Flammable extent of hydrogen jets close to surfaces

1. Flammable extent of hydrogen jets close to surfaces International Conference on Hydrogen Safety, San Francisco, September 12-15 2011 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 • 100-700 bars (H2)

5. Cases considered 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)

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 Methane Hydrogen 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 subsonichydrogen and methane jets wereperformedusing Fluent : • Froude numbers values for the jets for a givenleak 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. Fr=120 • 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= 1.905 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 5.17 % D=1.905 mm, v=133.9 m/s D=9.45 mm, v=134.5m/s

13. Results LFL contours of hydrogen Effectsof buoyancy 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