Search

This paper deals with simulation of the spreading and solidification of a fully molten particle impacting onto a preheated substrate under traditional plasma spraying conditions. The multiphase problem governing equations of mass, momentum and energy conservation taking into account heat transfer by conduction, convection and phase change are solved by using a Finite Element approach. The interface between molten particle and surrounding air, is tracked using the Level Set method. The effect of the Reynolds number on the droplet spreading and solidification, using a wide range of impact velocities (40-250m/s), is reported. A new correlation that predicts the final spread factor of splat as a function of Reynolds number is obtained. Thermal contact resistance, viscous dissipation, wettability and surface tension forces effects are taken into account.

In the current study, a 3-dimensional lattice Boltzmann model which can tolerate high density ratios is employed to simulate the impingement of a liquid droplet onto a flat and a spherical target. The four phases of droplet impact on a flat surface, namely, the kinematic, spreading, relaxation and equilibrium phase, have been obtained for a range of Weber and Reynolds numbers. The predicted maximum spread factor is in good agreement with experimental data published in the literature. For the impact of the liquid droplet onto a spherical target, the temporal variation of the film thickness on the target surface is investigated. The three different temporal phases of the film dynamics, namely, the initial drop deformation phase, the inertia dominated phase and the viscosity dominated phase are reproduced and studied. The effect of the droplet Reynolds number and the target-to-drop size ratio on the film flow dynamics is investigated.

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu. In this paper, a numerical modeling of the impact, spreading, and eventually rebound of a viscoelastic droplet is reported. The numerical model is based on the volume of fluid (VOF) method coupled with the FENE-CR constitutive equations, and accounts for both the surface tension and the substrate wettability. The FENE-CR constitutive equations are used to model the polymer solution, while taking advantage of its rheological characterization. The comparison is performed between droplets of Newtonian solvent and a monodisperse polymer solution. The droplet impact on both hydrophilic and superhydrophobic substrate is analyzed through a detailed analysis of the spreading diameter evolution. It is found that while the droplet kinematic phase seems independent of the substrate and fluids properties, the recoiling phase is highly related to all of them. In addition the model infers a critical polymer concentration above which the droplet rebound from a superhydrophobic substrate is suppressed. The simulation is of particular interest to ink-jet processing, and demonstrates the capability of the model to handle complex non-Newtonian droplet dynamics.

(:unav)

Combustion, spray and flow dynamics for aerospace propulsionCombustion, spray and flow dynamics for aerospace propulsion; International audience; In aero-engines, droplet/wall interaction phenomena have a considerable influence on the mixture formation process and on wall heat fluxes. Impinging droplets may rebound, splash into secondary droplets or form a liquid film onto the solid surface. Droplet rebound and splashing is also a mechanism for the back penetration of the fuel vapor in the central region of the combustion chamber where the gas temperature is high enough for ignition. This work is an experimental study aiming at characterizing the heat transfers induced by the impingement of water droplets (diameter 80ֱ80 ֭) on a thin nickel plate heated by electromagnetic induction. The temperature of the rear face of the nickel sample is measured by means of an infrared camera and the heat removed from the wall due to the presence of the droplets is estimated using a semi-analytical inverse heat conduction model. In parallel, the temperature of the droplets is measured using the two-color Laser-Induced Fluorescence Thermometry which has been extended to imagery for the purpose of these experiments. The measurements of the variation in the droplet temperature occurring during an impact allow determining the liquid sensible heat. Measurements are performed at surface conditions well above the Leidenfrost temperature. A wide range of Weber numbers corresponding to the bouncing and splashing regimes are tested. Comparison between the heat flux removed from the wall and the sensible heat gained by the liquid allows estimating the heat flux related to liquid evaporation. Results reveal that the respective level of the droplet sensible heat and the heat lost due to liquid vaporization can vary significantly with the droplet sizes and the Weber number.

International audience; The present study focuses on the modeling and simulation of the process of impact and solidification of a molten alumina particle onto a substrate under plasma spray conditions. It aims at a better understanding of the different mechanisms that govern the formation of a single splat resulting from the impact of a particle onto a substrate. The successive stacking of these splats leads to the coating formation. The main objective is to examine the influence of the nature of the substrate and its initial temperature on the formation of a splat. The model is based on the Navier-Stokes and energy equations coupled with the "Level Set" function which permits tracking of the interface between molten particle and surrounding air. These equations are solved by the finite element method using Comsol Multiphysics software. The effects of surface tension forces, wettability and thermal contact resistance are taken into account.; La présente étude porte sur la modélisation et la simulation numérique du processus multi-physique de l’écrasement et de la solidification d’une particule d’alumine fondue sur un substrat dans les conditions de la projection plasma. Elle vise une meilleure compréhension des différents mécanismes qui régissent la formation d’une lamelle individuelle résultant de l’écrasement d’une particule sur un substrat qui par empilement successif constitue un dépôt. L’objectif majeur est l’examen de l’influence de la nature du substrat et de sa température initiale sur la formation d’une lamelle. La modélisation du problème est fondée sur les équations de Navier-Stokes et de l’énergie couplées avec une équation de la fonction «Level Set» qui permet de tracer et de suivre l’interface entre la particule et l’air qui l’entoure. Ces équations sont résolues par la méthode de discrétisation en éléments finis à l’aide de l’outil numérique Comsol multiphysiques. Les effets des forces de tension de surface, de la mouillabilité et de la résistance thermique de contact sont pris en considération.

The gel-supported precipitation (GSP) process can be used to make spherical particles (spherules) of nuclear fuel, particularly for very high temperature reactors (VHTR) and even for implementing the process called SPHEREPAC. In these different cases, the main characteristics are the sphericity of the particles to be manufactured and the control over their grain size. Nonetheless, depending on the specifications defined for these spherical particles, the GSP process has intrinsic limits, particularly when fabricating very small particles. This paper describes the use of secondary fragmentation (water, water/PVA and uranyl nitrate) on solid surfaces under varying temperature and vibration conditions to assess the relevance of using this new technique to manufacture very small spherical particles by means of a modified GSP process. The fragmentation mechanisms are monitored and analysed, before the trends for its subsequent optimised application are described., {"references":["S.M. Tiegs, P.A. Haas, and R.D. Spence, \"The Sphere-Cal Process :\nFabrication of Fuel Pellets from Gel Microspheres\", ORNL/TM-6906.","R.L. Beatty, R.E. Norman, and K.J. Notz, \"Gel-Sphere-Pac Fuel for\nthermal Reactors - Assessment of fabrication Technology and\nIrradiation Performance\", ORNL-5469 (Nov. 1979).","R. Spence, \"Sol Gel Spherical Fuel, Conf on metallurgical techn. of\nuranium and uranium alloys\", American Society for metals (1981).","P. Naefe, and E. Zimmer, \"Preparation of UO2 kernels by an external\ngelation process\", Nuclear Technology, V. 42 (1979).","Patent GB2094771.","M.R. Simpson, C.Z Stockwell, \"Improvements in or relating to\ngelation\", patent GB 1401962 (1 August 1975).","C.H.R. Mundo, M. Sommerfeld, and C. Tropea, \"Droplet-wall\ncollisions : experimental studies of the deformation and breakup\nprocess\", Int. J. Multiphase Flow Vol. 21 n┬░.2, 1995.","N. Zainoun, J-M Chicheportiche, J-P. Renaudeaux, \"le vibro-générateur\nd'aérosols homogènes\", Proceedings of the annual conference ASFERA,\ndec 2004.","A.L. Yarin, \"Drop impact dynamics : splashing, spreading, receding,\nbouncing\", Annual Review of fluid mechanics, vol. 38, 2006.\n[10] M. Bussmann, S. Chandra, and J. Mostaghimi, \"Modeling the splash of\na droplet impacting a solid surface, Physics of fluids\", Vol.12, n┬░12,\n2000.\n[11] M. Rein, \"Phenomena of liquid drop impact on solid and liquid\nsurfaces\", Fluid Dynamics Research, 12 (1993) 61-93.\n[12] J. Dewitte, \"Modelisation de l-impact d-un brouillard de gouttes en\nevaporation et sous pression sur une paroi chauffée\", Thesis of PhD,\n2006.\n[13] K.J. Baumeister, F.F. Simon and R.E. Henry, \"Role of the surface in the\nmeasurement of the Leidenfrost temperature, Augmentation of\nConvective Heat and Mass Transfer\", ASME, pp. 91-101, 1970.\n[14] L. Bolle and J.C. Moureau, \"Spray cooling of hot surfaces, in\nMultiphase Science and Technology\" (ed. By G.F. Hewitt, J.M. Delhaye\nand N. Zuber) pp. 1-92. Hemisphere, New York, 1976.\n[15] H.Y. Kim, Z.C. Feng, and J.H. Chun, \"Instabillity of a liquid jet\nemerging from a droplet upon collision with a solid surface\", Physics of\nfluids, vol. 12, number 3, march 2000.\n[16] M.H. and J.C. Chen-s., J.C. Chen, \"Behavior of a liquid droplet\nimpinging on a solid surface\". ASME. 83-WA/HT-104\n[17] F. Akao, K. Araki, S. Lori and A. Moriyama, \"Deformation behaviors of\na liquid droplet impinging onto hot metal surface\", Trans. Int. Steel Inst.\nJapan 20, 737-743 (1980)\n[18] M. Kurokawa and S. Toda, \"Heat Transfer of an impacted single droplet\non the wall\", in Proceedings of the ASME/JSME, Thermal Engineering\nJoint Conf, Vol. 2, pp 141-146, 1991\n[19] A.L. Biance, F. Checy, C. Clanet, G. Lagubeau, D. Quere, \"On the\nelasticity of an inertial liquid shock\", Journal of Fluid Mechanics 554\n(2006) 47-66\n[20] R. Bhola and S. Chandra, \"Parameters controlling solidification of\nmolten wax droplets falling on a solid surface\", J. Mater. Sci. 34, 4883\n(1999)\n[21] G.E. Cossali, A. Coghe, and M. Marengo, \"The impact of a single drop\non a wetted solid surface\", Exp. Fluids 22, 463"]}

International audience; The splat formation is one of the basic processes in thermal spray coatings.The performance of these coatings is strongly related to the process of spreading and solidification of molten droplets. The aim of the present paper is to simulate the fluid flow, heat transfer and phase-change that occur when a micrometric molten droplet impacts onto a rigid substrate and to examine the effect of the substrate conditions, such as initial temperature and material on the solidification time and spreading process. The effect of thermal contact resistance is also investigated.The simulation model used is based on the Navier-Stokes equations and the energy equation which includes convection and phase change. These equations are coupled with the Level Set function to track the interface between molten particle and surrounding air. The numerical model is solved using Finite Element Method and Comsol multiphysics 3.5a software

International audience; In this paper, a Finite Element Analysis is carried out in order to simulate the process of spreading and solidification of a micrometric molten droplet impinging onto a cold substrate. This process is a crucial key to have a good understanding of coatings obtained by means of thermal spraying. The effect of thermal contact resistance (TCR) on the droplet spreading and solidification was investigated using different values of TCR and different droplet sizes. The solidification time was found to be a linear function of the droplet diameter square. Viscous dissipation, wettability and surface tension effects are taken into account. The Level Set method was employed to explicitly track the free surface of molten droplets.

Inkjet technology has been recognized as one of the most successful and promising micro-system technologies. The wide application areas of printer heads and the increasing demand of high quality prints are making ink consumption and print see-through important topics in the inkjet technology. In the present study we investigate numerically the impact of ink droplets onto a porous material that mimics the paper structure. The mathematical framework is based on a free energy formulation, coupling the Cahn-Hilliard and Navier Stokes equations, for the modelling of the two-phase flow. The case studied here consists of a multiphase flow of air-liquid along with the interaction between a solid structure and an interface. In order to characterize the multiphase flow characteristics, we investigate the effects of surface tension and surface wettability on the penetration depth and spreading into the paper-like structure. Qc 20120215

In order to study in-flight ice adhesion at the droplet-scale, a strategy is presented to simulate the impact and solidification of a super cooled water droplet on a cooled substrate. Upon impact, nucleation is assumed to occur instantaneously, and properties of the droplet are chosen to account for the nucleation process. Simulations are performed in ANSYS Fluent using a coupled Volume of Fluid and Level-Set method to capture the air-water interface and an Enthalpy-Porosity method to capture the liquid-solid interface. Calibration of a simulation parameter, Amush, is performed in order to match experimental data for different surface types and surface temperatures. The calibrated simulation strategy is applied to low-speed, in-flight icing conditions. The effects of surface variation and droplet diameter variation are investigated, providing insight into the icephobicity of super hydrophobic surfaces. Numerical results suggest that large droplets (approximately 200 micron-diameter) will freeze and adhere to a super hydrophobic surface.

An aircraft may experience in-flight ice accretion and corresponding reductions in performance and control when the vehicle encounters clouds of super-cooled water droplets. The EADS-IW Surface Engineering Group is investigating passive anti-icing possibilities, such as functional and ice phobic coatings. Ice-resistant coatings require investigating droplet impact on dry surfaces and wet films, including microscopic effects such as droplet splashing. To investigate droplet impacts, a volume of fluid (VOF) flow solver was used for droplets impacting dry and wetted hydrophobic and superhydrophobic surfaces, focusing on meso-scale simulations. The effects of structured, micro-scale surface roughness and the effects of a thin wet film on the surface, corresponding to a saturated surface under high humidity conditions, were investigated. Axisymmetric domains produced acceptable results for smooth, dry surfaces. It was determined that in order to properly predict behavior of droplets impacting surfaces with structured micro-scale roughness, three-dimensional simulations are recommended.

Droplet impact onto a solid surface is modeled and numerically simulated using an ALE formulation of the Navier-Stokes free surface equations. The triple line dynamics aspect is modeled implicitely through the Shikhmurzaev theory. Those equations involve a generalized Navier slip boundary conditions with the variation of the surface tension near the triple line (thus a local Marangoni effect). Comsol Multiphysics 3.2 potentialities are investigated to implement the different components of this ALE free surface problem. Finally, some numerical results of the spreading phase are obtained using the algorithm elaborated and implemented into Micralef, a home-developed software; Nous nous intéressons à la modélisation numérique d'une gouttelette impactant un solide. Les équations sont celles de Navier-Stokes surface libre en formulation ALE. La dynamique de la ligne triple est implicitement modélisée par les équations de Shikhmurzaev. Ces équations introduisent une condition de glissement de Navier généralisée, avec un gradient de tension de surface (effet Marangoni localisé). Les potentialités du logiciel Comsol Multiphysics 3.2 pour résoudre ce problème sont étudiées. Finalement, nous obtenons des résultats basés sur les algorithmes établis mais implémentés au sein de Micralef, un code de calcul spécialement développé pour cette étude.

Superhydrophobic surfaces for repelling impacting water droplets are typically created by designing structures with capillary (antiwetting) pressures greater than those of the incoming droplet (dynamic, water hammer). Recent work has focused on the evolution of the intervening air layer between droplet and substrate during impact, a balance of air compression and drainage within the surface texture, and its role in affecting impalement under ambient conditions through local changes in the droplet curvature. However, little consideration has been given to the influence of the intervening air-layer thermodynamic state and composition, in particular when departing from standard atmospheric conditions, on the antiwetting behavior of superhydrophobic surfaces. Here, we explore the related physics and determine the working envelope for maintaining robust superhydrophobicity, in terms of the ambient pressure and water vapor content. With single-tier and multitier superhydrophobic surfaces and high-resolution dynamic imaging of the droplet meniscus and its penetration behavior into the surface texture, we expose a trend of increasing impalement severity with decreasing ambient pressure and elucidate a previously unexplored condensation-based impalement mechanism within the texture resulting from the compression, and subsequent supersaturation, of the intervening gas layer in low-pressure, humid conditions. Using fluid dynamical considerations and nucleation thermodynamics, we provide mechanistic understanding of impalement and further employ this knowledge to rationally construct multitier surfaces with robust superhydrophobicity, extending water repellenc y behavior well beyond typical atmospheric conditions. Such a property is expected to find multifaceted use exemplified by transportation and infrastructure applications where exceptional repellency to water and ice is desired. ISSN:0027-8424 ISSN:1091-6490

Background A variety of imaging methods are available to obtain kinematic data at an interface, with a widely varying range of spatial and temporal resolution. These methods require a trade-off between imaging rate and resolution. Objective A deep learning framework trained on synchronous profilometry data acquired using two imaging modalities at two different spatial resolutions to enhance spatial resolution while maintaining temporal resolution is desired. Methods Fizeau interferometry (FIF) and frustrated total internal reflection (FTIR) are used to overcome the resolution-rate trade-off via a deep learning framework. The FTIR imaging data are recorded at high resolution, while the FIF imaging data are recorded with a lesser resolved, larger field of view. We apply a deep learning framework using a multi-layer convolutional neural network to enhance the FIF image resolution. Results With the deep learning framework, we achieve the high spatial resolution of measurements obtained by FTIR imaging in all three dimensions from the lower resolution FIF data. A high-order overset technique ultimately yields full up-scaled images from the network outputs without losing precision. The accuracy of the super-resolved image is evaluated using test data. Conclusions This hybrid framework, called HOTNNET, is implemented in its entirety on high-speed imaging profilometry data acquired in the study of droplet impacts on a smooth, solid surface, and is used to recover full, high-resolution images at high rates by unwrapping the phase of the interferometry. This framework can be readily adapted to other paired datasets by retraining the network on the novel data.

Superhydrophobic surfaces for repelling impacting water droplets are typically created by designing structures with capillary (antiwetting) pressures greater than those of the incoming droplet (dynamic, water hammer). Recent work has focused on the evolution of the intervening air layer between droplet and substrate during impact, a balance of air compression and drainage within the surface texture, and its role in affecting impalement under ambient conditions through local changes in the droplet curvature. However, little consideration has been given to the influence of the intervening air-layer thermodynamic state and composition, in particular when departing from standard atmospheric conditions, on the antiwetting behavior of superhydrophobic surfaces. Here, we explore the related physics and determine the working envelope for maintaining robust superhydrophobicity, in terms of the ambient pressure and water vapor content. With single-tier and multitier superhydrophobic surfaces and high-resolution dynamic imaging of the droplet meniscus and its penetration behavior into the surface texture, we expose a trend of increasing impalement severity with decreasing ambient pressure and elucidate a previously unexplored condensation-based impalement mechanism within the texture resulting from the compression, and subsequent supersaturation, of the intervening gas layer in low-pressure, humid conditions. Using fluid dynamical considerations and nucleation thermodynamics, we provide mechanistic understanding of impalement and further employ this knowledge to rationally construct multitier surfaces with robust superhydrophobicity, extending water repellenc y behavior well beyond typical atmospheric conditions. Such a property is expected to find multifaceted use exemplified by transportation and infrastructure applications where exceptional repellency to water and ice is desired., Proceedings of the National Academy of Sciences of the United States of America, 117 (44), ISSN:0027-8424, ISSN:1091-6490

Fortunately, the human being has already started to be environmentally concerned and the search for new alternatives to reduce pollution increased. Transports are responsible for a significant portion and it is extremely necessary to bet on alternatives to oil. The introduction of biofuels in aero-engines could be an example. In order to modify and optimize piston engines and gas turbines to operate efficiently with alternative fuels, this work used Jet Fuel and Biofuel mixtures. The focus of these studies was to visualize the dynamic behavior of single droplets impinging upon liquid films with variable thickness. The existence of splash as well as its characteristics were reported and the differences and similarities between the outcomes according to the impact conditions and the fluid properties were catalogued. To achieve that an experimental facility was designed and built. Four fluids were tested: water (as reference), 100% Jet A-1, 75%/25% and 50%/50% mixtures of Jet A-1 and HVO (Hydro-processed Vegetable Oil), respectively, since civil aviation only accept mixtures with at least 50% Jet Fuel in volume. The fluid properties were measured to ensure accuracy. The liquid film depths considered were 10%, 50% and 100% of the droplet diameter. A high-speed digital camera was used to image acquisition and the droplet was released by a syringe pump connected to the needle at a specific pumping rate. The impact surface was a perspex container filled with fluid. Five needles with different inner diameters and three impact heights were employed to provide a variety of Weber and Reynolds numbers., Fundação para a Ciência e a Tecnologia

Bearing Chambers in Aero-Engines are located near the rolling-element type of bearings which support the shafts and accomodate the resulting thrust loads. One of the main task of the bearing chambers is, beside an efficient scavenging of the lubricating oil, the cooling of the hot compartments. A very complex two-phase air-oil flow takes usually place in these bearing chambers consisting of oil droplet-laden air flows and shear-driven liquid wall films. The interaction of the droplets with the wall films is significantly influencing the wall heat transfer and the cooling performance of these systems. For this reason, a detailed characterization and modelling of the mass and momentum exchange between droplets and wall films for the unique impingement parameter range in bearing chambers is inevitable. \ud This scientific report investigates the oil droplet impact dynamics for typical impingement regimes relevant to aero-engine bearing chambers. The application of a Direct Numerical Simulation (DNS) technique based on the Volume-of-Fluid (VOF) method and coupled with a gradient-based adaptive mesh refinement (AMR) technique allowed to characterize the drop impact dynamics during various single micro- and millimeter drop impacts onto thin and thick films. With the help of a special numerical treatment, a self-perturbing mechanism is installed that leads to the correct resolution of the crown disintegration process. The numerical methodology was thoroughly validated using the experimental results of millimeter sized drop impacts onto deep liquid pools. These results were developed with an enhanced back-illuminated high-speed imaging and Particle Tracking Velocimetry (PTV) technique. \ud New insights into the cavity penetration, the crown’s breakup dynamics and the secondary droplet characteristics following a single drop impact have been developed with the help of the isolated variation of different parameters of influence. Particularly the influence of the Froude number, the impingement angle, and the cavity-wall interaction delivered results to date not reported in scientific literature. Beside the advances in fundamental physics describing the drop impact dynamics with the help of the numerical and experimental results, a set of correlations could also be derived. From these correlations, a drop-film interaction model was formulated that is suitable for the parameter range found in bearing chambers.

Liquid penetration analysis in porous media is of great importance in a wide range of applications such as ink jet printing technology, painting and textile design. This article presents an investigation of droplet impingement onto metallic meshes, aiming to provide insights by identifying and quantifying impact characteristics that are difficult to measure experimentally. For this purpose, an enhanced Volume-Of-Fluid (VOF) numerical simulation framework is utilised, previously developed in the general context of the OpenFOAM CFD Toolbox. Droplet impacts on metallic meshes are performed both experimentally and numerically with satisfactory degree of agreement. From the experimental investigation three main outcomes are observed—deposition, partial imbibition, and penetration. The penetration into suspended meshes leads to spectacular multiple jetting below the mesh. A higher amount of liquid penetration is linked to higher impact velocity, lower viscosity and larger pore size dimension. An estimation of the liquid penetration is given in order to evaluate the impregnation properties of the meshes. From the parametric analysis it is shown that liquid viscosity affects the adhesion characteristics of the drops significantly, whereas droplet break-up after the impact is mostly controlled by surface tension. Additionally, wettability characteristics are found to play an important role in both liquid penetration and droplet break-up below the mesh., Fluids, 5 (2), ISSN:2311-5521