1. E.Vyazmina / S.Jallais October 2015 ICHS 2015
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. due to the flame-acoustic oscillations
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)
Filtering is necessary

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

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

9. Effect of obstacles inside the enclosure
Obstacles increase the first peak and decrease the second peak !!!! dP
time
2013

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
Vent area m2
Concentration of H2,%
Obstacles
Experimental overpressure (barg)
FLACS overpressure (barg)
5.4
~17.9
0.15
0.18
~18.1 8
0.43
0.69
2.7
~18
0.32
0.53
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
Pext (barg)
P (barg) at 2m
P (barg) at 5m
2mp
16.5% H2 mixture (vent area of 0.49 m2)
Experiment
0.114
0.087
0.056
0.029
FLACS
0.128
0.107
0.021
15.5% H2 mixture (vent area of 0.25 m2)
0.202
0.0767
0.0836
0.0245
0.210
0.072
0.034
0.010

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.