Validation and Recommendations for CFD Modeling of Hydrogen Vented Explosions

1. VALIDATION AND RECOMMENDATIONS FOR CFD MODELING OF HYDROGEN VENTED EXPLOSIONS:EFFECTS OF CONCENTRATION, OBSTRUCTION AND VENT AREA E.Vyazmina / S.Jallais October 2015 ICHS 2015

2. Phenomenogy • Explosion in a closed volume P PAICC • 29.6 % H2 in air • P(AICC) = 7.38 bar

3. Phenomenology • Vented explosion – Back Wall ignition

4. P1 or P_ext

5. Phenomenology P2 or P_vib due to the flame-acoustic oscillations To compute P_vib the simulation mesh must be extremely fine (smaller than flame thickness) which is not possible nowadays

6. Example of experimental pressure signal • Two peaks : • P1 (or P_ext – external explosion ) • P2 (P_vib – flame accoustics oscillations ) • High-frequency oscillations are present (especially for P2) • Filteringisnecessary

7. Ignition location • No one ignition location is the most severe for all cases

8. Effect of vent area S_vent • Smaller vent size increases peak pressure for all configurations and ignition locations !!!!! S_vent dP time

9. Effect of obstacles inside the enclosure • Obstacles increase the first peak and decrease the second peak !!!! dP time

10. Cases to model • BWI • Large vent areas • Obstacles inside the combustion chamber • 3 experiments are modelled FMGlobal, 63m3, vent 5.4m2, ~18% INERIS, 4m3, vent 0.25m2 and 0.49m2, 16.5% KIT, 1m3, vent 0.25m2, 18%

11. KIT: 1m3, 18%, BWI, vent area 0.25 m2 • Good agreement with experimental data for the for 17.9% mixture • Slight overestimation of the overpressure maximum • Tend to anticipate the spike

12. FMGlobal: 63m3, 18%, BWI, vent area 5.4 m2 • Good agreement with experimental data for the for 17.9% mixture • Slight overestimation of the overpressure maximum • Tend to anticipate the spike

13. FMGlobal: Effect of obstacles and vent area • Obstacles effect : • FLACS is conservative without obstacles, with an overestimation of the overpressure by 30%. • The overpressure is overestimated by ~50% in the presence of obstacles. • Vent area effect : • For smaller vent the overpressure is overestimated by 50%

14. INERIS: 4m3, ~18%, BWI, influence of vent area • Good agreement with experimental for both test cases for the overpressure inside the chamber • Tend to anticipate the spike Vent 0.49 m2 and 16.5% Vent 0.25 m2 and 15.5%

15. INERIS: 4m3, ~18%, BWI, influence of vent area • Outside the combustion chamber the computed overpressure is underestimated due to the stretching of the computational grid outside the combustion chamber

16. CFD validation for Vented explosion • Various scales of the experimental set up are considered: • Small or laboratory scale : very bad agreement of FLACS with experiment • Middle–scale (from 1m3 up to 64 m3): good agreement with experimental results for the first overpressure spike

17. Conclusions • In the case of CI and FI Pvib (vibration of the flame) can be dominant compare to P1 (vented explosion). • FLACS is not able to compute Pvib • there is no a CFD tool, which is able properly capture Pvib • Thus for CI and FI ignitions CFD simulations can lead to misleading conclusions. • For BI without obstacles • FLACS slightly overestimates the overpressure. • Outside of the combustion chamber (in a far field) it is recommended to use square grid (no-stretched). • Presence of obstacles inside the chamber, • FLACS overestimates the overpressure by ~60%. • The second overpressure peak disappears, leading to a better general match between simulations and experimental data.