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 H34: Drops X: Splashing on Heated Surfaces |
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Chair: Vladimir Ajaev, Southern Methodist University Room: 405 |
Monday, November 25, 2013 10:30AM - 10:43AM |
H34.00001: Splash transition of droplets impacting on heated surfaces Hendrik J.J. Staat, Tuan Tran, Bart Geerdink, Chao Sun, Detlef Lohse For large enough velocities, droplets impacting on a dry solid surface make a splash. How does the surface temperature affect the transition towards the splashing regime? We answer this questions by high-speed interferometric imaging for millimeter-sized droplets. We find that for moderate surface heating when the droplet still touches the surface, the velocity threshold towards splashing increases with increasing surface temperature. In contrast, for strong surface heating when the droplet is in the Leidenfrost regime and does not touch the superheated surface due to the formation of a vapor layer, the velocity threshold towards splashing is much lower. We theoretically explain both findings within a pressure balance model. [Preview Abstract] |
Monday, November 25, 2013 10:43AM - 10:56AM |
H34.00002: Microdroplet impact on superheated surfaces: Vapor triggers splashing Tuan Tran, Yoshiyuki Tagawa, Yanbo Xie, Chao Sun, Detlef Lohse In many engineering and technological applications that involve impact of microdroplets on a superheated solid surface, the small size of the droplets and the vaporization of the liquid as the droplets approach the surface pose a challenge to visualize and understand the splash mechanism. In particular, the spontaneously generated vapor contributes to destabilize the spreading stage of the liquid and potentially influences the onset of splash. In this study, we experimentally determine the dependence of the splash transition on the surface temperature. We also provide a model that incorporates the liquid-vapor phase transition to explain this experimentally-observed transition. [Preview Abstract] |
Monday, November 25, 2013 10:56AM - 11:09AM |
H34.00003: The Effect of Disturbances and Surrounding Air on the Droplet Impact Phenomena Andrew Work, Yongsheng Lian, Mark Sussman Supercooled Large Droplets (SLDs) represent an icing hazard in a number of areas, most obviously in aviation. SLDs pose a hazard above smaller supercooled droplets because they don't freeze completely on impact, and can spread or splash. Experiments have demonstrated that surrounding air plays an important role in the droplet impact phenomena: a low ambient pressure can suppress the droplet splashing. However, the effect of surrounding air on the droplet impact has not been adequately addressed. Numerical simulations are conducted to systematically investigate the interplay between the droplet and the surrounding air in the droplet splashing regime. Disturbances originating from the experimental droplet generator are also studied in the simulation. We investigate whether these disturbances are responsible for the fingering observed in experimentation. We compare the results of several perturbations on the droplet, as well as the effect of surface roughness. Simulations are conducted using the Moment of Fluid numerical method, and the grid features adaptive mesh refinement. [Preview Abstract] |
Monday, November 25, 2013 11:09AM - 11:22AM |
H34.00004: How drops bounce and dance on ice: the role of sublimating surfaces Carlo Antonini, Ilaria Bernagozzi, Stefan Jung, Dimos Poulikakos, Marco Marengo Drop rebound is a spectacular event that appears after impact on superhydrophobic surfaces, due to low drop-solid substrate adhesion, and on hot substrates in Leidenfrost conditions, thanks to a vapor layer forming at the liquid-substrate interface, caused by drop evaporation. However, at temperatures below water freezing temperature, i.e. 0C, even superhydrophobicity can get lost. In the present work, we demonstrate that drop rebound can also be originated by another physical phenomenon, i.e. the solid substrate sublimation, at temperatures as low as -79C. To prove this mechanism, drop impact experiments were conducted on solid carbon dioxide, commonly known as dry ice. Drop dynamics and rebound were analyzed, together with the cases of drop impacting on a superhydrophobic surface and on a hot plate, to show how three different physical mechanisms, which apparently share nothing in common, i.e. superhydrophobicity, evaporation and sublimation, can all lead to drop rebound, in an extremely wide temperature range, from 300C down to even below -79C. Additional glycol drop impact tests proved the independence of the observed phenomena from the chosen liquid. Finally, the formation and visualization of an air vortex ring around an impacting drop is also reported. [Preview Abstract] |
Monday, November 25, 2013 11:22AM - 11:35AM |
H34.00005: Quantitative visualization of droplet hot-surface interaction Nejdet Erkan, Koji Okamoto Up to this date liquid droplet impingement phenomenon onto hot surfaces has drawn massive attention from a broad spectrum of research fields, since its hydrodynamic and thermodynamic characteristics has profound importance for various industrial applications Although tremendous experimental and computational work exist in the literature, thermal-hydraulic mechanism of droplet impingement boiling on hot surfaces received several contradictory approaches due to the parametric sensitivity of the problem. To understand and to predict the physical mechanism, an experimental database including large amount of spatio-temporal data, which is formed by the tests performed under well-controlled BCs and high sensitive devices, is still a necessity. This study investigates the parametric variation of droplet boiling regimes due to the experimental BCs (e.g surface roughness, ambient pressure) by performing separate effect tests employing high-speed visualization system. Differences in the impingement boiling characteristics of water droplets on solid (with surface roughness) and liquid metal (without surface roughness) in film boiling regime are investigated. A unique quantitative velocity data inside the droplet at several surface temperatures including (Leidenfrost temperatures) captured by Particle Tracking Velocimetry (PTV). This data is a unique component for the validation of CFD simulations which are performed to resolve the phenomena. [Preview Abstract] |
Monday, November 25, 2013 11:35AM - 11:48AM |
H34.00006: Contribution of enhanced heat transfer in individual droplet impact cavities to overall heat transfer in spray cooling John Kuhlman, Nicholas Hillen The thickness of the thin liquid film beneath the cavity formed by impact of an individual water droplet into a static liquid film over an unheated, horizontal surface (termed the sub-cavity liquid film thickness) was measured using a non-contacting optical thickness sensor, versus both time and radial distance away from the impact cavity centerline. Sub-cavity liquid film thickness data were obtained for ranges of Reynolds and Weber numbers expected for a commercial spray nozzle of interest, based on PDPA velocity and diameter data. These film thickness data were numerically integrated to determine the sub-cavity liquid volume time variation. The measured liquid film thickness decreases away from the cavity centerline over much of the cavity lifetime, for all test conditions. Computed sub-cavity volumes are typically between 50{\%} to 100{\%} of the droplet volume, and remain near this plateau value over much of the cavity lifetime. The measured sub-cavity liquid volume and cavity lifetime are used to estimate values for both the local cavity heat flux and the overall heat flux, averaged over the heater surface, that would be required to dry out the cavity prior to cavity fill in. These computed overall average heat flux values are compared with measured overall critical heat fluxes from the literature. [Preview Abstract] |
Monday, November 25, 2013 11:48AM - 12:01PM |
H34.00007: Dropwise Condensation on a Radial Gradient Surface Ashley Macner, Susan Daniel, Paul Steen In transient dropwise condensation from steam onto a cool surface, distributions of drops evolve by nucleation, growth, and coalescence. This study examines how surface functionalization affects drop growth and coalescence. Surfaces are treated by silanization to deliver either a spatially uniform contact-angle (hydrophilic, neutral, and hydrophobic) or a radial gradient of contact-angles. The time evolution of number-density and associated drop-size distributions are reported. For a typical condensation experiment on a uniform angle surface, the number-density curves show two regimes: an initial increase in number-density as a result of nucleation and a subsequent decrease in number-density as a result of larger scale coalescence events. Without a removal mechanism, the fractional coverage, regardless of treatment, approaches unity. For the same angle-surface, the associated drop-size distributions progress through four different shapes along the growth curve. In contrast, for a radial gradient surface where removal by sweeping occurs, the number-density increases and then levels off to a value close to the maximum number-density that is well below unity coverage and only two shapes of distributions are observed. Implications for heat transfer will be discussed. [Preview Abstract] |
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