Presentation Slide Title

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First International Conference on HYDROGEN SAFETY
Hydrogen-air deflagrations in open atmosphere
LES analysis of experimental data
V. Molkov, D. Makarov, H. Schneider
- University of Ulster, UK - Fraunhofer
Institut Chemische Technologie, GERMANY
7-10 September 2005, Pisa - Italy
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• Contents
• Experiment in 2094-m3 hemisphere 1
• Theoretical background for modelling 2
• The Large Eddy Simulation model 3
• Theory versus experiment 4

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EXPERIMENT
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Experimental details
Experiment Schneider H., Pförtner H.
PNP-Sichcrheitssofortprogramm, Prozebgasfreisetzun
g-Explosion in der gasfabrik und auswirkungen von
Druckwellen auf das Containment, Dezember 1983.
20 meters
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Side and top view movies
10 m
20 m
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Distributed flame front

• Estimate of turbulent flame front (distributed)
  thickness
• 1. The pocket (mole) of size 0.2 m behind a
  leading edge of the flame front will burn inward
  during 0.2m2m/s0.1s (0.2 m divided by burning
  velocity 2 m/s)
• 2. During this time leading edge will propagate
  as far as 0.1sx40m/s4 m! (8 m for mole 0.4 m)?

4 m
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Experimental results

• Flame propagation velocity was independent upon
  ignition energy in the investigated energy range
  (10-1000 J or pyrotechnical charge).
• The resulting flames propagated in almost
  hemispherical form with a developed structure.
• The maximum visible flame velocity occurs between
  the original radius of the balloon R0 and radius
  1.5R0.
• The maximum flame radius reached about 2R0.
• No transition to detonation was observed.
• The maximum visible flame velocity reached 84
  m/s.
• At a sufficient distance from the explosion the
  maximum pressure decayed inversely proportional
  to the distance.
• The positive pressure wave was followed by a
  negative pressure phase.

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THEORETICAL BACKGROUND FOR MODELLING
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Self-similarity (fractals)
Gostintsev et al (1988) analysed about 20
experiments on large-scale unconfined
deflagrations and concluded that the hydrodynamic
flame instability leads to accelerating,
self-similar regime of fully developed turbulent
flame propagation. According to this analysis,
the flame front surface obeys the fractal theory
after self-similar regime is established. The
authors found that the transition to the
self-similar turbulent regime of flame
propagation occurs after the critical value of
the flame front radius R is achieved, which was
found to be R1.0-1.2 m for near stoichiometric
premixed hydrogen-air flames.
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Flame generated turbulence
The study performed by Karlovits et al (1951)
using burner flames led to the conclusion that a
flame front itself generates turbulence. The
maximum theoretical value of the flame front
wrinkling due to flame induced turbulence was
found to be where Ei combustion
products expansion coefficient. LES of large
scale problems can not at foreseen future resolve
all details of flame front structure and this can
be modelled only.
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• S. Pope (2004)
• Physical LES (filter size is ARTIFICIAL parameter
  D)
• Numerical LES (filter size is cell size)

The Ulster LES model
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Ulster LES model (1/3)

• Conservation of mass
• Conservation of momentum
• Conservation of energy

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Ulster LES model (2/3)

• Premixed flame front propagation (progress
  variable)
• Gradient method for the source term
• Yakhots RNG like turbulent premixed combustion
  (inflow)
• 
  where u residual SGS velocity
• Karlovitz turbulence generated by flame front
  itself (SGS)
• Chemistry is in burning velocity (dependence on
  T, p, j)

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Ulster LES model (3/3)

• RNG SGS turbulence model
• Dilution of initial H2-air mixture by atmospheric
  air

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Three main FAQ

• Why gradient method? Decoupling physics and
  numerics
• Integral of source term through numerical flame
  front is always equal to physical value ruSt
  (physically correct heat release, given up
  structure of turbulent flame front)
• Why RNG (renormalization group) turbulence model?
• No turning. Validated for both laminar and
  turbulent flows.
• No cut-off at D but scaling down at inertial
  range.
• Why turbulence generated by flame front itself?
• LES of large scale accidental combustion can not
  resolve phenomena at scales comparable with
  flamelets thickness.
• Existence of a theoretical maximum and critical
  radius

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Domain and grid
Structured hexahedral (SHH)
200x200x100 m

• Characteristic size of control volumes (CV) for
  309494 CVs grid
• Radius, m CV size, m
• 0 - 22 0.4 - 1.2
• 22-30 (UTH zone) 1.2 - 4.0
• 30-200 (SHH zone) 4.0 (2.0 in direction of
  pressure gauges)

Unstructured tetrahedral
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Numerical details

• Initial conditions
• initial temperature T283 K initial pressure
  p98.9 kPa
• quiescent mixture progress variable c0.
• hydrogen concentration YH20.0287 at R?10.0m
  (Ya1 for Rgt10.0 m)
• Boundary conditions
• no-slip impermeable adiabatic boundary on the
  ground
• non-reflecting boundary conditions in atmosphere
• Ignition 15 ms increase of progress variable in
  1 CV
• Numerical details
• code FLUENT
• explicit linearisation of the governing equations
• explicit time marching procedure
• second order accurate upwind scheme for
  convection terms, central-difference scheme for
  diffusion terms
• Courant-Friedrichs-Lewy number CFL0.8

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THEORY versus EXPERIMENT
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Flame shape 1
EXPERIMENT
SIMULATION (averaging c0.2-0.8)
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Flame shape 2
Experiment
Simulation
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Flame propagation
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Flame radius
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Burning velocity St
Total flame wrinkling factor is about 5,of
which RNG SGSis only St/Su1.2
Balloon rupture at 5 m is a reason for flame
acceleration?
Ei7.2
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Pressure dynamics 1
Gauge affected by combustion
Gauge affected by combustion
Gauge affected by combustion
Flame zone 2 m, 5 m, 8 m, 18 m
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Pressure dynamics 2
Similar to experiment the positive pressure
wave was followed by a negative pressure phase.
Usually the negative pressure wave was somewhat
shorter than the positive one providing larger
negative pressure peak.
Far-field 35 m, 80 m
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Conclusions

• The Ulster LES model has been applied to study
  the dynamics of the largest unconfined
  deflagration of stoichiometric hydrogen-air
  mixture. The model has no adjustable parameters
  and reasonably reproduced the experimental data
  on dynamics of flame and pressure wave
  propagation.
• Effects of the hydrodynamic flow instabilities
  and the turbulence induced by turbulent flame
  front itself on the burning velocity acceleration
  are accounted separately in the model. It is
  demonstrated that the main contributor to the
  turbulent flame propagation is the turbulence
  generated by flame front itself.
• Further studies have to model under resolved
  fractal structure of large-scale flames to
  reproduce in more detail the observed monotonous
  acceleration of the flame front.

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THANK YOU