A . Kotchourko Karlsruhe Institute of Technology, Germany

1. Combustion – Part 2 Deflagration andExplosion A. Kotchourko Karlsruhe Institute of Technology, Germany

2. Hydrogen combustionaccidents Hindenburg 1937 Motivation to study combustion! Stockholm 1984 Fukushima 2011 Most of the accidents involving (H2) combustion were really dangerous

3. Human injury threshold

4. Industrial accident initial conditions • Various H2-air mixture compositions (4% - 75%) • Various geometry environment • Open geometry (external explosions): outside buildings, little influence of partial confinement and obstacles (in containment such as dome area) • Partially confined: limitation in space, geometrical limitations (semi-open configurations; near wall, roof; etc) • Confined explosions (internal explosions) inside rooms, compartments, pipes, etc • Various congestion • High: high space blockage by pipework, cabling, repeating obstacles, etc • Medium: intermediate level of space obstruction • Low: open space with little blockage of the flame propagation path • Ignition source

5. Pressurewavegeneratedbycombustion • Slow flame • Turbulent flame • Fast turbulent flame • DDT • Detonation Combustion regime is important for the pressure loads determination

6. Practicalneeds • Safety engineer needs to know which combustion regime is to be expected • Numerous scientific studies on FA and DDT still do not provide complete understanding of these complicated events, which usually exhibit probabilistic nature and complicate their confident prediction both in the experiments and in detailed CFD simulations • Complexity of the modeling for turbulent combustion • Simplified outdated models as EBU, EDM, etc • Widely using now in engineering simulations BVM • A number of different correlations • Poor accounting of flame instabilities • Complexity of adequate modeling of turbulence (RANS vs. LES) • PDF approach requires better theory

7. Detailed modeling of FA and DDT Requires high enough resolution for chemistry and turbulence, including • Reproduction of the interaction of generated acoustic waves with flame • Multiple reflections from obstacles and walls • Possible development of powerful instabilities such as K-H and R-M Gamezo, et al, 2007

8. 1 1 2 2 3 Soot track image Schlieren image 2 3 Modeling of the decay and re-initiation of the detonation interacting with obstacles

9. Local grid refinement is not enough Schlieren image Hydrogen concentration Structure of the wave front during re-initiation of the detonation interacting with obstacles

10. The principles of combustion and detonation processes can be basically considered as known, however distinct prediction of the flame propagation regime is still challenging task • The technical recipes often used, can be subjective or even not fully consistent, such as e.g., • LFL and UFL for combustion possibility • Concentration limits for explosion possibility • Evaluation of the combustion speed in comparison with sound speed as criterion for transition to detonation • Methodology which is considered below is based on the evaluation of the possibility for FA and DDT depending on engineering characteristics of the system, such as geometrical properties and composition of the mixture

11. Possible combustion regimes during accident Flame speed along combustion tube for different tube configurations and mixture compositions • In respect to sound speed in products Cpthree different regimes can be distinguished: • Slow deflagration (v < Cp) • Fast deflagration (v ~ Cp) • Detonation (v ~ 2Cp) Not full variety of the accident conditions, starting from simple to more complex

12. Expansion ratio as main governing parameter

13. FA criterion for closed obstructed volumes Alekseev V., et al., 2000Dorofeev S., et al., 2001, 2004

14. Vented combustion FA criterion Combustion in closed volumes Alekseev V., et al., 2000Dorofeev S., et al., 2001, 2004 Combustion in vented volumes Dependence of the critical expansion ratio on vent ratio

15. d Semi-confined flat layer • Critical conditions for FA in closed volumes and under transverse venting conditions : σ/σ0 ~ 1+2·α • Tunnel 2D-geometry of gas mixture with one wall (semi-confined volume) can be assumed to be an enclosure with venting ratio α= 0.5 Combustible H2-air flat layer

16. Semi-confined layer combustion FA criterion Grüne, et al, 2009 Kuznetsov, et al, 2010 Dependence of the terminal flame speed on H2 concentration Dependence of the critical expansion ratio on D/H ratio H – layer thickness D – obstacle distance

17. Flat layer FA criterion New outlook: numerical parametric study of the possibility of FA on geometrical characteristics of the system Main outcome: Broader look at the controlling dependencies Expressing in conventional form against expansion ratio of the mixture K = 0.17

18. Summary • The studies targeted to formulation of the principles (criteria) permitting reliably (or at least conservatively) predict possible combustion regimes have to be highly appreciated • An acceptance and approval of the standards based on such criteria has to be considered as significant improvement in safety regulations