The Phase Doppler Method
PDPA

Phase Doppler Particle Analyzers can measure, simultaneously, the size, velocity and concentration of spherical particles (typically liquid sprays, but also some bubbles and solid spheres). Simultaneous measurement of both size and velocity allows correlations to be made between these two quantities. The characteristics of moving particles are obtained by utilizing laser Doppler effect. It was developed on the basis of Laser Doppler Velocimetry (LDV) systems.

These analyzers are point sampling devices and flux-sensitive instruments. Points sampling refers to an instrument that focuses on a portion of the total spray pattern and requires targeting several test points within the spray in order to obtain a composite sample of the spray flux distribution.

The PDPA is a highly specialized technology used to measure any liquid spray with transparent/semitransparent droplets. The equipment combines a laser-based optical transmitter, an optical receiver, an electronic signal processor and software for capturing and analyzing data. The laser beams of the analyzer intersect at the sample volume location. Liquid is discharged through pump. As it passes through a nozzle or sprinkler, it produces a distinctive spray pattern. When a drop passes through the intersection region of the PDPA's laser beams, the scattered light forms an interference fringe pattern. Since the drop is moving, the scattered interference pattern sweeps past the receiver aperture at the Doppler difference frequency, which is proportional to the drop velocity. The spatial frequency of the fringe pattern is inversely proportional to the drop diameter.

The Phase Doppler method requires no calibration because the measured particle size and velocity are dependent only on the laser wavelength and optical configuration. PDPA measurements are not based on light intensity, and consequently are not subject to errors from beam attenuation or deflection which occur in dense particle and combustion environments.

Courtesy:

1. Spray Analysis and Research Services
2. Underwriters Laboratories Inc.
3. Spray Research, Inc.

Edited by: Mr. Bhupesh Doda, IIT Bombay

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