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

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

Phenomenology Vented explosion – Back Wall ignition

P1 or P_ext

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

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

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

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

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

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%

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

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

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%

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%

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

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

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.