1. 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

2. 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

3. 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

4. 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

5. Conservation Equations5 5

6. 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

7. Qualitative results

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

9. Time concentration profile – 5mm case

10. Time concentration profile – 20mm case

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

12. 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