Laser anemometry

Abstract: This chapter will address the measurement of velocity with non-intrusive optical methods based on a laser in a fluid stream seeded with submicron light-scattering particles.

Key words: laser Doppler anemometer (LDA), particle image velocimetry (PIV), two focus velocimeter (L2F).

4.1 Introduction

In the previous chapters, methods for measuring the velocity of a fluid involving the use of probes immersed in the stream have been described; the perturbations due to the probe itself can be reduced but not eliminated by using small probes. On the other hand, there are cases where it is quite impossible to introduce a probe into the stream: this may be due to high temperatures (flames, plasmas), to a too high dynamic pressure or to an abrasive or chemically aggressive fluid. In all these cases the use of an optical (and hence non-intrusive) method is mandatory.

The air and plenty of fluids, however, are transparent to light radiation, therefore the optical anemometers may be used only if the fluid is seeded with light-scattering particles. Because these particles act as indicators of stream velocity, even when it changes rapidly in time (turbulent flow), they must have a low inertia which implies a low density and/or a very small volume (diameter of the order of pm). Table 4.1 lists some types of particles used in various fluids and their maximum allowed diameter at two frequencies of speed fluctuation.

Since the particles have a diameter of the same order of magnitude as the wavelength of light, when they are hit by a light beam, they spread it in all directions (Mie’s theory): the back-scattered light (Figure 4.1) is hundreds of times smaller than the forward-scattered light.

Particle	Fluid	f = 1 kHz	f = 10 kHz
Polystyrene, polyamide, hollow glass spheres	Water	5-100
Water + glycerin (50% +50%)	Air	2-5
Silicone oil	Air	2.6	0.8
CM О і—	Air	1.3	0.4
MgO	Methane-air flame 1800 [K]	2.6	0.8
CM о І—	Oxygen plasma 2800 [K]	3.2	0.8

Table 4.1

Maximum diameter (mm) of tracer particles

Given the small size of the particles, only a small fraction of the total light is diffused, so only a laser can be used as a light source, as in it all the light output is concentrated in a beam approximately 1 mm in diameter.

The anemometers using a laser are:

■ the laser-Doppler anemometer (LDA): the speed of the particle is calculated from the difference between the frequency of the scattered light as perceived by an observer and that of the incident light;

■ the laser 2 focus (L2F) or laser transit anemometer (LTA): the speed is measured from the time the particle takes to travel the distance between two focused laser beams;

■

the PIV (particle image velocimetry): the velocity of many particles is measured from the distance that they trail in the time interval between two consecutive images of the test chamber.

dp = 0.2X dp = 1 .OX dp= 10X

Note: dp is the diameter of the particles, l the wavelength of the incident light.

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