Investigation of Thermal Mixing in the Control Rod Top Tube Using Large Eddy Simulations

  • Reijo Pegonen KTH Royal Institute of Technology, Roslagstullsbacken 21, SE-10691 Stockholm, Sweden
  • Nicolas Edh Forsmarks Kraftgrupp AB, SE-74203 Östhammar, Sweden
  • Kristian Angele Vattenfall Research & Development AB, SE-81426 Älvkarleby, Sweden
  • Henryk Anglart KTH Royal Institute of Technology, Roslagstullsbacken 21, SE-10691 Stockholm, Sweden


Thermal mixing and thermal fatigue has led to component failures in the nuclear industry. The thermal fatigue phenomenon is intimately linked with the mixing of streams of different temperatures in proximity to a solid wall. Due to conjugate heat transfer, temperature fluctuations are induced in the wall. One of the key issues is to predict the amplitude and the frequency of the fluctuations. This paper presents pre-calculations of the thermal mixing experiments that are under preparation at the KTH Royal Institute of Technology as part of the THEMFE project (Thermal Mixing and Fatigue Experiment).The proposed geometry is a simplification of a reactor control rod and consists of a top-tube and control rod stem, which are modeled as concentric cylinders. In addition there are only two hot inlet jets and two cold inlet jets, whereas in reality there are 8 upper inlets and 4 lower inlets for hot bypass water and the cold flow is annular.Thermal mixing was studied by using a transient Computational Fluid Dynamics (CFD) solver for the incompressible filtered Navier-Stokes equations and employing a Large Eddy Simulation model of turbulence implemented in OpenFOAM. The aim was to verify that the proposed simplified geometry and the flow conditions of the experiment will lead to low frequent temperature fluctuations of the order of 0.1-1 Hz, as seen in previous experiments with the real geometry. Such low frequencies are typical for the thermal fatigue phenomenon. The study was focused on the region near the control rod stem and therefore a refined grid was used in that region. The final mesh consisted of over one million cells.The results did indeed reveal low frequent temperature fluctuations in the lower part of the mixing region near the control rod stem. The results of this paper indicate that the length of the mixing region is 23 cm, which is large enough to be resolved in the experiment. It was also found that the most dangerous region, where the dominant high amplitude temperature fluctuations have a frequency of the order of 0.1 Hz, is 4 cm long. As expected, the instant flow field is asymmetric with large secondary flows. The present results verify that the proposed geometry and flow conditions can be applied in the experiment.


[1] U.S.N.R. Commission, Information on Control Rod Cracking Found at Swedish BWRs., ML083330227.html, 2008. Accessed 5 February 2012.
[2] H. Tinoco, H. Lindqvist, Thermal Mixing Instability of the Flow In-side a Control-Rod Guide Tube, Conference paper N13P1245, The 13th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-13), Kanazawa City, Ishikawa Prefecture, Japan, 2009.
[3] K. Angele, Y. Odemark, M. Cehlin, B. Hemstrom, C.-M. Hogstrom, M. Henriksson, H. Tinoco, H. Lindqvist, Flow mixing inside a control-rod guide tube- Experimental tests and CFD simulations, Nuclear Enineering and Design 241 (2011) 4803-4812.
[4] A. Bakker, Information on Computational Fluid Dynamics, Lecture 7-Meshing,, 2008. Ac¬cessed 13 April 2012.
[5] S. N. Acharya, A general survey of hexahedral mesh generation, SeminarGrid_04/Refs/seminar.pdf, 2004. Accessed 13 April 2012.
[6] R. Courant, K. Friedrichs, H. Lewy, Uber die partiellen Differenzen-gleichungen der mathematischen Physik, Mathematische Annalen 100 (1928) 32-74.
[7] E. de Villiers, The Potential of Large Eddy Simulation for the Modeling of Wall Bounded Flows, PhD dissertation, Imperial College of Science, Technology and Medicine, Department of Mechanical Engineering, 2006.
[8] ANSYS Inc., ICEM CFD Help Manual,, 2011. Accessed 11 September 2013.
[9] K. Abe, A hybrid LES/RANS approach using an anisotropy-resolving algebraic turbulence model, International Journal of Heat and Fluid Flow 26 (2005) 204-222.
[10] Y. Gong, F. X. Tanner, Comparison of RANS and LES Models in the Laminar Limit for a Flow Over a Backward- Facing Step Using Open-FOAM, Conference paper, Nineteenth International Multidimensional Engine Modeling Meeting at the SAE Congress, Detroit, Michigan, USA, 2009.
[11] L. Davidson, Fluid mechanics, turbulent flow and turbulence modeling,, Chalmers University of Technology, Goteborg, Sweden, 2011.
[12] OpenFOAM Foundation, OpenFOAM User Guide, http://www., 2011. Accessed 15 April 2012.
[13] OpenFOAMWiki, The SIMPLE algorithm in OpenFOAM, in_0penF0AM, 2010. Accessed 14 April 2012.
[14] OpenFOAMWiki, The PISO algorithm in OpenFOAM,, 2009. Accessed 14 April 2012.
[15] T. Ming, J. Zhao, Large-eddy simulation of thermal fatigue in a mixing tee, International Journal of Heat and Fluid Flow 37 (2012) 93-108.
[16] N. Edh, radialProfile, Private communications, 2012.
[17] OpenFOAMWiki, BuoyantBoussinesqPisoFoam,, 2010. Accessed 14 April 2012.
[18] H. Jasak, Error Analysis and Estimation for the Finite Volume Method with Application to Fluid Flows, PhD dissertation, University of London, Department of Mechanical Engineering, 1996.
[19] J. Westin, P. Veber, L. Andersson, High-Cycle Thermal Fatigue in Mixing Tees: Large-Eddy Simulations Compared to a New Validation Experiment, Conference paper ICONE16-48731 pp. 515-525, 16th International Conference on Nuclear Engineering (ICONE16), Orlando, Florida, USA, 2008.
How to Cite
PEGONEN, Reijo et al. Investigation of Thermal Mixing in the Control Rod Top Tube Using Large Eddy Simulations. Journal of Power Technologies, [S.l.], v. 94, n. 1, p. 67--78, oct. 2013. ISSN 2083-4195. Available at: <>. Date accessed: 18 june 2024.


Thermal mixing, Mixing region, Low frequency temperature fluctuations, Computational Fluid Dynamics, Large Eddy Simulation, Incompressible flow.

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