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Prandtl number effects in abruptly separated flows: LES and experiments on an unconfined backward facing step flow - Sophia Buckingham - Ph.D. Thesis - Free download

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VKI PHDT 2018-11, Sophia Buckingham, Prandtl number effects in abruptly separated flows: LES and experiments on an unconfined backward facing step flow, ISBN 978-2-87516-139-0, 205 pgs


https://doi.org/10.35294/phdt201811

Prandtl number effects in abruptly separated flows: LES and experiments on an unconfined backward facing step flow - Sophia Buckingham - Ph.D. Thesis - Free download

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Prandtl number effects in abruptly separated flows: LES and experiments on an unconfined backward facing step flow
By Sophia Buckingham

PhD Thesis from the von Karman Institute / Université catholique de Louvain, August 2018, ISBN 978-2-87516-139-0, 205 pgs

https://doi.org/10.35294/phdt201811


Abstract

Liquid metal cooled reactors have been relying on Computational Fluid Dynamics (CFD) analyzes for their design and safety evaluation. However, 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. In this thesis, Prandtl number effects on the turbulent heat transfer are investigated by carrying out Large-Eddy Simulations (LES) and experiments in abruptly separated flows. To represent simply the detached flow areas and wall interactions that dominate the upper plenum, an unconfined Backward-Facing Step (BFS) is chosen as benchmark test-case.

An essential contribution of this work has been to propose an efficient and flexible approach to generate inflow turbulence for LES. The use of the temperature perturbation method (TPM), up to now restricted to large scale atmospheric flows, has been extended to all types of wall-bounded flows. The key modification relies on artificially increasing buoyancy effects up to levels that are strong enough to generate turbulence, via a suitable perturbation function that will avoid any stratification effects from increasing the flow recovery distance. The optimized TPM enables to minimize this distance needed to obtain “realistic” turbulence, without the need to impose second order statistics.

To evaluate the accuracy of the LES, tests are performed in a He-Xe gas mixture (Pr ≅0.2), that is close to the lowest achievable value for a transparent medium, thus enabling to measure turbulent quantities while providing an insight into moderate Prandtl number effects. PIV measurements are carried out in air to validate the LES velocity field. A double sensor probe, consisting of a hot-wire and a thermocouple, is used to correlate velocity and temperature fluctuations in order to characterize the transport of heat due to turbulence. This work has demonstrated the importance of integrating the probe’s limitations while processing the LES data, resulting in a meaningful and satisfactory comparison between LES and experiment.

Based on the LES results obtained at Pr = 0.025, 0.2 and 0.71, Prandtl number effects are investigated. As opposed to heat transport in moderate Prandtl number fluids that is strongly dominated by turbulence effects, in low Prandtl number fluids, the heat is transported much further away due to a strong molecular contribution that is comparable to the turbulent part. More generally, this work confirms the necessity of resorting to anisotropic turbulent heat transfer models for RANS, that can now be assessed by comparison to this newly generated validation data.

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