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Arab Journal Of Nuclear Science And Applications, 46(2), (276-286) 2013
was, thus, important to survey over the available turbulence models in terms of their criteria of
applicability, limitations and restrictions in order to choose the one that might be appropriate to model
this complex system. Moreover, literature surveys were conducted to explore on what other researcher
have recommended for similar systems. It was found that substantial amount of research work have
been done on rod bundle geometry using the standard k turbulence models over the past few
decades for a recent in-depth review on CFD analysis on rod bundle geometries ( Tzanos 2001 (1) ). In
general, there is a great deal of agreement between researchers that the standard k model may not
be the turbulence model of choice in rod bundle geometries. On the other hand, most of these
researches have considered periodic arrays of rods which allowed them to solving the flow in
elementary sections and assuming symmetry across the boundaries (Rapley and Gosman 1986 (2) ;
Baglietto and Ninokata, 2005 (3) ; and many others). However, as Chang and Tavoularis, 2007 (4) pointed
out, this approach restricts the solution based on the fact that symmetry in geometrical configurations
does not necessarily imply symmetry in the flow. In other words, simulating a full sector may be
required to better capture the essential features of the flow field. It was reported the use of the shearstress
transport (SST) k
model to perform CFD analysis of flow field in a triangular rod bundle
by Toth and Aszodi (2008) (5) . Chang and Tavoularis, 2005 (6) , on the other hand, used the unsteady
Reynolds averaged Navier-Stokes equations supplemented by a standard Reynolds stress model to
simulate the experimental work of Guellouz and Tavoularis 2000 (7) a,b and reported good agreement.
In this work, we consider the use of the shear-stress transport (SST)
Toth and Aszodi, 2008 (5) . Moreover, the two approaches to dealing with the near wall region were
considered with the aim that if it is founded that the wall function approach provides reasonable
approximation to the measured inner clad surface temperature, then it might be recommended for
further investigation since it usually requires less computing resources.
2-The Shear-Stress Transport (SST)
k model
281
k model as suggested by
k and SST models, which are basically two equation eddy-viscosity models, the
In both the
Reynolds stresses can be calculated from the eddy-viscosity hypothesis introduced by Boussinesq,
(Pope, 2000 (8) ):
2
U
uiu j k
ij
t
3
x
j
i
U
x
i
j
As suggested by Menter, if it is possible to combine the standard
shows to be relatively accurate if applied to the near wall region with the
accurate in the far field, one may obtain a model that may be used in a variety of applications
involving confining walls. This model is called the shear-stress transport
basically the same formulation as the standard
activate the
region. In addition the definition of turbulent viscosity is modified to account for the transport of the
turbulent shear stress.
(1)
k model, Wilcox, which
k model which is also
k model. It has
k model but it includes blending function to
k model in the near-wall region or the transformed
k away from the wall
In this model, the turbulent kinetic energy, k, and the specific dissipation rate, , may be
obtained from the following transport equations (FLUENT, 2006 (9) ):
k
~
( k)
( kui
) (
k
) Gk
Yk
S k
(2)
t
xi
x
j
x
j