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COMPARISON OF RELAP5-3D/ATHENA AND ANSYS/CFX THERMAL-HYDRAULIC
RESULTS
Juan J. Carbajo, Joel McDuffee, and A. Louis Qualls
Reactor and Nuclear Systems Division
Oak Ridge National Laboratory
P.O. Box 2008 MS-6167
Oak Ridge, TN 37831-6167
[email protected]
A heat exchanger (HX) for a space reactor has
been modeled by the codes RELAP5-3D/ATHENA (Ref.
1) and ANSYS/CFX (Ref. 2), and some thermal-
hydraulic results have been compared. The parameters
that have been compared are the total heat transferred
within the component and the pressure drop across the
component.
The HX modeled is a shell and tube HX made
of stainless steel with 61 tubes, 9.65 mm OD and 0.55 m
long. The overall dimensions of the HX are 0.16 m in
diameter and 0.8 m long. Both the primary and the
secondary fluids are liquid NaK, which is a eutecticmixture of
22% sodium and 78% potassium. Different
hot and cold fluid inlet temperatures were investigated.
The transient analysis 3-dimensional (3-D)
computer code RELAP5-3D/ATHENA was one of the
codes used in this modeling. This code can employ a
variety of coolants in addition to water, the original
coolant employed in early versions of the code. Liquid
metals (sodium, potassium, NaK, lithium) and cryogenic
fluids (hydrogen, helium, nitrogen) are some of the
available coolants. The thermo-physical properties for
NaK in RELAP5 have been modified to correct some
observed property errors. The code can also use 3-D
volumes and 3-D junctions, thus allowing for morerealistic
representation of complex geometries.
This HX has been modeled using simple 1-D
volumes with 14 axial nodes for the volumes inside the
shell and headers and 10 axial nodes for the volume
inside the tubes. The primary flows inside the 61 tubes
and the secondary flows inside the shell, outside the
tubes. The tube walls are modeled as a heat structure that
separates the primary and the secondary volumes but
transmits heat between both volumes. The primary and
the secondary fluids flow counter-currently. Fig. 1 shows
this model with the NaK temperature distribution inside
the tubes. This model has been used as part of the system
design to calculate flows, temperatures, and pressuredrops.
ANSYS is a multipurpose finite-element
code that can perform a variety of calculations, including
stress analysis, temperature distributions, and thermal
expansions in solid materials. CFX is one of the
computational fluid dynamics packages of the code and
can perform thermal-hydraulic fluid calculations. The
ANSYS/CFX representation of this HX with the
temperatures inside the tubes is shown in Fig. 2. The
temperatures inside the shell fluid are shown in Fig. 3.
Every tube is modeled. This model has been used for
component design to calculate material stresses.
The results from both codes (RELAP5 and
ANSYS) for different input conditions are shown in
Table 1. The primary NaK flow is 1.1 kg/s, the
secondary NaK flow is 1.75 kg/s. Two different hot inlet
temperatures were investigated: 875 K and 880 K; and
two different cold inlet temperatures: 814 K and 818 K.
The manufacturers design value of the HX is 48.2 kW
when a hot temperature of 875 K and a cold temperature
of 818.6 K are used.The maximum heat transferred in the HX
is
calculated by RELAP5 for the case with the maximum
hot temperature (880 K) and the minimum cold
temperature (814 K) with a value of 55.4 kW. The
minimum heat transferred is calculated for the lowest hot
temperature of 875 K and the highest cold temperature of
818 K with a value of 48 kW. The difference between
the maximum and the minimum heat transferred is 15%.
Small changes in the inlet temperatures (~5 K) result in
significant changes in the heat transferred (15%). The
RELAP5 calculated value of 48 kW is very close to the
design value of 48.2 kW using about the same inlet
temperatures. ANSYS/CFX calculated 51.6 kW which isabove the
design value of 48.2 kW and above the
RELAP5 calculated value.
Pressure drops calculated by RELAP5 and
ANSYS/CFX for the primary side were in good
agreement (0.6 kPa for both). For the secondary side,
ANSYS predicted 2.4 kPa while RELAP5 predicted only
0.2 kPa. Other calculations showed that RELAP5 does
not predict accurate pressure drops in bundles with cross-
flows since it uses correlations for flow in parallel with
the bundles.
In conclusion, both RELAP5-3D/ATHENA and
ANSYS/CFX results appear to be in reasonably good
agreement. RELAP5 does not calculate correctlypressure drops for
cross-flow in bundles.
REFERENCES
1. INEEL, RELAP5-3D Code Manual, INEEL-EXT-
98-00834, Rev. 2.4, Idaho National Laboratory
(2006).
2. ANSYS Multiphysics 8.1 Code, ANSYS Inc., 275
Technology Drive, Canonsburg, PA 1531.
mailto:[email protected]:[email protected]:[email protected]
