4th International Conference on Hydrogen Safety, San Francisco, USA, 12 - 14 September, 20111 A. Kotchourko Karlsruhe Institute of Technology, Germany

4th International Conference on Hydrogen Safety, San Francisco, USA, 12 - 14 September, 20114th International Conference on Hydrogen Safety, San Francisco, USA, 12 - 14 September, 2011 11

A. Kotchourko

Karlsruhe Institute of Technology, Germany

Combustion – Part 2Combustion – Part 2Deflagration and ExplosionDeflagration and Explosion

Combustion – Part 2Combustion – Part 2Deflagration and ExplosionDeflagration and Explosion

Hydrogen combustion accidentsHydrogen combustion accidents


Hindenburg 1937

Fukushima 2011Stockholm 1984

Most of the accidents involving (H2) combustion were really dangerous

Motivation to study combustion!

Human injury thresholdHuman injury threshold


Industrial accident initial conditionsIndustrial accident initial conditions


Various HVarious H22-air mixture compositions (4% - 75%)-air mixture compositions (4% - 75%) Various geometry environmentVarious geometry environment

Open geometry (external explosions): outside buildings, Open geometry (external explosions): outside buildings, little influence of partial confinement and obstacles (in little influence of partial confinement and obstacles (in containment such as dome area)containment such as dome area)

Partially confined: limitation in space, geometrical Partially confined: limitation in space, geometrical limitations (semi-open configurations; near wall, roof; limitations (semi-open configurations; near wall, roof; etc)etc)

Confined explosions (internal explosions) inside rooms, Confined explosions (internal explosions) inside rooms, compartments, pipes, etc compartments, pipes, etc

Various congestionVarious congestion High: high space blockage by pipework, cabling, High: high space blockage by pipework, cabling,

repeating obstacles, etcrepeating obstacles, etc Medium: intermediate level of space obstructionMedium: intermediate level of space obstruction Low: open space with little blockage of the flame Low: open space with little blockage of the flame

propagation pathpropagation path Ignition sourceIgnition source

Pressure wave generated by combustionPressure wave generated by combustion


Slow flameSlow flame Turbulent Turbulent

flameflame Fast turbulent Fast turbulent

flameflame DDTDDT DetonationDetonation

0

2

4

6

8

10

12

14

16

18

0.01 0.1 1 10t, s

P,

bar

Slow

Fast

Detonation

Combustion regime is important for the pressure loads determination

Practical needsPractical needs

- Safety engineer needs to know which Safety engineer needs to know which combustion regime is to be expectedcombustion regime is to be expected

- Numerous scientific studies on FA and DDT still Numerous scientific studies on FA and DDT still do not provide complete understanding of these do not provide complete understanding of these complicated events, which usually exhibit complicated events, which usually exhibit probabilistic nature and complicate their probabilistic nature and complicate their confident prediction both in the experiments confident prediction both in the experiments and in detailed CFD simulations and in detailed CFD simulations

- Complexity of the modeling for turbulent Complexity of the modeling for turbulent combustioncombustion- Simplified outdated models as EBU, EDM, etcSimplified outdated models as EBU, EDM, etc- Widely using now in engineering simulations BVMWidely using now in engineering simulations BVM

- A number of different correlationsA number of different correlations- Poor accounting of flame instabilitiesPoor accounting of flame instabilities- Complexity of adequate modeling of turbulence (RANS vs. LES)Complexity of adequate modeling of turbulence (RANS vs. LES)

- PDF approach requires better theoryPDF approach requires better theory4th International Conference on Hydrogen Safety, San Francisco, USA, 12 - 14 September, 20114th International Conference on Hydrogen Safety, San Francisco, USA, 12 - 14 September, 2011 66

Detailed modeling of FA and DDTDetailed modeling of FA and DDT

Requires high enough Requires high enough resolution for chemistry resolution for chemistry and turbulence, including and turbulence, including Reproduction of the Reproduction of the interaction of generated interaction of generated acoustic waves with flameacoustic waves with flameMultiple reflections from Multiple reflections from obstacles and wallsobstacles and wallsPossible development of Possible development of powerful instabilities such powerful instabilities such as K-H and R-Mas K-H and R-M


Gamezo, et al, 2007


1

1 2

2 3

3 2

Soot track imageSchlieren image

Modeling of the decay and re-initiation of the detonation interacting with obstacles


Schlieren image

Hydrogen concentration

Structure of the wave front during re-initiation of the detonation interacting with obstacles

Local grid refinement is not enough


- The principles of combustion and detonation The principles of combustion and detonation processes can be basically considered as known, processes can be basically considered as known, however distinct prediction of the flame however distinct prediction of the flame propagation regime is still challenging taskpropagation regime is still challenging task

- The technical recipes often used, can be The technical recipes often used, can be subjective or even not fully consistent, such as subjective or even not fully consistent, such as e.g.,e.g.,- LFL and UFL for combustion possibilityLFL and UFL for combustion possibility- Concentration limits for explosion possibilityConcentration limits for explosion possibility- Evaluation of the combustion speed in comparison with sound Evaluation of the combustion speed in comparison with sound

speed as criterion for transition to detonation speed as criterion for transition to detonation

- Methodology which is considered below is based Methodology which is considered below is based on the evaluation of the possibility for FA and on the evaluation of the possibility for FA and DDT depending on engineering characteristics DDT depending on engineering characteristics of the system, such as geometrical properties of the system, such as geometrical properties and composition of the mixtureand composition of the mixture

Possible combustion regimes during accidentPossible combustion regimes during accident


Flame speed along combustion tube for different tube configurations and mixture compositions

In respect to sound speed in products Cp three 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

Expansion ratio as main governing parameterExpansion ratio as main governing parameter


FA criterion for closed obstructed volumesFA criterion for closed obstructed volumes


Alekseev V., et al., 2000Alekseev V., et al., 2000Dorofeev S., et al., 2001, 2004Dorofeev S., et al., 2001, 2004

Vented combustion FA criterionVented combustion FA criterion


Combustion in closed volumes

Combustion in vented volumes

Alekseev V., et al., 2000Alekseev V., et al., 2000Dorofeev S., et al., 2001, 2004Dorofeev S., et al., 2001, 2004

Dependence of the critical expansion ratio on vent ratio

Semi-confined flat layerSemi-confined flat layer

2nd International Conference on Hydrogen Safety, San Sebastian, Spain, 11 - 13 September, 20072nd International Conference on Hydrogen Safety, San Sebastian, Spain, 11 - 13 September, 2007 1515

Critical conditions for FA in closed volumes and under transverse venting conditions : Critical conditions for FA in closed volumes and under transverse venting conditions : σσ//σσ00 ~ 1+2· ~ 1+2·αα

Tunnel 2D-geometry of gas mixture with one wall (semi-confined volume) can be Tunnel 2D-geometry of gas mixture with one wall (semi-confined volume) can be assumed to be an enclosure with venting ratio assumed to be an enclosure with venting ratio αα = 0.5= 0.5

Combustible H2-air flat layer

Semi-confined layer combustion FA criterionSemi-confined layer combustion FA criterion


Dependence of the terminal flame speed on H2 concentration

Dependence of the critical expansion ratio on D/H ratio

H – layer thicknessH – layer thicknessD – obstacle distanceD – obstacle distance

Grüne, et al, 2009Grüne, et al, 2009Kuznetsov, et al, 2010Kuznetsov, et al, 2010

Flat layer FA criterionFlat layer FA criterion


0 10 20

0

2

4

6

0 10 20D/h [-]

0

2

4

6

Exp

lana

tory

Var

iabl

e[]

Fast

Slow

0.136 0.575H DBR f

h h

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

SummarySummary

- The studies targeted to formulation of the The studies targeted to formulation of the principles (criteria) permitting reliably (or at principles (criteria) permitting reliably (or at least conservatively) predict possible least conservatively) predict possible combustion regimes have to be highly combustion regimes have to be highly appreciatedappreciated

- An acceptance and approval of the standards An acceptance and approval of the standards based on such criteria has to be considered based on such criteria has to be considered as significant improvement in safety as significant improvement in safety regulations regulations


Documents

4th International Conference on Hydrogen Safety, San Francisco, USA, 12 - 14 September, 20111 A. Kotchourko Karlsruhe Institute of Technology, Germany