Transfer efficiency and area coverage of spray droplets impacting planar surfaces

Abstract

An experimental study was conducted to measure spray transfer efficiency, defined as the mass fraction of sprayed droplets that adhere to a target surface, and the rate of surface coverage by impacting droplets. The objective was to determine how transfer efficiency and surface coverage rates vary with droplet size distribution and air velocity, which is important in selecting spray parameters in painting and coating applications. The study was conducted using a wind tunnel consisting of a 6.5-cm-diameter tube connected to a tubular fan, producing controlled airflow velocities from 2 to 9 m/s. Sprays of canola oil or a 33 vol% glycerin–water mixture were introduced into the airstream and directed toward a 10-cm-diameter target disk, where the mass of deposited droplets was measured to evaluate transfer efficiency. Transfer efficiency was calculated by dividing the total target weight change by the weight of liquid sprayed. Droplet diameter distributions were measured using a direct imaging method. A high-speed camera was used to photograph droplets landing on the substrate and the rate of area coverage by impacting droplets measured. Transfer efficiency and surface coverage rates increase with airstream velocity. Larger droplets, whose motion is dominated by inertia, have a higher probability of reaching the target and a higher transfer efficiency. Below a critical Stokes number (St < 0.25), droplets fail to reach the target, irrespective of velocity. Droplet trajectories were modeled using an analytical solution to the inviscid stagnation flow problem to determine air velocities and calculate drag forces on droplets. A stochastic model of droplet transport and deposition accurately predicts transfer efficiencies and rates of surface coverage, except at higher (> 5 m/s) velocities where turbulence in the flow increases.