7 - Single-Point Laser Measurement

Abstract

This chapter addresses the recent developments of laser-Doppler anemometry (LDA) and phase-Doppler anemometry (PDA). The chapter concentrates on single-point measurements for two-phase flows using elastic light scattering, scattering, with a focus on the laser-Doppler and phase-Doppler techniques. It discusses their application to measurements in particulate two-phase flows. Both measurement techniques are optical non-intrusive single point techniques permitting local instantaneous and ensemble-averaged measurements of particle velocities with high spatial resolution. The basic principles of both methods are introduced and guidelines for selecting an optimal optical system are provided, especially for PDA measurements in two-phase flows. Laser-Doppler anemometry has been first applied to measurements of mean velocities and turbulence properties in single-phase flows. In this case small particles, which follow the flow and the turbulent fluctuations, need to be present in the flow or must be added to it. The principle of phase-Doppler anemometry (PDA) relies on the Doppler difference method used for conventional laser-Doppler anemometry. By using an extended receiving optical system with two or more photodetectors it is possible to measure simultaneously size and velocity of spherical particles. For obtaining the particle size the phase shift of the light scattered by refraction or reflection from the two intersecting laser beams is used. The chapter discusses several principles and layout of PDA, particle concentration and mass flux measurements by PDA, signal processing and signal detection using an amplitude level. The chapter concludes by stating that field of phase-Doppler anemometry or PDA-like instruments appear to have larger potential for new developments.