Droplet Splashing Research Articles

Experimental study of water vaporization occurring inside a channel of a smooth plate-type heat exchanger at subatmospheric pressure

In order to be able to design properly low-pressure evaporators for sorption chillers, expertise on vaporization of the refrigerant under conditions that might occur in these evaporators is fundamental. However, few studies focus on this subject and there is a lack of knowledge about vaporization (boiling or evaporation) phenomena occurring in compact evaporators at low pressure. The objective of this article is thus to go further in the understanding of phenomena occurring in compact plate-type evaporators. In that goal, an experimental test setup was designed and built. It allows the observation of the water vaporization in a channel mimicking that of a plate heat exchanger of standard dimension (0.2m width×0.5m height) under various operating conditions (working pressure ranging from 0.85kPa to 16kPa, secondary fluid temperature ranging from 10.9°C to 23.1°C, filling ratio ranging from 1/2 to 1/10 of the whole channel height). During these experiments, three main flow regimes were observed and three different working areas were identified: a pool boiling area, a vapor area, and a film evaporation area. In the latter, the creation of a liquid film due to the splashing of droplets is observed. These droplets result from the break-up of the membrane of a previously formed large bubble. The bubble formed is of several centimeters in diameter and appears in the pool boiling area, few centimeters below the free surface. It was shown that the major part of the cooling capacity is achieved in the film evaporation area. As a matter of fact, it is possible to predict the overall cooling capacity obtained experimentally with a maximum relative difference of 30% directly from the evaluation of the overall heat transfer coefficients in that area.

Influence of liquid formulation and impact conditions on the wetting of hydrophobic surfaces by aqueous polymeric solutions

Impacts of single droplets on hydrophobic surfaces were investigated using a high speed camera and image analysis. Low viscous aqueous solutions of polyvinylpyrrolidone (PVP) were prepared for this investigation. Results revealed that increasing polymer concentration favours splashing and rebound inhibition. In addition, increasing polymer mass fraction leads to a decrease in the maximum extent of spreading and slows down the retraction movement. The effect of droplet inertia was also investigated; higher impact velocity was found to promote both spreading and receding of droplets. However, increasing droplet inertia leads to droplet rebound and disintegration in the case of low polymer mass fraction. Two available models for splashing and rebound prediction were used. Results showed that the existing models, originally based on Newtonian fluid experimental results, are not able to predict droplet behaviour for non-Newtonian PVP solutions. Finally, a regime map based on We and Oh numbers was established; three regions corresponding to deposition, rebound and splashing of droplets were delimited.

A mass-conserving lattice Boltzmann method with dynamic grid refinement for immiscible two-phase flows

A mass-conserving lattice Boltzmann method (LBM) for multiphase flows is presented in this paper. The proposed LBM improves a previous model (Lee and Liu, 2010 [21] ) in terms of mass conservation, speed-up, and efficiency, and also extends its capabilities for implementation on non-uniform grids. The presented model consists of a phase-field lattice Boltzmann equation (LBE) for tracking the interface between different fluids and a pressure-evolution LBM for recovering the hydrodynamic properties. In addition to the mass conservation property and the simplicity of the algorithm, the advantages of the current phase-field LBE are that it is an order of magnitude faster than the previous interface tracking LBE proposed by Lee and Liu (2010) [21] and it requires less memory resources for data storage. Meanwhile, the pressure-evolution LBM is equipped with a multi-relaxation-time (MRT) collision operator to facilitate attainability of small relaxation rates thereby allowing simulation of multiphase flows at higher Reynolds numbers. Additionally, we reformulate the presented MRT-LBM on nonuniform grids within an adaptive mesh refinement (AMR) framework. Various benchmark studies such as a rising bubble and a falling drop under buoyancy, droplet splashing on a wet surface, and droplet coalescence onto a fluid interface are conducted to examine the accuracy and versatility of the proposed AMR-LBM. The proposed model is further validated by comparing the results with other LB models on uniform grids. A factor of about 20 in savings of computational resources is achieved by using the proposed AMR-LBM. As a more demanding application, the Kelvin–Helmholtz instability (KHI) of a shear-layer flow is investigated for both density-matched and density-stratified binary fluids. The KHI results of the density-matched fluids are shown to be in good agreement with the benchmark AMR results based on the sharp-interface approach. When a density contrast between the two fluids exists, a typical chaotic structure in the flow field is observed at a Reynolds number of 10000, which indicates that the proposed model is a promising tool for direct numerical simulation of two-phase flows.

Effect of preparing method of ZnO powders on electrical arc erosion behavior of Ag/ZnO electrical contact material

Abstract

Investigation of splashing phenomena during the impact of molten sub-micron gold droplets on solid surfaces.

The dynamics of splashing accompanying the impact of molten 800 nm diameter gold droplets on silicon, gold coated silicon, gold coated glass and polished solid gold surfaces has been studied. A novel method based on laser induced forward transfer has been developed to generate single submicron molten gold droplets. Splashing morphology has been characterized using Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB) techniques. It is found that the splashing of submicron gold droplets upon impact is enhanced by high droplet impact energy achieved by reducing the droplet flight distance and that an air layer resulting in a bubble becomes trapped under the impacting droplets even when the size of the droplet is less than one micron. Our results show that, under these conditions, heat transfer between the submicron droplet and the solid substrate is more important than surface roughness and surface tension in the evolution of splashing. A theoretical model has been developed to simulate the splashing characteristics of submicron gold droplets during impact. Both the experimental data and the analytical model show that splashing is enhanced by high heat transfer rates to the surface.

Uptake of phosphorus from surfactant solutions by wheat leaves: spreading kinetics, wetted area, and drying time.

The delivery and uptake of nutrients at the surface of plant leaves is an important physicochemical phenomenon that depends on leaf surface morphology and chemistry, fertilizer formulation chemistry (including adjuvant and associated surfactants), wetting dynamics, and many other physical, chemical and biological factors. In this study, the role of spreading dynamics in determining uptake of the macronutrient phosphorus from phosphoric acid fertilizer solution in combination with three different adjuvants was measured in the absence of droplet run-off and splashing. When run-off and splashing losses were zero, spreading and drying rates had a small to negligible effect on the uptake efficiency. The results suggest that uptake may be much less sensitive to the specific choice of adjuvant and long time-scale spreading behaviour than one might intuitively expect.

A two-dimensional splashing model for investigating impingement characteristics of supercooled large droplets

In this article, a two-dimensional (2D) splashing model is proposed to investigate the dynamics when supercooled large droplets (SLD) impinging on a wall surface in the aircraft-icing field. Energy conservation for droplet motion and impingement is used to capture the properties of the splashed droplets. A new statistical treatment of the droplet impinging energy and angle during the droplet-wall interaction is introduced in order to calculate the average dynamics of the SLD within a micro-control volume on wall surface. Based on the LEWICE predictions of droplet collection efficiencies and the available experimental ones, a new criterion for droplet splashing/deposition as well as a new formulation for the splashed mass is suggested. Lagrangian approach is adopted to describe the movement and impingement of SLD. The proposed model together with the previously developed droplet tracking method (DTM) for calculating droplet collection efficiency with the effect of droplet reimpingement constitute a relatively complete predicting approach of SLD impingement characteristics. Comparisons between the current predictions and the experimental observations, including SLD impingement over clean and contaminated airfoil surfaces as well as shapes of ice accretion in typical icing conditions, are carried out. Further, results obtained with the LEWICE splashing model are also plotted on the same graphs in order to assess the accuracy of the current splashing model in predicting SLD impingement. Results show that good agreement is achieved between the current predictions, including SLD impingement and ice accretion shapes, and the experimental ones. The predictions of the impingement distribution over contaminated surfaces obtained with the current splashing model show a much closer agreement with the experimental results than the ones obtained with LEWICE splashing model. For further investigation of SLD impingement, the properties of the droplet splashing and reimpingement during the ice accretion process are also addressed.

Supercooled Large Droplet Modeling for Aircraft Icing Using an Eulerian–Eulerian Approach

Previous models employed in the computations of collection efficiencies for supercooled large droplets illustrate that the droplets’ interaction with an airfoil surface, such as bouncing and splashing of droplets, should be taken into account in the simulation to obtain accurate results. The previous supercooled large droplet models have been modified to compute the collection efficiencies for supercooled large droplets in such a way that the modified models can be adopted more effectively and suitably in the Eulerian–Eulerian approach. The modified supercooled large droplet models are implemented into the Eulerian–Eulerian approach within CFD++, which is the commercial computational fluid dynamics software suite from Metacomp Technologies, Inc. In addition, in order to account for aerodynamic droplet breakup in the Eulerian–Eulerian approach, a number density equation has been derived that includes a source term representing the rate of change of the number density during breakup. To validate the present supercooled large droplet modeling, simulations to compute icing collection efficiencies have been carried out for droplets of small and large diameters over clean airfoils and airfoils with glaze ice shapes.

An improved multiphase lattice Boltzmann flux solver for three-dimensional flows with large density ratio and high Reynolds number

An improved multiphase lattice Boltzmann flux solver (MLBFS) is proposed in this work for effective simulation of three-dimensional (3D) multiphase flows with large density ratio and high Reynolds number. As a finite volume scheme, the MLBFS originally proposed in [27] applies the finite volume method to solve for macroscopic flow variables directly. The fluxes are reconstructed locally at each cell interface by using the standard LBM solutions. Due to the modeling error of the standard LBM, the reconstructed fluxes deviate from those in the Navier–Stokes equations; and to compensate this error, a complex tensor is introduced in the original MLBFS. However, the computation of the tensor introduces additional complexity and usually needs a relatively thicker interface thickness to maintain numerical stability, which makes the solver be complex and inefficient in the 3D case. To remove this drawback, in this work, a theoretical analysis to the formulations obtained from the Chapman–Enskog expansion is conducted. It is shown that the modeling error can be effectively removed by modifying the computation of the equilibrium density distribution function. With this improvement, the proposed 3D MLBFS not only avoids the calculation of the compensation tensor but also is able to maintain numerical stability with very thin interface thickness. Several benchmark cases, including the challenging droplet impacting on a dry surface, head-on collisions of binary droplets and droplet splashing on a thin film with density ratio 1000 and Reynolds number up to 3000, are studied to validate the proposed solver. The obtained results agree well with the published data.

CASE STUDY: THE SYNDROME BLOCH-SULZBERGER

The paper presents a clinical case of hereditary disease of the newborn - a syndrome BLOCH-SULZBERGER (familial incontinentia pigmenti). The peculiarity of this case can be considered as the prevalence of cutaneous syndrome, lack of comorbidities CNS, the eyes, the presence of congenital heart disease, quick change of stages of rash during therapy. The clinical diagnosis was made for admission of the child to the hospital, and later fully confirmed by experts of medical genetic center. Girls age for admission was 1 month. On examination the child drew the attention of the polymorphism of cutaneous syndrome: lots erythematous-papular rash, sometimes with crusts and erosions, as well as the effects of previous eruptions in the form of pigmentation. Brown spots brown with lighter edges were in the form of droplets (splashing mud), arranged in a flat, parallel, semi-circular or winding tapes; they are bilateral, symmetrical and located mainly in the abdomen and legs. In peripheral blood - leukocytosis (48.3 thousand) with marked eosinophilia (from 48 to 76%). Eosinophilia developed in parallel with the skin inflammatory changes and decreased at the same time setting the stage pigment. Additionally diagnosed with a heart defect. The girl was discharged on day 30 with normalized blood counts and the presence of bilateral, linear, symmetrically arranged (mostly on the abdomen and extremities) areas of skin pigmentation.

Eulerian Modeling of Supercooled Large Droplet Splashing and Bouncing

A conservative Eulerian numerical approach for modeling postimpact Supercooled Large Droplets undergoing splashing and bouncing on aircraft surfaces is presented. The approach introduces the effect of the postimpact droplets by successive solutions of the conservation equations. Two models have been selected to identify the droplet splashing and bouncing conditions, and to provide initial conditions for the reinjected water. The method has been applied to droplet impingement in Supercooled Large Droplet conditions on clean and iced NACA 23012 geometries, as well as the MS(1)-0317 airfoil, and the results have been compared to experimental data. Good agreement is observed for both impingement limits and collection efficiency. Additionally, the method has been applied to a three-element high-lift configuration operating in one of the proposed Appendix O Supercooled Large Droplet environments to demonstrate the danger posed by the re-impingement of splashing and bouncing droplets on complex interacting aerodynamic components.

Healing of the Interface Between Splashed Particles and Underlying Bulk Coating and Its Influence on Isothermal Oxidation Behavior of LPPS MCrAlY Bond Coat

The thermally grown oxide formed on the bond coat surface plays an important role in determining the lifetime of thermal barrier coatings (TBCs). The splashed particles on the thermally sprayed MCrAlY bond coat surface are weakly bonded to the underlying bulk coating, leading to the formation of mixed oxides and contributing to the TBC failure. In this study, the healing behavior of the weakly bonded interface between splashed particles and underlying MCrAlY bulk coating deposited by low pressure plasma spraying was examined, and the influence of interface healing on the isothermal oxidation behavior of the bond coat was discussed. Results show that the granular particles resulting from splashing of molten droplets were exposed on smooth splats which make up the surface of bulk coating. After the pre-diffusion treatment in vacuum, the small granular splashed particles are immersed into the bulk coating resulting from the element diffusion on the interface between splashed particles and underlying bulk coating. After the vacuum heat treatment, the formation of mixed oxides was effectively restrained due to the healing of the splashed particle/underlying bulk coating interface.

Estimation and prediction for a unified hybrid-censored Burr Type XII distribution

The unified hybrid censoring is a mixture of generalized Type I and Type II hybrid censoring schemes. This paper presents the statistical inference and prediction of the Burr Type XII distribution for unified hybrid-censored data. The maximum likelihood estimators for estimating the unknown parameters are developed using the expectation-maximization algorithm. The Bayesian estimates of the unknown parameters under the assumption of independent gamma priors are obtained using two approximations, namely Lindley's approximation and the Markov Chain Monte Carlo (MCMC) technique. Also, MCMC samples are used to construct the highest posterior density credible intervals as well. Simulation experiments are performed to compare the different proposed methods. The prediction of the future observations based on a unified hybrid-censored sample is also provided. A real dataset representing the droplet splashing reported in 90° spray angle is used to illustrate the derived results. Furthermore, a useful interval say tracking interval is introduced for comparing the distance of the two rival models from the true model.

A coupled level-set and volume-of-fluid simulation for splashing of single droplet impact on an inclined liquid film

A coupled level set and volume of fluid (CLSVOF) method is utilized to simulate the droplet evolution, during splashing after impacting on an inclined liquid film, and the bubble entrainment by the droplet impacting of the liquid. The influence of impact angles of bubble entrainment, the processes of splashing pressure field and velocity field after impingement are studied and discussed. Compared with the results obtained by experiment, the validity of CLSVOF method is verifiled by simulating the droplet splashing on an inclined liquid film. Results indicate that the mechanisms of front and back splashing are different. The front splashing is caused by pressure difference in the neck region, and the back splashing is produced by radial flow in the liquid film. It is also found that along with the impact angle moving towards the large angle direction, the bubble entrainment number shows a reducing trend.

Multiphase lattice Boltzmann flux solver for incompressible multiphase flows with large density ratio

A multiphase lattice Boltzmann flux solver (MLBFS) is proposed in this paper for incompressible multiphase flows with low- and large-density-ratios. In the solver, the flow variables at cell centers are given from the solution of macroscopic governing differential equations (Navier–Stokes equations recovered by multiphase lattice Boltzmann (LB) model) by the finite volume method. At each cell interface, the viscous and inviscid fluxes are evaluated simultaneously by local reconstruction of solution for the standard lattice Boltzmann equation (LBE). The forcing terms in the governing equations are directly treated by the finite volume discretization. The phase interfaces are captured by solving the phase-field Cahn–Hilliard equation with a fifth order upwind scheme. Unlike the conventional multiphase LB models, which restrict their applications on uniform grids with fixed time step, the MLBFS has the capability and advantage to simulate multiphase flows on non-uniform grids. The proposed solver is validated by several benchmark problems, such as two-phase co-current flow, Taylor–Couette flow in an annulus, Rayleigh–Taylor instability, and droplet splashing on a thin film at density ratio of 1000 with Reynolds numbers ranging from 20 to 1000. Numerical results show the reliability of the proposed solver for multiphase flows with high density ratio and high Reynolds number.

Numerical investigation on super-cooled large droplet icing of fan rotor blade in jet engine

Icing (or ice accretion) is a phenomenon in which super-cooled water droplets impinge and accrete on a body. It is well known that ice accretion on blades and vanes leads to performance degradation and has caused severe accidents. Although various anti-icing and deicing systems have been developed, such accidents still occur. Therefore, it is important to clarify the phenomenon of ice accretion on an aircraft and in a jet engine. However, flight tests for ice accretion are very expensive, and in the wind tunnel it is difficult to reproduce all climate conditions where ice accretion can occur. Therefore, it is expected that computational fluid dynamics (CFD), which can estimate ice accretion in various climate conditions, will be a useful way to predict and understand the ice accretion phenomenon. On the other hand, although the icing caused by super-cooled large droplets (SLD) is very dangerous, the numerical method has not been established yet. This is why SLD icing is characterized by splash and bounce phenomena of droplets and they are very complex in nature. In the present study, we develop an ice accretion code considering the splash and bounce phenomena to predict SLD icing, and the code is applied to a fan rotor blade. The numerical results with and without the SLD icing model are compared. Through this study, the influence of the SLD icing model is numerically clarified.

A density-scaled continuum surface force model within a balanced force formulation

We propose a numerical framework which can simulate free surface flows with complex moving interfaces like droplet splashing as minimizing spurious currents. The numerical framework is based on the CLSVOF (coupled level set and volume-of-fluid) method, the THINC/WLIC (tangent of hyperbola for interface capturing/weighted line interface calculation) scheme, multi-moment methods (CIP-CSL and VSIAM3) and density-scaled CSF (continuum surface force) model within a balanced force formulation. In this paper, we propose a level set based algorithm of the density-scaled balanced CSF model and show that the density-scaled balanced CSF model can reduce spurious currents more than the standard balanced CSF model without using the density-scaling when the exact curvature is not given. We also show that the numerical framework can well capture the physics of droplet splashing.

Nonlinear preconditioning for efficient and accurate interface capturing in simulation of multicomponent compressible flows

Single fluid schemes that rely on an interface function for phase identification in multicomponent compressible flows are widely used to study hydrodynamic flow phenomena in several diverse applications. Simulations based on standard numerical implementation of these schemes suffer from an artificial increase in the width of the interface function owing to the numerical dissipation introduced by an upwind discretization of the governing equations. In addition, monotonicity requirements which ensure that the sharp interface function remains bounded at all times necessitate use of low-order accurate discretization strategies. This results in a significant reduction in accuracy along with a loss of intricate flow features. In this paper we develop a nonlinear transformation based interface capturing method which achieves superior accuracy without compromising the simplicity, computational efficiency and robustness of the original flow solver. A nonlinear map from the signed distance function to the sigmoid type interface function is used to effectively couple a standard single fluid shock and interface capturing scheme with a high-order accurate constrained level set reinitialization method in a way that allows for oscillation-free transport of the sharp material interface. Imposition of a maximum principle, which ensures that the multidimensional preconditioned interface capturing method does not produce new maxima or minima even in the extreme events of interface merger or breakup, allows for an explicit determination of the interface thickness in terms of the grid spacing. A narrow band method is formulated in order to localize computations pertinent to the preconditioned interface capturing method. Numerical tests in one dimension reveal a significant improvement in accuracy and convergence; in stark contrast to the conventional scheme, the proposed method retains its accuracy and convergence characteristics in a shifted reference frame. Results from the test cases in two dimensions show that the nonlinear transformation based interface capturing method outperforms both the conventional method and an interface capturing method without nonlinear transformation in resolving intricate flow features such as sheet jetting in the shock-induced cavity collapse. The ability of the proposed method in accounting for the gravitational and surface tension forces besides compressibility is demonstrated through a model fully three-dimensional problem concerning droplet splash and formation of a crownlike feature.

An improved free surface modeling for incompressible SPH

In Incompressible Smooth Particle Hydrodynamics (ISPH), a pressure Poisson equation (PPE) is solved to obtain a divergence free velocity field. When free surfaces are simulated using this method a Dirichlet boundary condition for pressure at the free surface has to be applied. In existing ISPH methods this is achieved by identifying free surface particles using heuristically chosen threshold of a parameter such as kernel sum, density or divergence of the position, and explicitly setting their pressure values. This often leads to clumping of particles near the free surface and spraying off of surface particles during splashes. Moreover, surface pressure gradients in flows where surface tension is important are not captured well using this approach. We propose a more accurate semi-analytical approach to impose Dirichlet boundary conditions on the free surface. We show the efficacy of the proposed algorithm by using test cases of elongation of a droplet and dam break. We perform two dimensional simulations of water entry and validate the proposed algorithm with experimental results. Further, a three dimensional simulation of droplet splash is shown to compare well with the Volume-of-Fluid simulations.

Concentration of environmental fungal and bacterial bioaerosols during the monsoon season

Rain has been known to remove aerosol particles in air environments. The aerosol particles were captured and removed from the air by rain and the concentration of aerosol particles significantly decreased after rain events. Therefore, rain is regarded as having a good effect on air environments in terms of the respiratory health of the general public. However, humid environments produced by long-term rain events such a monsoon may be a sufficient condition for the growth of microorganisms and vibrations because of the splashing of droplets may facilitate the aerosolization of ground microorganisms. We therefore hypothesize that the rain may increase concentrations of bioaerosols in outdoor air environments, thereby possibly influencing respiratory diseases.To verify this hypothesis, at the initial stepwise approach, we measured the concentration of airborne biological particles before, after, and during rain in a monsoon season. The measurement data of the concentration of fungal particles and bacterial particles show quantitatively that the bioaerosol concentrations during the rain event are several times higher than the concentration of the bioaerosols in the condition of no rain.