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 L22: Drops: Heat Transfer and Evaporation II |
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Chair: Paul Steen, Cornell University Room: 604 |
Monday, November 25, 2019 1:45PM - 1:58PM |
L22.00001: Evaporation of sessile drops on soft membranes with capillary origami Yuhong Chen, Daniel Orejon, Prashant Valluri, Vasileios Koutsos, Khellil Sefiane Drops evaporating on soft substrates are common in nature, such as raindrops on tree leaves. If the substrate is thin enough, it can get folded by the action of surface tension of a drying drop. This folding is known as `capillary origami', which provides a simple and cheap method for fabricating predetermined 3D structures at the micro/nano-scale. Here, we explore the influence of capillary origami on drop evaporation. Our experiments concern with evaporation of sessile microliter water drops on soft PDMS membranes. We consider the effect of substrate thickness noting that bending stiffness is a cubic function of thickness. Thus, drying sessile droplets of the same size can lead to either absent, partial or complete folding of the substrate. The evolution of evaporation fluxes is obtained through a microbalance with a resolution of \textpm 0.01 mg. Simultaneously the profiles of drops and membranes are recorded by a side-view CCD camera and a top-view optical camera, respectively. These videos are processed using ImageJ to extract the data and quantify the folding extent of the membranes. Our results show that foldable membranes reduce evaporation rates of sessile droplets thereby lengthening the drop lifetimes. [Preview Abstract] |
Monday, November 25, 2019 1:58PM - 2:11PM |
L22.00002: Inclination effect on evaporation of colloidal droplets Jin Young Kim, Marta Goncalves, Hyoungsoo Kim, Byung Mook Weon When colloidal droplets including micro- and nanoparticles evaporate on flat solid substrates, commonly ring-like depositions are generated by continuous outward capillary flows. When the substrate is inclined, droplets become asymmetric and non-spherical by gravity and it affects evaporation dynamics and particle deposition. However, inclination effects on evaporation and deposition are not clear yet. Here we observe evaporation and deposition of colloidal droplets by changing particle sizes and tilting angles. We find that there is competition between downward sedimentation flows by gravity and upward capillary flows by evaporation, inducing that uniformity of coffee-rings depends on particle sizes and tilting angles. For small particles, evaporation-driven upward flows are more dominant than gravity-driven downward flows. [Preview Abstract] |
Monday, November 25, 2019 2:11PM - 2:24PM |
L22.00003: A non-equilibrium multi-component evaporation model for blended diesel/alcohol droplets. Hongyuan Zhang, Ping Yi, Suo Yang A non-equilibrium Langmuir-Knudsen model for multi-component pure diesel and blended diesel/alcohol droplets and sprays is developed. This model takes into account most of the key processes during the droplet lifetime, including the finite heat conduction and limited mass diffusion inside the droplet, the differential diffusion in gas phase, and the non-equilibrium Langmuir-Knudsen evaporation law for multi-component droplets. The present model shows good agreements with experimental measurements for pure ethanol, diesel, and blended diesel/ethanol droplets. The non-equilibrium effects become significant when the initial droplet diameter is smaller than 20 $\mu $m, and these effects are enhanced with increasing ambient temperature and forced convection intensity. The non-equilibrium effects are more significant for the blended diesel/alcohol droplets than pure diesel, especially during the evaporation period of ethanol. The present evaporation model has also been applied to calculate the evaporation processes of single- and multi-component fuel sprays under various ambient conditions. The non-equilibrium effects for the blended diesel/alcohol sprays are significant in terms of the fuel vapor component concentrations. [Preview Abstract] |
Monday, November 25, 2019 2:24PM - 2:37PM |
L22.00004: Direct Numerical Simulations of Fully Developed Turbulent Channel Flow with Evaporation of Spatially Resolved Droplets Giandomenico Lupo, Andrea Gruber, Christophe Duwig We perform direct numerical simulations (DNS) of more than 14000 cold spherical droplets evaporating in hot turbulent channel flow ($Re_\tau$ = 180, 5\% initial liquid volume fraction), at conditions representative of industrial applications. Four-way coupling of the droplet motion with the turbulent carrier phase and interface-resolved evaporation dynamics allow us to fully describe the intra- and inter-phase exchange of heat, species and momentum transfer, at all relevant scales of motion. We analyze the modulation of turbulence by the dispersed phase, and the associated migration of the droplets towards the channel centreline. The redistribution of the droplets has a strong impact on the evaporation dynamics, which we characterize by the joint probability density function of the evaporation rate and the droplet distance from the wall. This leads to an inquiry of the feasibility of film theory for modelling the present evaporation regime, and the definition of an appropriate nondimensional parameter for the scaling of the evaporation rate (correlation of Sherwood number). Finally, we investigate the influence of the energy boundary condition (adiabatic vs. isothermal walls) on the evaporation dynamics. [Preview Abstract] |
Monday, November 25, 2019 2:37PM - 2:50PM |
L22.00005: Experimental and numerical study of the flow instabilities during sessile droplet evaporation in microgravity. Sanjeev Kumar, Marc Medale, David Brutin Our current research is focused on thermal Marangoni instabilities in sessile ethanol and Hydrofluoroether (HFE7100) droplets, which develop spontaneously during forced evaporation. We will present the numerical modelling in 3D unsteady with moving interface of a sessile droplet under forced evaporation and showing internal flow instabilities. We assumed a pinned contact line and a spherical-cap shape of the liquid-gas interface. Our computations contribute to figure out the internal 3D flow structure in the droplet and also to determine the driving mechanism and energy sources of the observed thermo-convective instability and thus clarifies its nature. We will compare the numerical results with experimental results obtained with ARLES experiment in microgravity during the ESA SSC MASER-14 rocket campaign for the HFE7100 droplets and the parabolic flight campaign CNES VP139 and ESA VP140 for the ethanol, methanol and pentane droplets. [Preview Abstract] |
Monday, November 25, 2019 2:50PM - 3:03PM |
L22.00006: The role of substrate geometry on droplet evaporation John McCarthy, Dominic Vella, Alfonso A. Castrejon-Pita The coffee ring effect is the name given to the phenomenon where particles suspended in an evaporating droplet migrate towards the contact line, producing characteristic dense rings of particles. It is observed in many everyday and technological settings, from stains after coffee spills to imperfections in inkjet printed objects. Methods to control and even suppress this - often undesired - effect include the use of tailored mixtures of solvents to control evaporation, the use of elongated particles, the use of electrowetting and surface acoustic waves to induce internal flows, etc. All these methods have the objective of reducing, eliminating or overcoming the capillary flows leading to the deposition of suspended particles along the contact line during (differential) evaporation. Each method limits to some extent the type of droplets for which it can be used. We explore the role played by substrate geometry in controlling the evaporation of droplets and its role on the formation, or otherwise, of a coffee ring. [Preview Abstract] |
Monday, November 25, 2019 3:03PM - 3:16PM |
L22.00007: Effects of contact lines on evaporative heat transfer of liquid films Jungtaek Kim, Hanseul Choi, Yun Seog Lee, Ho-Young Kim Falling film evaporators cool liquids that flow through a bundle of tubes, over which refrigerants are sprayed and evaporated. A key parameter to determine the heat transfer coefficient is the film Reynolds number(Re) that depends on the spraying amount of refrigerant. Because dry-out of tube walls occurs when Re is too low while the liquid film becomes too thick for too high Re, an optimal Re exists that maximizes the heat transfer coefficient. Here we show that the optimal Re for evaporative heat transfer over geometrically patterned tube walls is also critically affected by the enhanced mass transfer rate at the solid-liquid-gas three phase contact lines. It is experimentally found that the heat transfer of partially filled grooves with contact lines is more efficient than completed wet grooves, despite the decreased heat transfer area. We experimentally measure the heat transfer coefficient from various surface patterns under contact line effects. Then we provide a mathematical model to optimize enhanced surface features maximizing evaporative heat transfer coefficient. [Preview Abstract] |
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