1. Flammable extent of hydrogen jets close to surfaces
International Conference on Hydrogen Safety,
San Francisco, September
Flammable extent of hydrogen jets close to surfaces
Benjamin Angers*, Ahmed Hourri*, Luis Fernando Gomez, Pierre Bénard and Andrei Tchouvelev**
*Hydrogen Research Institute, Université du Québec à Trois-Rivières, Trois- Rivières, Québec Canada (G9A 5H8)
**A.V.Tchouvelev & Associates, Mississauga, Ontario, Canada

2. Project objective
Molar concentration envelopes can be used to define clearance distances and exclusion zones based on the prevention of ignition (typical values: 2%, 4%, 8%)
Jet releases constitute one of the more basic process through which hydrogen releases disperses in air
The behavior of the expanded region of vertical jets is well understood and can be treated analytically
Vertical jet release properties (concentration profile) can be calculated from the storage conditions
The effect of surfaces will significantly alter their predictions
The objective of this work is to quantify the effect of surfaces on unignited hydrogen jets and find engineering correlations that could be used to establish the flammable extent of jet releases in the presence of surfaces
Side
Top

3. Surface jet studies In this work we consider
Vertical transient jets
Subsonic horizontal jets
Approach: CFD sims of Hydrogen and Methane jets using FLACS (numerical efficiency) and Fluent (more control over models)
In terms of the cases considered
Vertical jets
GexCon FLACS, k-, sonic release using a pseudo-diameter approach based on conservation laws and the Hugoniot relations
Horizontal jets
Fluent, RNG k-, subsonic

4. Vertical jets
Hydrogen & methane jets
Flacs
bars (H2)

5. Cases considered Vertical jets (d=6.38 mm, T=298 K) Gas
Storage pressure
(barg)
Mass Flow rate
(kg/s)
Jet exit distance from the surface (m) H2
100
0.20
from m to 10 m
250
0.49
from m to 10 m
400
0.78
from m to 10 m
550
1.07
from m to 10 m
700
1.36
from m to 12 m
CH4
0.54
from m to 4 m
1.34
from m to 4 m
2.14
from m to 5 m
2.94
from m to 5 m
3.74
from m to 10 m
Vertical jets (d=6.38 mm, T=298 K)

6. Results for hydrogen stored @700 bars
0.077 m from surface
free jet

7. Summary results for hydrogen
Steady decrease to free jet values for centerline
Crossover behaviour for maximum extent due to proximity of the surface

8. Normalized relative extent
NRE(h) = (Xmax(h) - Xfree_jet)/(Xabs_max - Xfree_jet)
Pressure (barg)
Xfree_jet (m)
Xabs_max - Xfree_jet (m)
h (m) at NRE(h) = 0.5
100
19.4
37.3
1.63
250
29.2
53.9
2.51
400
39.7
65.0
3.28
550
42.7
74.2
3.93
700
47.2
84.8
4.23

9. Results for methane
Larger effect of the surface for methane (relative)
Reversed amplitude in the crossover region
NRE (Max extent) > NRE (Centerline)
Reserved effect not observed in 0 G simulations

10. Normalized extent of zero G jet
Hydrogen
Methane
Centerline extent and max extent normalized axes for 100 bar, 250 bar, 400 bar, 550 bar and 700 bar release, with no gravity.

11. Horizontal subsonic jets : Surface jets studies of hydrogen using Fluent
Subsonic releases performed to avoid issues with notional approximations and for easier comparison with planned experiments
Simulation of horizontal surface effects on horizontal subsonic hydrogen and methane jets were performed using Fluent :
Froude numbers values for the jets for a given leak orifice diameter of 6. mm, of : 50, 250, 500, 750 and 1000.
Variable distance between the centerline of the jet and the wall : 5cm , 20 cm and 50 cm
Profiles of 50 % LFL contours of hydrogen free jets for various Froude numbers

12. Validation simulations using Fluent (k-ε realizable)
.Swain et al ont effectué une étude expérimentale. Le diamètre de l'orifice de la fuite était de 9.45 mm. L'orifice était situé à 1.22 m du sol. La vitesse de la fuite était de m/s à la sortie, ce qui correspondait à un débit de 7.910-4 kg/s. La densité de l'hydrogène à la sortie était de kg/m3
Swain et al. Fr=120
Location
Exp. values
Swain et al.
This work
deviation (%) 1
4.16
23.7 2
5.81
7.8 3
10.99
8.8 4
7.84
10.4 5
5.7
6.6 6
5.075
25.3
Average deviation
13.8
Cross validation of a simulation leak of hydrogen (D= 5 mm, 31.2 scfm, Fr=1000 ) by Houf et al.
Inverse hydrogen mole fraction along jet centerline versus streamwise distance along jet centerline deviate from our simulation values using Fluent by an average deviation of 0. 8%
Validation of experimental leak of hydrogen (D= mm, 22.9 slm, Fr=268) by Houf et al.
Inverse hydrogen mole fraction along jet centerline versus streamwise distance along jet centerline deviate from our simulation values using Fluent by an average deviation of %
D=1.905 mm, v=133.9 m/s
D=9.45 mm, v=134.5m/s

13. Results Effects of buoyancy LFL contours of hydrogen
5 cm from the ground
20 cm from the ground
50 cm from the ground
Free jets
Hydrogen free jets
LFL contours of methane
5 cm from the ground
20 cm from the ground
50 cm from the ground
Free jets
Methane free jets

14. Effect of buoyancy on free jets
Pitts

15. Surface effects on horizontal jets

16. Normalized relative extent

17. Lower Flammability extents
Hydrogen
Methane

18. Conclusions
Vertical surfaces lead to a collapse of the curves of the flammable extent as a function of height when expressed as normalized relative extent when using isotropic turbulence models (both k- and Realizable) as reported earlier for vertical and zero g simulations
True for both centreline and maximum flammable extent
Crossover behaviour observed for maximum extent
likely due to the proximity of the surface (law of the wall vs scaling behavior of the jet)
Reversed behaviour for H2 and methane in the crossover region (NRE (Max extent) > NRE (Centerline)) which is not observed in 0 G simulations
Similar collapse observed when the results are expressed as the NRE when performing subsonic simulations using Fluent and a different turbulence model

19. Conclusions
Even if two different turbulent models concord qualitatively, the use of isotropic turbulence models (1) for jet and (2) close to a surface is a potential issue
Comparison of LES simulations & experiments are needed before definitive conclusions

20. Acknowledgements Natural Resources Canada
Natural Sciences and Engineering Council of Canada