High-Speed Imaging provides the ability to capture and playback videos of the spray process that may happen very quickly and are unable to be evaluated in real-time. By utilizing recorded high-speed video of the spray process, the transient spray characteristics may be closely evaluated whether the area of interest is at the exit of the nozzle or near the spray target.
There are many manufacturers of high-speed cameras; Spraying Systems Co. uses an Olympus i-SPEED TR camera for high-speed imaging purposes, which provides a maximum frame rate of 10,000 fps, a user-friendly interface, and a stand-alone video playback screen.
The High-Speed Imaging system uses a high sensitivity custom CMOS sensor with a very high shutter speed to acquire up to 10,000 image frames per second. Images may be acquired at a pixel resolution of up to 1280x1024 (up to 2,000 fps). Typically, an external high-intensity light source is used to back-light the spray region, resulting in darkened regions where spray droplets or particles are present.
High-Speed Imaging provides a unique view of the instantaneous spray distribution, concentration, shape, trajectory and allows for the assessment of very fast spray development. The image based measurement makes the results mostly qualitative in nature, but the images are easily evaluated and provide a simple way to visualize the spray.
Due to very high frame rate and memory usage, the image field of view is limited. Also, adequate background light is require for the images to be collected with sufficient contrast, therefore this instrument works best with light droplet density sprays.
High-Speed Imaging provides a unique combination high-temporal resolution and high pixel resolution for visual spray evaluation. Examples of past projects using high-speed imagery at Spraying Systems Co. include transient spray development evaluation with the PulsaJet® nozzle, and overspray evaluation for surface coating.
PulsaJet® spray transients
The PulsaJet® line of nozzles is able to manipulate the total spray flow rate by quickly pulsing the nozzle on/off with a prescribed duty cycle. However, when spraying a highly viscous material, the spray characteristics were observed to deteriorate. Using the high-speed camera, the spray with this high viscosity material was imaged during a many cycles. During the low-speed playback, it was clearly observed that the start of each spray included extra large droplets. This was then resolved with a slight nozzle design change, resulting in a well atomized spray during the entire spray pulse cycle.
A customer approached Spraying Systems Co. and was experiencing excessive overspray in a surface coating application. It was not clear where the overspray was originating as the nozzle spray angle was sized appropriately for the target width. High-speed videos where acquired near the target edge and it was observed that the spray material was only partially adhering to the target surface; and the remainder was splashing off the target. By reducing the nozzle spray angle and using two side-by-side nozzles, the splash-off was nearly eliminated, and benefited the customer in material costs and cleanup time.