Removing the disrupting wind effect in single-vented enclosure exposed to external wind S.G. Giannissi1,2, I.C.Tolias1,2, A.G. Venetsanos1 1Environmental Research Laboratory, National Centre for Scientific Research Demokritos, Aghia Paraskevi, Athens, 15310, Greece, sgiannissi@ipta.demokritos.gr, venets@ipta.demokritos.gr 2School of Chemical Engineering, National Technical University of Athens, Heroon Polytechniou 9, 15780, Athens, Greece,

Scope of the study Hydrogen is flammable. Therefore, an accidental release inside confined spaces can form a flammable cloud. Passive ventilation is a common mitigation practice for reducing the gas concentration inside an enclosure. In single vented enclosure bidirectional flow occurs through the vent with hydrogen escaping through the upper part of the enclosure. Previous work1 has shown that wind blowing onto the opposite side of the vent in a single-vented facility hinders the exchange flow through the vent and more hydrogen is accumulated inside the enclosure. This is attributed to the turbulent eddies formed around the vent and block the bidirectional flow. This wind disrupting effect is studied here and several flow deflectors around the vent are investigated to eliminate this effect. 1 Giannissi, S.G., Hoyes, J.R., Chernyavskiy, B., Hooker, P. and J. Hall (2015). CFD benchmark on hydrogen release and dispersion in a ventilated enclosure : passive ventilation and the role of an external wind, Int. J. Hydrogen Energy, 40, pp. 6465-6477 21 Oct. 2015

Geometry and conditions Facility, release and wind conditions The facility is the GAMELAN facility2. Injection rate: 60 NL/min, injection point: 21 mm above ground, nozzle diameter: 20 mm, temperature: 26oC. Helium is released (for safety reasons). No external wind. This experiment is already simulated by Giannissi et al.3 and showed good agreement with the experiment. Therefore, it is considered trustworthy to use only simulations for the analysis. 2 Cariteau, B. and Tkatschenko, I., (2013). Experimental study of the effects of vent geometry on the dispersion of a buoyant gas in a small enclosure, Int. J. Hydrogen Energy, 38, No. 19, pp. 8030–8038 3 Giannissi, S.G., Shentsov, V., Melideo, D., Cariteau, B., Baraldi, D., Venetsanos, A.G. and Molkov, V. (2015). CFD benchmark on hydrogen release and dispersion in confined, naturally ventilated space with one vent, Int. J. Hydrogen Energy, 40, No. 5, pp. 2415-2429. 21 Oct. 2015

Geometry and conditions Facility, release and wind conditions The same facility and release conditions used for the present analysis imposing hypothetical external wind. 1.8 m/s at 3 m height (weak wind) 3.6 m/s at 3 m height (strong wind) 21 Oct. 2015

Geometry and conditions Description of the flow deflectors 5 different flow deflectors were tested to eliminate the wind disrupting effect and assist the bidirectional flow (only with weak wind) A. up plate B. down plate C. vertical plate D. middle plate E. complex 21 Oct. 2015

Modeling approach The CFD code ADREA-HF is used. The transient, 3D conservation equations for the mixture (mass, momentum, helium mass fraction). Isothermal conditions K-epsilon turbulence model with extra buoyancy terms 1st order fully implicit scheme for the time integration MUSCL scheme (2nd order) for the convective terms Central differences scheme for the diffusive terms. CFL=10 for the time step control (~0.025 sec and 0.013 sec for the simualtions with wak and strong wind respectevely) 21 Oct. 2015

Modeling approach Domain is extended only downwind the vent wall in the cases without wind, whereas the domain is extended at all directions around the facility in the case with wind. Symmetry assumption along y-axis. Source is modeled as a solid surface, on which the helium inlet conditions (velocity etc.) are imposed. 4 cells placed along the source diameter. Refinement near the source, the walls, the ceiling and the vent (maximum expansion ratio 1.12). 21 Oct. 2015

Results Replication of the wind disrupting effect No wind Weak wind 21 Oct. 2015

Results Eliminating the effect (weak wind) A. up plate D. middle plate Optimal flow deflector for practical considerations B. down plate E. complex C. vertical plate 21 Oct. 2015

Results Eliminating the effect (weak wind) 21 Oct. 2015

Results Eliminating the effect (weak wind) middle plate 21 Oct. 2015

Conclusions (1/2) The wind disrupting effect is reproduced: wind blowing onto the opposite side of the vent in single-vented facility opposes passive ventilation. This behavior is due to the turbulent eddies that are formed around the vent and hinder the hydrogen to flow out through the upper part of the vent and the fresh air to flow in through the lower part of the vent (bidirectional flow). The stronger the wind the higher the effect is. Flammable mass inside the enclosure is more than doubled when wind is blowing compared to the case without wind. 21 Oct. 2015

Conclusions (2/2) 5 flow deflectors around the vent were tested to eliminate the wind disrupting effect, up plate, down plate, vertical plate, middle plate, and both middle and vertical plate (complex). Only the middle plate and the complex layout assist the bidirectional flow. With these geometrical configurations both flammable and total mass of hydrogen inside the enclosure are lower compared to the case without any deflector. Small differences are observed between the results with these two flow deflectors with the complex one to be best. These vent configurations predict lower concentrations level even with the case without wind. Consequently, if the facility is equipped with these flow deflectors wind blowing windward the facility is now desired. However, for practical reasons the simpler flow deflector (middle plate) is proposed here. 21 Oct. 2015

Future work The performance of the proposed flow deflector can be tested imposing several release flow rates and using different vent sizes. Publications S.G. Giannissi, I.C. Tolias, A.G. Venetsanos (2015). Mitigation of buoyant gas releases in single-vented enclosure exposed to external wind: removing the disrupting wind effect, Int. Journal of Hydrogen Energy, under review. Strong wind also examined with the flow deflectors. A size parameterization of the proposed flow deflector is included. 21 Oct. 2015

Thank you very much for your attention Any Questions? The study was performed within the HyIndoor project. The authors would also like to acknowledge the H2FC project. H2FC European Infrastructure