Page 1 SIMULATIONS OF HYDROGEN RELEASES FROM STORAGE TANKS: DISPERSION AND CONSEQUENCES OF IGNITION By Benjamin Angers 1, Ahmed Hourri 1 and Pierre Bénard.

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SIMULATIONS OF HYDROGEN RELEASES FROM STORAGE TANKS:

DISPERSION AND CONSEQUENCES OF IGNITION

By Benjamin Angers1, Ahmed Hourri1 and Pierre Bénard1

Jérôme Perrin2 and Pascal Tessier3 1Hydrogen Research Institute, UQTR2Air Liquide3Membrane LP, Membrane Separation Systems Air Liquide

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Introduction• Overall objectives of the project:

– To estimate the risks associated with the releases from hydrogen high pressure storage systems

– Examine the consequences of ignition for late ignition

– Validate standard simulation techniques relevant to this work

• Specific objectives of this work:– Validate dispersion simulation models used for this

approach– Validate and adapt AutoReaGas (ARG)

overpressure calculations for hydrogen jet releases

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• The dispersion simulations were performed using FLUENT

• The ignitions and subsequent deflagration simulations were performed using a customized version of AutoReaGas (Century Dynamics & TNO) taking into account the initial velocity distribution of the gas mixture.– ARG does not simulate dispersion from an

outlet

CFD tools

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Methodology dispersion simulations• The hydrogen concentration profile was

obtained from CFD dispersion simulations performed by solving the steady-state Navier-Stokes equations in the presence of turbulence using FLUENT

• Turbulence was modelled using the RNG k- model with standard parameters

• The choice of the modelling assumptions were validated by comparing with hydrogen horizontal jet experiments by M. Swain et al.

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Validation: dispersion simulations

Experimental data from M. Swain

RNG k-ε Simulation (this work)After 45 seconds

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Validation – Dispersion model(RNG k-ε)

Sensor location

Experimental concentration (% (vol))

Simulation results (45 sec)

1 5.0-5.9 5.04

2 5.6-7.0 6.96

3 9.4-10.8 13.99

4 8.1-9.4 8.25

5 5.6-6-6 5.29

6 3.5-4.6 5.37

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Explosion simulation• The incident overpressure is calculated from the gas

explosion solver AutoReaGas.• The steady-state velocity and concentration profile obtained

from the dispersion solver is imported for use in ARG. • Turbulence is modelled in AutoReaGas using the standard k-

epsilon approach. • The laminar combustion process is based on a one step

irreversible reaction.• A turbulent reaction rate calculated from the Bray turbulent

flame velocity is used for the turbulent combustion. • Transition of laminar to turbulent flame occurs when the

turbulent burning velocity exceeds the laminar burning velocity.

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Methodology

• The concentration and velocity profiles obtained from Fluent are averaged over the coarser grid used in ARG and imported into the latter.

• The simulations were performed using the default values for hydrogen in the solver (3.5 m/sec) and using the laminar burning velocity of hydrogen as an adjustable parameter.– ARG is calibrated for methane.

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Validation – Overpressure from hydrogen jet explosions

• Vertical hydrogen jets were compared to data obtained by H. Seifert and H. Giesbrecht from BASF.

– Maximum overpressure observed: 80 Pascals at 2 meters from the point of ignition.

• The pressure waves immediately following the ignition of hydrogen were studied for propane and methane jets resulting from subsonic outflows: 140, 190 and 250 m/sec (10 mm diameter outlet).

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140 m/s 190 m/s 250 m/s

Validation studies (Seifert et al): ignition of a vertical jet

Vertical jet:– outlet: 10 mm diameter, 1 m above the ground– Outflow velocities: 140, 190 and 250 m/sec

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Validation studies : ignition of a vertical jet

Flow velocity (m/s)

Extension of the hydrogen cloud along the x axis (m) perpendicular to the jet at concentrations of

Extension of the hydrogen cloud along the y axis (m) perpendicular to the jet at concentrations of

Extension of the hydrogen cloud along the z axis (m) parallel to the jet at concentrations of

2% 4% 15% 2% 4% 15% 2% 4% 15%

140 0.52 0.36 0.12 0.5 0.33 0.11 5.88 4.15 1.04

190 0.58 0.39 0.12 0.58 0.36 0.11 6.27 4.63 1.11

250 0.6 0.41 0.12 0.6 0.36 0.12 6.82 4.89 1.16

Extent of flammable cloud

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Flow velocity (m/s)

Laminar burning velocity (m/s)

Overpressure at 2 m (pascal)

Overpressure at 5 m (pascal)

Overpressure at 10 m (pascal)

140 3.50 236.7 140.9 50.2

1.00 57.3 25.8 12.9

190 3.50 418.1 202.8 66.9

1.15 64.3 31.6 15.6

250 3.50 429.7 214.1 70.0

1.35 87.6 40.2 19.1

Flow velocity (m/s)

Overpressure at 2 m (pascal)

Overpressure at 5 m (pascal)

Overpressure at 10 m (pascal)

140 57 ± 22 19± 8 12 ± 4

190 61 ± 22 25 ± 7 16 ± 4

250 75 ± 25 35 ± 10 22 ± 6

Validation : incident overpressureExperimental results Seifert et al

Simulation results

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Volume of hydrogen as a function of flow velocity and fluid mechanics

solver • The grids of the fluid mechanics solver and the

explosion module Autoreagas being different, an averaging program was used to map the data from the solver onto the Autoreagas (ARG) grid.

• Due to the averaging process performed to import the Fluent Data into the ARG mesh, there is a problem with conservation of mass during data transfert.

Flow Velocity (m/s)

Volume of H2 in AutoReaGas (m3)

Volume of H2 in Fluent (m3)

140 1.09 0.75

190 1.29 0.99

250 1.47 1.31

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• Horizontal jet, dispersion simulation:– Outlet: 6 mm diameter, 0.5 m above

ground– Outflow velocities: 140, 190 and 250 m/sec– Ignition after sec for a 350 bars reservoir– Size of the CFD domain: 8 by 8 by 8 meters– Unstructured mesh: 279,026 cells

Horizontal jets

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Horizontal jets : dispersionFlow velocity (m/s)

Extension along the x axis (m) perpendicular to the axis of the jet

Extension along the y axis (m) perpendicular to the axis of the jet

Extension along the z axis (m) parallel to axis of the the jet

2% 4% 15% 2% 4% 15% 2% 4% 15%

140 1.74 1.35 0.24 0.62 0.37 0.10 1.91 0.82 0.07

190 2.03 1.55 0.27 0.49 0.29 0.09 2.05 0.78 0.06

250 2.35 1.77 0.27 0.79 0.39 0.09 2.11 0.80 0.07

2% 4% 15%

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Overpressure generated by the ignition of the flammable cloud from a 250 m/sec outflow from a 6mm PRD device of a cylinder as a function of distance at 0.036 seconds after ignition.

Horizontal jets : overpressure

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Flow velocity (m/s)

Laminar burning velocity (m/s)

Overpressure at 2 m (pascal)

Overpressure at 5 m (pascal)

Overpressure at 10 m (pascal)

140 3.50 72.0* 26.5* 9.0*

1.00 13.5 4.8 1.9

190 3.50 118.4* 44.62* 14.4*

1.15 25.4 9.71 3.9

250 3.50 138.0* 52.0* 16.3*

1.35 35.1 13.8 6.1*Reflected peaks were larger

Horizontal jets : overpressure

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Discussion• For vertical jets, the use of the standard values

in Autoreagas (burning velocity=3.5 m/sec) leads to larger overpressure peaks than experimentally observed by a factor of 4 to 8.

• Good agreement with the available experimental data from jets could be obtained by adjusting the burning velocity to about 1.5 m/sec.

• In the case of a 700 bars reservoir, this would correspond to an ignition delay of the order of a minute after release.

• Initial velocity profile of the release has an effect on the calculated overpressure.

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Conclusions• Case considered represent essentially late ignition

of a release from high pressure reservoir• Issues:

– Lack of experimental data for large, chocked,outflows expected when ignition occurs immediatly after release

– Simulation issues • Mesh: Importing dispersion data from Fluent to AutoReaGas

(mass of hydrogen must be conserved when Fluent data is imported into ARG)

• Unavailability of outflow boundary condition in explosion solver may be a problem for early ignition of the release when large, sonic leaks occur.

• (Cannot predict steady state jet fire)

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Further work• Obstacles• The next stage of this project is to

estimate the size and time dependant concentration profile from the release through a PRD of a fully filled hydrogen tank for late stage ignition, as well as the overpressure resulting from its ignition

• Investigate other explosion solvers. • Effects of obstacles• … Tackle early ignition

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References1. Seifert H. and H. Giesbrecht, Safer

design of inflammable gas vents, Loss prevention and safety promotion in the process industries: 5th International symposium of the European Federation of Chemical Engineering, 1986.

2. Swain, M., Codes and standards analysis, 2004 annual program review meeting of the hydrogen, fuel cells & infrastructure program of the US Department of Energy, 2004.

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Acknowledgements• Air Liquide

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Overpressure from a 250 m/sec outflow from a 10mm outlet from a

vertical pipe as a function of distance.

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Overpressure as a function of cubic cell size for a 250m/s leak, using a laminar

burning velocity of 1.35 m/s Cubic cell size (m)

Overpressure at 2 m (pascal)

Overpressure at 5 m (pascal)

Overpressure at 10 m (pascal)

Volume of H2

in AutoReaGas (m3)

0.1 87.6 40.2 19.1 1.47

0.15 72.4 36.6 16.5 1.94

0.2 55.9 29.6 14.9 2.64

Volume of hydrogen in Fluent: 1.31 m3