Conference Paper

Numerical Investigation of Electrostatic Spray Painting Transfer Processes for Vehicle Coating

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Abstract

In this study we examined numerically the electrostatic spray transfer processes in the rotary bell spray applicator, which is this case implemented in a full 3D representation. The algorithm implemented and developed for this simulation includes airflow, spray dynamics, tracking of paint droplets and an electrostatic modularized solver to present atomization and in-flight spray phenomena for the spray forming procedure. The algorithm is implemented using the OpenFOAM package. The shaping airflow is simulated via an unsteady 3D compressible Navier-Stokes method. Solver for particle trajectory was developed to illustrate the process of spray transport and also the interaction of airflow and particle that is solved by momentum coupling. As the numerical results in this paper indicates dominant operating parameter voltage setting, further the charge to mass ratio and air-paint flow rate deeply effect the spray shape and the transfer efficiency (TE). The spin of the bell forced the paint to fall off from the bell edge into the high-velocity airflow. By increasing the shaping airflow more uniform distribution of mass of paint is produced but the TE decreases. The size distribution is highly sensitive to the bell rotational speed. Our results demonstrate the validity of the numerical procedure and also the applicability of it in reducing the production costs for painting. Unsteady and dynamic treatment of the flow is described using Large eddy simulation (LES) turbulence models, whereas the movement of the paint particles is modeled by the approach of tracking droplet size distribution, also the electrical potential effect on its size and form can be predicted. Using this advanced numerical simulation the efficiency and quality of the program can be evaluated precisely for industrial robots of paint over Off-Line Programming (OLP). Further, the paint spray structure obtained from the numerical simulations is compared with the experimental data at the same working condition with appropriate quantitative accuracy.

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... In industries, the applications of a robot include loading, finishing, welding, assembly, spray painting, etc. Spray painting involves directing atomized paint particles toward a surface that needs to be coated [1]. The paint particles are carried to the surface in a gaseous medium, which is usually compressed air [2]. Robots are employed for this process primarily because of the hazardous nature of the process, especially because of the involvement of atomized paint particles, which are damaging to the human body. ...
... Normalization was done to ensure that the performance values conformed to the same standard. The overall performance values of alternatives were calculated using Equation (2) and are shown in Table 2. The performance value of each alternative upon removing each criterion was calculated using Equation (3). ...
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Robots are being increasingly utilized for various operations in industrial and household applications. One such application is for spray painting, wherein atomized paint particles are sprayed on a surface to coat the surface with paint. As there are different models of robots available for the job, it becomes crucial to select the best among them. Multi-criteria decision-making (MCDM) techniques are widely used in various fields to tackle selection problems where there are many conflicting criteria and several alternatives. This work focuses on selecting the best robot among twelve alternatives based on seven criteria, among which payload, speed, and reach are beneficial criteria while mechanical weight, repeatability, cost, and power consumption are cost criteria. Five MCDM techniques, namely combination distance-based assessment (CODAS), complex proportional assessment (COPRAS), combined compromise solution (CoCoSo), multi-attributive border approximation area comparison (MABAC), and višekriterijumsko kompromisno rangiranje (VIKOR) were used for the selection while a weight calculation was performed using an objective weight calculation technique called MEREC. HY1010A-143 was found to be the most suitable robot for spray-painting applications by four of the five techniques used. Correlation studies showed a significant level of correlation among all the MCDM techniques.
... Since the blades are cyclically experiencing up-strokes and down-strokes, flow shedding and separations are highly affecting the blade surfaces, thus, k-ω-S S T model better captures the near-wall effects as well. Pendar and Pascoa [42,43] simulated a turbulent flow with high rotational speed and provided a thorough understanding of the fluid dynamic characteristics. Their results for those rotational cases were also better obtained by the k-ω-S S T model. ...
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... Since the blades are cyclically experiencing up-strokes and down-strokes, flow shedding and separations are highly affecting the blade surfaces, thus, k-ω-S S T model better captures the near-wall effects as well. Pendar and Pascoa [42,43] simulated a turbulent flow with high rotational speed and provided a thorough understanding of the fluid dynamic characteristics. Their results for those rotational cases were also better obtained by the k-ω-S S T model. ...
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The present study demonstrates an improved performance of cycloidal rotors by actively controlling the pitching oscillations and rotational speeds. The computational fluid dynamics (CFD) coupled with artificial neural network (ANN) were the methodologies used in the optimization analysis for the hover-state operation rather than the take-off mode under ground effects [1]. The former is carried out to obtain numerical predictions at various operating conditions for an UAV-scale cyclorotor. The oscillating-rotating blades and the corresponding flowfield is computed unsteadily along the complete circular trace for performance considerations. From CFD simulations, the optimum operational state is predicted for a 30◦ and 500 (rpm) pitch angle and rotating speed, respectively. On a second step, by training the ANN with the CFD database at various operating conditions and parameters, the ANN was then capable of analyzing the optimum states for operating at different conditions. The pitching oscillation schedule is then optimized for each rotational speed by using ANN and for each azimuthal location over the traversing trace. This will imply to perform onboard control in active mode for the blades, rather than assigning constant pitching oscillations for all operating states. This active control concept showed to be a potential approach to enhance the cyclorotor efficiency by 12 percent in average.
... By this, these plasma devices create a body force that can be used for active flow control purposes. This electro-hydrodynamic phenomenon has been also studied for other type of applications [8,9].The works conducted by Post & Corke [10] and Corke et al. [11], are examples of studies in which the authors used DBD plasma actuators for separation flow control in airfoils. In these studies the authors operated DBD plasma actuators in the flow separation region and concluded that these devices are able to accelerate the neutral gas flow pulling the flow toward the surface and re-attaching it. ...
Conference Paper
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Dielectric Barrier Discharge (DBD) plasma actuators are simple devices with great potential for active flow control applications. Further, it has been recently proven their ability for applications in the area of heat transfer, such as film cooling of turbine blades or ice removal. The dielectric material used in the fabrication of these devices is essential in determining the device performance. However, the variety of dielectric materials studied in the literature is very limited and the majority of the authors only use Kapton, Teflon, Macor ceramic or poly(methyl methacrylate) (PMMA). Furthermore, several authors reported difficulties in the durability of the dielectric layer when the ac-tuators operate at high voltage and frequency. Also, it has been reported that, after long operation time, the dielectric layer suffers degradation due to its exposure to plasma discharge, degradation that may lead to the failure of the device. Considering the need of durable and robust actuators, as well as the need of higher flow control efficiencies, it is highly important to develop new dielectric materials which may be used for plasma actuator fabrication. In this context, the present study reports on the experimental testing of dielectric materials which can be used for DBD plasma actuators fabrication. Plasma actuators fabricated of poly(vinylidene fluoride) (PVDF) and polystyrene (PS) have been fabricated and evaluated. Although these dielectric materials are not commonly used as dielectric layer of plasma actua-tors, their interesting electrical and dielectric properties and the possibility of being used as sensors, indicate their suitability as potential alternatives to the standard used materials. The plasma actuators produced with these nonstandard dielectric materials were analyzed in terms of electrical characteristics, generated flow velocity and mechanical efficiency, and the obtained results were compared with a standard actuator made of Kapton. An innovative calorimetric method was implemented in order to estimate the thermal power transferred by these devices to an adjacent flow. These results allowed to discuss the ability of these new dielectric materials not only for flow control applications but also for heat transfer applications. NOMENCLATURE C Capacitance. C p Specific heat at constant pressure. I Current. l Plasma actuator length. n Number of cycles. m Mass. m Mass flow rate. P el Electrical power. P m Mechanical power. P t Thermal power.
... The accurate implementation of the electric field is a crucial issue in an electrostatic spray simulation to correctly capture the flow details. Some of the more recently published numerical investigations on the electrostatic effect on the paint particles during coating are the ones of Domnick et al. (2005), Ye et al. (2005), Colbert (2007), Toljic et al. (2011), Pendar and P ascoa (2019a;2020d), and Pendar and P ascoa (2020a). ...
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This paper presents a numerical performance evaluation of the electrostatic rotary bell sprayer (ERBS) with a particular focus on droplet charge, electric field, and ambient conditions through the implementation of a high-voltage control-ring field pattern effect into the fully turbulent airflow and by including the atomized droplets discrete phase. The simulation shows that the inclusion of droplet charging and electric field coupling, with different parametric values, significantly impacts the atomized droplet distribution over the spray plume and the deposition rate. This analysis was conducted using a three-dimensional (3D) Eulerian–Lagrangian model to describe the two-phase spraying flow by extending the base OpenFOAM package. The procedure includes an unsteady compressible Navier–Stokes solver combined with a large Eddy simulation approach to model turbulence effects on the air flowfield. This is coupled to the spray dynamics by including droplet trajectory tracking, wall film dynamics, and electric field charge. The approach is further extended to include the evaporation phenomenon and the transport of its products. Compared to a conventional ERBS, herein, we provide an in-depth analysis of the fluid dynamic characteristics around the ERBS with a control-ring field pattern for vorticity, velocity, and electrical fields. The results indicate that the control-ring operation improves the performance and transfer efficiency of the ERBS, and it also helps to harmonize the direction of the charged paint droplets. For the first time, finding a balance between the effect of the inside bell cup surface and control-ring voltage and charged droplet has been conducted.
... During the last years, some works had been implemented on the coating procedure by the ERBS that are focused on (a) the electrical charge process [1,2,3], (b) droplet size and disintegration procedure [4,5] and (c) the high-pressure airflow director' effect on the spray cloud [6,7]. As a recent numerical publication about the electrical assistance effect on the paint particles during the painting, references [1,[8][9][10][11][12] can be named. Im et al. [13] showed that using the electrical field in combination with the shaping airflow considerably improves the painting efficiency. ...
Conference Paper
Electrostatic Rotary Bell Sprayers (ERBSs) have been widely used in the painting industry, especially in the automotive and aerospace industries, due to their superior performance. The effects of the applied voltage and paint droplet charge values on the spraying pattern and coating Transfer Efficiency (TE) in the ERBS, including a high-voltage ring for spray cloud control, have been studied numerically in a wide range of droplet size distribution. A 3D Eulerian-Lagrangian numerical analysis is implemented under the framework of the OpenFOAM package. The fluid dynamics of turbulence, primary and secondary breakup procedures are modeled using a large eddy simulation (LES) model, Rosin-Rammler distribution, and modified TAB approach, respectively. Compared to the conventional ERBSs, in the current work an exhaustive exhibition of the airflow dynamic and spray pattern characteristics around the ERBS with outer high-voltage control-ring field pattern, i.e., velocity, vorticity, electric potential field, overspray, film thickness are reported. Our findings indicate that the controlling around the rotary bell rim improves the coating procedure TE and produces a more harmonized and narrower spray plume. Our in-depth investigation clearly shows the importance of the droplet charge values, voltage quantity, and considered droplet size range on the film thickness and its corresponding quality that is obtained from the workpiece.
... In order to have an accurate simulation of the turbulence airflow, selecting an appropriate turbulence model is a critical issue [20,21]. The Large Eddy Simulation (LES) turbulence model, as described in detail in [22,23], is appropriate to capture the flow features with high turbulent and gradient like present work and analyze the internal flow principle [24,25,26]. ...
Conference Paper
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... When a signal with these characteristics is applied to the exposed electrode, a plasma discharge is generated in the top of the dielectric surface. The adjacent air is ionized and, due to the presence of the electric field, a body force is generated which will accelerate the charged particles [27,28]. An ionic wind is then created with a tanget direction to the surface in which the device is applied [29,30]. ...
Conference Paper
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... When a signal with these characteristics is applied to the exposed electrode, a plasma discharge is generated in the top of the dielectric surface. The adjacent air is ionized and, due to the presence of the electric field, a body force is generated which will accelerate the charged particles [7,8]. An ionic wind is then created with a tangent direction to the surface in which the device is applied [9,10]. ...
Conference Paper
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An experimental investigation was conducted in order to understand the ability of plasma actuators to operate in three different modes: flow control, ice formation detection and ice accumulation prevention. When plasma actuators are operated with voltage levels, above the breakdown voltage, a plasma discharge surface is generated and with that, an ionic wind is produced. By using this phenomena, plasma actuators may be used to manipulate flow fields and control adjacent flows to the surface in which they are applied. However, a big part of the power applied to the device is dissipated as heat. Due to heat dissipation, the actuator surface temperature rises and the adjacent air is heated. Considering this, actuators may operate as ice prevention devices by heating the surface where they are applied and preventing the ice formation and accumulation. On the other hand, plasma actuators present a behavior similar to a capaci-tor and they may operate as a capacitive sensor. In the presence of water or ice on the top of the surface, the electric field changes and with that, several plasma actuator electrical features change as well. By monitoring that changes, the presence of water or ice on the top of the surface can be detected and the plasma actua-tor may be used as an ice sensor device. Therefore, in the present study a plasma actuator was experimentally tested operating in these three different operation modes and its feasibility to perform these different tasks is shown.
... Selection of a suitable turbulence approach is a determinant issue to provide accurate and precise results in a simulation. Pendar and Pascoa (2019) simulated the electrostatic spray transfer processes in the rotary bell spray applicator in a full 3D representation. As their results indicate dominant operating parameter voltage setting, further the charge to mass ratio and air-paint flow rate deeply affect the spray shape and the transfer efficiency (TE). ...
... The challenges faced in developing and implementing effective automotive coating processes are evident by research performed to develop new paint formulations [1], new applicator designs [2], and optimum operating parameters for coating apparatus [3,4]. Researchers have performed studies concerning the effect of the speed of the rotary bell atomizers on the evaporation of the fluid [5], shaping air holes effect on atomization for pneumatic atomizers [6], the transfer process for electrostatic spray coating [7], and non-Newtonian liquid with shear-thinning behavior [8]. Furthermore, a substantial amount of research has concentrated on simulating the effects of operating parameters on the performance of electrostatic rotary bell sprayers (ERBS) [9][10][11][12][13]. ...
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A rotary atomizer, which is generally used in car body painting, atomizes liquid paint into micro-particles at a rate of several hundred ml/min. The diameter of the micro-particles is not uniform, but has a broad size distribution ranging from near zero to over several hundreds of microns. It is known that this distribution affects transfer efficiency and paint finish quality. This paper discusses the relationship between liquid paint flow and atomization when the inner shape of the bell was changed, focusing on paint flow just before discharge from the bell edge.
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The reliability of numerical simulations of turbulence depend on ones ability to quantify and control discretization errors. In the classical literature on error analysis, typically, simple linear equations are studied. Estimates of errors derived from such analyses depend on the assumption that each dependent variable can be characterized by a unique amplitude and scale of spatial variation that can be normalized to unity. This assumption is not valid for strongly nonlinear problems, such as turbulence, where nonlinear interactions rapidly redistribute energy resulting in the appearance of a broad continuous spectrum of amplitudes. In such situations, the numerical error as well as the subgrid model can change with grid spacing in a complicated manner that cannot be inferred from the results of classical error analysis. In this paper, a formalism for analyzing errors in such nonlinar problems is developed in the context of finite difference approximations for the Navier-Stokes equations when the flow is fully turbulent. Analytical expressions for the power spectra of these errors are derived by exploiting the joint-normal approximation for turbulent velocity fields. These results are applied to large-eddy simulation of turbulence to obtain quantitative bounds on the magnitude of numerical errors. An assessment of the significance of these errors in made by comparing their magnitudes with that of the nonlinear and subgrid terms. One method of controlling the errors is suggested and its effectiveness evaluated through quantitative measures. Although explicit evaluations are presented only for large-eddy simulation, the expressions derived for the power spectra of errors are applicable to direct numerical simulation as well. 21 refs., 10 figs.
Article
Charge to mass ratio is a crucial parameter that governs the behavior of particle trajectories in a charged cloud of particles. The complex nature of the charging process limits our ability to accurately determine the charging level when particles of varying size are present. Using a numerical approach, it is possible, however, to take into account predefined values for this parameter. In this paper, the average charge to mass ratio and the distribution of the charge to mass ratio in the coating of a flat target were systematically varied to demonstrate their effect on the motion of the charged particles. The results show that the transfer efficiency increases as the average charge to mass ratio increases. It was found that the transfer efficiency is a weak function of the average particle size in the range tested and that it increases as the width of the size distribution increases.
Article
High-speed rotary bell atomizers are widely used in the painting industry for high-quality applications. They provide a highly uniform film thickness with reasonable transfer efficiency due to the additional electrostatic field supporting the droplet transport toward the target. The present contribution summarizes results related to high-speed rotary atomization of Newtonian fluids without electrical field. Water-sucrose-alcohol solutions have been used to vary viscosity and surface tension independently. The atomization processes observed could be divided into two distinct modes, that is, jet disintegration and transition from jet to sheet disintegration, the latter one being characterized by the presence of both radial and lateral fluid structures close to the bell edge. The separation line between these modes is expressed in terms of the We number and the flow number q. For jet disintegration, the Sauter diameter has been found to be a function of the three dimensionless parameters Re, We, and the flow number q. In the transition region from jet to sheet disintegration, the Sauter diameter is a function of the We number only. Finally, measured size distributions have been fitted by the three-parameter log-hyperbolic function and the location and shape of the distributions discussed as functions of the dimensionless parameters. Compared to other types of atomizers, it was found that the size distributions are not particularly narrow, as expected from the well-organized disintegration pattern, especially in the jet mode.
Article
A numerical study of the spray transfer processes in an electrostatic rotary bell applicator (ESRB) has been conducted utilizing code for a newly developed simulation code. This code consists of three modularized solvers: a fluid flow solver a spray dynamics solver and an electrostatic solver. The development of the code consisted of the following steps. First, the flow solver designed for an unsteady three-dimensional Navier-Stokes equation was developed to simulate the shaping airflow with the initial condition and the boundary condition supported by experimental data. Second, the particle trajectory solver which interacts with the airflow by momentum coupling, was developed to apply the spray transport processes. Finally, the electrostatic solver was developed to calculate the electrostatic field within the two phase flow field. The integrated code created by combining those three solvers was then applied to simulate the paint spray transport processes according to the operating conditions of interest. The numerical results show that the spray shape is very sensitive to changes in the charge to mass ratio. The voltage setting is a dominant operating parameter affecting the numerical transfer efficiency The voltage range studied was 0 kV to 90 kV In addition, the transfer efficiency decreases as the shaping airflow rate increases. However a high shaping airflow rate produces a more uniform distribution of spray mass on the target plane.
Article
Numerical calculations of gas-particle flows involving an electrical field, as they are found in the powder painting process, are presented based on the Euler/Lagrange approach. The gas flow is calculated by solving the Reynolds-averaged Navier–Stokes equations including the k–ε turbulence model. In order to solve the Laplace equation for the electrostatic field, a finite-volume approach is applied. The particle phase is simulated by using a Lagrangian treatment where a large number of particles are tracked through the flow and the electrostatic field. In addition to the drag and gravity, also, the electric field force is considered in the equation of motion. Average properties of the particle phase are obtained by ensemble averaging. The method is applied to the powder painting process. Two types of powder guns were considered, a slit nozzle and a round nozzle with a dispersion cone. The calculated general flow structure in the vicinity of the painting gun and the large-scale flow within the painting booth agreed reasonably well with the experimental observations. Additionally, the coating layer thickness was used for comparison, which is also relevant to judge the quality of the coating process. The results showed a reasonable good agreement between calculations and measurements for the slit nozzle. However, the extent of the paint layer was remarkably under-predicted for the round nozzle, which is mainly caused by the complex flow in this case. Nevertheless, the results demonstrate that numerical calculations are useful and effective for supporting the design and optimization of powder painting booths.
Article
Although electrostatic spray coating (E-spray) is widely used, its complexity requires optimization based on an empirical understanding of the spray dynamics. The project goal is to develop a mathematical model of the electrostatic field, continuum flow-field, and particle trajectories in an E-spray process. By restricting the use of empirically based equations to the atomization phase of the spray process, this model should have the flexibility to tolerate “real-world” system complexities (i.e. multiple applicators, complicated geometries, etc.) and the ability to be used with any type of E-spray gun sharing the same atomization characteristics.This model predicts coupling between three components: the fluid mechanics of the continuum flow field, the electrostatic field, and the particle trajectories. The system is a vertical bell-cup sprayer and a grounded disc centered on the gun axis. An axisymmetric electrostatic model is assumed, while the fluid mechanics and particle trajectories are solved in 3-D.A dilute spray assumption (i.e. no direct particle–particle interactions) allows modeling single-particle trajectories resulting from a balance of electrostatic force, drag and inertia. Varying the particle size generates volume-averaged properties of individual paths to simulate the charge density and fluid drag of a sprayed particle distribution. A turbulence energy–dissipation rate (k–ɛ) model provides the continuum velocity for the particle drag. These individual systems are solved sequentially and that sequence is iterated to convergence.Results include the effect of charged particles on the electrostatic field and identification of the dominant factors affecting coating thickness distribution.
Article
The ability to simulate complex unsteady flows is limited by the current state of the art of subgrid-scale (SGS) modeling, which invariably relies on the use of Smagorinsky-type isotropic eddy-viscosity models. Turbulent flows of practical importance involve inherently three-dimensional unsteady features, often subjected to strong inhomogeneous effects and rapid deformation, which cannot be captured by isotropic models. Although some available improved SGS models can outperform the isotropic eddy-viscosity models, their practical use is typically limited because of their complexity. Development of more-sophisticated SGS models is actively pursued, and it is desirable to also investigate alternative nonconventional approaches. In ordinary large eddy simulation (LES) approaches models are introduced for closure of the low-pass filtered Navier–Stokes equations (NSE). A promising LES approach is the monotonically integrated LES (MILES), which involves solving the unfiltered NSE using high-resolution monotone algorithms; in this approach, implicit SGS models, provided by intrinsic nonlinear high-frequency filters built into the convection discretization, are coupled naturally to the resolvable scales of the flow. Formal properties of the effective SGS modeling using MILES are documented using databases of simulated free and wall-bounded inhomogeneous flows, including isotropic decaying turbulence, transitional jets, and channel flows. Mathematical and physical aspects of (implicit) SGS modeling through the use of nonlinear flux limiters are addressed using a formalism based on the modified LES equations.
CFD Analysis of the Electrostatic Spray Painting Process with a Rotating Bell Cup
  • V Viti
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Stevenin, C., Béreaux, Y., Charmeau, J.-Y., and Balcaen, J., "Shaping Air Flow Characteristics of a High-Speed RotaryBell Sprayer for Automotive Painting Processes," J. Fluids Eng. 137:111304, 2015, doi:10.1115/1.4030703.
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