On On numerical simulation of numerical simulation of liquefied and gaseous liquefied and gaseous

hydrogen releases at large scaleshydrogen releases at large scales

V. Molkov, D. Makarov, V. Molkov, D. Makarov, EE. . ProstProst

8-10 September 2005, Pisa, Italy8-10 September 2005, Pisa, Italy

First International Conference onFirst International Conference on

HYDROGEN SAFETYHYDROGEN SAFETY

• Introduction of hydrogen as an energy carrier makes great demands on hydrogen safety. Development of robust and reliable risk assessment methodologies requires all-round validation of models and tools.

• The need to model non-uniform hydrogen-air mixture formation at real scales is important to have realistic initial conditions for subsequent modelling of partially premixed hydrogen combustion.

• The aim of this study is validations of the LES model in application to large-scale hydrogen release scenarios and formulation of tasks for future research in this area.

ContentsContents•The LES modelThe LES model

•LH2 release in open LH2 release in open

atmosphereatmosphere

•GH2 release in a closed vesselGH2 release in a closed vessel

Large Eddy Simulation (LES) Large Eddy Simulation (LES) modelmodel

• Conservation of massConservation of mass

•

• Conservation of momentumConservation of momentum

• Conservation of energyConservation of energy

LES model (1/LES model (1/22))

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• RNG SGS turbulence modelRNG SGS turbulence model

• Conservation of “Conservation of “HH22”” concentrationconcentration

LES model (2/LES model (2/22))

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Liquefied hydrogen release in Liquefied hydrogen release in open atmosphereopen atmosphere

Chirivella J.E., Witcofski, R.D. Am. Inst. Chem.

Eng. Symp., 82, No 251, 1986, pp.120-142:

- Spill 5.11 m3 (361.8 kg) of LH2 in 38 s

- LH2 pool radius between 2 and 3 m

- Total evaporation time 43 s

- Wind speed ~2.2 m/s at height 10 m

NASA experimentNASA experiment

Calculation domain (1/2)

180 m

70 m

Spill area and instrumentation towers area

Cloud propagation area

Characteristic size of CV: Numerical grid: 156133 CV– tower location 1.0 - 2.0 m– cloud area 2.0 - 3.0 m – the rest of domain up to 10 m

Calculation domain (2/2)

70 m

180 m

Spill areaCloud propagation area

Characteristic size of CV: Numerical grid: 103163 CV– spill area 0.6 - 1.0 m– cloud area 2.0 - 3.0 m – the rest of domain up to 10 m

Initial conditionsatmosphere velocity profile:where (provided u=2.2 m at H=10 m)

Boundary conditionsvelocity profile at inflow

prescribed pressure conditions at outflow boundaries, p=0 Pa

H2 injectionmass injection rate

Run 1: injection area radius

Run 2: injection area radius

average injection velocity

instant injection velocity

Run 1: turbulence

Run 2: turbulence

Geomerty: Run 1 (no pool border, no obstacles), Run 2 (+)

0* ln yykuzu ,40.0k ,03.00 my 2

* 1015.15 u

0* ln yykuzu

skgConstm 41.8

115.0 tR mR 5.2sts 100 st 10

poolaver AmV )2sin())(2sin(1( inf zntvxnIVV averinj 5.0n

99.0I

Numerical details

mR 5.2

99.0I sts 100 10.0I st 10

H2 concentration (Run 1)

4%

52%

52% 4%

Texp = 21.33 sTsim = 21.36 s

H2 concentration (Run 2)

4%52%

Texp = 21.33 sTsim = 21.35 s

Simulated temperature (Run 1)

)

Simulated temperature (Run 2)

)

Visible cloud (Run 1)

Visible cloud (Run 2)

Cloud propagation (Run 1)

Cloud propagation (Run 2)

Phenomena to be addressed

• Condensation of air in temperature range Condensation of air in temperature range 20-90 K (with heat release) and evaporation 20-90 K (with heat release) and evaporation above 90 Kabove 90 K

• Two phase flow Two phase flow (gas: hydrogen-air; solid: air ice)(gas: hydrogen-air; solid: air ice)

• Detailed spill modelling (initial fractions of Detailed spill modelling (initial fractions of GH2 and LH2; heat transfer to the ground: GH2 and LH2; heat transfer to the ground: initial violent evaporation stage, etc)initial violent evaporation stage, etc)

Gaseous hydrogen release in Gaseous hydrogen release in 20-m20-m33 closed vessel closed vessel

5.5

m2.2m

ExperimentExperiment

1.4m

H2

Time of release = 60 seconds

Volume injection rate: V=4.5 l/s

Non-uniform tetrahedral grid

CV number: 54004CV size: 0.01-0.10 m close to place of H2 injection CV size: up to 0.20 m in the rest of domain

“Uniform” grid

CV number: 28440CV size: 0.14-0.20 m

0-180 s0-180 s 3-251 min3-251 min

Calculation domainCalculation domain

• Initial conditions– quiescent air, u=0 m/s,– initial air concentration Yair=1.0,– initial temperature T=293K

• Boundary conditions– t=0-1s: Vinj increased from 0 to 57.5 m/s– t =1-59s: Vinj=57.5 m/s– t=59-60s: decrease from 57.5  to 0 m/s– t=60s-251min: Vinj=0 m/s– YH2=1.0, Tinj=293K

• Numerical details– explicit linearisation of the governing equations– implicit method for solution of linear equation set– second order accurate upwind scheme for convection terms, central-

difference scheme for diffusion terms– Time steps: t=0-180 s: t=0.01 s; t=3-251 min: t=0.01-1.0 s

Numerical details

Simulation results

2 min2 min

50 min50 min 100 min100 min 250 min250 min

Hydrogen distribution 1

Hydrogen distribution 2

Vessel depth, m

H2

vol.

con

cen

trat

ion

0 1 2 3 4 5 60

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Experiment: 2 minExperiment: 50 minExperiment: 100 minExperiment: 250 minSet 2 simulations: 2 minSet 2 simulations: 50 minSet 2 simulations: 100 minSet 2 simulations: 250 min

50min:50min:VVmaxmax=10 cm/s=10 cm/s

100min:100min:VVmaxmax=8 cm/s=8 cm/s

250min:250min:VVmaxmax=5 cm/s=5 cm/s

Residual velocities

Conclusions

• The LES model has been applied to analyse large-scale experimental LH2 and GH2 releases.

• The simulation of non-uniform flammable cloud formation, resulting from a LH2 spill, reproduced a characteristic structure of the turbulent eddies and the direction of cloud propagation.

• The simulation results were found to depend on initial and boundary conditions.

• The air condensation-evaporation sub-model may improve predictive capabilities of the LES model

Conclusions

• Good agreement was achieved with experimental data on GH2 release in 20-m3 closed vessel up to t=250 min after the 1 minute release.

• The LES results demonstrated that random flow field remains in the vessel long time after the injection and this is presumably responsible for H2 transport.

• Further experiments with observation of velocity field after release and simulations with higher accuracy are required to give final answer to this question.