On numerical simulation of liquefied and gaseous hydrogen releases at large scales

1. First International Conference on HYDROGEN SAFETY On numerical simulation of liquefied and gaseous hydrogen releases at large scales V. Molkov, D. Makarov, E. Prost 8-10 September 2005, Pisa, Italy

2. Introduction of hydrogen as an energy carrier makes great demands on hydrogen safety. Development of robust and reliable risk assessment methodologies requires all-round validation of models and tools. • The need to model non-uniform hydrogen-air mixture formation at real scales is important to have realistic initial conditions for subsequent modelling of partially premixed hydrogen combustion. • The aim of this study is validations of the LES model in application to large-scale hydrogen release scenarios and formulation of tasks for future research in this area.

3. Contents • The LES model • LH2 release in open atmosphere • GH2 release in a closed vessel

4. Large Eddy Simulation (LES) model

5. LES model (1/2) • Conservation of mass • Conservation of momentum • Conservation of energy

6. LES model (2/2) • RNG SGS turbulence model • Conservation of “H2”concentration

7. Liquefied hydrogen release in open atmosphere

8. NASA experiment Chirivella J.E., Witcofski, R.D. Am. Inst. Chem. Eng. Symp., 82, No 251, 1986, pp.120-142: - Spill 5.11 m3 (361.8 kg) of LH2 in 38 s - LH2 pool radius between 2 and 3 m - Total evaporation time 43 s - Wind speed ~2.2 m/s at height 10 m

9. Calculation domain (1/2) Spill area and instrumentation towers area Cloud propagation area 70 m 180 m Characteristic size of CV: Numerical grid: 156133 CV • tower location 1.0 - 2.0 m • cloud area 2.0 - 3.0 m • the rest of domain up to 10 m

10. Calculation domain (2/2) Cloud propagation area Spill area 70 m 180 m Characteristic size of CV: Numerical grid: 103163 CV • spill area 0.6 - 1.0 m • cloud area 2.0 - 3.0 m • the rest of domain up to 10 m

11. Numerical details • Initial conditions • atmosphere velocity profile: • where(provided u=2.2 m at H=10 m) • Boundary conditions • velocity profile at inflow • prescribed pressure conditions at outflow boundaries, p=0 Pa • H2 injection • mass injection rate • Run 1: injection area radius • Run 2: injection area radius • average injection velocity • instant injection velocity • Run 1: turbulence • Run 2: turbulence • Geomerty: Run 1 (no pool border, no obstacles), Run 2 (+)

12. H2 concentration (Run 1) Texp = 21.33 s Tsim = 21.36 s

13. H2 concentration (Run 2) Texp = 21.33 s Tsim = 21.35 s

14. Simulated temperature (Run 1)

15. Simulated temperature (Run 2)

16. Visible cloud (Run 1)

17. Visible cloud (Run 2)

18. Cloud propagation (Run 1)

19. Cloud propagation (Run 2)

20. Phenomena to be addressed • Condensation of air in temperature range 20-90 K (with heat release) and evaporation above 90 K • Two phase flow (gas: hydrogen-air; solid: air ice) • Detailed spill modelling (initial fractions of GH2 and LH2; heat transfer to the ground: initial violent evaporation stage, etc)

21. Gaseous hydrogen release in 20-m3 closed vessel

22. 5.5m 2.2m Experiment Volume injection rate: V=4.5 l/s Time of release = 60 seconds 1.4m H2

23. Calculation domain Non-uniform tetrahedral grid CV number: 54004 CV size: 0.01-0.10 m close to place of H2 injection CV size: up to 0.20 m in the rest of domain “Uniform” grid CV number: 28440 CV size: 0.14-0.20 m 3-251 min 0-180 s

24. Numerical details • Initial conditions • quiescent air, u=0 m/s, • initial air concentration Yair=1.0, • initial temperature T=293K • Boundary conditions • t=0-1s: Vinj increased from 0 to 57.5 m/s • t =1-59s: Vinj=57.5 m/s • t=59-60s: decrease from 57.5  to 0 m/s • t=60s-251min: Vinj=0 m/s • YH2=1.0, Tinj=293K • Numerical details • explicit linearisationof the governing equations • implicit method for solution of linear equation set • second order accurate upwind scheme for convection terms, central-difference scheme for diffusion terms • Time steps: t=0-180 s: t=0.01 s; t=3-251 min: t=0.01-1.0 s

25. Simulation results

26. Hydrogen distribution 1 2 min 50 min 100 min 250 min

27. Hydrogen distribution 2

28. Residual velocities 50min: Vmax=10 cm/s 100min: Vmax=8 cm/s 250min: Vmax=5 cm/s

29. Conclusions • The LES model has been applied to analyse large-scale experimental LH2 and GH2 releases. • The simulation of non-uniform flammable cloud formation, resulting from a LH2 spill, reproduced a characteristic structure of the turbulent eddies and the direction of cloud propagation. • The simulation results were found to depend on initial and boundary conditions. • The air condensation-evaporation sub-model may improve predictive capabilities of the LES model

30. Conclusions • Good agreement was achieved with experimental data on GH2 release in 20-m3 closed vessel up to t=250 min after the 1 minute release. • The LES results demonstrated that random flow field remains in the vessel long time after the injection and this is presumably responsible for H2 transport. • Further experiments with observation of velocity field after release and simulations with higher accuracy are required to give final answer to this question.