Joseph C. Simmer

Formation of liquid sheets by deposition of droplets on a surface

Experiments were done to observe the coalescence of highly viscous liquid droplets (87 wt% glycerin-in-water solutions) deposited onto a flat, solid steel plate. Droplets were deposited sequentially in straight lines or square droplet arrays. Droplet center-to-center distance was varied and the final dimensions of lines and sheets measured from photographs. When overlapping droplets were deposited surface tension forces pulled impacting droplets towards those already on the surface, a phenomena known as drawback. A dimensionless drawback index, quantifying the extent of droplet displacement, was calculated from experimental measurements for different values of droplet overlap. At large overlaps droplets deposited in a line or square array coalesced to form a circular film. When the droplet center-to-center distance increased, leading to less interaction, long, thin lines and square sheets were formed. As overlap was further decreased lines and sheets became discontinuous. A simple model was developed to predict the conditions under which rupture occurred. The lowest droplet overlap ratio (defined as droplet overlap distance divided by droplet spread diameter) at which a continuous liquid film could be formed was λ=0.293. At large overlap ratios (λ>0.6) droplets deposited in a square array formed a circular film. The minimum thickness of a continuous film formed by coalescence of droplets was shown to vary from 5% to 70% of the initial droplet diameter while increasing impact Weber and Reynolds number reduced the film thickness.

Charging Level of a Ligament-Droplet System Atomized in a Uniform Electric Field

In this paper, a numerical algorithm has been formulated for calculating the charge magnitude on a spherical droplet created by breakup of a cylindrical ligament in an external electric field. The algorithm is based on the finite-element method and it determines the droplet charge as a function of the droplet radius and ligament length. It has been found that the droplet charge is a function of particle radius to some exponent, which is equal to 2 when the droplet is in direct contact with the atomizer and decreases dramatically to approach 1.1, as the ligament length increases. The effect of the ligament radius on the charging level has been found to be significant and increases with the ligament length. It was determined that the charging level is affected by the presence of adjacent ligaments and previously ejected droplets. To investigate the dynamics of the droplet atomization, the Navier–Stokes equations, describing the fluid motion, as well as the level-set equation for tracking the interfaces between air and liquid have been solved. A comparison between the charging level of the dynamically formed droplets and an assumed spherical one has been presented. It has been found that the predicted charging level of the spherical droplet is lower than the one of the actual shape in the range 15%–26%.

Comparison Between the Numerical and Experimental Deposition Patterns for an Electrostatic Rotary Bell Sprayer

In this paper, a full 3D numerical model using ANSYS commercial software has been created to simulate the particle deposition profile for stationary and moving flat targets, assuming multiple injections of charged poly-dispersed particles. Different injection angles along three virtual rings were assumed to form a shower injection pattern. The experimental and the numerical results of deposition thickness have been presented and compared for different injection patterns. It has been found that there are some parameters, such as the total number of injection points, the radii of the rings and the fractional mass flow rate in each injection ring, which affect the numerical results of the deposition thickness and uniformity.Copyright

Estimation of droplet charge forming out of an electrified ligament in the presence of a uniform electric field

The charge on a liquid droplet is a critical parameter that needs to be determined to accurately predict the behaviour of the droplet in many electrostatic applications, for example, electrostatic painting and ink-jet printing. The charge depends on many factors, such as the liquid conductivity, droplet and ligament radii, ligament length, droplet shape, electric field intensity, space charge, the presence of adjacent ligaments and previously formed droplets. In this paper, a 2D axisymmetric model is presented which can be used to predict the electric charge on a conductive spherical droplet ejected from a single ligament directly supplied with high voltage. It was found that the droplet charging levels for the case of isolated electrified ligaments are as much as 60 times higher than that in the case of ligaments connected to a planar high voltage electrode. It is suggested that practical atomization systems lie somewhere between these two extremes and that a better model was achieved by developing a 3D approximation of a linear array of ligaments connected to an electrode having variable width. The effect on droplet charge and its radius was estimated for several cases of different boundary conditions.

The charging level of a ligament-droplet system atomized in a uniform electric field

In this paper, a numerical algorithm has been formulated for calculating the charge magnitude on a spherical droplet created by breakup of a cylindrical ligament in an external electric field. The algorithm is based on the finite-element method and it determines the droplet charge as a function of the droplet radius and ligament length. It has been found that the droplet charge is a function of particle radius to some exponent, which is equal to 2 when the droplet is in direct contact with the atomizer and decreases dramatically to approach 1.1, as the ligament length increases. The effect of the ligament radius on the charging level has been found to be significant and increases with the ligament length. It was determined that the charging level is affected by the presence of adjacent ligaments and previously ejected droplets. To investigate the dynamics of the droplet atomization, the Navier-Stokes equations, describing the fluid motion, as well as the level-set equation for tracking the interfaces between air and liquid have been solved. A comparison between the charging level of the dynamically formed droplets and an assumed spherical one has been presented. It has been found that the predicted charging level of the spherical droplet is lower than the one of the actual shape in the range 15%-26%.

Measuring Paint Droplet Size, Velocity, and Charge-to-Mass Ratio Distribution for Electrostatic Rotary Bell Spray Simulation

High-speed electrostatic rotary bells are widely used in the automotive industry as they provide high quality paint films with better transfer efficiency compared to air-atomizing guns. However, due to its highly turbulent spray pattern, transfer efficiency is still not ideal, i.e. some portion of paint will not reach intended target surfaces and becomes overspray. Numerical simulation of the electrostatic spraying process provides a tool to model this process as well as a way to optimize transfer efficiency. Currently, the state-of-the-art simulation model can simulate the flying trajectories of paint droplets from the edge of the rotating bell cup to the target surfaces. It requires some input information to start the simulation. The input information includes paint droplet size, velocity, and charge-to-mass ratio. Due to its large number of droplets, distributions based on droplet diameters are used to represent the entire droplet population. This paper describes experimental and mathematical methods to measure and calculate paint droplet size, velocity, and charge-to-mass ratio distributions. The resulting information can then be organized and used as the input data files for Electrostatic spray painting simulation.Copyright

Measurement of condensate- and smoke-causing emissions from automotive paints and sealers

Two major problems that periodically arise during the surface coating of automobiles are oven plume opacity (smoke that violates local opacity ordinances) and oven condensate that can drip on cars and leave an oily residue. Both of these problems are caused by the evaporation of semi-volatile organic compounds in bake ovens. A new test procedure has been developed to standardize the analysis and reporting of condensate/smoke-causing compounds that are emitted from paint, sealer, and coating materials when they are baked. Use of this test will allow identification and quantification of potential sources of condensate, smoke, and opacity in the laboratory. The information is essential to material suppliers and formulators, and to users who prescreen materials in order to avoid costly problems in production.