Application of shadow Doppler velocimetry to paint spray: potential and limitations in sizing optically inhomogeneous droplets

Abstract

This paper reports size measurement of droplets with optically inhomogeneous media by shadow Doppler velocimetry (SDV), which can provide spatially and temporally precise in situ readings of the size and velocity of a single particle with irregular shape and with arbitrary optical properties of the particle medium. In this work, water, instant coffee solution and water-based paint with various solid contents were measured to evaluate the capability and limitations of the measurement. The experiment with instant coffee solutions of 2 and 5% (wt:wt), which contained 10 µm discrete particles, atomized by a standard paint spray gun, demonstrated that the accuracy of sizing was not affected by the optical properties of the medium. Insensitivity to the optical properties is one of the primary advantages of SDV over the other optical, single-particle sizing methods. As a further demonstration, paint samples atomized by the same gun containing solid flakes of nominal diameter 25 µm were also measured. The results revealed a spatially uniform arithmetic mean diameter of 30 µm and suggested that the atomization characteristics were influenced in the highest flake volume fraction case (red paint containing mica, 1.5%) with the result that the mean diameter was 20% larger than that of a similar paint with a smaller flake volume fraction (green paint containing aluminium, 0.4%). It was also found that the measurable number density is limited to no more than 1000 droplets cm −3 in the case of droplets with an average size of 20 µm.

Effects of automotive paint spray technology on the paint transfer efficiency – a review

“…Their results showed that the PDA technique was capable of providing useful information about paint-spraying applications. Using shadow Doppler velocimetry (SDV), Morikita and Taylor 61 measured the droplet size distributions at 30 cm from a paint gun nozzle for water, an instant coffee solution, and a water-based paint. It was suggested that the droplet size distributions obtained using SDV were insensitive to the optical properties of the sprayed liquid.…”