Laser anemometer signals: visibility characteristics and application to particle sizing

Area of Science:

• Fluid mechanics
• Optical physics
• Particle sizing

Background:

• Laser anemometry is a non-intrusive technique for measuring fluid velocity.
• Signal visibility is a critical parameter in laser anemometry, affecting measurement accuracy.
• Previous studies primarily focused on forward scatter configurations.

Purpose of the Study:

• To investigate the signal visibility characteristics of a dual beam laser anemometer in backscatter mode.
• To analytically and experimentally determine the factors influencing backscatter signal visibility.
• To assess the applicability of the visibility sizing technique for particle diameter measurements in backscatter.

Main Methods:

• Experimental investigation of signal visibility.
• Analytical modeling based on Mie's electromagnetic scattering theory for spherical particles.
• Evaluation of dependencies on particle diameter, refractive index, beam polarization, and aperture characteristics.

Main Results:

• Signal visibility is not solely dependent on the particle diameter to fringe spacing ratio.
• Visibility is influenced by Mie scattering size parameter, refractive index, beam polarization, and light collection aperture.
• Backscatter signal visibility characteristics differ significantly from forward scatter.

Conclusions:

• Backscatter measurements of particle diameters using the visibility sizing technique may not always be feasible.
• The study highlights limitations and dependencies of the visibility sizing method in both backscatter and forward scatter modes.
• Accurate particle sizing in backscatter requires careful consideration of multiple optical and system parameters.