The development of a turbulent mixing layer at mixing of two horizontal water streams with slightly different densities is studied by the means of numerical simulation. The mixing of such flows can ...be modelled as the flow of two components, where the concentration of one component in the mixing region is described as a passive scalar. The velocity field remains common over the entire computational domain, where the density and viscosity difference due to the concentration mainly affects the turbulent fluctuations in the mixing region. The numerical simulations are performed with the open source code OpenFOAM using two different approaches for turbulence modelling, Reynolds Averaged Navier Stokes equations (RANS) and Large Eddy Simulation (LES). The simulation results are discussed and compared with the benchmark experiment obtained within the frame of OECD/NEA benchmark test. A good agreement with experimental results is obtained in the case of the single liquid experiment. A high discrepancy between the simulated and the experimental velocity fluctuations in the case of mixing of the flows with the slightly different densities and viscosities triggered a systematic investigation of the modelling approaches that helped us to find out and interpret the main reasons for the disagreement.
•A fluid-dynamics model within an open-source CFD code was improved to simulate experiments on containment atmosphere mixing.•The model was validated against 10 experiments performed in PANDA and ...SPARC facilities.•Various possibilities to reduce the computational domain for practical reasons were considered.•Effects of reduced numerical domain is assessed and the suitability of the reductions is discussed.
Several experiments of interaction of air or steam jet with a horizontal helium rich layer in a closed vessel representing a NPP containment, which were performed in PANDA and SPARC experimental facilities, are simulated on the local instantaneous scale using the Unsteady Reynolds Averaged Navier Stokes approach. Axisymmetric two-dimensional and three-dimensional numerical models of cylindrical vessels are developed, and the effects of numerical domain reduction are studied. The mixing process is simulated as a single-phase flow with common momentum and energy equations. Consequently, an additional term accounting for enthalpy diffusion due to compositional changes is implemented into the energy equation. Included gas species mass fractions are considered as passive scalars and are calculated using the transport equation. However, the term describing the molecular diffusion cannot be neglected and had to be added to the diffusion equation implemented in the default OpenFOAM computational fluid dynamics code that was used. The k-ε turbulence model with additional turbulence generation and dissipation due to buoyancy is used for turbulence modelling. The comparison of simulation and experimental results shows that the improved physical model is able to successfully describe the phenomena in several experiments performed in different facilities with different initial and boundary conditions.
•Defined local Froude number (Frl) to asses jet characteristics during calculations.•A model to dynamically prescribe turbulent Schmidt and Prandtl numbers is proposed.•Model identifies regions with ...possible occurrence of Kelvin-Helmholtz instability.•Improved results in case with occurrence of Kelvin-Helmholtz instability.
To theoretically investigate atmosphere mixing in an enclosure, three experiments of interaction of a vertical air jet with a helium-rich layer performed in SPARC, PANDA and MiniPanda experimental facilities, were simulated using the URANS approach. For the purposes of this work, firstly, a local Froude number is introduced, which can be directly used in computational fluid dynamics calculations to specify the regions with possible occurrence of Kelvin-Helmholtz instability. Namely, it is shown that in these regions, turbulent diffusion is underpredicted and turbulence model fails to replicate some phenomena. Secondly, a model for dynamically prescribe turbulent Schmidt and Prandtl numbers is proposed. The proposed model improves the results in isothermal case with possible occurrence of Kelvin-Helmholtz instability, while its effects are negligible in cases with higher temperature jets with lower local Froude number values, where the conditions for the Kelvin-Helmholtz instability are not established.
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