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9 pages, 2915 KiB

Open AccessArticle

Analysis of Paint Flow Pulsations during High-Speed Rotary Bell Atomization

by Walter Oswald, Jan Lauk, Lutz Gödeke, Peter Ehrhard and Norbert Willenbacher

Coatings 2019, 9(10), 674; https://doi.org/10.3390/coatings9100674 - 17 Oct 2019

Cited by 4 | Viewed by 3578

High-speed rotary bell atomization is the preeminent coating technique in the automotive industry. It is widely accepted that a narrow droplet size distribution and constant spray are necessary in order to guarantee uniform film thickness and high-quality appearance. This may be deteriorated by paint flow pulsations. So far, however, no studies exist regarding such fluctuations quantitatively for this type of atomizers. We fill this gap using image analysis of high-speed recordings close to the bell edge. We could show that the fundamental pulsation frequency increases linearly with rotational speed. A ratio of pulsation frequency and true rotational speed of about 3 was found, indicating that pulsations were initiated mainly by the three struts of the distributor disc. The coefficient of variation, i.e., the amplitude of fluctuation increased with decreasing liquid volume rate and rotational speed. Beyond that, we could show that the formation of droplets larger than 100 μm, which are assumed to cause paint defects, is promoted by the degree of fluctuation. These findings may stimulate development of bell cups showing less paint flow pulsations. Full article

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19 pages, 3616 KiB

Open AccessFeature PaperArticle

Spatial Positioning and Operating Parameters of a Rotary Bell Sprayer: 3D Mapping of Droplet Size Distributions

by Adnan Darwish Ahmad, Binit B. Singh, Mark Doerre, Ahmad M. Abubaker, Masoud Arabghahestani, Ahmad A. Salaimeh and Nelson K. Akafuah

Fluids 2019, 4(3), 165; https://doi.org/10.3390/fluids4030165 - 5 Sep 2019

Cited by 22 | Viewed by 6168

Abstract

In this study, we evaluated the fundamental physical behavior during droplet formation and flow from a rotary bell spray in the absence of an electrostatic field. The impact of a wide range of operating parameters of the rotary bell sprayer, such as flow rates, rotational speeds, and spatial positioning, on droplet sizes and size distributions using a three-dimensional (3-D) mapping was studied. The results showed that increasing the rotational speed caused the Sauter mean diameter of the droplets to decrease while increasing flow rate increased the droplet sizes. The rotational speed effect, however, was dominant compared to the effect of flow rate. An increase in droplet size radially away from the cup was noted in the vicinity of the cup, nevertheless, as the lateral distances from the cup and rotational speed were increased, the droplet sizes within the flow field became more uniform. This result is of importance for painting industries, which are looking for optimal target distances for uniform painting appearance. Furthermore, the theoretical formulation was validated with experimental data, which provides a wider range of applicability in terms of environment and parameters that could be tested. This work also provides an abundance of measurements, which can serve as a database for the validation of future droplet disintegration simulations. Full article

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5 pages, 194 KiB

Open AccessArticle

Reduction of Order: Analytical Solution of Film Formation in the Electrostatic Rotary Bell Sprayer

by Mark Doerre and Nelson K. Akafuah

Symmetry 2019, 11(7), 937; https://doi.org/10.3390/sym11070937 - 18 Jul 2019

Cited by 2 | Viewed by 3887

Abstract

This brief paper explains the slight differences in governing equations for a fluid film in a spinning cone, and the mechanism that reduces the order of a solution. Spinning cones with a centrally supplied fluid that spreads over its inner surface as a thin film have been the subject of interest for many years. Though often cast as a mathematical analysis, understanding this process is important, especially in the application of automotive painting. The analysis consists of a system of equations obtained from the Navier–Stokes equations along with simple boundary conditions that describe radial and tangential momentum conservation. Solutions to this system of equations are shown using several techniques. The connection between these techniques is slightly subtle. However, the conditions that enable reduction of order are clear once they are exposed. Directional velocity profiles in the film can be a combination of four roots in the complex plane. This system of roots also contains two diagonal axes of symmetry that are offset by 90 degrees. Alternatively, if the radial and tangential velocity profiles are expressed as a single complex function, a reduced order solution that is a combination of one set of diagonal set of roots can be found. Full article

13 pages, 7683 KiB

Open AccessArticle

Schlieren Visualization of Shaping Air during Operation of an Electrostatic Rotary Bell Sprayer: Impact of Shaping Air on Droplet Atomization and Transport

by Adnan Darwish Ahmad, Ahmad M. Abubaker, Ahmad A. Salaimeh and Nelson K. Akafuah

Coatings 2018, 8(8), 279; https://doi.org/10.3390/coatings8080279 - 11 Aug 2018

Cited by 35 | Viewed by 9106

Abstract

Electrostatic rotary bell sprayers (ERBSs) are widely used in the automotive industry. In ERBS, atomization is facilitated using centrifugal forces which disintegrate the paint film inside the cup into droplets at the cup edge. The droplets are then transported by the flow of a shaping air (SA) and electrostatic forces to a target surface; the characteristics of these droplets dramatically influence the quality of a painted surface and the painting transfer efficiency. In the current paper, a novel Schlieren-based visualization of the shaping air in the absence of paint droplets was performed during a qualitative investigation to delineate shaping air flow behavior and its interaction with droplets and droplet transport. An infrared thermographic flow visualization (IRFV) method and droplet size measurement were used to complement the Schlieren data for providing insight into shaping air-droplet interactions. The results demonstrated the impact of different operating conditions on the SA flow pattern, and the influence SA has on the secondary atomization and transport of droplets. Hence, these experimental methods combine with a useful tool for optimizing SA configurations that improve spray quality, droplet transport, and the efficiency of ERBS operations. Full article

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17 pages, 7903 KiB

Open AccessArticle

Study of Near-Cup Droplet Breakup of an Automotive Electrostatic Rotary Bell (ESRB) Atomizer Using High-Speed Shadowgraph Imaging

by Jacob E. Wilson, Stephen W. Grib, Adnan Darwish Ahmad, Michael W. Renfro, Scott A. Adams and Ahmad A. Salaimeh

Coatings 2018, 8(5), 174; https://doi.org/10.3390/coatings8050174 - 3 May 2018

Cited by 31 | Viewed by 7172

Abstract

Electrostatic Rotary bell (ESRB) atomizers are used as the dominant means of paint application by the automotive industry. They utilize the high rotational speed of a cup to induce primary atomization of a liquid along with shaping air to provide secondary atomization and transport. In order to better understand the fluid breakup mechanisms involved in this process, high-speed shadowgraph imaging was used to visualize the edge of a serrated rotary bell at speeds varying between 5000 and 12,000 RPM and with a water flow rate of 250 ccm. A multi-step image processing algorithm was developed to differentiate between ligaments and droplets during the primary atomization process. The results from this experiment showed that higher bell speeds resulted in a 26.8% reduction in ligament and 22.3% reduction in droplet Sauter Mean Diameters (SMD). Additionally, the ligament (ranging from 40 to 400 μm) diameters formed bimodal distributions, while the droplet (ranging from 40 to 300 μm) diameters formed a normal distribution. Velocities were also measured using particle tracking velocimetry, in which size-dependent velocities could then be computed. Droplet velocities were affected more by rotational speed than droplet SMD, while ligaments were affected by other factors than the rotational speed and ligament SMD. Full article

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9 pages, 254 KiB

Open AccessArticle

Effects of Bell Speed and Flow Rate on Evaporation of Water Spray from a Rotary Bell Atomizer

by Rajan Ray, Paul Henshaw, Nihar Biswas and Chris Sak

Coatings 2015, 5(2), 186-194; https://doi.org/10.3390/coatings5020186 - 29 May 2015

Cited by 8 | Viewed by 7464

Abstract

A phase doppler anemometer (PDA) was used to determine the effects of evaporation on water spray for three rotary bell atomizer operational variable parameters: shaping air, bell speed and liquid flow. Shaping air was set at either 200 standard liters per minute (L/min) or 300 L/min, bell speed was set to 30, 40 or 50 thousand rotations per minute (krpm) and water flow rate was varied between 100, 200 or 300 cubic centimeters per minute (cm³/min). The total evaporation between 22.5 and 37.5 cm from the atomizer (cm³/s) was calculated for all the combinations of those variables. Evaporation rate increased with higher flow rate and bell speed but no statistically significant effects were obtained for variable shaping air on interactions between parameters. Full article

(This article belongs to the Special Issue Innovative Coatings for Automotive Industry)

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