1. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy NUMERICAL STUDY OF A HIGHLY UNDER-EXPANDED HYDROGEN JET B P Xu, J P Zhang, J X WEN, S Dembele and J Karwatzki Faculty of Engineering, Kingston University Friars venue, Roehampton Vale, London, SW15 3DW, UK

3. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy

4. Very High Pressure Hydrogen Storage The Fuel cell vehicles (FCV) currently in trial use are mounted with hydrogen containers pressurized up to 400 bar and yield a driving range of 300-350 km per filling - roughly half of the gasoline vehicle’s driving range. Industry is developing containers for up to 700 bar pressurization. Need to gain insight of such release and its potential for ignition

5. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy Schematic diagram of free jet flow shock structure

6. Two Modelling Approaches Pseudo-source approach (Ewan and Modie 1985) –Leak modelled from downstream as a sonic jet with the same mass flow rate Numerically solving the under-expanded shock structure Results used as inflow for the subsequent large eddy simulation of the jet

7. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy Simulation of the Under-expanded Shock Structure Commercial code CFX –Total energy model take into the kinetic energy of high speed flows – The k-ω based shear stress turbulence (SST) model –A TVD type high resolution discretisation scheme to represent sharp gradients without numerical oscillations –A global 2nd accuracy, which switches to a 1st order upwind scheme locally to prevent non- physical oscillations –The 2nd order backward Euler scheme to define the discretisation algorithm for the transient term. Validation of the code for supersonic application is available through CFX Vendor, now part ANSYS Europe

8. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy Simulation of the free hydrogen jet KIVA-LES (modified for LES from KIVA-3V) –Finite volume based ALE (Arbitrary Lagrangian- Eulerian) method –the 2 nd order Crank-Nicolson scheme for the diffusion terms and the terms associated with pressure wave propagation; –The 2 nd order MacCormack method for the convective terms in the rezone phase; – A 2 nd order centred scheme for the convection term in the momentum equation. 17. B B P Xu, J X Wen, S Dembele, Large eddy simulation of plane impinging jets, submitted to Physics of Fluids. 18. B P Xu and J X Wen, Validation of a new droplet collision model in LES of non-evaporating diesel fuel sprays, submitted to Int. J of Multiphase Flow.

9. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy Real Gas Property Abel-Noble EOS (Equation of State) (1) For hydrogen, b=1.55×10-5 (m3/mole), valid for P<1600atm and 200<T< 350K Van der Waals EOS (3) (2) This EoS has been reported to reproduce a large part of the experiment thermodynamic data on hydrogen within 0.1% and practically all data within 0.5%.

10. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy Comparison of density as a function of pressure for constant temperatures

11. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy Choked Flow Nozzle Dynamics Assuming isentropic flow T* from Van der Waals EoS Table 1. Initial data in the high-pressure jet simulation Vessel pressure (MPa)20 Release temperature (K)267 Vessel temperature (K)300 Release velocity (m/s)1020 Orifice diameter (m)0.01 Discharge coefficient0.85 Release pressure (MPa)10.6 Density in the vessel (Kg/m3) 14.1 Density at nozzle exit (kg/m3) 8.93

12. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy 530 K Mohamed and M Paraschivoiu, Real gas simulation of hydrogen release from a high-pressure chamber, Int J of Hydrogen Energy, 30 (2005).

13. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy Release pressure: 200 Bar

14. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy Release pressure: 20 bar At very high pressure ratios (such as the previous one), only one Mach disk Several Mach disks at relatively lower pressure ratios

15. Predicted pressure, velocity, temperature and hydrogen mass concentrations at the centreline

16. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy

17. The free hydrogen jet (with KIVA-LES) Instantaneous densityMean velocity

18. Normalized values of main axial velocity, axial turbulent intensity and hydrogen mass fraction on the centreline Normalized values of main axial velocity and hydrogen mass density versus distance to the centreline at different Z positions.

19. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy

20. Conclusion The predicted flow pattern and Mach number distribution within the shock structure are in line with previous experimental observation and theoretical analysis. Apparent air entrainment is found after these shock structures, implying that the widely used pseudo-source approach may incur some errors for such jet simulations. The hydrogen release temperature is lower than the vessel temperature when the container pressure is below a certain value (e.g. 530 bar in the current configuration). The situation is different for higher vessel pressures. A combustible cloud could be formed above the leak source within a very short period of time (about 0.1s).

21. International Conference on Hydrogen Safety, Sep. 8-10, Pisa, Italy ACKNOWLEDGEMENT We gratefully acknowledge the helpful discussion with Vincent Tam, Peter Cumber and Marius Paraschvoiu. Jet flame simulation is already ongoing Work will continue jointly with BP and HSL through the EC funded HYFIRE project in several areas concerning fire and explosion safety of hydrogen. FUTURE WORK