Page 83 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

58 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Mirror

Laser

Light sheet
Flow direction

Illuminated
particles

Imaging optics
Image plane y

x
(A)

D 2z R

W o

(B) F

Fig. 3.1.3 (A) Layout of a PIV system. (B) Top view of a laser beam sheet. D, lens
diameter; F, focal length; w 0 , beam waist; 2z R , interval where the laser thickness does not
p ffiffiffi
vary more than 2z R .

repetition rate are achievable with such type of laser. Nd:YLF (neodimium-doped
yttrium lithium fluoride) lasers can reach higher repetition rates (of the order of 1
kHz) but with less energy per pulse. When operating in dual-head mode, the over-
lapping of the two laser beams is crucial to have good PIV results.

Image requirements
The image of a seeding particle appears as a light dot in the photograph. Its shape is
given by the geometric shape of the particle and also by the contribution of light dif-
fraction (which is the main limitation to the image quality of a particle). If the imaged
diameter of the particle is too small (i.e., smaller than 1 pixel), the particle