Measurement of Spray Droplet Diameter

Methods to Measure Droplet Diameter

Spray droplet diameter is an important element in selecting nozzles and designing nozzle-related equipment.
IKEUCHI, "The Fog Engineers", uses the immersion sampling method and the laser analyzer to measure spray droplet diameters.

Immersion sampling method
Laser analyzer

Immersion Sampling Method

In an immersion sampling method, droplets are collected on a glass plate thickly coated with silicone oil and are immediately photographed at high magnification for subsequent count of those number by size.
The collected droplets quickly settle in the silicone oil and do not evaporate even in strong light while being photographed. Remaining suspended in the silicone oil, they are measured as perfect spheres.
IKEUCHI, "The Fog Engineers", uses the immersion sampling method to measure droplet diameter sprayed mainly with hydraulic nozzles.

Fraunhofer Diffraction Method

A laser beam scatters at the surface of droplets in the laser beam path and the diffraction pattern caused by interference of scattered light is focused behind the droplets. This method can simultaneously measure all droplets in the laser beam path.

Laser Doppler Method

This method forms an interference fringe by crossing two laser beams. Two or more receivers located at a certain distance from the spray detect the phase difference and calculate diameter of droplets having passed through this interference fringe.
As this method measures droplets one by one, it is relatively less affected by droplet density and, as another advantage, it can measure droplet velocity simultaneously.
IKEUCHI, "The Fog Engineers", uses the laser Doppler method to measure droplet diameter sprayed mainly with pneumatic nozzle.

Shadowgraph Method

Backlight illuminated shadows of droplets in various sizes are photographed and converted to circular shapes, from which the droplet diameters are calculated.
This method enables the measurement of non-spherical coarse droplets that cannot be measured by the laser analyzer.
On the other hand, it is not suitable for measuring fine droplets due to the low magnification of the camera. Also, when the droplets are dense, the overlapped multiple droplets could be measured as a single droplet, thus its droplet size may appear larger than the actual size.

Mean Droplet Diameter

Mean droplet diameter is one of the important elements in selecting nozzles and designing nozzle-related equipment.
The following three are commonly used.

In chemical processes such as cooling, evaporation, combustion and drying, it is common to determine efficiency using the ratio of volume-surface area, i.e. specific surface.
Because a small portion of large droplets is more influential over the rate of reaction than a large portion of small droplets, it is advisable to use Sauter Mean Droplet Diameter as representative droplet sizes.