Page 91 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

66 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

3.1.3.1 Mean flow and turbulence characteristics

One of the first experimental studies on the mean flow in a rod bundle has been per-
formed by Eifler and Nijsing (1967). They measured axial pressure drops along the
bundle and local mean velocity at different wall distances with the help of a movable,
very small (0.5 mm) Pitot tube (see Fig. 3.1.6).
Pitot tubes were used by Seale (1979) as well, but measurements were performed in
air instead of water. Seale combined the Pitot tube with a temperature sensor so that
mixing of thermal energy could be analyzed as well. Obviously, putting a probe (such
as a Pitot tube) inside the flow disturbs the flow itself and, therefore, influences the
outcome. For this reason, Rowe (1973) applied, by then, the very new technique called
laser Doppler velocimetry (LDV) or LDA. Much later, this technique has successfully
been applied in rod bundles by, for example, McIlroy et al. (2008) and Mahmood et al.
(2011). Besides LDA, McIlroy et al. used another optical technique, called PIV.
Results can be found in the same reference (i.e., McIlroy et al., 2008). The major draw-
back of optical techniques, such as LDV/LDA and PIV, is that the internals of the rod
bundle need to be optically accessible and predictable. Light cannot pass through com-
ponents like metal rods and is refracted when crossing two transparent materials of
different refractive index (e.g., water and Perspex). Both the laser-based optical tech-
niques and the solution to obtain proper optical access, called RIM, have been
explained in Sections 1.2.4.1, 3.1.2.4.2, and 3.1.2.4.4, respectively.
Turbulence, obviously, is a very important phenomenon in rod bundles, as heat
transfer from the surface of the fuel to the bulk of the coolant flow is strongly

Fig. 3.1.6 Test channel as used by Eifler and Nijsing in 1967 to measure local mean
velocities.
(Adopted from Eifler, W., Nijsing, R., 1967. Experimental investigation of velocity
distribution and flow resistance in a triangular array of parallel rods. Nucl. Eng. Des. 5(1),
22–42.)