ENGINEERING AND CFD

This project, which was made possible through the Partnership for Advanced Computing in Europe, PRACE, is part of the NURESAFE initiative for nuclear safety. The objective is to develop a global modelling framework for multi-scale core thermal-hydraulics in Pressurized Water Reactors (PWR) as understanding heat transfer phenomena in turbulent bubbly flows is of great interest for the scientist community and for the industry.

Detailed computational science in this context amounts at using advanced CPU-demanding Computational Fluid Dynamics (CFD) simulations to understand basic heat and fluid flow phenomena and derive practical and useful models to be implemented in (i) CFD codes for component scale, and (ii) system codes for the global reactor behaviour. The numerical models need to be capable of dealing with thermal-hydraulics phenomena acting at several levels: from system-level and component-level scales down to the length scales of the bubble layer adjacent to the hot fuel-rod, and further down to the entrained droplets from ligaments and sheets forming at wall liquid films.

The researchers separately employed detailed computational techniques/models (namely direct numerical simulation, DNS, and large eddy simulation, LES, for turbulence, Interface Tracking Methods (ITM), and Lagrangian particle tracking for multiphase flow representation) to infer coarse-grained interfacial momentum and heat transfer models for phase-averaged Eulerian prediction techniques (Homogeneous Algebraic Slip (ASM), or Two-Fluid). The work consisted in (i) developing flow databases and (ii) exploring these databases to infer practical coarse-grained models.

The simulations included resolution of all the important phenomena including how bubbles grow, depart and move along and away from the wall, and how and where they re-condense as they move into the colder liquid in the core of the channel. This entailed calculating the temperature and velocity distributions as well as tracking the vapor/liquid interface of each bubble within the whole domain. For these scale-resolving unsteady simulations, tremendous computational resources and storage such as provided by the HPC-platform of the High Performance Computing Center Stuttgart (HLRS) was inevitable.

The researchers’ goals were two-fold: (1) to develop beyond state-of-the-art computational techniques (multiscale, multiphysics) which have never been undertaken hitherto, and (2) develop new models for heat transfer and interphase momentum exchange or revisit existing ones by means of detailed predictions techniques (LES, DNS and ITM) based on the new computational techniques developed within (1).

The project delivered very promising results and enabled the researchers to develop beyond state-of-the-art computational techniques (multiscale, multiphysics) that have never been undertaken hitherto. A rich flow database was generated and can now be used for model validation. The detailed simulations and benchmarks provided data that are beyond reach of experiments, e.g. turbulent stresses very near the walls, correlations between turbulence and interface dynamics, etc.

Work is in progress to develop new models for heat transfer and interphase momentum exchange or revisit existing ones by means of detailed predictions techniques (LES, DNS and ITM) based on the new computational techniques developed and the flow database.

The results obtained during this first phase of the project laid the foundation for a more general study that will be carry out in the upcoming year by ASCOMP AG.

Acknowledgements:

This work has been accomplished in the frame of the FP7 project NURESAFE under grant agreement no 323263. ASCOMP acknowledges PRACE for awarding access to resource Hermit based in Germany at HLRS.

References:

D. Lakehal, D, LEIS for the prediction of turbulent multifluid flows applied to thermalhydraulics applications, Nuclear Engineering and Design, 240, 9, pp. 2096-2106, (2010).

J. Lu and G. Tryggvason. Effect of bubble deformability in turbulent bubbly upflow in a vertical channel. Physics of Fluids, 20 (4): 040701, (2008).

D. Métrailler, S. Reboux and D. Lakehal, Near-Wall Turbulence-Bubbles Interactions in a Channel Flow at Reτ=400: a DNS/LES Investigation, (NURETH-16) Chicago, USA, Aug. (2015)

D. Métrailler, S. Reboux, D. Caviezel and D. Lakehal, DNS of Turbulent Convective Flow Boiling in a Channel, (NURETH-16), Chicago, USA, Aug. (2015)

R.D. Moser, J. Kim and N.N. Mansour, Direct numerical simulation of turbulent channel flow up to Reτ= 590. Phys. Fluids, 11(4), 943-945, (1999).

Contact Information:

Dr. Sylvain Reboux
Head of Research, ASCOMP AG
Technoparkstr. 1, CH-8005 Zurich (Switzerland)
E-mail: reboux @ ascomp.ch
www.ascomp.ch