Transcript Validation of CFD Calculations Against Impinging Jet

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
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Validation of tool primary requirement
Important to focus on “realistic” scenarios while carrying out
validation of CFD tool

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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
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Total hydrogen inventory fixed (10 g)
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Experimental Details (2)
 Two different geometrical configurations
10 00
H2-release
nozzle
Plate Geometry

1500
1500
50 0
10
0
10
00
0
1000
H2-release
nozzle
Hood Geometry
Released H2 ignited using at two different ignition positions (0.8
and 1.2 m above the release nozzle)
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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-e 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
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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
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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
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
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:
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Identify scenarios for ignition (cloud size & reactivity)
Optimal ignition position and time
Expected overpressures
=> Avoid un-interesting tests, optimise use of resources
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Secondary purposes:
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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 for
low momentum
Small nozzle (4mm)
=> ”no flammable cloud”
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Dispersion Simulations: Hood geometry
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Dispersion Results: Comparison with Experiments
Concentration dependence on distance from nozzle
100mm nozzle
21mm nozzle
Plate Geometry
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Dispersion Results: Comparison with Experiments
Lateral distribution of concentration
100mm nozzle
(0.7 g/s)
Plate Geometry
21mm nozzle
(3.0 g/s)
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Dispersion Results: Comparison with Experiments
Photograph of plume vs. Predicted shape
Plate Geometry, 21mm nozzle (3.0 g/s)
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Dispersion Results: Comparison with Experiments
Concentration dependence on distance from nozzle
Hood Geometry, 21mm nozzle
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Dispersion Results: Comparison with Experiments
Concentration dependence on distance from nozzle
Hood Geometry, 100mm nozzle
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Dispersion Results: Comparison with Experiments
Photograph 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 ?
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Aim: Development of QRA methodology
Concept of ”equivalent stoichiometric cloud size”
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Obtained using reactivity- and expansion-based weighting
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Expected to give similar explosion loads as the real cloud
Q9 = V  BV  E / (BV  E)stoich
Real gas cloud (Hood)
100
Explosion Pressure (mbar)
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Real gas cloud (Plate)
Stoichiometric gas cloud (Hood)
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Stoichiometric gas cloud (Plate)
60
40
20
0
0
0.1
0.2
0.3
0.4
0.5
3
Q9 (m )
Cloud Size
Overpressures
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Explosion Results: Comparison with Experiments
Ignition 1.2m from release nozzle
(Calculations
performed subsequent to experiments to match ignition position)
Combution experiments with hood (I=ca.10gH , H =1.2m)
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1.6
ign
1,6
plate
1.4
1,4
Hign=1.2m
Height above release (m)
Height above release [m]
1.2
sidew all
1
0.8
0.6
0.4
1,2
1
0,8
0,6
0,4
0.2
100mm (plate), 3.5g/s
100mm (hood), 3.5g/s
21mm (hood), 3g/s
21mm (hood), 6g/s
P lC05(plate): d=100mm, m=3.5g/s
P lC23(ho o d): d=100mm, m=3.5g/s
0,2
P lE13(ho o d): d=21mm, m=3g/s
0
P lF07(ho o d): d=21mm, m=6g/s
0
-0.2
0
10
20
30
40
Overpressure [m bar]
Experiments
50
60
0
10
20
30
40
50
60
70
80
Overpressure (mbar)
Simulations
 Possible different time of ignition for 100mm hood leads to higher simulated pressure
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Explosion Results: Comparison with Experiments
Ignition 0.8m from release nozzle
(Calculations
performed subsequent to experiments to match ignition position)
Combution experiments with hood (I=ca.10gH , H =0.8m)
2
1.6
ign
1,6
plate
1.4
1,4
1,2
sidew all
1
0.8
Hign=1.2m
0.6
0.4
0.2
Height above leak (m)
Height above release [m]
1.2
1
0,8
0,6
0,4
P lE04(plate): d=21mm, m=3g/s
21mm (plate), 3g/s
P lE14(ho o d): d=21mm, m=3g/s
21mm (hood), 3g/s
0,2
P lF03(plate): d=21mm, m=6g/s
0
21mm (plate), 6g/s
P lF08(ho o d): d=21mm, m=6g/s
21mm (hood), 6g/s
-0.2
0
0
10
20
30
40
Overpressure [m bar]
Experiments
50
60
0
10
20
30
40
50
60
Overpressure (mbar)
Simulations
 Local pressure transient around ignition influences simulated pressures near ignition location
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Conclusions
 Leak scenarios well predicted in general
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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
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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
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