Planar Droplet Sizing (D₃₂) in Sprays

The planar droplet sizing technique reveals a full 2-dimensional map of the Sauter Mean Diameter (D₃₂) with Laser Imaging. This approach is complementary to measurements with Phase Doppler Interferometry (PDI).The D₃₂-laser imaging technique measures relative droplet sizes in a plane, while PDI measures calibrated size-velocity distributions of individual droplets in a point.
The basic principle behind planar D₃₂ measurements consists of recording two images from the spray, one with laser induced fluorescence (LIF) and simultaneously a corresponding one with direct light scattering (Mie).
While the LIF image represents the droplet’s volume, the Mie scattered light is basically proportional to the total droplet surface. The ratio of both is proportional to the definition of the Sauter Mean Diameter.

The Sauter Mean Diameter is a droplet ensemble average. The same is the LIF/Mie ratio: it does not reveal the size of individual droplets, but the local average of the size distribution.
By meaningful selection of fluorescent tracers and its concentration, it is possible to record appropriate LIF and Mie images. This could be done at the same time by the use of 2 cameras or LaVision’s Image Doubler on a single camera. For time averaged results the measurements can be taken subsequently.
For quantitative size information, a calibration at least at 1 point is necessary. This can be either a known spray or with another instrument, like the ParticleMaster or a Phase Doppler Interferometer (PDI).