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Validation of CFD Calculations Against Impinging Jet Experiments
Prankul Middha and Olav R. Hansen, GexCon, Norway Joachim Grune, ProScience, Karlsruhe, Germany
Alexei Kotchourko, FZK, Karlsruhe, Germany
September 11, 2007
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Motivation CFD calculations increasingly used for quantitative risk
assessments Validation of tool primary requirement
Important to focus on “realistic” scenarios while carrying out validation of CFD tool Need to reproduce the complex physics of the accident scenario Validation of tools for combined release and ignition scenarios
Recent experiments performed at FZK present an opportunity to perform “real” validation against a complex experiment Possibility to develop risk assessment methods for hydrogen
applications (Caution: Not large scale)
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Experimental Details (1) Release of hydrogen in a ”workshop” setting followed by
ignition Nine different release scenarios
Total hydrogen inventory fixed (10 g)
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Experimental Details (2) Two different geometrical configurations
Released H2 ignited using at two different ignition positions (0.8 and 1.2 m above the release nozzle)
1500
1000
H -releasenozzle
2
10 00
500
1500
H -releasenozzle
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Plate Geometry Hood Geometry
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CFD Tool FLACS (1) Solution of 3D compressible Navier-Stokes equations using
a finite volume method over a cartesian grid Implicit method (SIMPLE algorithm) for pressure correction 2nd order scheme in space and 1st order scheme in time (2nd order available)
Standard k- model with several important modifications Model for generation of turbulence behind sub-grid objects Turbulent wall functions for adding production terms to the relevant CV
across the boundary layer Model for build-up of proper turbulence behind objects of a particular size
(about 1 CV) for which discretization produces too little turbulence
A “distributed porosity concept” which enables the detailed representation of complex geometries using a Cartesian grid Large objects and walls represented on-grid, and smaller objects
represented sub-grid Necessary as small details of “obstacles” can have a significant impact on
flame acceleration, and hence explosion pressures
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CFD Tool FLACS (2) Combustion Model
Flame in an explosion assumed to be a collection of flamelets 1-step reaction kinetics, with the laminar burning velocity being a
measure of the reactivity of a given mixture A “beta” flame model normally used that gives the flame a constant
flame thickness (equal to 3-5 grid cells) Burning velocity model:
A model that describes the laminar burning velocity as a function of gas mixture, concentration, temperature, etc. Le effects accounted for H2.
A model describing quasi-laminar combustion (increase in burning rate due to flame wrinkling, etc.)
A model that describes ST as a function of turbulence parameters (intensity and length scale) and laminar burning velocity (based on Bray et al.)
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Purpose of Simulations Simulations performed prior to experiments with the primary
purpose of aiding the design of experiments, if possible: Identify scenarios for ignition (cloud size & reactivity) Optimal ignition position and time Expected overpressures=> Avoid un-interesting tests, optimise use of resources
Secondary purposes: Evaluate prediction capability (topic of current presentation) Demonstrate efficiency of calculations Development of risk assessment methods
Presented at LPS, Houston Connection with HyQRA (HySafe) and IEA Task 19
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Representation of geometry and grid
Grid used:• 5 cm standard grid (2.5cm for explosion)• Stretch outside interesting region• Refine towards leak (21mm and 4mm leaks)
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Dispersion Simulations: Plate geometry
Small flammable volume with plate only
Small nozzle (4mm) => ”no flammable cloud”
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Dispersion Simulations: Plate geometry
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Dispersion Simulations: Hood geometry
Flammable cloud inside confinement forlow momentum
Small nozzle (4mm) => ”no flammable cloud”
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Dispersion Simulations: Hood geometry
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Dispersion Results: Comparison with Experiments
100mm nozzle 21mm nozzle
Concentration dependence on distance from nozzle
Plate Geometry
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Dispersion Results: Comparison with Experiments
100mm nozzle(0.7 g/s)
21mm nozzle(3.0 g/s)
Lateral distribution of concentration
Plate Geometry
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Dispersion Results: Comparison with ExperimentsPhotograph of plume vs. Predicted shape
Plate Geometry, 21mm nozzle (3.0 g/s)
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Dispersion Results: Comparison with ExperimentsConcentration dependence on distance from nozzle
Hood Geometry, 21mm nozzle
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Dispersion Results: Comparison with ExperimentsConcentration dependence on distance from nozzle
Hood Geometry, 100mm nozzle
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Dispersion Results: Comparison with ExperimentsPhotograph of plume vs. Predicted shape
Hood Geometry, 21mm nozzle (3.0 g/s)
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Explosion Simulations (Pre-calculations) ”Worst-case” explosion overpressures (quiescent)
Plate geometry Hood geometry
Ignition of non-homogeneous clouds
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Possible to scale overpressures with cloud size ? Aim: Development of QRA methodology Concept of ”equivalent stoichiometric cloud size”
Obtained using reactivity- and expansion-based weighting
Expected to give similar explosion loads as the real cloud
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20
40
60
80
100
0 0.1 0.2 0.3 0.4 0.5Q9 (m3)
Expl
osio
n Pr
essu
re (m
bar)
Real gas cloud (Hood)Real gas cloud (Plate)
Stoichiometric gas cloud (Hood)Stoichiometric gas cloud (Plate)
Cloud Size Overpressures
stoich9 = BV / (BV )Q V E E
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Explosion Results: Comparison with Experiments
Combution experiments with hood (I=ca.10gH2, Hign=1.2m)
plate
sidew all
Hign=1.2m
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 10 20 30 40 50 60
Overpressure [mbar]
Hei
ght a
bove
rele
ase
[m]
P lC05(plate): d=100mm, m=3.5g/sP lC23(hood): d=100mm, m=3.5g/sP lE13(hood): d=21mm, m=3g/sP lF07(hood): d=21mm, m=6g/s
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
0 10 20 30 40 50 60 70 80
Overpressure (mbar)
Hei
ght a
bove
rele
ase
(m)
100mm (plate), 3.5g/s100mm (hood), 3.5g/s21mm (hood), 3g/s21mm (hood), 6g/s
Experiments Simulations
Ignition 1.2m from release nozzle (Calculations performed subsequent to experiments to match ignition position)
Possible different time of ignition for 100mm hood leads to higher simulated pressure
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Explosion Results: Comparison with Experiments
Experiments Simulations
Ignition 0.8m from release nozzle(Calculations performed subsequent to experiments to match ignition position) Combution experiments with hood (I=ca.10gH2, Hign=0.8m)
plate
sidew all
Hign=1.2m
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 10 20 30 40 50 60
Overpressure [mbar]
Hei
ght a
bove
rele
ase
[m]
P lE04(plate): d=21mm, m=3g/sP lE14(hood): d=21mm, m=3g/sP lF03(plate): d=21mm, m=6g/sP lF08(hood): d=21mm, m=6g/s
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
0 10 20 30 40 50 60
Overpressure (mbar)
Hei
ght a
bove
leak
(m)
21mm (plate), 3g/s21mm (hood), 3g/s21mm (plate), 6g/s21mm (hood), 6g/s
Local pressure transient around ignition influences simulated pressures near ignition location
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Conclusions Leak scenarios well predicted in general
Less interesting scenarios simplified somewhat with respect to grid definition to save time, which led to some underprediction
Predicted pressure levels with FLACS similar to those observed in experiments
Possible to scale predicted overpressures with equivalent gas cloud size
Work important to build confidence in CFD tools for QRA calculations
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Acknowledgements FZK and coauthors for interesting experiments and access
to experimental data Look forward to larger scale controlled studies in similar setups
European Union for support through the NoE HySafe Norwegian Research Council for support for hydrogen
modelling activities