LES investigation of Prandtl number effects over a backward facing step and consequences in terms of best practice guidelines for RANS
(2019) 18th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 2019) (18.August.2019)
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- Liquid metal cooled reactors have been relying on Computational Fluid Dynamics (CFD) analyzes for their design and safety evaluation. However, for industrial simulations using the RANS framework, the major challenge is the modeling of the turbulent heat transfer in the low Prandtl number coolant (Pr ≃ 0.001 − 0.01). More advanced models require to be tested, calibrated and validated on a wide variety of flows to make them applicable to Liquid Metal Reactors (LMR). In this work, Prandtl number effects on the turbulent heat transfer are investigated by carrying out Large-Eddy Simulations (LES) in abruptly separated flows. To simply represent the detached areas and wall interactions that dominate the upper plenum of a LMF, an unconfined Backward-Facing Step (BFS) is chosen as benchmark test-case. To evaluate the accuracy of the LES, wind tunnel tests were performed in air (Pr ≃ 0.71) and in a He-Xe gas mixture (Pr ≃ 0.2), before extending the database to Lead Bismuth-Eutectic (LBE). At such low Prandtl number (Pr = 0.025), large differences in scale are observed between the momentum and the thermal field. The molecular effects previously negligible become significant, to the extent where conduction becomes a major heat transport mechanism. Regarding RANS modeling, a critical review of the widely used Reynolds analogy is proposed by extracting the turbulent Prandtl number from the LES; this parameter varies significantly across the domain. Finally, the performance of several RANS model is assessed by comparison to this reference data and a Best practice guideline is proposed for this type of flows. © 2019 American Nuclear Society. All rights reserved.
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Buckingham, S., Koloszar, L., Villa Ortiz, A., Bartosiewicz, Y., & Winckelmans, G. (2019). LES investigation of Prandtl number effects over a backward facing step and consequences in terms of best practice guidelines for RANS. 18th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 2019). https://hdl.handle.net/2078.5/169645