4th International Conference on Hydrogen Safety, San Francisco, USA, September, A. Kotchourko Karlsruhe Institute of Technology, Germany

Hydrogen combustion accidents 4th International Conference on Hydrogen Safety, San Francisco, USA, September, Hindenburg 1937 Fukushima 2011 Stockholm 1984 Most of the accidents involving (H2) combustion were really dangerous Motivation to study combustion!

Human injury threshold 4th International Conference on Hydrogen Safety, San Francisco, USA, September, 20113

Industrial accident initial conditions 4th International Conference on Hydrogen Safety, San Francisco, USA, September,  Various H 2 -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

Pressure wave generated by combustion 4th International Conference on Hydrogen Safety, San Francisco, USA, September,  Slow flame  Turbulent flame  Fast turbulent flame  DDT  Detonation Combustion regime is important for the pressure loads determination

Practical needs - 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in the experiments and in detailed CFD simulations - Complexity of the modeling for turbulent combustion - Simplified outdated models as EBU, EDM, etc - Widely using now in engineering simulations BVM - A number of different correlations - Poor accounting of flame instabilities -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 4th International Conference on Hydrogen Safety, San Francisco, USA, September, 20116

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 4th International Conference on Hydrogen Safety, San Francisco, USA, September, Gamezo, et al, 2007

4th International Conference on Hydrogen Safety, San Francisco, USA, September, Soot track image Schlieren image Modeling of the decay and re-initiation of the detonation interacting with obstacles

4th International Conference on Hydrogen Safety, San Francisco, USA, September, Schlieren image Hydrogen concentration Structure of the wave front during re-initiation of the detonation interacting with obstacles Local grid refinement is not enough

4th International Conference on Hydrogen Safety, San Francisco, USA, September, 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

Possible combustion regimes during accident 4th International Conference on Hydrogen Safety, San Francisco, USA, September, Flame speed along combustion tube for different tube configurations and mixture compositions In respect to sound speed in products C p three different regimes can be distinguished:  Slow deflagration (v < C p )  Fast deflagration (v ~ C p )  Detonation (v ~ 2C p ) Not full variety of the accident conditions, starting from simple to more complex

Expansion ratio as main governing parameter 4th International Conference on Hydrogen Safety, San Francisco, USA, September,

FA criterion for closed obstructed volumes 4th International Conference on Hydrogen Safety, San Francisco, USA, September, Alekseev V., et al., 2000 Dorofeev S., et al., 2001, 2004

Vented combustion FA criterion 4th International Conference on Hydrogen Safety, San Francisco, USA, September, Combustion in closed volumes Combustion in vented volumes Alekseev V., et al., 2000 Dorofeev S., et al., 2001, 2004 Dependence of the critical expansion ratio on vent ratio

Semi-confined flat layer 2nd International Conference on Hydrogen Safety, San Sebastian, Spain, September,  Critical conditions for FA in closed volumes and under transverse venting conditions : σ/σ 0 ~ 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 H 2 -air flat layer

Semi-confined layer combustion FA criterion 4th International Conference on Hydrogen Safety, San Francisco, USA, September, 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 Grüne, et al, 2009 Kuznetsov, et al, 2010

Flat layer FA criterion 4th International Conference on Hydrogen Safety, San Francisco, USA, September, Expressing in conventional form against expansion ratio of the mixture K = 0.17 New outlook: numerical parametric study of the possibility of FA on geometrical characteristics of the system Main outcome: Broader look at the controlling dependencies

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 4th International Conference on Hydrogen Safety, San Francisco, USA, September,