NUMERICAL STUDY OF SPONTANEOUS IGNITION OF PRESSURIZED HYDROGEN RELEASE INTO AIR

1. NUMERICAL STUDY OF SPONTANEOUS IGNITION OFPRESSURIZED HYDROGEN RELEASE INTO AIR B. P. Xu1, L. EL Hima1, J. X. Wen1, S. Dembele1 and V.H.Y. Tam2 1Faculty of Engineering, Kingston University Friars Avenue, London, SW15 3DW, UK 2EPTG, bp Exploration, Chertsey Road, Sunbury-on-Thames, TW16 7LN, UK

2. Analysis of Hydrogen Accidental Database Compiled by Kingston University

3. Postulated Ignition Mechanisms(Astbury & Hawksworth, 2005) • Reverse Joule-Thomson effect • Static electricity • Sudden adiabatic compression • Hot surface ignition • Mechanical friction and impact • Diffusion ignition Self-ignition and explosion during discharge of high-pressure hydrogen, Toshio Mogi, Dongjoon Kim, Hiroumi Shiina, Sadashige Horiguchi, J of Loss Prevention, 2007.

4. Sudden rupture of a pressure boundary Shock wave (primary) Shock-heated air or oxidizer (behind the shock wave) Cooling hydrogen – flow acceleration or divergence Formation of combustible mixture at contact surface – molecular diffusion Ignition (after a delay time) Diffusion Ignition(Wolanski and Wojcicki, 1973) Temperature Schlieren Density

5. Mixing at Contact Surface • Mass and energy exchange between shock-heated air and cooled hydrogen through molecular diffusion • Molecular diffusivity is inversely proportional to local pressure and proportional to local temperature • The thickness of the contact surface – extremely thin and increasing with release time For the release case of 100 bar through a 1mm hole at t=3us

6. Mixing time the time for the temperature of the mixture to reach the autoignition temperature Chemical delay time due to the slow hydrogen combustion rate under low temperature Ignition & its Delay Time An increase in the release pressure will lead to a rapid decrease in both mixing time and chemical delay time

7. Numerical Methods • 2-D Unsteady Compressible Navier-Stokes equations solved with anILES method • Detailed chemical-kinetic scheme - 8 reactive species and 21 elementary steps –third body and “fall off” behavior considered (Williams 2006) • Multi-component diffusion approach for mixing - thermal diffusion • ALE numerical scheme: convective term solved separately from diffusion terms • In Lagrangian stage, 2nd-order Crank-Nicolson scheme + 2nd-order central differencing • In rezone phase, 3rd-order Runge-Kutta method + 5th - order upwind WENO scheme Numerical diffusion resulting from the use of lower order schemes could lead to overprediction of the contact surface thickness, and hence artificially increase the chance of autoignition.

8. Problem Description • Three hole sizes: 1mm, 3mm, 5mm • Three release Pressures: 100 bar, 200 bar, 300 bar • Non-slip and adiabatic wall boundary • Minimum cell size: 10 microns • Only early stage of release simulated

9. Overview of the Under-expanded jet 1. Mach Number 2. Temperature 3. Mass Fraction 3. Density Schlieren

10. The Very Early Release Moment 1. Axial Velocity 2. Temperature 3. Density Schlieren

11. Changing Patterns for the release case of 300 bar through a 3mm hole at t=9us

12. Contour of OH for the release case of 300 bar through a 5mm hole at t=35us

13. Contour of Mach Number for the release case of 300 bar through a 5mm hole at t=35us

14. Contour of Temperature for the release case of 300 bar through a 5mm hole at t=35us

15. Contour of Mass Fraction for the release case of 300 bar through a 5mm hole at t=35us

16. Density Schlieren for the release case of 300 bar through a 5mm hole at t=35us

17. Release through a 1mm, 3mm and 5mm holes

18. Grid Sensitivity - fine mesh (10 microns), coarse mesh (20 microns)

19. Concluding Statement The release of highly pressurised hydrogen into air could lead to spontaneous ignition depending on pressure, hole sizes, etc.   Current work demonstrated the conditions leading to autoignition. A flame was found to be sustained over a period of 50 s and still stable when the simulation was terminated due to limitation of computer power.  Further work is underway to establish whether this flame could be maintained, leading to a jet fire, fire ball or an explosion.

20. Conclusions • Diffusion ignition has been numerically proved to be a possible mechanism for spontaneous ignition of a sudden high pressure hydrogen release direct into air • Ignition first occurs at the tip of a very thin contact surface and then moves downward along the contact surface • The pressure at the contact surface is higher than the ambient pressure; this high pressure produces a high heat release rate to overcomes the flow divergence and sustain the very thin flame • As the jet develops downstream, the front of the contact surface becomes distorted • The distorted contact surface can increase the chemical reaction interface and enhance the mixing process, and it may be the key factor for the final diffusive turbulent flame • Finally, an increase in the release pressure or the hole size can facilitate the ignition.

21. ACKNOWLEDGEMENT • B P XU’s Post-doctoral Fellowship is funded by the Faculty of Engineering at Kingston University. • The authors would like to acknowledge EU FP6 Marie Curie programme for funding hydrogen research at Kingston University through the HYFIRE (HYdrogen combustion in the context of FIRE and explosion safety) project. • BP and HSL are supporting groups for HYFIRE.