324 research outputs found
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Non-dimensionalisation parameters for predicting the cooling effectiveness of droplets impinging on moderate temperature solid surfaces
The conjugate problem of fluid flow and heat transfer during the impact of water droplets onto a heated surface is studied numerically using the Volume of Fluid (VOF) methodology; adaptive grid refinement is used for increased resolution at the droplet moving interface. The phenomenon is assumed to be 2D-axisymmetric and the wall temperature is moderated to prevent the onset of nucleate boiling. Parametric studies examine the effect of Weber number, droplet size, wall initial temperature and liquid thermal properties on the cooling process of the heated plate during the impaction period. The main variables describing the evolution of the phenomenon are non-dimensionalised with expressions arising from the transient conduction theory. It is proved that for all cases examined, these non-dimensional expressions can be grouped together for describing the hydrodynamic and thermal behavior in a similar manner. Additionally, semi-analytic expressions are derived, which, for a given range of variation, describe the spatial distribution and the temporal evolution of the temperature of the wall as well also the heat flux absorbed from the droplet, cooling effectiveness and mean droplet temperature
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Single droplet impacts onto deposited drops. Numerical analysis and comparison
The impact of a spherical water droplet onto a stationary sessile droplet lying on a solid wall is studied numerically using the volume-of-fluid methodology. The governing Navier-Stokes equations are solved both for the gas and liquid phase coupled with an additional equation for the transport of the liquid interface. An unstructured numerical grid is used along with an adaptive local grid refinement technique, which enhances the accuracy of the numerical results along the liquid-gas interface and decreases the computational cost. The stationary sessile droplet has been created from the prior impact of one or two water droplets falling onto the solid wall, while two solid walls have been studiedâan aluminum substrate and a glass substrate. The material of the wall plays an important role because it has an impact on the droplet's wetting behavior. The numerical model is validated against corresponding experimental data presented in the first part of the present work (Nikolopoulos et al., 2010), showing good agreement. Furthermore, the numerical investigation sheds light on the governing physics of the phenomenon
Numerical Investigation of Droplet Impact on Smooth Surfaces with Different Wettability Characteristics:Implementation of a dynamic contact angle treatment in OpenFOAM
[EN] The âDirect Numerical Simulationsâ (DNS) of droplet impact processes is of great interest and importance for a
variety of industrial applications, where laboratory experiments might be difficult, costly and time-consuming.
Furthermore, in most cases after validated against experimental data, they can be utilised to further explain the
experimental measurements or to extend the experimental runs by performing âvirtualâ numerical experiments. In
such âDNSâ calculations of the dynamic topology of the interface between the liquid and gas phase, the selected
dynamic contact angle treatment is a key parameter for the accurate prediction of the droplet dynamics. In the
present paper, droplet impact phenomena on smooth, dry surfaces are simulated using three different contact
angle treatments. For this purpose, an enhanced VOF-based model, that accounts for spurious currents
reduction, which has been previously implemented in OpenFOAM CFD Toolbox, is utilised and further enhanced.
Apart from the already implemented constant and dynamic contact angle treatments in OpenFOAM, the dynamic
contact angle model of Kistler, that considers the maximum advancing and minimum receding contact angles, is
implemented in the code. The enhanced VOF model predictions are initially compared with literature available
experimental data of droplets impacting on smooth surfaces with various wettability characteristics. The constant
contact angle treatment of OpenFOAM as well as the Kistlerâs implementation show good qualitative and
quantitative agreement with experimental results up to the point of maximum spreading, when the spreading is
inertia dominated. However, only Kistlerâs model succeeds to accurately predict both the advancing and the
recoiling phase of the droplet impact, for a variety of surface wettability characteristics. The dynamic contact angle
treatment fails to predict almost all stages of the droplet impact. The optimum version of the model is then applied
for 2 additional series of parametric numerical simulations that identify and quantify the effects of surface tension
and viscosity, in the droplet impact dynamics.Vontas, K.; Andredaki, M.; Georgoulas, A.; Nikas, K.; Marengo, M. (2017). Numerical Investigation of Droplet Impact on Smooth Surfaces with Different Wettability Characteristics: Implementation of a dynamic contact angle treatment in OpenFOAM. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 58-65. https://doi.org/10.4995/ILASS2017.2017.5020OCS586
Numerical Investigation of Droplet Impact on Smooth Surfaces with Different Wettability Characteristics: Implementation of a dynamic contact angle treatment in OpenFOAM
The âDirect Numerical Simulationsâ (DNS) of droplet impact processes is of great interest and importance for a variety of industrial applications, where laboratory experiments might be difficult, costly and time-consuming. Furthermore, in most cases after validated against experimental data, they can be utilised to further explain the experimental measurements or to extend the experimental runs by performing âvirtualâ numerical experiments. In such âDNSâ calculations of the dynamic topology of the interface between the liquid and gas phase, the selected dynamic contact angle treatment is a key parameter for the accurate prediction of the droplet dynamics. In the present paper, droplet impact phenomena on smooth, dry surfaces are simulated using three different contact angle treatments. For this purpose, an enhanced VOF-based model, that accounts for spurious currents reduction, which has been previously implemented in OpenFOAM CFD Toolbox, is utilised and further enhanced. Apart from the already implemented constant and dynamic contact angle treatments in OpenFOAM, the dynamic contact angle model of Kistler, that considers the maximum advancing and minimum receding contact angles, is implemented in the code. The enhanced VOF model predictions are initially compared with literature available experimental data of droplets impacting on smooth surfaces with various wettability characteristics. The constant contact angle treatment of OpenFOAM as well as the Kistlerâs implementation show good qualitative and quantitative agreement with experimental results up to the point of maximum spreading, when the spreading is inertia dominated. However, only Kistlerâs model succeeds to accurately predict both the advancing and the recoiling phase of the droplet impact, for a variety of surface wettability characteristics. The dynamic contact angle treatment fails to predict almost all stages of the droplet impact. The optimum version of the model is then applied for 2 additional series of parametric numerical simulations that identify and quantify the effects of surface tension and viscosity, in the droplet impact dynamics
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