[ID 204] Evaluation of ADREA-HF LES against Hydrogen release dispersion Mr. Nektarios Koutsourakis1,2 Dr. Alexander Venetsanos2 Prof. John G. Bartzis1 4th International Conference on Hydrogen Safety San Francisco, California, September 12-14, 2011 1 University of West Macedonia Greece 2 NCSR “Demokritos” Greece ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

ADREA-HF CFD CODE Constantly developing Latest additions: LES (Large Eddy Simulation) Parallel solver Arbitrary number of species Combustion Back tracking GUI pre and post processor (called EDes) Advantages: Robust, powerful and general Specialization in environmental applications ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

ADREA-HF CFD CODE ADREA specializes in dispersion applications ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

ADREA-HF CFD CODE GUI environment for pre and post processing ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

LARGE EDDY SIMULATION Between DNS and RANS Navier-Stokes math. analysis: impossibly difficult DNS: Fully-resolved NS Cost  Re2.75 (for wall-bounded flows  Re3.5) RANS: Time-averaged NS Fast, widely tested, usually accurate “enough” LES: Spatially-filtered NS Cost near wall:  Re2.4 ! (at outer layer  Re0.5) Converges to DNS. As in DNS, LES needs: a) averaging b) demanding boundary conditions Suggested were RANS fails: transient, separated flow DNS: Thousands of years to simulate 1s of flight RANS: Good for steady state problems, with no big separations ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

ENERGY CASCADE LES solves most of the turbulence ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

LES EQUATIONS AT ADREA-HF Compressible volume-filtered Navier-Stokes ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

LES EQUATIONS AT ADREA-HF Alternative subgrid model: RNG-LES ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

LES AT ADREA-HF Numerics ADREA/SIMPLER algorithm Solver: Parallel BiCGstab with parallel Schwarz preconditioner. Speedup with 2 CPUs up to 1.7 Numerics: Deferred correction central scheme for convection Crank-Nicolson 2d order for time advancement Linear upwind with VanLeer limiter for concentration For more: Venetsanos, A. G., E. Papanikolaou and J. G. Bartzis, 2010: The ADREA-HF CFD code for consequence assessment of hydrogen applications. Int. J. Hydrogen Energy, 35, 3908. ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

TEST CASE HySafe INERIS-6C experiment 1g/s H2 for 240s Jet 20mm Room 3.78 * 7.2 * 2.88m Jet exit velocity 38m/s ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

TEST CASE SETUP Simulation details Standard Smagorinsky with Cs=0.1 Total number of cells in the room: 130000 CFLmax=0.3 Minimum mesh resolution: 0.02*0.02*0.053 m Simulation time in a workstation: 15 days for the release phase (240s); dtav1= 0.0005s + 2 days for the first 260s of the post-release phase ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

TEST CASE SETUP Geometry, sensors, part of the grid ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

RESULTS Comparison with experiment Photo from experiment ADREA LES An asymmetry on the ceiling can be noticed (higher C towards the sensors’ side of the room). The plume at the higher parts of the jet seems to be slightly wider in the simulation. ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

RESULTS Video of the Hydrogen propagation ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

RESULTS Concentration contours animation A flow field develops that transfers the hydrogen along the ceiling and then downwards along the side walls, which have higher concentrations than the neighbouring regions of the same height. Thus the plume takes a mushroom shape. ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

RESULTS Comparison with sensors Each plot should be explained, along hydrogen route: Starting from 16 to 1 to 7 to 12. Why the slope increases along route, why the difference of release and diffusion decreases, etc. ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

DISCUSSION Some remarks In general very good agreement High turbulence during release phase, only Release overestimated, especially near source. Very difficult for CFD and for turbulence modelling to capture the physical processes next to the source Diffusion underestimated (possible experimental asymmetry?) - improvement with better grid Better agreement is prevented from experimental inaccuracies and numerical discrepancies I think release is overestimated due to inadequacy of CFD to see what exactly happens at the emission area. That is a very difficult point that needs very advanced models ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

FURTHER EXAMINATION Cs value influence (release phase -coarse grid) Jet axis sensors Ceiling sensors Side wall sensors Sensor 16 Sensor 14 Sensor 13 Sensor 1 Sensor 4 Sensor 6 Sensor 7 Sensor 8 Sensor 9 Sensor 10 Sensor 12 Only 80s-160s measurements taken into account! Max diffusion close to 0.12 ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

DISCUSSION Cs affects the results Cs (SGS modelling in general) important near jet At the ceiling, no Cs influence (buoyancy dominates) Non-monotonic behaviour (increasing the Cs gives better results, but up to a limit) Worst Cs by far: 0.2 ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

FURTHER EXAMINATION C variances for each Cs C variance = mean (Ci’2) VARIANCE IS A MEASURE OF RESOLVED TURBULENCE - if total turbulence is higher (while variance being same), it means that the SGS (unresolved) is higher…. To calculate the variance I only used values from 74s till 166s for all cases. ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

DISCUSSION A critical balance: resolved vs. SGS turbulence As we go away from the jet, variance (and resolved turbulence intensity) drops Two mechanisms compete: - Modelled SGS turbulence - Resolved turbulence (generally low if SGS is high) With Cs=0.2, very high artificial viscosity causes more “laminar” flow and lower fluctuations Resolved turbulence is higher with Cs=0.1, but total turbulence seems to be higher with Cs=0.12 ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

FURTHER EXAMINATION Evaluation of RNG-LES (run not finished yet) ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

DISCUSSION RNG LES seems to be an improvement RNG LES provides in general better results Improvement towards the right side for sensors both close to the jet (turbulent flow) and far from it (laminar flow). That was not the case by varying Cs Resolved turbulence seems to decrease close to the jet and increase far from it RNG LES is believed to better model the subgrid scales for both the turbulent and the laminar flow Suggestible when the turbulence intensity varies ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

CONCLUSIONS I Acceptable results and physical insight LES results are good, given all numerical and experimental inaccuracies Overestimation during release phase (close to jet) Underestimation during diffusion phase (and far from jet) Attempt to investigate the balance between resolved and SGS turbulence with parametric analysis of Cs RNG LES seems to behave slightly better in both turbulent and laminar parts of the flow ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

CONCLUSIONS II LES is good, but not for everyday use ADREA-HF has now tested LES in hydrogen release LES can provide peak concentrations (good for flammable mixtures or for calculation of dosages) LES can provide concentration statistics LES many times more expensive than RANS LES is not RANS++ There is not good LES – there is good CFD In general, in any good CFD code LES can be added ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE

THANK YOU Acknowledgements: Greek Scholarships Foundation, Municipality of Aghia Varvara ICHS 2011, #204 LES MODELLING OF HYDROGEN RELEASE AND ACCUMULATION WITHIN A NON-VENTILATED AMBIENT PRESSURE GARAGE USING THE ADREA-HF CFD CODE