faster Fan Yang, IvanUriev Vakarelski, Yuansi Tian, Erqiang Li, Derek Chan, Sigurdur Thoroddsen Increased hydrodynamic interfacial mobility of bubbles or droplets in multiphase systems are expected to reduce the characteristic coalescence time and thereby change the stability of gas or liquid emulsions, which are of great importance across many industrial and biological fields. High-speed imaging of a bubble bouncing from a pool surface shows that it bounces more strongly from a mobile than an immobile one. We demonstrate this with a controlled collision within a pure fluorocarbon liquid, which produces a series of rebounds prior to a rapid coalescence event. The bubble shows a weaker bounce if the interface is immobile, whereas the final coalescence takes longer. Experiments with a buoyant droplet show similar results. The stronger rebound is due to lower viscous dissipation in the intervening thin film, during the collision when the surfaces are mobile. We also perform Volume-of-Fluid simulations with the Gerris software using extreme local grid refinement, which can reproduce the experiments. This allows us to simulate the head-on collision of two drops, where for certain parameter regime, we counter-intuitively see a full rebound for mobile surfaces, while immobile surfaces can lead to coalescence owing to a second collision driven by the added mass. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H24.00008: Gravity Effects on Pool Boiling Heat Transfer Akash Dhruv, Elias Balaras, Amir Riaz, Jungho Kim Effects of gravity on boiling heat transfer efficiency is of special interest due to its application in two-phase cooling systems for spacecraft and satellite components. Experimental investigations have identified trends in heat flux scaling that demonstrate two distinct boiling regimes dominated by buoyancy (BDB) in high gravity and surface tension (SDB) in low gravity. Regression models constructed from experimental data show that the transition between the two regimes is dependent on heater size and degree of sub cooling. However, the bubble dynamics in this intermittent region are not very well understood. This serves as a motivation to identify heat flux mechanisms associated with bubble shape, size and merger using high fidelity numerical techniques to increase scientific understanding of the process. In this talk, we will present results from our three-dimensional simulations, show a quantitative and qualitative agreement with experiments, and discuss bubble statistics that govern the scaling of heat flux in BDB, SDB and transition regimes. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H24.00009: Numerical Study of Effect of Vapor Bubble Behavior on Wetting Condition in Boiling Flow Junnosuke Okajima, Peter Stephan The effect of vapor bubble behavior on wetting condition was evaluated by numerical simulation. The wetting properties were calculated by moving-contact-line evaporation model and that model was coupled with macro-scale simulation, in which the vapor bubble growth on the solid surface and heat conduction in solid were calculated by Finite Volume Method. The working fluid was assumed as FC-72 at 0.1013 MPa. The dynamic contact angle and the evaporation rate on the contact line were calculated and their dependencies on the wall superheat and contact line speed were found. In the macro-scale simulation, the heat transfer effect and bubble dynamics were evaluated by changing the wall temperature and mainstream velocity. Due to the evaporation rate on contact line and the convective heat transfer around the contact area, the temperature distributions at the front-side and backside of bubble was different. Additionally, it was found the trend of time variation of contact angle was varied by the mainstream velocity. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H24.00010: A Fundamental Study of the Channel Shape Impact on Microchannel Flow Boiling via Direct Numerical Simulations Mirco Magnini, Omar Matar Microchannel flow boiling is an attractive cooling solution for high-power density electronic devices. The coolant flows in a microevaporator, where many parallel channels are etched on a wafer substrate directly bonded on the surface to be refrigerated. Despite an extensive literature, there is still disagreement about the optimal shape of the channels cross-section which maximises heat transfer. Therefore, we have performed a fundamental study of the impact of the channel shape on the dynamics of elongated bubbles growing in microchannels and the associated heat transfer. We use a customised version of the Volume-Of-Fluid method in OpenFOAM, which includes a non-equilibrium evaporation model. Elongated bubbles are seeded at the upstream of a long microchannel heated with a constant heat flux. Channel aspect-ratios from 1 (square) to 8 (rectangular) are tested. We observe an essential impact of the perimetral distribution of the liquid film surrounding the bubble on the heat transfer patterns. Square channels exhibit higher heat transfer rates at low flow rates, where very thin liquid films are observed; flattened channels yield the best performance at larger flow rates, as they promote the formation of an extended liquid film covering a large fraction of the channel wall. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H24.00011: Optimising Boiling by Surface Design from the Nanoscale Upwards Edward Smith, Tassos Karayiannis, Prashant Valluri, Alessio Lavino, Omar Matar Molecular Dynamics (MD) is a promising approach for capturing the fundamental mechanism underpinning bubble nucleation, a challenge which cannot be overcome by traditional computational fluid dynamics (CFD) models. By constructing a heated surface at the molecular scale, we model the creation and growth of a bubble. In this talk, we present an introduction to MD, before showing nucleation results for a range of different surfaces. Nucleation rates are seen to depend on the details of the surface and we discuss insights provided by the MD approach. The challenge is to make these MD data relevant to larger scales, by linking these nanoscale bubbles to CFD simulations and optimising surface design to maximise nucleation. We discuss potential solutions to this problem, using multi-scale methods guided by a programme of experimental measurements. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H24.00012: Optimal control of the nonspherical oscillation of encapsulated microbubbles for biomedicine Fathia F. Arifi, Michael L. Calvisi Encapsulated microbubbles (EMBs) consist of a gas core surrounded by a stabilizing shell comprised of lipid, polymer, or protein and are used for ultrasound imaging and medical therapies, such as drug and gene delivery. The nonspherical oscillation of EMBs is essential for their function as it can enhance the acoustic signature in medical imaging as well as the onset of rupture, which can affect drug/gene release. Therefore, the ability to control such oscillations can improve the efficacy of diagnosis and treatment mediated by EMBs, and reduce unwanted side effects. This talk discusses the use of optimal control theory to optimize the acoustic driving for a specific objective, such as maximizing the nonspherical subharmonic response to improve blood-tissue contrast, or exciting shape modes to incite bubble rupture or the formation of microjets, which can facilitate drug/gene uptake. These objectives are achieved through prescribing various cost functions that enhance the dynamic response of shape modes while minimizing overall acoustic energy input to improve patient safety. Single frequency, dual frequency, and broadband acoustic forcing schemes are explored and compared. [Preview Abstract] |
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