A Comparative CFD Assessment Study of Cryogenic Hydrogen and Liquid Natural Gas Dispersion

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, sgiannissi@ipta.demokritos.gr, venets@ipta.demokritos.gr ICHS2017 International Conference on Hydrogen Safety 11-13, 2017Hamburg,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

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. 2015. • 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.

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.

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.

6. Problem setup/Accident Scenarios

7. Modeling approachRelease 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’, JofHydrogen Energy,42(11), p. 7672-7682 (2017).

8. Modeling approachDispersion 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

9. Modeling approachComputational 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.

10. Results-Humidity phase change effect • LH2: Fr = 16.7, Re = 3.7∙105 Reduction of horizontal LFL distance by 27%

11. Results-Humidity phase change effect • LNG: Fr = 11.4, Re = 1.8∙104 0% 99%

12. Results-Humidity phase change effect 0% 99% • LNG: Fr = 11.4, Re = 1.8∙104 13 Sep. 2017 12/16

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 = 14475.4 Re = 1.4 ∙105 Reduction of horizontal LFL distance by 30% Fr = 849 Re = 6.0 ∙107

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

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 288.15 and relative humidity 99%). • Higher specific humidity levels results in more enhanced buoyancy effect.

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.

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 (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° FCH-JU-325386.