Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D12: Drops: Heat Transfer and Evaporation IIDrops
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Chair: Jeremy Marston, Texas Tech University Room: 505 |
Sunday, November 19, 2017 2:15PM - 2:28PM |
D12.00001: Flow within an evaporating glycerol-water binary droplet: Segregation by gravitational effects Yaxing Li, Pengyu Lv, Christian Diddens, Herman Wijshoff, Michel Versluis, Detlef Lohse The flow within an evaporating glycerol-water binary droplet with Bond number Bo $\ll$ 1 is studied both experimentally and numerically. First, we measure the flow fields near the substrate by micro-PIV for both sessile and pendant droplets during evaporation process, which surprisingly show opposite radial flow directions -- inward and outward, respectively. This observation clearly reveals that gravitational effects play a crucial role in controlling flow fields within the evaporating droplets. We theoretically analyse that this gravity-driven effect is caused by density gradients due to the local concentration difference of glycerol within the droplet triggered by different volatilities of the two components during evaporation. Finally, for confirmation, we numerically simulate the process, revealing a good agreement with experimental results. [Preview Abstract] |
Sunday, November 19, 2017 2:28PM - 2:41PM |
D12.00002: Simultaneous Dropwise and Filmwise Condensation on a Microstructured Surface without the Assistance of a Hydrophobic Coating Daniel Orejon, Orest Shardt, Naga Siva Kumar Gunda, Tatsuya Ikuta, Koji Takahashi, Sushanta K. Mitra, Yasuyuki Takata We demonstrate micropillar surfaces on which condensation occurs in a new mode with simultaneous dropwise/filmwise condensation (DWC/FWC). This is achieved without the assistance of a hydrophobic coating; the pillars and base surface are hydrophilic. By considering thermodynamic principles of droplet wetting and spreading, we designed microstructured surfaces where the condensate is able to spread through the structures. The geometry of the microstructures constrains the condensate between the pillars, the rise of condensate above the structures is not thermodynamically favorable and condensation takes place as FWC between pillars. At the same time, the continuous nucleation, growth and departure of droplets at the pillars' tops in a DWC fashion is observed. We propose a simple resistance based heat transfer model to support the greater heat transfer performance of the simultaneous DWC/FWC when compared to solely FWC. In addition we propose rational guidelines for the design of an optimum configuration that maximizes the heat transfer performance in the simultaneous DWC/FWC mode. The authors acknowledge the support of WPI-I2CNER and KAKENHI JSPS. [Preview Abstract] |
Sunday, November 19, 2017 2:41PM - 2:54PM |
D12.00003: Solutal Marangoni flow as the cause of ring stains from drying salty colloidal drops Alvaro Marin, Stefan Karpitschka, Massimiliano Rossi, Christian J. Kaehler, Diego Noguera-Marin, Miguel A. Rodriguez-Valverde Salts can be found in different forms in almost any evaporating droplet in nature, our homes and in laboratories. The transport processes in such apparently simple systems differ strongly from 'sweet' evaporating droplets since the liquid flows in the inverse direction due to Marangoni stresses at the surface. Such an effect has crucial consequences to salt crystallization processes and to the evaporation itself. In this work we show measurements that not only confirm clearly the details of the inverted flow patterns, but also permit us to calculate the surface tension gradients responsible for the reversal. Such a reversal does not prevent the coffee-stain effect; on the contrary, particles accumulate and get trapped at the liquid-air interface driven by the surface flow. In order to prove this, we show measurements of the full three-dimensional flow inside the evaporating salty droplet, confocal imaging is used to quantify the growth of the particle deposits for different salt concentrations, and we compare the experimental results with numerical simulations that capture the solvent evaporation, the evaporation-induced liquid flow and the quasi-equilibrium liquid-gas interface. [Preview Abstract] |
Sunday, November 19, 2017 2:54PM - 3:07PM |
D12.00004: Transient dynamics in dynamic Leidenfrost phenomenon SangHyeon Lee, Sang Jun Lee, Ji San Lee, Kammel Fezzaa, Junh Ho Je When a liquid droplet impinges on a superheated surface, the droplet can form a vapor layer toward the surface and bounce back, known as dynamic Leidenfrost phenomenon. Transient dynamics of the vapor layer before the droplet bounces back can play a critical role in many industrial technologies such as power plant cooling and engine combustion, but is not fully understood mostly due to lack of appropriate visualization methods. Here, we successfully visualize the transient dynamics using ultrafast X-ray imaging. We experimentally reveal that the initial vapor dome flattens to a vapor disk, which then grows in thickness ($\delta ) $following the Fourier`s heat conduction law $\delta \propto \Delta T^{\mathrm{0.5}}t^{\mathrm{0.5}}$, where $\Delta T$ is the temperature difference across the vapor disk and $t$ is the time after impact. Ripples generate near the periphery of the vapor disk as long as the thickness reaches a certain value (\textasciitilde 12.16 $\mu $m), afterwards rapidly growing in amplitude while propagating to the center. Interestingly, rippling enhances droplet evaporation rate significantly The transient dynamics will provide import insight in understanding and modelling of power plant cooling or engine performance. [Preview Abstract] |
Sunday, November 19, 2017 3:07PM - 3:20PM |
D12.00005: Universality of oscillating boiling in Leidenfrost transition Tuan Tran, Mohammad Khavari The Leidenfrost transition leads a boiling system to the boiling crisis, a state in which the liquid loses contact with the heated surface due to excessive vapor generation. Here, using experiments of liquid droplets boiling on a heated surface, we report a new phenomenon, termed oscillating boiling, at the Leidenfrost transition. We show that oscillating boiling results from the competition between two effects: separation of liquid from the heated surface due to localized boiling, and rewetting. We argue theoretically that the Leidenfrost transition can be predicted based on its link with the oscillating boiling phenomenon, and verify the prediction experimentally for various liquids. [Preview Abstract] |
Sunday, November 19, 2017 3:20PM - 3:33PM |
D12.00006: Towards a sharp-interface volume-of-fluid methodology for modeling evaporation Ashish Pathak, Mehdi Raessi In modeling evaporation, the diffuse-interface (one-domain) formulation yields inaccurate results. Recent efforts approaching the problem via a sharp-interface (two-domain) formulation have shown significant improvements. The reasons behind their better performance are discussed in the present work. All available sharp-interface methods, however, exclusively employ the level-set. In the present work, we develop a sharp-interface evaporation model in a volume-of-fluid (VOF) framework in order to leverage its mass-conserving property as well as its ability to handle large topographical changes. We start with a critical review of the assumptions underlying the mathematical equations governing evaporation. For example, it is shown that the assumption of incompressibility can only be applied in special circumstances. The famous $D^2$ law used for benchmarking is valid exclusively to steady-state test problems. Transient is present over significant lifetime of a micron-size droplet. Therefore, a 1D spherical fully transient model is developed to provide a benchmark transient solution. Finally, a 3D Cartesian Navier-Stokes evaporation solver is developed. Some preliminary validation test-cases are presented for static and moving drop evaporation. [Preview Abstract] |
Sunday, November 19, 2017 3:33PM - 3:46PM |
D12.00007: Assessing Quasi-Steady State in Evaporation of Sessile Drops by Diffusion Models Cameron Martin, Hoa Nguyen, Peter Kelly-Zion, Chris Pursell The vapor distributions surrounding sessile drops of methanol are modeled as the solutions of the steady-state and transient diffusion equations using Matlab’s PDE Toolbox. The goal is to determine how quickly the transient diffusive transport reaches its quasi-steady state as the droplet geometry is varied between a Weber’s disc, a real droplet shape, and a spherical cap with matching thickness or contact angle. We assume that the only transport mechanism at work is diffusion. Quasi-steady state is defined using several metrics, such as differences between the transient and steady-state solutions, and change in the transient solution over time. Knowing the vapor distribution, the gradient is computed to evaluate the diffusive flux. The flux is integrated along the surface of a control volume surrounding the drop to obtain the net rate of diffusion out of the volume. Based on the differences between the transient and steady-state diffusive fluxes at the discrete points along the control-volume surface, the time to reach quasi-steady state evaporation is determined and is consistent with other proposed measurements. By varying the dimensions of the control volume, we can also assess what regimes have equivalent or different quasi-steady states for different droplet geometries. [Preview Abstract] |
Sunday, November 19, 2017 3:46PM - 3:59PM |
D12.00008: Thermocapillary flow contribution to dropwise condensation heat transfer Akshay Phadnis, Konrad Rykaczewski With recent developments of durable hydrophobic materials potentially enabling industrial applications of dropwise condensation, accurate modeling of heat transfer during this phase change process is becoming increasingly important. Classical steady state models of dropwise condensation are based on the integration of heat transfer through individual droplets over the entire drop size distribution. These models consider only the conduction heat transfer inside the droplets. However, simple scaling arguments suggest that thermocapillary flows might exist in such droplets. In this work, we used Finite Element heat transfer model to quantify the effect of Marangoni flow on dropwise condensation heat transfer of liquids with a wide range of surface tensions ranging from water to pentane. We confirmed that the Marangoni flow is present for a wide range of droplet sizes, but only has quantifiable effects on heat transfer in drops larger than 10 \textmu m. By integrating the single drop heat transfer simulation results with drop size distribution for the cases considered, we demonstrated that Marangoni flow contributes a 10---30{\%} increase in the overall heat transfer coefficient over conduction only model. [Preview Abstract] |
Sunday, November 19, 2017 3:59PM - 4:12PM |
D12.00009: DNS of unstable bubble growth in a superheated liquid Arash Asadollahi, Asghar Esmaeeli, Robert Ferris, Jim Hermanson Direct Numerical Simulations are performed to study growth of a vapor bubble in a pool of superheated liquid. At sufficiently low superheats, the bubble growth is stable and isotropic and the bubble surface is smooth. In this case the growth rate conforms to the classical theory for diffusion-controlled growth regime. However, as the degree of superheat is increased, the growth becomes unstable, as a result of protrusions that develop at the bubble surface and grow with time. This leads to evaporative mass fluxes that are much higher than those for the stable growth. This study is to investigate the transition from stable to unstable growth and to shed some insight into the key aspects of the unstable growth, such as correlation of the evaporative mass fluxes with the increase in the surface area of the bubble. Experiments are also performed by the University of Washington to compliment the computational modeling. [Preview Abstract] |
Sunday, November 19, 2017 4:12PM - 4:25PM |
D12.00010: Framework for simulating droplet vaporization in turbulent flows John Palmore, Olivier Desjardins A framework for performing direct numerical simulations of droplet vaporization is presented. The work is motivated by spray combustion in engines wherein fuel droplets vaporize in a turbulent gas flow. The framework is built into a conservative finite volume code for simulating low Mach number turbulent multiphase flows. Phase tracking is performed using a discretely conservative geometric volume of fluid method, while the transport of mass fraction and temperature is performed using the BQUICK scheme. Special attention is given to the implementation of transport equations near the interface to ensure the consistency between fluxes of mass, momentum, and scalars. The effect of evaporation on the flow appears as a system of coupled source terms which depend on the local thermodynamic equilibrium between the phases. The sources are implemented implicitly using an unconditionally stable, monotone scheme. Two methodologies for resolving the system's thermodynamic equilibrium are compared for their accuracy, robustness, and computational expense. Verification is performed by comparing results to known solutions in one and three dimensions. Finally, simulations of droplets vaporizing in turbulence are demonstrated, and trends for mass fraction and temperature fields are discussed. [Preview Abstract] |
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