High-speed Imaging

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
  PulsaJet nozzles manipulate the total spray flow rate by quickly pulsing the electrically-actuated nozzle on and off with a prescribed duty cycle. However, when spraying a highly viscous material for a customer, the spray characteristics deteriorated. Spraying Systems Co. used the high-speed camera to record the spray for many cycles. During the low-speed playback, extra-large droplets were clearly observed at the start of each spray. This problem was resolved with a slight nozzle design change, resulting in a well-atomized spray during the entire spray pulse cycle.

• Overspray Evaluation
  A customer 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 were 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, reducing material costs and cleanup time.

High-speed imaging provides a unique view of the instantaneous spray distribution, concentration, shape and trajectory, and also 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. However, the image’s field of view is limited, and because adequate background light is required, this instrument works best with light droplet density sprays.

The high-speed imaging system uses a high-sensitivity, custom CMOS sensor with a high shutter speed to acquire over 600,000 image frames per second. Images may be acquired at a maximum pixel resolution of 1280 x 800 (1 MP) up to 22,000 frames per second. High intensity lights are used to illuminate the spray plume and droplets using front or back-lighting.