Charging Level and Deposition of Droplets in Electrostatic Painting

The process of electrostatic painting has become a very important method of coating in a wide range of industrial applications including those used in the automobile industry. The general principle of spray coating is to deposit liquid droplets or solid powder particles on coated targets having various shapes. The electrostatic coating process consists of three main stages: droplet formation and charging, transportation and deposition. The complication of this process is caused by various factors, such as the physical properties of the material to be used, the appropriate electrical and mechanical conditions and the target surface to be coated, which affects significantly the deposition uniformity and the finish quality, especially when it contains some sharp edges and recessed areas. In this thesis, a numerical investigation of the charging level on a spherical droplet formed out of a cylindrical ligament in an external uniform electric field is presented. The droplet charge on a single ligament was predicted for different droplet sizes, ligament lengths, ligament diameters and electrode widths. The effect of these model parameters on the charge levels was found to be significant. A mathematical approximation for the charge magnitude as a function of the droplet radius to some exponent and ligament length is also formulated. The value of the radius exponent decreases dramatically with increasing the ligament length. Also, the estimated values of the droplet charge were compared for linear and circular arrays of ligaments, which show a great influence of the geometry of the sprayer on the charge levels. A very good agreement between the experimental and numerical results in the case of a circular array of ligaments, including a specified space charge, was obtained. All numerical simulations were performed using COMSOL, a Finite Element commercial software. A further study is carried out to investigate the deposition thickness profile on a stationary and moving flat target by incorporating a numerical simulation of the industrial electrostatic coating process via ANSYS, a CFD commercial software. A modified injection pattern was suggested to achieve a closer agreement with the experimental data. The injection pattern includes 15 bands of different particle sizes (i.e. polydispersed particles) and charge to mass ratios. A combination of different injection angles and fractions of mass flow rates was suggested in each size band. A very good agreement between the experimental and numerical deposition patterns was obtained in both cases of a stationary and moving target. Also, the deposition thickness profile was calculated on a target surface with a small perturbation at the center using ANSYS numerical model. Different model parameters of a perturbed surface, such as the size of the indentation or the protrusion and the radius of the corner were investigated in this study. The numerical results reveal a very low particle concentration inside the indentation, which is caused by the Faraday cage effect and it is strongly affected by the depth of the indentation, while the edge effect, which shows the high concentration of deposited particles at the corner, increases with decreasing the radius of curvature. The predicted deposition patterns were very consistent with the calculated values of the electric field for different surface perturbations

INKJET PRINTING: FACING CHALLENGES AND ITS NEW APPLICATIONS IN COATING INDUSTRY

This study is devoted to some of the most important issues for advancing inkjet printing for possible application in the coating industry with a focus on piezoelectric droplet on demand (DOD) inkjet technology. Current problems, as embodied in liquid filament breakup along with satellite droplet formation and reduction in droplet sizes, are discussed and then potential solutions identified. For satellite droplets, it is shown that liquid filament break-up behavior can be predicted by using a combination of two pi-numbers, including the Weber number, We and the Ohnesorge number, Oh, or the Reynolds number, Re, and the Weber number, We. All of these are dependent only on the ejected liquid properties and the velocity waveform at the print-head inlet. These new criteria are shown to have merit in comparison to currently used criteria for identifying filament physical features such as length and diameter that control the formation of subsequent droplets. In addition, this study performs scaling analyses for the design and operation of inkjet printing heads. Because droplet sizes from inkjet nozzles are typically on the order of nozzle dimensions, a numerical simulation is carried out to provide insight into how to reduce droplet sizes by employing a novel input waveform impressed on the print-head liquid inflow without changing the nozzle geometry. A regime map for characterizing the generation of small droplets based on We and a non-dimensional frequency, Ω is proposed and discussed. In an attempt to advance inkjet printing technology for coating purposes, a prototype was designed and then tested numerically. The numerical simulation successfully proved that the proposed prototype could be useful for coating purposes by repeatedly producing mono-dispersed droplets with controllable size and spacing. Finally, the influences of two independent piezoelectric characteristics - the maximum head displacement and corresponding frequency, was investigated to examine the quality of filament breakup quality and favorable piezoelectric displacements and frequencies were identified

Smart electrostatic crop spraying using remote sensing technology

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University LondonFor this thesis, smart spraying robot was designed, constructed and tested to validate the concept of smart pest control. Electrostatic charging of sprayed pesticide was realized in a spray nozzle design that improved plant coverage and reduced wasted pesticide as well as soil pollution. A thorough investigation into electrostatic spraying was conducted, which was accompanied by extensive simulations and experimentation. The results obtained from the simulation experimentation on industry standard electrostatic spray system (ESS) nozzles along with laboratory testing of these nozzles, detecting spray coverage using water sensitive paper and additional optical spray visualization methods gave the necessary insight and experience required to develop a new spray nozzle. Additional COMSOL simulation and experimentation were carried out on a Fan Hydraulic Spray Nozzle (FHSN), the results of which allowed for the effective addition of electrostatic induction capabilities, thereby transforming the (FHSN) into Electrostatic Induction Spray Nozzle (EISN) which is one of the prime parts of the smart spraying system. SOLIDWORKS software was used in the designing parts of this nozzle which were then manufactured using a 3D printer. An AL05D robotic manipulator and a TTRK tracked platform from Lynxmotion ™ were the mini mobile robot components selected for the feasibility study of the smart electrostatic crop spraying system. This mobile robot was equipped with a CCD digital camera, a range detector, and path mark detector to provide the necessary sensors required by the smart electrostatic spray system. A Windows™ based mobile computer in addition to an ARDUINO™ based orksmicrocontroller system were chosen to provide the computational power required by the system. These were arranged in a master – slave configuration, with the main processing for images and motion being conducted inside the master computer using programs created by Matlab smart ™ software. The execution of motion commands and the operation of the range and path mark detection along with operating the spray nozzle were performed on the slave computer using C as the programming language. The manufactured smart electrostatic spray system moves along cotton crop rows with a camera that scans the selected plant for pest infestation on the upper and lower surfaces of plant leaves. When a pest is detected, the spray nozzle is targeted on it at the appropriate distance, and a burst of pesticide destroys it. The results of experiments have shown that using the electrostatic induction system improves coverage 3 to 4 fold and reduces soil contamination by 2 to 4 fold. The system has plenty of room for performance improvement, and future development will make it adaptable for application to other crops and applications.The Iraqi government, the Ministry of Higher Education and Scientific Research and Kerbala Universit

Characterization of Electrified Water Sprays

The research is focused on water sprays electrified by induction charging through an external electric field. The aim of this work is to verify and understand how the breakup mechanism of a liquid jet and the droplets’ size population are influenced by the electric field. Two experimental rigs were tested, and they will be named in the text as high-flow rate electrified spray (≈ L/min) and low-flow rate electrified spray (≈ mL/h). The choice to carry experiments on different scaled setup is due to the geometrical simplicity that the low-flow rate water spray system displays, which helped to better understand the physics beyond the electrification of a liquid jet. In both systems, the liquid was water pumped in a grounded nozzle. The liquid jet crossed an electric field generated by a toroidal ring connected to the HV power supply and placed around the jet itself. The experiments were conducted to estimate: i) the electric current (or the specific charge) acquired by the droplets, ii) the droplets’ size population at different electric potentials. For what concerns the high-flow rate electrified spray, hollow cone hydraulic nozzles were used. We found that the droplets current had a non-monotonic trend with the applied electric voltage: it increased linearly until to reach the optimum value and, after that point, it started to decrease fast. At same time, the breakup mechanism was influenced by the electric field. Indeed, for the primary breakup parameters, we observed that the breakup length reduced with the potential of about 20 %compared with the uncharged value until to reach an asymptotic value, while the spray angle enlarged due to the repulsion between equally charged droplets. In the secondary atomization, the droplets’ size distribution shifted toward smaller diameters as the electric potential increased. The percentile diameter, d70, chosen to represent the distribution, was quite constant for all the potentials from 0 kV until the potential at which the electric current was maximum. As soon as the current overcome the optimum point, the d70 started to reduce swiftly. In these conditions, the dimensionless ratio between the electrical stress and the surface tension stress, R, rise and become larger than 1, as confirmation of the effective influence of the electric field on liquid jet dynamics. We envisage that the insurgence of corona discharge on liquid jet surface could explain these phenomena. This hypothesis was confirmed by the experiments made on the low-flow rate electrified sprays or electrospray. In fact, this experimental rig was used to carry studied on the simple jet mode with whipping breakup, where the whipping is an off-axe instability that generates droplets much smaller that the nozzle inner diameter. The tests revealed that as soon as the electric current increased with the potential and the whipping instability took place on the liquid jet surface, the droplets’ size population were composed by droplets of size smaller than 0.5 mm. When it happens, the electrical stress overcomes the surface tension, as observed for the high-flow rate spray, and the corona discharge glow took place. It was confirmed by ad-hoc experiments made in a black-room. The results confirmed that the presence of the electric field modifies significantly the liquid jet atomization dynamic. It could be used to manipulate the droplets’ distribution accordingly to the require application

MODELING THIN FLUID FILM ON A ROTARY BELL

A component of the mission in the Institute of Research for Technology Development at the University of Kentucky is advancing research and development and bringing it to the factory floor for continuous improvement. This dissertation delves into the art and science of rotational fluid mechanics in the context of rotary bell atomizers. One outcome proves that an approximation for calculating fluid film thicknesses on high-speed spinning surfaces inferred while working in cylindrical and spherical coordinate systems can be applied to an arbitrary bell profile. However, the analytical limits of this approximation were not investigated. In all cases, a restriction exists that the bell profile curvature is much greater than the fluid film thickness. The validity of this approximation was supported by findings in publications employing curvilinear coordinates created by axisymmetric revolution of a planar curve. This validation enabled rigorous analyses of bell profiles beyond the common cylindrical or spherical profiles. A curvilinear system containing a coordinate of arc length along the bell profile is the arbitrary case, and the most reduced high-speed case collapses to an approximation from an observed pattern in simpler coordinate systems. The jump to varying curvature curvilinear coordinates requires additional mathematics to calculate metric tensor coefficients and spatial derivatives of directional unit vectors, and to develop lengthy vector invariants. Another outcome was to explain the underlying symmetry in the reduced order solution for a coupled rotary system that included centrifugal and Coriolis effects on a conical rotary bell

Evaluation of low-volume herbicide application technology for no-till soybeans

A study performed at The University of Tennessee Milan Experiment Station during 1983 evaluated and compared preemergence and postemergence herbicide application techniques in no-tillage and conventionally-cultivated soybean plots. The equipment utilized for herbicide application included conventional medium-volume hydraulic flat fan nozzles, low-volume flat fan nozzles, controlled droplet applicators (CDA), and an ultra low-volume electrostatic sprayer. Various application parameters and techniques were compared: low-volume versus medium-volume spray rates, ultra-low versus medium-volume application rates, flat fan nozzles versus controlled droplet applicators (CDA) for low-volume spraying, and water-only carrier versus oil-in-water diluents for low-volume application. Sprayed plots were evaluated for percentage control of selected weed species and for soybean yields. Results showed that spray application volume (5 gal/acre versus 20 gal/acre) applied either preemergence or postemergence did not affect the level of weed control in no-till soybeans. Moreover, low-volume hydraulic nozzles and controlled droplet applicators (CDA) at 5 gal/acre were equally effective in controlling weeds. Roundup plus Lorox plus Lasso applied preemergence in wheat stubble gave significantly better control of Pennsylvania smartweed than a tank mix of paraquat plus Lorox plus Lasso. Both full and half-label rates of Basagran plus Blazer applied postemergence in wheat stubble were significantly more effective in controlling cocklebur than Pennsylvania smartweed. Addition of a crop oil concentrate did not increase weed control in the low-volume treatments. Application of Fusilade in conventionally-cultivated soybeans using an electrostatic spray system at 0.8 pt/acre total spray volume gave similar levels of weed control to applications of the same herbicide rates at 20 gal/acre using conventional hydraulic nozzles. Soybean yields overall in 1983 were much below normal due to drought conditions during the growing season. Crop yields within a given production practice were not significantly affected by spray application techniques

Response of Electrified Micro-Jets to Electrohydrodynamic Perturbations

The breakup of liquid jets is ubiquitous with rich underpinning physics and widespread applications. The natural breakup of liquid jets originates from small ambient perturbations, which can grow exponentially until the amplitude as large as the jet radius is reached. For unelectrified inviscid jets, surface energy analysis shows that only the axisymmetric perturbation is possibly unstable, and this mode is referred as varicose instability. For electrified jets, the presence of surface charge enables additional unstable modes, among which the most common one is the whipping (or kink) instability that bends and stretches the charged jet that is responsible for the phenomena of electrospinning. A closer examination of the two instabilities suggests that due to mass conservation, the uneven jet stretching from whipping may translate into radial perturbations and trigger varicose instabilities. Although the varicose and whipping instabilities of electrified micro-jets have both been extensively studied separately, there is little attention paid to the combined effect of these two, which may lead to new jet breakup phenomena. This dissertation investigates the dynamic response of electrified jets under transverse electrohydrodynamic (EHD) perturbations which were introduced by exciters driven by alternating voltage of sweeping frequency. Three different jetting mechanisms are used to generate jets with various ranges of jet diameters: ~150 micrometer inertial jets from liquid pressurized through a small orifice, ~50 micrometer flow focused jets, and ~20 micrometer electrified Taylor-cone jets. The transverse perturbations enable systematic triggering of varicose and whipping instabilities, and consequently a wide range of remarkable phenomena emerge. For inertial jets with zero or low charge levels, only varicose instability is observable due to suppressed whipping instability. At modest charge levels, inertia jets can respond to the fundamental perturbation frequency as well as the second harmonic of the perturbation frequency. Highly charged jets such as fine jets generated from Taylor cones exhibit distinct behavior for different perturbation wavenumber x. Typical behavior include: whipping jets with superimposed varicose instability at small x, jet bifurcation from crossover of whipping and varicose instabilities at x~0.5, Coulombic fission owing to the surge of surface charge density as the slender liquid segments recover spherical shapes at x~0.7, and simple varicose mode near wave numbers of unity. The phenomena observed in this work may be explained by a linear model and rationalized by the phase diagram in the space of wave number and dimensionless charge levels. The experimental apparatus used in this dissertation is simple, non-intrusive, and scalable to a linear array of jets. The rich phenomena combined with the versatile apparatus may spawn new research directions such as regulated electrospinning, generating strictly monodisperse micro/nano droplets, and manufacturing of non-spherical particles from drying droplets that undergo controlled Coulombic fissions

Numerical Methods for Simulating Multiphase Electrohydrodynamic Flows with Application to Liquid Fuel Injection

One approach to small-scale fuel injection is to capitalize upon the benefits of electrohydrodynamics (EHD) and enhance fuel atomization. There are many potential advantages to EHD aided atomization for combustion, such as smaller droplets, wider spray cone, and the ability to control and tune the spray for improved performance. Electrohydrodynamic flows and sprays have drawn increasing interest in recent years, yet key questions regarding the complex interactions among electrostatic charge, electric fields, and the dynamics of atomizing liquids remain unanswered. The complex, multi-physics and multi-scale nature of EHD atomization processes limits both experimental and computational explorations. In this work, novel, numerically sharp methods are developed and subsequently employed in high-fidelity direct numerical simulations of electrically charged liquid hydrocarbon jets. The level set approach is combined with the ghost fluid method (GFM) to accurately simulate primary atomization phenomena for this class of flows. Surface effects at the phase interface as well as bulk dynamics are modeled in an accurate and robust manner. The new methods are implemented within a conservative finite difference scheme of high-order accuracy that employs state-of-the-art interface transport techniques. This approach, validated using several cases with exact analytic solutions, demonstrates significant improvements in accuracy and efficiency compared to previous methods used for EHD simulations. As a final validation, the computational scheme is applied in direct numerical simulation of a charged and uncharged liquid kerosene jet. Then, a detailed numerical study of EHD atomization is conducted for a range of relevant dimensionless parameters to predict the onset of liquid break-up, identify characteristic modes of liquid disintegration, and report elucidating statistics such as drop size and spray dispersion. Because the methodologies developed and validated in this work open new, simulations-based avenues of exploration within a broader category of electrohydrodynamics, some perspectives on extensions or continuations of this work are offered in conclusion

The flow of a Newtonian liquid on rotating inclined surfaces with application to atomization

Spinning cup atomizers are used industrially to produce an airborne spray. Often the size distribution of the spray is important. The size distribution of the spray depends on the physical dimensions of the cup, the operating-conditions and the physical properties of the fluid to be atomized. A spinning cup atomizer can be considered in the following way. The liquid to be atomized is fed centrally on to the surface of the cup where it begins to thin into a liquid film under the centrifugal action of the rotating cup. As the liquid flows film-wise to the lip of the cup, where atomization occurs, it obtains energy from the cup. Obviously, efficient atomization will occur if the liquid reaches the lip of the cup after it has picked up sufficient energy and if it was evenly distributed on the cup initially. [Continues.