Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D31: Drops: Leidenfrost Effects |
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Chair: Guillaume Riboux, Escuela Superior de Ingenieros, Universidad de Sevilla, Spain Room: 312 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D31.00001: Direct Numerical Simulation of the Leidenfrost Effect Sebastien Tanguy, Lucia Rueda Villegas The development of numerical methods for the direct numerical simulation of two-phase flows with phase changes, is the main topic of this study. We propose a novel numerical method which allows dealing with both evaporation and boiling at the interface between a liquid and a gas. For instance it can occur for a Leidenfrost droplet; a water drop levitating above a hot plate which temperature is much higher than the boiling temperature. In this case, boiling occurs in the film of saturated vapor which is entrapped between the bottom of the drop and the plate, whereas the top of the water droplet evaporates in contact of ambient air. Thus, boiling and evaporation can occur simultaneously on different regions of the same liquid interface or occur successively at different times of the history of an evaporating droplet. Usual numerical methods are not able to perform computations in these transient regimes, therefore, we propose in this paper a novel numerical method to achieve this challenging task. Finally, we present several accurate validations against experimental results on Leidenfrost Droplets to strengthen the relevance of this new method. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D31.00002: Leidenfrost drops on liquid baths : experiments Laurent Maquet, Baptiste Darbois-Texier, Alexis Duchesne, Martin Brandenbourger, Stéphane Dorbolo, Benjamin Sobac, Alexey Rednikov, Pierre Colinet In the Leidenfrost effect, a liquid drop stands above a very hot substrate and levitates over a bed of its own vapor. Recently, the use of these drops has shown rather interesting possibilities, and better understanding of this effect thus appears necessary. Roughness generally leads to an increase of the Leidenfrost temperature. Therefore, the idea of our work is to use the smoothest substrate possible: a liquid bath. Indeed, we observed stable Leidenfrost drops with superheat (difference between the temperature of the bath and the boiling temperature of the drop's liquid) down to $1^\circ$C. This remarquable behavior has been seen notably for ethanol drops on silicon oil baths. However, the viscosity of the liquid of the bath seems to play an important role as no ethanol drop can be in the Leidenfrost state over high viscosity baths (kinematic viscosity $\nu \sim 200$ cSt). This may be due to local cooling of the substrate under the drop. We also investigate the evaporation of these drops, and find scalings markably different from those applying in the case of a solid substrate. We also observe that the drop can enter in contact with the bath before its complete evaporation if the temperature is not high enough. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D31.00003: Leidenfrost drops on liquid baths: theory Benjamin Sobac, Alexei Rednikov, Laurent Maquet, Baptiste Darbois-Texier, Alexis Duchesne, Martin Brandenbourger, Stéphane Dorbolo, Pierre Colinet It is well known that a liquid drop released over a very hot surface generally does not contact the surface nor boils but rather levitates over a thin vapor film generated by its own evaporation (Leidenfrost effect). In particular, the case of a hot (and flat) solid substrate has been extensively studied in recent years. In contrast, we here focus on Leidenfrost drops over a superheated liquid bath, addressing the problem theoretically and comparing our predictions with experimental results, detailed in a separate talk. We predict the geometry of the drop and of the liquid bath, based on the hydrostatic Young-Laplace and lubrication equations. A good agreement is observed with the available experimental data concerning the deformation of the liquid bath. The modeling also yields a rather complete insight into the shape of the drop. As in the case of a solid substrate, the vapor layer generally appears to be composed of a vapor pocket surrounded by a circular neck. The influences of the superheat and of the drop size are parametrically investigated. A number of scaling laws are established. Unlike the case of a solid substrate, no chimney instability was found in the range of drop size studied. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D31.00004: Vapor layer evolution during drop impact on a heated surface SangHyeon Lee, Sangjun Lee, Jisan Lee, Kamel Fezzaa, Jung Ho Je When a liquid drop impacts on a sufficiently hot surface above the boiling point, a vapor layer is formed between the drop and the surface, preventing direct contact between them and as a result levitating the drop, known as the Leidenfrost effect. Understanding the evolution of the vapor layer is largely unexplored despite its importance in estimating heat transfer in cooling systems of thermal or nuclear power plants. The side-profile visualization of the vapor layer, as absolutely required for investigating its evolution, has been however unavailable by conventional optical microscopy. In this study, by employing ultrafast X-ray phase contrast imaging, we directly visualize the profiles of the vapor layers during liquid drop impact on a hot surface and elucidate the evolution of the vapor layers during spreading and retraction of the drop as functions of impact height and surface temperature. We reveal that the evolution is governed by the propagation of capillary waves generated in retraction and the wavelength of capillary waves $\mathrm{\lambda }$ is inversely proportional to the impact height h with a relation $\propto {\frac{\sigma}{\rho h}} \propto {\mathrm{We}}^{-1}$ where We is weber number. Capillary waves that converge at the center of the vapor layers are linked to the bouncing behavior of the drop. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D31.00005: A Leidenfrost Engine Gary Wells, Ridrigo Ledesma-Aguillar, Glen McHale, Khellil Sefiane The Leidenfrost effect, the sustained levitation of evaporating liquid droplets by a cushion of their on vapour on very hot surfaces, has received increased attention over the past few years. On patterned surfaces, rectification of the vapour layer flow can lead to rich dynamics of evaporating drops or sublimating blocks of dry ice, including self-propulsion, orbiting and conjoint rotation. In this paper we show that the Leidenfrost effect can be exploited to drive the rotation of rigid objects, such as solid hydrophilic plates coupled to water droplets and blocks of dry ice, by using turbine-like substrates. Using a hydrodynamic model, we show that drag-based rotation is achieved at low-Reynolds number by a rectification mechanism of the flow in the vapour layer caused by the underlying turbine-like geometry. Our theoretical model determines the maximum weight of Leidenfrost rotors and the net torque driving their motion in terms of operational parameters, showing an excellent agreement with experiments using dry-ice blocks. We generalise the concept of rotation into a new concept for a heat engine capable of harvesting thermal energy using either thin-film boiling or sublimation as a phase-change mechanism. As a proof principle, we implement the new sublimation engine in the lab to create a simple electromagnetic generator. Our results support the feasibility of low-friction in situ energy harvesting from both liquids and ices in challenging situations such as deep drilling, outer space exploration or micro-mechanical manipulation. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D31.00006: Maximum drop radius and critical Weber number for splashing in the dynamical Leidenfrost regime Guillaume Riboux, Jose Manuel Gordillo At room temperature, when a drop impacts against a smooth solid surface at a velocity above the so called critical velocity for splashing, the drop loses its integrity and fragments into tiny droplets violently ejected radially outwards. Below this critical velocity, the drop simply spreads over the substrate. Splashing is also reported to occur for solid substrate temperatures above the Leidenfrost temperature, T, for which a vapor layer prevents the drop from touching the substrate. In this case, the splashing morphology largely differs from the one reported at room temperature because, thanks to the presence of the gas layer, the shear stresses on the liquid do not decelerate the ejected lamella. Our purpose here is to predict, for wall temperatures above T, the dependence of the critical impact velocity on the temperature of the substrate as well as the maximum spreading radius for impacting velocities below the critical velocity for splashing. This is done making use of boundary integral simulations, where the velocity and the height of the liquid layer at the root of the ejected lamella are calculated numerically. This information constitutes the initial conditions for the one dimensional mass and momentum equations governing the dynamics of the toroidal rim limiting the edge of the lamella. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D31.00007: Relevant time- and length scale of touch-down for drops impacting on a heated surface Michiel A.J. van Limbeek, Minori Shirota, Chao Sun, Andrea Prosperetti, Detlef Lohse The vapor generated from a liquid drop impacting a hot solid surface can prevent it to make contact, depending on the solid temperature. The minimum temperature when no contact is made between the drop and the solid is called the dynamic Leidenfrost temperature. The latent heat needed to generated the vapor is drawn from the solid, and in general the Leidenfrost temperature depends on the solid thermal properties. Here we show experiments conducted on a sapphire plate, to minimize the cooling of the solid and ensuring nearly isothermal conditions. By using high speed total internal reflection imaging, we observe the drop base during impact up to about 100nm above the substrate surface. By this technique we are able to study the processes responsible for the transition between fully wetting and fully levitating drop impact conditions as the solid temperature increases. We reveal the relevant length- and time-scales for the dimple formation under the drop and explain their relevance for the late-time dynamics. As the transition regime is traversed from low to high temperature, the liquid-solid contact gradually decreases which reduces the friction with the solid, enhancing the spreading of the drop considerably. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D31.00008: Pool impacts of Leidenfrost drop Baptiste Darbois Texier, Eline Dehandschoewercker, Zhao Pan, Todd Truscott, Laurent Maquet, Stephane Dorbolo This work concerns the impact of a droplet made of a volatile liquid (typically HFE) on a pool of an other liquid (typically silicone oil) which temperature is above the boiling point of the drop. Depending on the properties of the two liquids and the impacting conditions, four different regimes are observed. For low impacting speeds, the droplet bounces on the surface of the bath and finally levitates above it in a Leidenfrost state. Such a regime occurs as soon as the pool temperature exceeds the boiling point of the drop. This observation means that there is no threshold in temperature for a Leidenfrost effect on a liquid surface contrary to the case of a solid substrate. For intermediate impacting velocities, the pinch-off of the surface of the pool entraps the drop in the liquid bulk. The entrapped drop is separated from the pool by a layer of its own vapour in a similar way of antibulles. For increasing impacting speeds, the vapour layer between the drop and the pool does not hold during the pinch-off event. The contact of the drop with the hot liquid provokes a sudden and intense evaporation. At very large impacting speeds, the drop rapidely contacts the pool, spreads and finally induces a hemi-spherical cavity. In the end, these four different regimes are summarized in a Froud-Weber diagram which boundaries are discussed. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D31.00009: ''Cold'' Leidenfrost effect Philippe Bourrianne, Christophe Clanet, David Quere An evaporating Leidenfrost drop placed on a hot substrate can levitate on its own vapor if the temperature of the substrate is high enough. We discuss the possibility to decrease this critical Leidenfrost temperature using a super-hydrophobic coating. Measuring adhesion and observing the liquid-solid interface, we suggest a possible explanation for this ``cold'' regime of levitation. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D31.00010: Does buoyancy matter in the melting dynamics of ice? Jicheng Guo, Mustafa Ordu, Soumendra Basu, James Bird Ice in a horizontal cylindrical container will melt when placed in a sufficient warm environment. Because of the density difference between the ice and the continuously forming water, the ice can rise close to the boundary, separated by a thin gap of water. The melting dynamics of the ice appear qualitatively similar to the evaporation of a drop under Leidenfrost conditions; however, the extent of the analogy is unclear. Here we investigate the melting dynamics of ice in thin-walled cylindrical containers. Through a combination of experiments and physical modeling, we identify a characteristic melting time and gap thickness, which we compare to evaporating droplets. [Preview Abstract] |
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