Phase Doppler Anemometry (PDA)
Phase Doppler Anemometry (PDA), also known as Particle Dynamics Analysis (PDA), Phase Doppler Particle Analysis (PDPA), Phase Doppler Difference Method (PDDM) and Phase Doppler Interferometry (PDI), is a laser optical instrument for simultaneous measureme
- PDF / 1,647,006 Bytes
- 14 Pages / 439 x 666 pts Page_size
- 44 Downloads / 176 Views
Phase Doppler Anemometry (PDA)
Bernd Ofner
9.1
Introduction
Phase Doppler Anemometry (PDA), also known as Particle Dynamics Analysis (PDA), Phase Doppler Particle Analysis (PDPA), Phase Doppler Difference Method (PDDM) and Phase Doppler Interferometry (PDI), is a laser optical instrument for simultaneous measurement of the velocity and size of spherical particles. Additionally it can be used for concentration and mass flux measurements. PDA enables measurements of high accuracy and a very high spatial resolution. Since it is based on an absolute physical effect PDA does not require any calibration. The principle of velocity measurement is the same as that used in Laser Doppler Velocimetry (s. Chap. 8). The velocity is determined by analyzing the frequency shift of light which is scattered by a particle flowing through the interference volume of at least two intersecting laser beams. For particle size measurements PDA makes use of the fact that scattered light which is received from two different directions of observation has a phase difference, which is proportional to the diameter of the particle. Since this relation is valid for both, transparent and non-transparent particles PDA can be used for different applications, such as for the investigation of droplets in water-, fuel- or painting-sprays, of bubbles in liquids and of solid spheres like powders, glasses or metal balls.
9.2
General considerations for the application of PDA
Application of PDA requires the consideration of the following items: • The particles (droplets, bubbles, solids) have to be spherical; otherwise the diameter and consequently the mass flux cannot be determined correctly. Since the surface tension supports the formation of spheres, this requirement is usually fulfilled for small droplets and bubbles. • Generally, the size of the particle can be in the range of some microto several millimeters. However, the measurable size range depends on several factors including the optical configuration (e.g. laser power, focal length, beam separation and location of detectors), the particle material, the photo detectors as well as the signal processor.
F. Mayinger et al. (eds.), Optical Measurements © Springer-Verlag Berlin Heidelberg 2001
140
B.Ofner
• The velocity of the particles generally can be analyzed up to 500 m/ s, depending on the signal processor and on the measurement configuration. But since some optical parameters, such as the focal length or the beam separation control the measurable diameter and velocity range in opposite ways, either the maximum velocity or the diameter range will be limited. • If transparent particles are analyzed, refraction is commonly used as the predominant scattering mode. In this case, the phase-diameter relationship is dependent on the refractive indices of the particle and the surrounding medium which therefore have to be known. In addition to that, the transparent particles have to be homogeneous. Any present impurities have to be much smaller than the wavelength of the laser light and their quan
Data Loading...
