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On surface area coverage by an electrostatic rotating bell atomizer

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Abstract

An electrostatic rotating bell atomizer was used to produce a spray of a 20 wt% aqueous glycerol solution. Droplet size distributions were measured by photographing droplets in-flight and using image analysis software to measure their diameters. Increasing the rotational speed of the bell-cup resulted in a decrease in the average droplet diameter. The fraction of area covered by spray droplets was measured for various bell-cup speeds using a high-speed camera positioned behind a vertical glass substrate on which the liquid was sprayed. An analytical model, based on probability theory, was used to predict the fraction of surface covered solely using the droplet flux and the droplet size. Results from the model were compared to experimentally measured fractions of surface coverage and showed good agreement. The minimum theoretical film thickness assuming full coverage was derived from the probabilistic model. When the minimum theoretical film thickness was divided by the droplet diameter, it was found to be only dependent on the spread factor. Experiments to measure the fraction of area covered were also conducted using an automotive paint. Increasing the paint flow rate increased the fraction of area covered. Increasing rotational speed reduced splat size but had little effect on coverage. Increasing shaping air 1 flow rate reduced splat size and increased surface coverage significantly. Application of an electrostatic potential increased splat diameter but enhanced surface coverage only slightly.

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Abbreviations

A :

Area (µm2)

c :

Shape parameter for Burr distribution

D :

Diameter (µm)

E :

Eccentricity

F b :

Fraction of area not covered

F c :

Fraction of area covered

h :

Thickness of liquid film (µm)

k :

Shape parameter for Burr distribution

ME:

Margin of error

\(\dot{N}\) :

Local droplet flux (droplet/s)

n :

Number of droplets

P :

Perimeter

\(\hat{p}\) :

Estimate of proportion

s :

Standard deviation

t :

Time (s)

U :

Droplet impact velocity (m/s)

V :

Volume (µm3)

x :

Random variable

z :

Confidence level

Re:

Reynolds number

We:

Weber number

α :

Scale parameter for Burr distribution

β :

Impact parameter

\(\epsilon\) :

\(= A_{\text{av}} /A_{\text{s}}\)

μ :

Viscosity (mPa s)

ξ :

Droplet spread factor

ρ :

Density (kg/m3)

σ :

Surface tension (mN/m)

τ :

Minimum time for full coverage

20:

Surface mean

98:

98% Coverage

av:

Arithmetic mean

d:

Droplet

f:

Fluid

i :

Number

n :

Number of droplets

s:

Substrate

splat:

Splat

th:

Theoretical

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Acknowledgments

The authors thankfully acknowledge that funding for this project was provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada and by Magna Exteriors. Experiments were carried out at the test facilities of Magna Exteriors. We are grateful for the assistance of Dr. Amirreza Amighi in carrying out droplet size measurements.

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Authors and Affiliations

  1. Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Rd, Toronto, ON, M5S 3G8, Canada

    K. Sidawi & S. Chandra

  2. Magna Exteriors, 65 Independence Pl, Guelph, ON, N1K 1H8, Canada

    P. Moroz

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  1. K. Sidawi
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  3. S. Chandra
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Correspondence to K. Sidawi.

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Sidawi, K., Moroz, P. & Chandra, S. On surface area coverage by an electrostatic rotating bell atomizer. J Coat Technol Res 18, 649–663 (2021). https://doi.org/10.1007/s11998-020-00430-4

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  • DOI: https://doi.org/10.1007/s11998-020-00430-4

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