Page 319 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Turbulent heat transport 289

of the SESAME project, this model has been implemented in the commercial TransAT
code. As a next step, this model will be validated against the variety of reference databases,
which will be generated within the SESAME project.
AHFM: Two different classes of AHFMs are available within the nuclear field:
1. Explicit AHFM: This class forms a four-parameter model, however, keeps the isotropic
nature of the eddy diffusivity model. One such model (Manservisi and Menghini, 2014)
has been implemented within OpenFOAM and will be validated within the SESAME and
the MYRTE projects.
2. Implicit AHFM: This class forms either a four- or a three-parameter model and contains
the nonisotropic formulation for the turbulent heat flux. Two different models of the
explicit AHFM formulation are being considered for the liquid-metal flows, that is,
AHFM-2000 (Kenjeres and Hanjalic, 2000) and AHFM-2005 (Kenjeres et al., 2005).
These models were originally developed for natural convection flows at Pr 1. How-
ever, within the THINS project, the AHFM-2005 was further calibrated for liquid-metal
flows and for all three flow regimes (natural, mixed, and forced convections). As a result,
a new model was proposed and is known as AHFM-NRG. Further validation of AHFM-
NRG model is foreseen to be performed within the SESAME and the MYRTE projects.
In the framework of the IVMR project, an extension of the AHFM-NRG for high Ray-
leigh number natural convection flows was proposed. The resulting new variant of the
model is called as AHFM-NRG+.
GGDH: A GGDH approach is a step below (wrt the AHFM formulation) in the hierarchy to
model the turbulent heat flux. This is a relatively simple closure and represents a non-
isotropic eddy diffusivity model. Such models show better predictions in comparison with
an isotropic eddy diffusivity model.
SGDH: Most of the CFD codes, by default, use a constant value of Pr t ¼0.85 or 0.9. How-
ever, for liquid-metal flows, the value of Pr t is not constant throughout the domain, and this
leads to a series of shortcomings. Hence, a number of alternatives have been proposed in
terms of correlations for the evaluation of Pr t . Several correlations are described in the chap-
ter. However, an analytic wall function, proposed by Duponcheel et al. (2014), in combi-
nation with the Kays correlation has shown better results for the forced convection channel
flows. To gain confidence in such a method, further validation for more complex geometries
should be performed. In addition, it must be noted that the use of such approach cannot
easily be extended to the natural and mixed convection flow regimes.

References

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