Page 242 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

CFD—Introduction 6

F. Roelofs, A. Shams
Nuclear Research & Consultancy Group (NRG), Petten, The Netherlands

For the coolants envisaged in innovative nuclear systems, usually, experiments are
very expensive, and detailed measurements of flow and temperature fields are com-
plex or even impossible. To this respect, computational fluid dynamics (CFD) plays an
important role in the prediction of various (complex) flow and heat transport charac-
teristics. Hence, CFD becomes an attractive and complementary practice used in the
design and evaluation process of innovative nuclear systems. In general, CFD covers a
broad field that is often categorized by how the turbulence is modeled or resolved. In
the realm of innovative reactor systems, various methods of CFD are adopted and suc-
cessfully being used. These methods are depicted in Fig. 6.1. Short descriptions of
these methods are given in the following sections. What all these methods have in
common is that they solve the governing conservation equations of fluid dynamics
with respect to mass, momentum, and energy that can be found in all major textbooks
concerning fluid dynamics (e.g., Wilcox, 2006). Even though it is recognized that rel-
evant liquid-metal flows for nuclear applications may include free surface and dis-
persed two-phase modeling, incompressible flow phenomena, the scope of this
book is limited to single-phase incompressible flows that remain the key issue in
nuclear liquid-metal applications.

6.1 Direct numerical simulation

Turbulence is a nonlinear phenomenon with a wide range of spatial and temporal
scales. The large scales are usually defined by the geometry of the flow and the bound-
ary conditions, while the smallest scales are determined by the flow itself. Direct
numerical simulation (DNS) is a simulation method in CFD in which the Navier-
Stokes equations are numerically solved by resolving the whole range of spatial
and temporal scales of turbulence. This means without the use of any turbulence
model. Hence, DNS offering a high-fidelity solution for the simulation of fluid flows
is often considered a numerical experiment.
Highly accurate numerical methods are required for the DNS of turbulence to accu-
rately reproduce the evolution of turbulence over a wide range of length and timescales.
These scales rangefromthesmallestdissipative scales (Kolmogorovmicroscales)upto
the integral scales associated with the motions containing most of the kinetic energy.
The difference between the largest and smallest length scales in turbulence increases
as the Reynolds number (Re) increases. Since there are three spatial dimensions, the
9/4
number of grid points required to resolve turbulence increases as Re . The grid

Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors. https://doi.org/10.1016/B978-0-08-101980-1.00006-5
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