Page 304 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

274 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

predictions. For nonunity Pr fluids, the limitations of the eddy diffusivity approach
have become more evident, as underlined in Arien et al. (2004), Gr€ otzbach (2007
and 2011), OECD/NEA (2007), and Shams et al. (2014). In the recent past, the
CFD codes that have been developed within the nuclear community are directly ori-
ented toward the modeling challenges within the nuclear applications. In the European
project assessment of computational fluid dynamics codes for heavy liquid metals
(ASCHLIM), an extensive assessment of the performance of the available CFD codes
was performed (Arien, 2004). It was found that heat transfer modeling in some of the
codes from the nuclear community was superior to the commercially available soft-
ware tools. Accordingly, within the European sponsored project Thermal Hydraulics
of Innovative Nuclear Systems (THINS), one of the objectives was to push forward the
validation and adoption of more accurate closures for single-phase turbulent heat
transfer for liquid metals in engineering codes (Roelofs et al., 2015a). As a part of this
THINS project, Cd-adapco and ASCOMP have implemented an algebraic heat flux
model (AHFM) developed by Kenjeres and Hanjalic (2000) in their commercial codes
STAR-CCM+ and TransAT, respectively. This AHFM, hereafter, is called as AHFM-
2
2000, is based on four transport equations (k-ε-θ -ε θ ), and was originally developed
for unity Prandtl fluids in natural convection flow regime. Based on the initial testing
of AHFM-2000 in STAR-CCM+, it was found out that it is difficult to get a proper
convergence in some of the considered test cases. Consequently, a new variant of
AHFM (Kenjeres et al., 2005) was implemented in STAR-CCM+ (Shams et al.,
2014). This model, hereafter, will be called as AHFM-2005, consists of three transport
2
equations (k-ε-θ ), and has less coefficients compared with the AHFM-2000. Within
the framework of the THINS project, the AHFM-2005 was further calibrated and opti-
mized for liquid-metal flows. As a result, a new model was proposed that is called
AHFM-NRG. This model has shown improved results in all three flow regimes, that
is, natural, mixed, and forced convection (Shams et al., 2014). However, this valida-
tion was performed on a limited amount of available relatively simple test cases.
Hence, further validation of such models for more complex test cases needs to be per-
formed. However, due to the lack of a reference database for liquid-metal flows, it was
not perceivable at that time.
In this regard, within the thermal hydraulic Simulations and Experiments for the
Safety Assessment of Metal-cooled Reactors (SESAME) and the MYRRHA Research
and Transmutation Endeavor (MYRTE) projects, an extensive effort has been put for-
ward to generate a variety of reference databases (Roelofs et al., 2015b). It is worth
reminding that in Europe, consensus was achieved that further development of models
should be limited. The main focus should be on the further validation and if required
calibration of the available turbulent heat flux models for complex geometric config-
urations and their further extension to natural, mixed and forced convection regimes.
Hence, in the framework of the SESAME and the MYRTE projects, the following
promising turbulent heat flux models have been identified by Roelofs et al.
(2015b) for further validation:
l AHFM-NRG
l Local turbulent Prandtl number model
l Turbulence Model for Buoyant Flows (TMBF)