Resulting Deposition Patterns Research Articles

Runaway electron beam formation, vertical motion, termination and wall loads in EU-DEMO

Abstract Runaway electron loads onto material structures are a major concern for future large tokamaks due to the efficient avalanching at high plasma currents. 
Here, we perform predictive numerical studies using the JOREK code for a plausible plasma configuration in the European DEMO fusion power plant, focusing in this paper on the scenario where the highest multi-ampere runaway electron beam is formed.
The work first comprises axisymmetric predictions of runaway electron beam formation in a mitigated scenario and of the simultaneous vertical motion of the beam due to loss of position control. The subsequent runaway electron beam termination triggered by a burst of MHD activity during the course of the vertical motion is then simulated in 3D with the runaway electron fluid self-consistently coupled to the MHD modes. Finally, the resulting deposition pattern of the runaway electrons onto wall structures is calculated with a relativistic test particle approach. This way, the suitability of a possible sacrificial limiter concept for the protection of first wall components is assessed.

Resulting deposition patterns in various sessile droplets undergoing evaporation

Resulting deposition patterns in various sessile droplets undergoing evaporation

Understanding the formation of particle bands and fingering patterns during evaporation of a sessile droplet containing colloids

Sessile droplets containing dispersed colloids evaporate on substrates leaving distinct deposition patterns. Applications using the droplets technique require a critical understanding of the complex particle dynamics that affect the deposition pattern. The fingering-like pattern formed when surfactant-laden droplets containing dispersed colloids evaporate is not yet well understood. The lateral imbalanced capillary meniscus forces (CMFs) between particles moving along the Marangoni vortex (MV) cluster particles into bands that deposit to form the fingering-like pattern. This work investigates the effect of particle shape, particle, and surfactant concentration on the phenomenon. Further, the importance of the contact angle and contact line (CL) dynamics of the droplet is also discussed. The results demonstrate that manipulating these parameters can control the band formation and the resulting deposition pattern. The findings of this work would be pivotal for a critical understanding of particle self-assembly and pattern morphologies of evaporating surfactant-laden droplets.

Numerical investigation of detachment and transport of particulate structures in wall-flow filters using lattice Boltzmann methods

The exhaust from combustion engines contains particulate matter (PM), which poses potential health risks to human lungs. Current emission laws place increasingly strict limitations on both PM and particle number, leading to the necessity of using wall-flow filters to separate out a significant amount of the introduced PM. As this leads to an increase in the filter's loading, it is regenerated continuously or periodically, leading to the rearrangement of individual particulate structures inside the filter channels. Such rearrangement events cause the formation of specific deposition patterns, which affect the filter's pressure drop, its loading capacity and the separation efficiency. In order to derive predictions on the formation of specific deposition patterns, the transient behaviour of individual particle structures needs to be examined. The present work investigates the detachment and transport of particle structures during filter regeneration with three-dimensional surface-resolved simulations using a lattice Boltzmann method. The goal of this work is the determination of relevant key quantities and their interpretation with respect to predictions regarding the resulting deposition patterns. In this context, it is shown that lift forces are not the predominant detachment forces for non-spherical particle structures, and that the stopping distance of such structures is too long to avoid back-end deposition.

Unsteady simulations of migration and deposition of fly-ash particles in the first-stage turbine of an aero-engine

AbstractParticulate deposits in aero-engine turbines change the profile of blades, increase the blade surface roughness and block internal cooling channels and film cooling holes, which generally leads to the degradation of aerodynamic and cooling performance. To reveal particle deposition effects in the turbine, unsteady simulations were performed by investigating the migration patterns and deposition characteristics of the particle contaminant in a one-stage, high-pressure turbine of an aero-engine. Two typical operating conditions of the aero-engine, i.e. high-temperature take-off and economic cruise, were discussed, and the effects of particle size on the migration and deposition of fly-ash particles were demonstrated. A critical velocity model was applied to predict particle deposition. Comparisons between the stator and rotor were made by presenting the concentration and trajectory of the particles and the resulting deposition patterns on the aerofoil surfaces. Results show that the migration and deposition of the particles in the stator passage is dominated by the flow characteristics of fluid and the property of particles. In the subsequential rotor passage, in addition to these factors, particles are also affected by the stator–rotor interaction and the interference between rotors. With higher inlet temperature and larger diameter of the particle, the quantity of deposits increases and the deposition is distributed mainly on the Pressure Side (PS) and the Leading Edge (LE) of the aerofoil.

Analysis of Circulation Reversal and Particle Transport in Evaporating Drops

We report experimental evidence of circulation reversal in evaporating drops. The internal flow patterns and particle distribution inside the evaporating water droplet with suspended polystyrene particles was studied using the Particle Image Velocimetry (PIV) technique. Droplet placed on externally heated substrate showed two symmetric and counter rotating Marangoni convection cells at the beginning of evaporation process. Towards the final stages of evaporation, the two counter rotating Marangoni convection cells reversed the direction of circulation. Interestingly, it was observed that between these two stages four symmetric and counter rotating convection cells appeared inside the evaporating droplet. Our results are in support of recent theoretical investigations on circulation reversal in evaporating drops. The flow patterns were driven by the surface tension gradient caused by the surface temperature distribution along the droplet-air interface. The surface temperature distribution is not only dependent on the non-uniform evaporation flux but also on the conduction path from the substrate to the droplet surface. Towards the end of evaporation, the fluid flow becomes radially outward from the center to the pinned edge of the droplet. We have also investigated the evaporating droplets placed on a curved substrate. Unlike the case of flat substrate, the evaporation from curved surface resulted monotonic temperature profile. The particle distribution and resulting deposition patterns was strongly influenced by the internal fluid flow pattern and was observed to be different from that of a flat substrate.

Understanding of the role of dilution on evaporative deposition patterns of blood droplets over hydrophilic and hydrophobic substrates

Blood is a complex colloidal suspension which carries myriads of information about human health. Understanding the evaporation dynamics and its consequent deposition patterns have direct relevance in disease detection. We report evaporation dynamics of whole and diluted blood droplets over hydrophilic (glass) and hydrophobic (PDMS, polydimethylsiloxane) substrates. Our experiments show that blood drops evaporating on a hydrophilic substrate exhibit radial and orthoradial cracks in the coronal region and random cracks in the central region. Using Griffith’s energy criterion, we show that crack formation takes place when the capillary pressure and the resulting compressive stress inside the evaporating droplet exceeds critical stress which depends on the elastic modulus, interfacial energy, and the particle concentration of the system. The width of the coronal region (w), the film thickness (h) at the contact line, and the crack pitch (p) decrease with increasing blood dilution. In the dilution range of 2.0–0.8% HCT (hematocrit), the transition from the cracking to the non-cracking regime is observed, which can be attributed to inadequate compressive stress available even after the evaporation of the blood droplet is completed. For the hydrophobic substrate, buckling instead of cracking is observed for the whole blood droplets, which can be attributed to the distinct wetting and evaporation kinetics. The buckling of the blood drop on a hydrophobic surface is attributed to the competition between capillary pressure originated due to the formation of an elastic network of RBCs (red blood cells) and the menisci formed between adjacent RBCs, and the critical buckling pressure. With increasing blood dilution, a transition from buckling (between 21 and 42% HCT) to cracking (between 21 and 2.0% HCT) of the droplets, and eventually to the non-cracking regime (between 2.0 and 0.8% HCT) is observed. Our study unravels the interesting attributes about one of the important physico-chemical factors (i.e. % HCT) that affect the evaporation of blood droplets and the resulting deposition patterns on substrates with different hydrophobicity.

Multiphysics Eulerian-Lagrangian Electrostatic Particle Spray- And Deposition Model for OpenFOAM® and KaleidoSim® Cloud-Platform

A finite volume based Eulerian-Lagrangian model has been created within OpenFOAM® in order to predict the behavior of particle clouds as well as particle deposition thicknesses on substrates under the influence of electro-static effects. The model resolves close to electrode effects as well as phenomena within the entire deposition chamber. It considers fluid dynamic effects, particle inertia, gravity, electric- as well as mechanic particle-particle interaction, corona formation, dynamic particle charging mechanisms, and coupling of particle motion to Reynolds-Averaged Navier-Stokes (RANS) based flow simulations. Resulting deposition pattern predictions were experimentally validated. It is demonstrated qualitatively and quantitatively that the measured deposition thicknesses and patterns vary by; i) applied voltage, ii) airflow rate, pistol-substrate iii) distance and iv) angle. Furthermore, the software has been prepared such that it works on the cloud computing software KaleidoSim®, which enables the simultaneous browser-based running of hundreds of cases for large parameter studies.

Relaxation and relaxation exchange NMR to characterise asphaltene adsorption and wettability dynamics in siliceous systems

The fraction of asphaltenes in crude oils is among the major concerns in upstream and downstream petroleum engineering. At reservoir scale asphaltenes may cause compartmentalisation and at pore scale they may create barriers to flow, change wettability conditions and relative permeability; and as a result affect ultimate oil recovery. Rock core ageing in oil is a common step in laboratory core analysis. Where possible, crude oils relevant to the origin of cores are used, while the use of arbitrary oils, various hydrophobic chemicals and anti-wetting agents is not uncommon. Published ageing time and temperature vary broadly. This fact motivates us to evaluate the applicability of synthetic oils for studies requiring wettability alteration. We systematically study the precipitation kinetics of the heavy-end oil fraction and wettability alteration properties of mixtures comprised by various proportions of commercially available bitumen, aromatics and alkane components.Low-field NMR relaxation measurements have been applied to characterise wettability of rocks by introducing an NMR wettability index. The latter requires multiple reference measurements at end-point saturation states similar to a standard Amott-Harvey workflow. Furthermore, petrophysical interpretation of T2 relaxation data is prone to be affected by diffusional coupling effects. NMR correlation techniques have a higher prediction capacity, e.g. the T2-store-T2 (REXSY) experiment is naturally sensitive to the spatial variation of physical properties by detecting diffusion exchange between different relaxation environments. We applied a combination of relaxation and relaxation exchange techniques to study the effect of asphaltene deposition on pore-space morphology and wettability for two siliceous systems with different surface topology. The change of wettability over ageing time in different synthetic oils was tracked using T2 relaxation measurements, providing estimates of ageing dynamics useful in designing wettability-related experiments.Results show that the kinetics of asphaltene deposition and wettability alteration processes are strongly dependent on chemical composition of synthetic oils, asphaltene origin (light oil or bitumen) and solid phase morphology. Elements of the resulting deposition pattern and wetting state of the cores were inferred using the proposed approach, utilising low-field NMR T2 relaxation and T2-store-T2 relaxation exchange experiments combined with numerical simulation of relaxation responses. The knowledge of deposition pattern and dynamics obtained mainly by the mean of combination of NMR relaxation techniques contributes to the improved design of core wettability alteration steps and potentially to enhanced petrophysical application of low-field NMR technology.

Aerosol dynamics model for the simulation of hygroscopic growth and deposition of inhaled NaCl particles in the human respiratory tract

The purpose of the present work is the implementation of the aerosol dynamics model ADiC, specifically its heat/vapor transport and phase transition components, into the stochastic lung deposition model IDEAL. The combined ADiC/IDEAL model allows the simultaneous calculation of relative humidity, hygroscopic growth and deposition of inhaled NaCl aerosols in individual airways of the human lung along randomly selected particle paths and randomly selected times during the inhalation phase. Hygroscopic growth decreases deposition of submicron particles compared to hydrophobic particles with equivalent diameters due to a less efficient diffusion mechanism, while the more efficient impaction and sedimentation mechanisms increase total deposition for micrometer particles. Due to the variability and asymmetry of the human airway system, individual trajectories of inhaled particles are associated with individual growth factors. For a realistic breathing scenario, particles are inhaled at different times during the inspiratory phase and hence experience individual growth factors, thereby further enhancing the variability of individual growth factors and resulting deposition patterns. While nanometer particles adopt their equilibrium growth factor value already within the mouth and pharynx/larynx during the inspiratory phase, micrometer particles approach this value at the end of inspiration, thus further increasing in size during the exhalation phase. In summary, individual hygroscopic growth factors vary for different paths and different inhalation times. For model validation purposes, theoretical predictions were compared with human experimental deposition data and previous hygroscopic growth model simulations for total deposition of ultrafine and micrometer-sized NaCl particles.

Gold particle formation via photoenhanced deposition on lithium niobate

In this work, we report on a technique to reduce gold chloride into sub-micron particles and nanoparticles. We use photoelectron transfer from periodically polarized lithium niobate (PPLN) illuminated with above band gap light to drive the surface reactions required for the reduction and particle formation. The particle sizes and distributions on the PPLN surface are sensitive to the solution concentration, with inhibited nucleation and large particles (>150nm) for both low (2E−8M to 9E−7M) and high (1E−5M to 1E−3M) concentrations of gold chloride. At midrange values of the concentration, nucleation is more frequent, resulting in smaller sized particles (<150nm). We compare the deposition process to that for silver, which has been previously studied. We find that the reduction of gold chloride into nanoparticles is inhibited compared to silver ion reduction, due to the multi-step reaction required for gold particle formation. This also has consequences for the resulting deposition patterns: while silver deposits into nanowires along boundaries between areas with opposite signed polarizations, such patterning of the deposition is not observed for gold, for a wide range of concentrations studied (2E−8 to 1E−3M).

Global migration of impurities in tokamaks

The migration of impurities in tokamaks has been studied with the help of tracer-injection (13C and 15N) experiments in JET and ASDEX Upgrade since 2001. We have identified a common pattern for the migrating particles: scrape-off layer flows drive impurities from the low-field side towards the high-field side of the vessel. Migration is also sensitive to the density and magnetic configuration of the plasma, and strong local variations in the resulting deposition patterns require 3D treatment of the migration process. Moreover, re-erosion of the deposited particles has to be taken into account to properly describe the migration process during steady-state operation of the tokamak.

The role of wind in the dispersal of floating seeds in slow‐flowing or stagnant water bodies

AbstractQuestionWhat is the role of wind in the dispersal of waterborne seeds in slow‐flowing and stagnant water bodies at different temporal and spatial scales? (i) Is there a direct effect of wind on seed dispersal speed and distance? (ii) Are prevailing wind conditions reflected in the seed deposition patterns during a year? (iii) What are the long‐term (multiple year) effects of prevailing wind conditions on the pattern and composition of shoreline seed banks?LocationThe Westbroekse Zodden (52˚10N; 5˚07E) and De Weerribben (52°46N; 5°55E) fen reserves in The Netherlands.MethodsReal‐time seed movement tracking experiments were conducted at different wind speeds. Additionally, we performed a seed trap experiment using artificial grass mats and carried out seed bank analyses using a seedling emergence test.ResultsWind speed and direction strongly determined the dispersal process and the resulting deposition patterns of floating seeds in shallow lakes or ponds. Wind speed directly influenced dispersal speed and distance. Increasing wind speed increased dispersal speed but decreased dispersal distance. Over multiple seasons, more seeds were deposited at downwind shorelines than at upwind shorelines, showing that wind‐driven hydrochory resulted in directional transport according to the prevailing wind direction. The species composition of deposited seeds was also affected, with proportionally more water‐dispersed seeds being deposited at down‐wind shorelines. These effects of wind speed and directionality will have consequences for the colonization of riparian zones in lentic systems and, therefore, also influence management and restoration. In the long term, local seed banks in riparian zones reflected the prevailing wind conditions poorly, showing that additional processes, such as differential germination and predation, also play important roles at longer time scales.ConclusionsWind plays an important role in the dispersal of waterborne seeds in lentic systems and (prevailing) wind speed and direction are reflected in seed dispersal trajectories and deposition patterns.

Material deposition and migration processes with resonant magnetic perturbation fields at TEXTOR

Resonant Magnetic Perturbations (RMPs) are applied with the Dynamic Ergodic Divertor (DED) at TEXTOR to control the plasma edge transport and the plasma surface interaction. This leads to the formation of a three-dimensional (3D) topology of the scrape-off layer (SOL). To quantify the erosion/deposition balance and the material migration in this 3D boundary, spherical test limiters were exposed to plasmas with and without RMP fields applied. Methane doped with 13C as tracer element was injected through a gas inlet in the test limiter. The local gas source was monitored by spatially resolving spectroscopy and the resulting deposition patterns on the limiters were analysed with colourimetry and nuclear reaction analysis. These measurements were compared to simulations of the magnetic field topology simulations. The data provide evidence of a particle migration dominated by an ExB drift within stochastic zones of the 3D plasma boundary.

A Two-Species Drug Delivery Model is Required to Predict Deposition from Drug-Eluting Stents

Drug-eluting stents are the treatment of choice for reducing restenosis rates after plaque removal. The drugs in use today partition between the plasma phase and binding sites in the vessel wall, where preferential binding leads to buildup and an increase in tissue residence times. Computational modeling has been used extensively to reveal key relationships between geometry, flow, drug physicochemical properties and the resulting deposition patterns. Yet, in spite of its’ simplicity, very little work has been done to include the presence of two drug forms in the modeling framework. Therefore, this paper outlines a method for simulating the transport of bound and unbound drug in a vessel wall. The method is demonstrated with a three-dimensional model of a standard stainless steel stent of Palmaz design deployed in a coronary artery. Drug transport in the vessel wall is driven by both diffusion and convection. The drug transport model also includes a reversible equilibrium reaction to account for tissue binding. The relative reaction rates control the interconversion of drug between the bound and unbound states. Results include a parametric study of convection–diffusion and equilibrium coefficient, providing an understanding of how drug physicochemical properties affect retention and distribution in the tissue. They also provide a first look at the relative distribution of the two drug forms in a full three-dimensional model of the vessel wall. The model also reveals how a single species drug delivery model can not accurately predict the distribution of bound drug. We therefore conclude that a two-species approach that includes reversible binding is the way forward for future stent-based drug delivery simulations.

Evaporative Deposition of Bacteria and Microspheres on Mica from a Sessile Drop: The Use of Surface Conditioning in a Laboratory Atmosphere to Control Drop Spreading and Particle Deposition Patterns

AbstractEvaporative deposition from a sessile drop is an appealing way to deposit materials on a surface due to the simplicity of the technique. In this work we deposit aqueous solutions of two types of colloidal particles, namely bacteria and microspheres, on mica. We show that by controlling the extent of initial drop spreading through subtle changes in surface conditioning caused by exposure to the laboratory atmosphere in a laminar flow hood it is possible to systematically vary the particle deposition patterns. On freshly cleaved mica the contact angle of water is &lt; 5°. Drops of bacterial and microsphere solutions deposited on freshly cleaved mica spread to cover a large surface area. Drying occurs through pinning and depinning events leaving a series of colloidal particle rings. We found in our laboratory that the contact angle of water on mica exposed to a constant flow of filtered laboratory air in a laminar flow hood gradually increases with time. For drops of both bacterial and microsphere solutions there is a corresponding decrease in the extent of drop spreading with increasing exposure of the mica surface to laboratory air. This results in a profound change in the colloidal particle deposition pattern. Short exposures of minutes to hours are enough to decrease spreading and affect the resulting deposition pattern. For our longest mica surface exposure times (months to 1 year) the contact angle of water reaches values near 20°. Spreading of the bacterial and microsphere drops is substantially decreased. A portion of the colloidal particles are deposited in an outer deposition ring which marks the extent of drop spreading and the remainder of the particles are deposited in the drop interior as a honeycomb or cellular film. The fraction of the drop residue covered with the cellular film increases with particle concentration as well as the length of time the mica is exposed to the laboratory atmosphere. This work shows that evaporative deposition on mica is very sensitive to surface conditioning through atmospheric exposure and also suggests that particle deposition patterns can be tuned by small changes in drop spreading.

Influence of the Dynamic Ergodic Divertor on the heat deposition pattern in TEXTOR at different collisionalities

The Dynamic Ergodic Divertor on TEXTOR imposes three-dimensional stochastic magnetic field lines in the plasma boundary. The resulting heat deposition patterns on the divertor surface depend strongly on a magnetic topology, including radial penetration of magnetic field lines within the first few toroidal revolutions. The influence of the collisionality in the plasma edge is of importance for the resulting deposition patterns. The heat flux formed by the electrons streaming along the field lines with shallow penetration is only seen at low collisionalities, while these field lines reach areas of low electron temperature. The other class of field lines penetrates relatively deep within the first few toroidal turns reaching the plasma region with much higher electron temperature, which makes parallel transport much more effective, even at the higher level of collisionality.

Numerical Modeling of Deposition of Inhaled Particles in Central Human Airways

The objective of our research is to model physical and biological processes related to the development of adverse health effects, especially initiation of lung cancer in the central human airways following inhalation of aerosol particles. There is experimental evidence that bronchogenic carcinomas originate mainly at the dividing zone of large central airway bifurcations where primary hotspots of deposition have been found. However, current lung deposition models do not take into consideration the inhomogeneity of deposition within bronchial airways. The flow field within three-dimensional morphologically realistic geometries of airway generations 3–6 of the human tracheobronchial tree is computed by the FLUENT CFD (computational fluid dynamics) code for a wide range of flow rates during both inhalation and exhalation. A large number of particle trajectories has been simulated to determine the resulting deposition patterns, which were finally quantified by local deposition enhancement factors. Both the local airflow fields and deposition patterns strongly depend on the shape of the geometry, especially on the form of the carinal ridge. The computed enhancement factors indicate that local deposition densities may be two orders of magnitude higher than the average values for scanning on the surface by a 100 × 100 μm surface element, i.e. at approximately cellular dimensions.

Monte Carlo modeling of aerosol deposition in human lungs. Part II: Deposition fractions and their sensitivity to parameter variations

The Monte Carlo code IDEAL-2 described in Part I [(1989) J. Aerosol Sci. 21, 666–674] has been used for the computation of total, regional and differential particle deposition in a stochastic lung structure. The results of these simulations are compared to experimental data and to other theoretical predictions based on deterministic lung models, demonstrating the applicability of our stochastic modeling approach. Compared to more simplistic deterministic models, this stochastic deposition model can supply additional information on the differential distribution throughout the lung and on the variability of the deposition within a given lung segment. A subsequent sensitivity analysis studies the effect of the parameters mainly affecting the resulting deposition pattern.