Page 79 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

54 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Fig. 3.1.1 Laser Doppler anemometry system.
(Adopted from Blocken, B., Stathopoulos, T., van Beeck, J.P., 2016. Pedestrian-level wind
conditions around buildings: review of wind-tunnel and CFD techniques and their accuracy for
wind comfort assessment. Build. Environ. 100, 50–81.)

where λ is the laser wavelength and θ is the angle between the laser rays. The flow is
seeded with particles to scatter the light; if a seeding particle passes through the ellip-
soidal probe volume, the light is scattered back with a shifted wavelength (Doppler
effect principle; Doppler, 1842) and is collected by the photo detector. Here, the light
is converted into an electric charge by a photomultiplier and is recorded as an electric
signal (the Doppler burst). The rate at which the particle scatters the light f b (“beat”
frequency), which is recorded by the photo detector, is proportional to its velocity and
to the fringe distance. The velocity component of the particle normal to the plane of
the laser beams is then calculated as
u ¼ f b d f : (3.1.19)

Because only the velocity module is measured, there is no way to distinguish between
two particles moving with the same speed but in opposite directions (directional ambi-
guity problem). The Bragg cell can shift the frequency of light of one of the lasers by a
reference value f ref so that the interference fringe pattern moves with constant speed
and the velocity is then calculated as

u ¼ f b f ref d f :