1. A Comparative CFD Assessment Study of Cryogenic Hydrogen and Liquid Natural Gas Dispersion
S.G. Giannissi1 and A.G. Venetsanos1
1Environmental Research Laboratory, National Centre for Scientific Research Demokritos, Aghia Paraskevi, Athens, 15341, Greece,
ICHS2017 International Conference on Hydrogen Safety 11-13, 2017 Hamburg, Germany

2. Introduction
Hydrogen is a key-element of potential energy solution for 21st century.
Natural gas is widely used since 2nd world war.
Storage practice under cryogenic conditions (liquefaction or cryo-compressed technology)
Need in studying the physical phenomena associated with cryogenic releases
Due to extreme low temperature atmospheric humidity, nitrogen and oxygen are condensed and solidified
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3. Introduction
There are no experiments to study the humidity and air component’s phase change effect.
Few CFD studies have examined these effects in LH2 spills, Ichard et al. 2012, Giannissi et al. 2014,
and LNG spills, Giannissi et al. 2015, Zhang et al
However, no study on the level of influence depending on the relative humidity and the release conditions has not been conducted.
In cryo-compressed hydrogen (CCH2) the effect has not been examined yet.
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4. Scope of study
Study the humidity and air’s component phase change effect on mixture dispersion under cryogenic release conditions (both liquid hydrogen (LH2) and cryo-compressed hydrogen (CCH2) ).
Study the humidity effect on liquefied natural gas (LNG) dispersion
Identify any differences among the behavior of LH2, CCH2 and LNG under the influence of these phenomena.
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5. Problem setup
To conduct the study we performed several simulations with the following characteristics:
Cryogenic fluid (H2 or CH4) release horizontally
1 m above the ground.
Stagnant environment.
Sensitivity tests on relative humidity levels.
Sensitivity tests on release conditions.
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6. Problem setup/Accident Scenarios
Case
Released substance
Storage conditions
Nozzle conditions
Diameter (mm)
Mass flow rate (kg/s2)
Froude number
Reynolds number
Relative humidity (RH, %)
Examined effect
P (bar)
T (K)
LH2_1
Hydrogen 2
22.9
P=1.01 bar Tsat= K (two-phase, qv=6%)
26.6
0.07
16.7
372,283
Humidity, air phase change
LH2_2 20
LH2_3 50
LH2_4 99
CCH2_1a 10 30
P=7.3 bar, T=29.4 K (liquid phase) 1
0.004
1006
634,408
CCH2_2a
CCH2_3a
CCH2_4a
CCH2_1b 48
P=18 bar, T=36.3 K (supercritical)
0.0094
1,366,865
CCH2_3b
CCH2_4b
CCH2_1c
P=18 bar, T= K (supercritical)
0.1
9.4∙10-5
136,
Humidity phase change
CCH2_4c
CCH2_1d 55
P=4.16 bar, T= K (liquid phase)
22.5
9.8
849.3
60,543,
CCH2_4d
LNG_1
Methane
120.6
1.01 bar, T= (two-phase, qv=6%)
0.08
11.4
18,478.1
LNG_3
LNG_4
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7. Modeling approach Release modelling
In LH2 and LNG case:
Isenthalpic expansion from storage conditions to source conditions using the NIST EoS to calculate the flashed vapor fraction.
In CCH2 case:
Isentropic expansion from storage conditions to source conditions using the NIST EoS.
Modelling the under-expanded jet using notional nozzle approach. Mass and momentum balance from nozzle to notional nozzle conditions and assume temperature equal to the temperature at nozzle, Venetsanos and Giannissi (2017).
A.G. Venetsanos and S.G. Giannissi, ‘Release and dispersion modeling of cryogenic under-expanded hydrogen jets’, J of Hydrogen Energy, 42(11), p (2017).
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8. Modeling approach Dispersion modelling
ADREA-HF CFD code is employed
Mass conservation equation
Momentum conservation equation
Enthalpy conservation equation
Conservation equation for mass fraction of component p
Phase distribution with Raoult’s law for ideal mixture.
Ideal gas assumption
k-epsilon turbulence model with extra buoyancy terms
1st order fully implicit scheme for time integration, MUSCL numerical scheme (2nd order) for the convective terms, central differences numerical scheme for diffusion terms
CFL =10 for time step control
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9. Modeling approach Computational domain and grid
Cartesian grid
Minimum cell size is set equal to nozzle diameter for LH2 and LNG and equal to notional nozzle diameter for CCH2.
Refinement near the nozzle.
Symmetry along y-axis
Domain size (x,y,z) - m
Minimum cell size - m
Expansion ratios
Number of cells
LH2
YES
(16, 5, 40)
0.0266
253800
CCH2
Case a
(7.012, 0.5, 3)
0.0047
196560
Case b
(12.012, 0.5, 2)
0.0048
182160
Case c
(2.012, 0.25, 0.4)
445060
Case d
(215, 10, 100)
0.156
127650
LNG
(41, 50, 5)
277032
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10. Results-Humidity phase change effect
LH2: Fr = 16.7, Re = 3.7∙105
Reduction of horizontal LFL distance by 27%
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11. Results-Humidity phase change effect
LNG: Fr = 11.4, Re = 1.8∙104 0%
99%
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12. Results-Humidity phase change effect
LNG: Fr = 11.4, Re = 1.8∙104 0%
99%
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13. Results-Humidity phase change effect
CCH2
Fr = 1006
Re = 6.3∙105
Reduction of horizontal
LFL distance by 25%
Fr = 4577
Re = 1.4 ∙106
Fr =
Re = 1.4 ∙105
Reduction of horizontal
LFL distance by 30%
Fr = 849
Re = 6.0 ∙107
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14. Results-Air component’s phase change effect
LH2: Fr = 16.7, Re = 3.7∙105
CCH2: Fr = 1006, Re = 6.3∙105
Without air phase change
With air phase change
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15. Conclusions and Future work (1/2)
A CFD study of the humidity effect on liquid hydrogen (LH2), cryo-compressed hydrogen (CCH2) and liquid natural gas (LNG) dispersion in stagnant environment is carried out.
In LH2 and CCH2 releases the effect of air components’ phase change is also examined.
In hydrogen the condensation/freezing of humidity makes the cloud more buoyant hence the longitudinal Lower Flammability Limit (LFL) distance is significantly reduced (approximately 27% for ambient temperature equal to and relative humidity 99%).
Higher specific humidity levels results in more enhanced buoyancy effect.
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16. Conclusions and Future work (2/2)
In order the humidity effect to be significant we should have low Fr number at nozzle (<1000). That is achieved if storage conditions correspond to liquid state or supercritical state in the side of the saturated liquid line.
In LNG the buoyancy effect in presence of humidity is not pronounced. However, the LFL horizontal distance seems to be increased in presence of humidity. More study for the reasons of this behavior should be conducted.
The air component phase change is perceptible in LH2 spills, while it is negligible in CCH2 releases due to the high jet momentum.
In the future, a critical value of Fr number for the humidity effect to be significant could be investigated.
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17. Thank you very much for your attention
The first author would like to acknowledge the “IKY FELLOWSHIPS OF EXCELLANCE FOR POSTGRADUATE STUDIES IN GREECE-SIEMENS PROGRAM” for the financial support.
The authors would also like to thank the SUSANA project, which has received funding from the European Union's Seventh Framework Programme (FP7/ ) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° FCH-JU