Impact of Graphite Roughness on Heat Transfer in graphite moderated reactors

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Background

In certain nuclear reactors, such as the Molten Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory (ORNL), molten salt fuel flows between graphite moderator blocks. These blocks are effectively cooled by the molten salt as heat is deposited in them. Modern reactor designs also employ this principle. Graphite, used as a moderator, can be manufactured with tight tolerances and relatively smooth surfaces. However, during operation, both the dimensions and surface roughness of the graphite can change, impacting heat transfer. Accurately simulating salt flow and heat transfer requires accounting for these changes in roughness. The flow regimes in these systems can vary from laminar to transitional, reaching Reynolds numbers of several tens of thousands. The size of the flow channels can range from a few millimeters to centimeters, while graphite roughness can differ by grade, estimated either from grain size or direct measurements, ranging from micrometers to hundreds of micrometers.  

Methodology

While past decades have seen numerous experimental and numerical studies on roughened surfaces, much of the focus has been on fluid flow rather than heat transfer. Commonly used techniques involve solving Reynolds-Averaged Navier-Stokes (RANS) equations, where the accuracy of the simulations heavily depends on the turbulence model used. This project aims to conduct detailed flow and heat transfer numerical calculations, alongside evaluating the performance of advanced Large Eddy Simulation (LES) models using OpenFOAM [1-3]. You will investigate the impact of graphite surface roughness on heat transfer through wallresolved LES in simplified roughened geometries, such as sinusoidal or ellipsoid patterns in channels or pipes.  

Project Goals

  • Conduct a comprehensive literature review to define the parameter space for surface roughness and other relevant parameters.
  • Select and implement appropriate simulation techniques for analyzing roughness effects.
  • Evaluate the impact of graphite surface roughness on heat transfer coefficients. 

Presentation of friction factor on the rough surface and temperature contours overlapped with surface streamlines. Illustration.

Figure 1: presentation of friction factor on the rough surface and temperature contours overlapped with surface streamlines. 

Supervision and Collaboration

Himani Garg will be the main supervisor with Jens Klingmann as co-supervisor. They will provide technical guidance and support to the student throughout the project, including regular meetings and discussions. Seaborg Collaboration: Seaborg Technologies will provide additional support and context.

More information and contact

For more information, please contact Himani Garg, Assistant Professor at the Department of Energy Sciences: himani.garg@energy.lth.se.

References

  1. Himani Garg et al.; Large eddy simulations of flow over additively manufactured surfaces: Impact of roughness and skewness on turbulent heat transfer. Physics of Fluids 1 August 2024; 36 (8): 085143. https://doi.org/10.1063/5.0221006
  2. Himani Garg et al.; Heat transfer enhancement with additively manufactured rough surfaces: Insights from large-eddy simulations. Physics of Fluids 1 February 2024; 36 (2): 025109. https://doi.org/10.1063/5.01891153.
  3. Himani Garg et al.; Large eddy simulations of fully developed turbulent flows over additively manufactured rough surfaces. Physics of Fluids 1 April 2023; 35 (4): 045145. https://doi.org/10.1063/5.0143863
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