Laser Doppler velocimetry is a well-proven technique that measures fluid velocity accurately and non-invasively. Laser light illuminates the flow, and light scattered from particles in the flow is collected and processed. In practice, a single laser beam is split into two equal-intensity beams which are focused at a common point in the flow field. An interference pattern is formed at the point where the beams intersect, defining the measuring volume.
Phase Doppler Particle Analyzer (PDPA) Systems
Phase Doppler measurements allow for the sizing of spherical particles (typically liquid sprays, but also some bubbles and solid spheres). Along with size information, the velocity of the particle is also obtained, so in this sense the phase Doppler technique is an extension of LDV. The size of particles that can be measured is limited on the small end by the amount of light that is scattered by very small particles (which depends on particle size, laser power, and light collecting optics), and is limited on the large end by the Gaussian nature of the laser beams and the far field condition. Typical limits for common configurations might be 1 or 2 mm on the lower end and 500 mm to 1 mm on the upper end.
