Gilles Bernard-Michel Id 165 : HELIUM RELEASE IN A CLOSED ENCLOSURE: COMPARISONS BETWEEN SIMPLE MODELS, CFD CALCULATIONS AND EXPERIMENTAL RESULTS Gilles Bernard-Michel Benjamin Cariteau 1 1 1

Context and objectives A benchmark (Air liquide, INERIS, PSA, CEA): This work takes place in a benchmark (Dimithry ANR project) Code comparisons : FLUENT, FLACS, PHOENIX, CAST3M CFD Vs experimental results comparisons (GAMELAN exp. Set-up). Physical model validation : Turbulence : K-epsilon, LES, laminar, for what injection flux and diameter etc… Compressibility : Boussinesq, Low Mach, Fully compressible ? Numerical discretization validation : 1st or 2nd order time scheme ? Explicite/ implicite, what terms of the equation ? For what purpose Centered or upwind scheme for convection Qualitiy of the mesh, convergence Simple models validation : Worster and Hupper model works fine for 20 mm diameter injection with a 5Nl/min source, and under predict by a factor 2 helium concentrations fot a 5 mm diameter. => Need for a better understanding of the phenomenon which is not accessible with experiments/ 2 2

Experimental set-up We only present two cases : A closed box - 5mm diam. injection nozzle - 5Nl/min helium injection – jet/plume A closed box - 20 mm diam. injection nozzle - 5 Nl/min helium injection – plume - High injection rate (jets) are not interesting because helium concentration is rapidely homogeneous. 3 3

Non costy calculations => geometrical simplifications Modeling stategy Non costy calculations => geometrical simplifications Highly converged calculations => EF order 3, and BDF2 time discretization scheme. L.E.S for turbulence Numerical cost reduction strategy : Explicite convection Constant time step by blocs Algebraic projection method => constant matrix for pressure discretization Parallel direct solver for pressure equation Parallel iterative solver for velocity, concentration equations (ILU0 precond, conjugate gradient). 4 4

Conservation Equations 5 5

time steps : Discretization Grids dt lower than 10 times CFL condition dt around 5 ms for the 5mm injection dt around 25 ms fot the 20mm injection. 6 6

Qualitative results

Reversal vortex Reversal vortex is observed : => Confirms the theory predicting coexistence of stratification and homogeneous layer.

Time concentration profile – 5mm case

Time concentration profile – 20mm case

Vertical concentration profile – 20mm case Codes results are more diffusive : More stratification Almost no homogeneous layer at the top.

Conclusions and perpectives The model is accurate enough : To predict concentration maxima better than 10% and consequently the problems with Worster an Hupper model for the 5mm low injection rate case. To predict the reversal vortex To predict correct concentration profiles We will go for 3D modeling : Discretization scheme and grids are validated Solvers are fast enough to move to 3D We expect to model correctly enough the jet to understand what can be improved in the Worster and Hupper model. 12 12