Numerical Investigation of Mixed Convection in AGRsBy Amir Keshmiri Supervisors: Prof. Dominique Laurence and Dr. Mark Cotton School of Mechanical, Aerospace & Civil Engineering (MACE) The University of Manchester Internal Seminar at the University of Manchester – 07/11/2007

Outline Introduction to AGRs Ascending/Descending Flows The Geometry Studied Some Results Conclusions Future Work

[ [ Advanced Gas-Cooled Reactors (AGRs)

[The Safety of the AGR by J M Bowerman (1982)] Advanced Gas-Cooled Reactors (AGRs)

[The Safety of the AGR by J M Bowerman (1982)]

Ascending/Descending Flows; Enhancement/Impairment of Heat Transfer

Solution Methods Solution Methods In-House Code (CONVERT) In-House Code (CONVERT) Commercial CFD Package (STAR-CD) Commercial CFD Package (STAR-CD) Industrial Code (Code_Saturne) or Radius=0.1 m Ascending Flow Constant Heat Flux BC ‘Boussinesq’ Approximation Key Features of the Flow Problem

The Governing Equations Continuity: Momentum: Energy:

The Geometry Used in ‘CONVERT’ An in-house Fortran77 Code, ‘CONVERT’ (for Convection in Vertical Tubes) Finite Volume/Finite Difference Code Parabolic governing equations i.e. Marching problem

RANS Results The Turbulence Models Tested by CONVERT : Launder-Sharma k-ε model [1] Cotton-Ismael k-ε-S model [2] Suga NLEVM [3] The Results are validated against: DNS of You et al (2003) [4] LS of Kim et al (2006) [5]

The analysis focuses on 4 cases: Gr/Re^2=0.000  Forced Convection Gr/Re^2=0.063  Early onset Mixed Convection Gr/Re^2=0.087  Laminarization Gr/Re^2=0.241  Recovery RANS Results

Gr/Re^2=0 – Forced Convection

Gr/Re^2=0.087 – Laminarization

Budgets of Turbulent Kinetic Energy Gr/Re^2=0.087Gr/Re^2=0.0

Heat Transfer Enhancement/Impairment

Nu and Cf Developments

Effects of Reynolds Number

Conclusions Mixed convection in an ascending flow in a heated pipe, is a very complex phenomenon, despite its simplicity; Thus requires more research. Most of the turbulence models successfully predict the flow field at relatively low heat loading i.e. small Gr/Re^2 Only very few turbulence models (only Linear k-ε) can predict the Re- laminarization Phenomena. There is a close agreement between the results of Code_Saturne and STAR-CD for the tested models. The relatively more advanced turbulence models, such as Non-Linear k-  of Suga and V2f models are observed to suffer from convergence problems at high Gr/Re^2. The few available DNS data are not sufficient to carry out in depth validation of the RANS models, particularly at the maximum heat transfer impairment point.

Development of Code_Saturne by implementing some advanced wall functions such as Analytical and Numerical Wall Functions. Cross examination of Code_Saturne with TEAM and STREAM Codes. Testing more complex geometries such as rib roughened surfaces, etc. Future Work

Acknowledgements This work was carried out as part of the TSEC programme KNOO and as such we are grateful to the EPSRC for funding under grant EP/C549465/1

References [1] Launder, B.E. and Sharma, B.I., 1974, “Application of the energy dissipation model of turbulence to the calculation of flow near a spinning disc”, Lett. Heat Mass Transfer, 1, pp [2] Cotton, M.A., Ismael, J.O., 1998, “A strain parameter turbulence model and its application to homogeneous and thin shear flows”, Int. J. Heat Fluid Flow 19, pp. 326–337. [3] Craft, T.J., Launder, B.E. and Suga, K. 1996, “Development and application of a cubic eddy- viscosity model of turbulence”, Int. J. Heat Fluid Flow, 17, pp [4] You, J., Yoo, J.Y. and Choi. H., 2003, “Direct Numerical Simulation of Heated Vertical Air Flows in Fully Developed Turbulent Mixed Convection”, Int. J. Heat Mass Transfer, 46, pp [5] Kim, W.S., Jackson, J.D. and He, S. (2006), “Computational Investigation into Buoyancy- Aided Turbulent Flow and Heat Transfer to Air in a Vertical Tube”, Turbulence, Heat and Mass Transfer, 5, (Hanjalić, K., Nagano, Y. and Jakirlic, S. (Editors))

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