Point measurements: Phase-Doppler Anemometry
Point interferometric techniques are based on dividing a coherent laser beam into two beams which intersect in the spray with a given angle . The recombination of the two coherent beams creates an interference pattern in the probe volume due to superposition of the two electric fields. When a droplet crosses the interference fringes, it scatters intensity-modulated light. This light signal is collected by a lens and focused onto a photo-detector which converts the light intensity fluctuations into voltage fluctuations. The rate of intensity variation, named the Doppler frequency, indicates the time for a droplet to travel the distance between two fringes. By knowing the separation distance of the fringes, the velocity of the droplets can be deduced. The use of interference fringes to determine droplet velocity is known as Laser Doppler Velocimetry (LDV). A major extension of the LDV consists of the use of a second photo-detector in order to measure not only the droplet velocity but also the droplet diameter. This technique is called the Phase-Doppler Anemometry (PDA), or Phase-Doppler Interferometry (PDI). When two detectors record the Doppler signal at slightly different scattering angles, changes in the phase of modulation occurs.
This changes of phase, is found to be linearly dependent to the droplet diameter of spherically and homogeneous (constant index of refraction) particles when the signal detected is dominated by only one scattering mode. Generally, the mode of interest is the reflection (80-110 degrees angle) for opaque particles and the first order refraction (at 30-70 degrees angle) for droplets with significant transparency.
2D measurements: LIF/Mie droplet sizing
The LIF/Mie droplet sizing technique also called Laser Sheet Dropsizing (LSD) or Planar Drop Sizing (PDS) is based on the combination of planar imaging of Mie scattering and liquid Laser Induced Fluorescence (LIF). Planar imaging consists in the creation of a thin laser sheet which traverses the spray and is imaged via a camera. Photons that scatter or fluoresce from the illuminated droplets are collected perpendicularly to the incident illumination direction. This optical geometry results in a spatially resolved measurement across a plane in the spray and provides a description of the spray structure on a macroscopic scale is obtained. In the LIF/Mie droplet sizing technique the imaged field of view is fairly large (cm scale) in order to image the entire spray region. Thus, the droplets are not resolved and the sizing is made from the detection of the scattered and fluorescing signals. The approach is based on the simple concept that, for a doped droplet which is excited, the fluorescence signal will give a measure of the droplet volume; whereas, the Mie elastically scattered light will represent the surface area.
The approach is based on the simple concept that, for a doped droplet which is excited, the fluorescence signal will give a measure of the droplet volume; whereas, the Mie elastically scattered light will represent the surface area. By dividing the LIF signal by the Mie signal, the Sauter Mean Diameter (SMD) can be deduced:
Here CLIF and CMie include experimental factors such as scattering efficiency, detector response, signal collection solid angle, laser power, etc. The coefficient K is found to be a variable and a calibration curve is needed if one wants to obtained the appropriate value of K for each SMD value. One way of obtaining a calibration curve is to use phase doppler measurements.
Note that when applied in its conventional way the LIF/Mie technique is largely affected by errors introduced by multiple light scattering even in situations where the spray is fairly dilute. To remove this unwanted light intensity on both the LIF and Mie images the SLIPI approach must be used prior to ratioing the images.
Y. Nath Mishra, E. Kristensson and E. Berrocal, Reliable LIF/Mie droplet sizing in sprays using structured laser illumination planar imaging, Opt. Express, 22, 4480-4492, 2014.
Microscopic imaging
One challenge in directly imaging micrometric droplets is the fact that camera objectives cannot be too close as from the spray, droplets impinge on the collecting lens. Thus, to obtained high resolution images, a sufficiently long distance is needed, requiring the use of long range microscopic objectives. In this case, the full field of view of is only a few millimeters but the liquid breakups can be resolved. While microscopic imaging in sprays was initially used on a shadowgraphic configuration, Light Sheet Fluorescence Microscopic (LSFM) is becoming a promising alternative solution.
