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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

2

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

0

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