Page 92 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Rod bundle and pool-type experiments in water serving liquid metal reactors 67

determined by the presence of turbulent eddies. The study of turbulence in rod bundles
goes back to the 1970s when the above-mentioned Rowe et al. (1974) measured,
besides the mean flow, the axial and lateral turbulent intensities with the help of
LDA. Around the same time, Trupp (1973) and Trupp and Azad (1975) measured
the turbulence characteristics in air in a triangular rod bundle by using Pitot tubes
and hot wire anemometry. They measured the axial pressure profile as well, providing
the opportunity to develop a correlation for the wall Darcy friction factor as a function
of both Re and P/D. Recently, Sato et al. (2009) performed an impressive study on the
turbulent flow field in a bundle geometry, which mimics the one in LM reactors. They
used PIV combined with RIM to measure the velocities in a plane perpendicular to the
flow direction and from the front side of the bundle. Dominguez-Ontiveros and
Hassan (2014) studied the turbulent flow field in a 3 3 rod bundle geometry with
the help of PTV (see Section 3.1.2.4.2) and RIM. They could measure the mean veloc-
0
0
ities and turbulence intensities (u , v ) within 11% accuracy. Currently, a study by
Bertocchi and Rohde is performed in the framework of the SESAME project
(Roelofs et al., 2015) where LDA, combined with RIM, is applied to obtain the tur-
bulence characteristics in a triangularly arranged, wire-wrapped bundle geometry.

3.1.3.2 Secondary flow
The nonuniformity of the turbulence causes an interesting additional flow phenome-
non, being secondary flows of the second kind. Trupp and Azad (1975) tried to mea-
sure these flows, but were unsuccessful because these flows are very small with
respect to the mean bulk velocity (<1%). Vonka (1988a) experimentally determined
secondary flows in a triangularly arranged rod bundle with the help of LDA at differ-
ent Re numbers. He analyzed that, despite its small magnitude, secondary flows may
significantly contribute to the lateral transport of heat (Vonka, 1988b). Hosokawa
et al. (2012) found secondary flows as well with the help of PIV and an RIM tech-
nique. These flows were found to be about 1.5% of the mean bulk velocity.

3.1.3.3 Periodic flow pulsations

Besides turbulence and secondary flow, another phenomena may occur in rod bundle
geometries. Tapucu and Merilo (1977) experimentally investigated the presence of
axial pressure variations in a system consisting of two parallel channels connected
by a long, lateral gap. They found a pressure difference between the two channels that
was oscillating. Particles in the fluid showed a sinusoidal path with a wave length that
seemed to be a function of the gap size of the slot. Lexmond et al. (2005) performed an
experiment in a similar setup consisting of two parallel channels, interconnected by a
narrow gap. The measurements visualized a vortex street on each side of the gap
(called gap vortex street by Tavoularis (2011)), both moving in the same direction
as the main flow. Mahmood et al. (2009) and Mahmood (2011) performed a number
of experimental studies (LDA, PIV with RIM) to obtain insight in the dependency of
these gap vortex streets on gap width, Re number, and other parameters. Fig. 3.1.7
shows a gap vortex street obtained by PIV measurements.