Turbulent flame propagation in large unconfined H2/O2/N2 clouds Jérôme Daubecha, Christophe Prousta,b, Guillaume Lecocqa E-mail: jerome.daubech@ineris.fr a INERIS, Verneuil en Halatte, France b UTC, Compiègne, France

Context BARPPRO french R&D project – Protection of industrial facilities against explosion blast wave Turbulent flame velocity – Key aspect to evaluate unconfined explosion consequences The intensity of flow velocity fluctuations u’ The characteristic length of vortices Lt The laminar burning velocity Slad Link between flame and turbulence Turbulent flow field Laminar flame

Context Purposes of this presentation : Presentation of the experimental device Presentation of the overpressurse and the flame velocities for quiescent and turbulent mixtures Discussion about flame behavior

Experimental Conditions Test plateform Metallic structure in 3.5 cm thich IPN structure coverd with a 5 mm metal plate 1.5 m hemispheric frame formed with 8 flexible plastic tube 150 µm plastic sheet covers the dome

Experimental Conditions Mixture preparation Injection of gas thanks to 4 jetflows (coanda effect) Placed at 0.9 m from center of the plateform Homogeneous mixture Creation of turbulent motion

Experimental conditions Instrumentation Concentration and homogeneity : Controlled in 2 points at 0.1 m and 1.4 m from the test plateform Ignition source : Pyrotechnical match at the center of the hemisphere on the floor Two pressure gauges Kistler 0-10 bar At the center of the hemisphere -> Overpressure in the burnt gases At 10 m from the center of the hemisphere -> Pressure wave

Experimental conditions Characterization of turbulence in the dome Measurement of turbulence with 3 pitot probes in earlier version of test plateform : Simulation of this flow field with PIMPLEFOAM Solver Good agreement witj experimental measures Simulation of this flow field with PIMPLEFOAM Solver (RANS Solver) Good agreement with the experimental measures Turbulent intensity : 5 m/s Turbulence length scale : 0.15 m Simulation of experimental situation Turbulent intensity : 1.5 m/s Mean turbulent length scale : 0.16 m

Laminar flame speed Slad (m/s) Flammable mixtures Stoichiometric hydrogen/oxygen diluted with nitrogen Mixture 1 Mixture 2 % H2 40 45 % O2 20 22.5 % N2 32.5 Laminar flame speed Slad (m/s) 3.5 4.2 Expansion ratio E 7.5 8

Explosion in 25 i/s camera

Quiescent stoichiometric H2/O2 mixture diluted with N2 Mixture 1 : 40 % H2, 20 % O2, 40 % N2 Burnt gas overpressure and overpressure at 10 m Maximum overpressure : 250 mbar at 36 ms Fast camera

Quiescent stoichiometric H2/O2 mixture diluted with N2 Mixture 1 : 40 % H2, 20 % O2, 40 % N2 Flame trajectory and flame velocity Maximum overpressure : 250 mbar at 36 ms Part 1 Flame velocity ~50 m/s Creation of a turbulent/shear flow between the flame front and the plastic envelope Part 2 Flame velocity Increase by 50 to 115 m/s Experimental observation consistent with previous works Hydrodynamic instability = self-acceleration of flame Flame velocity vs distance, spherical explosion of stoichiometric hydrogen-air mixtures – Drenckhahn et al, 1985 Fast camera Expansion ratio E : 7.5 Laminar flame speed Slad : 3.5 m/s E.Slad = 26 m/s -> Ratio ~2 between measured and theoretical flame velocity Motion of plastic sheet at 22 ms Two distincts part in flame trajectory

Comparison between quiescent and turbulent mixture Mixture 1 : 40 % H2, 20 % O2, 40 % N2 Turbulence created during gas injection by the 4 jetflows Burnt gas overpressures for the quiescent and the turbulent mixtures Quiescent/Turbulent mixtures : Two parts in pressure signals Strong pressure rise up linked to the motion of plastic sheet

Comparison between quiescent and turbulent mixture Mixture 1 : 40 % H2, 20 % O2, 40 % N2 Quiescent mixture – Average flame velocity = 50 m/s Turburlent mixture – Average flame velocity = 85 m/s x 1.5 Turbulent flame velocity = 11 m/s

Comparison between quiescent and turbulent mixture Mixture 1 : 40 % H2, 20 % O2, 40 % N2 Intercomparison between different correlations : Bray correlation : Shy correlation : Gülder correlation : u’ - turbulent intensity, Lt – turbulent length scale, η – flame thickness, K – Karlovitz number Experiment Bray Shy Gülder 11 25 33 14 Gülder correlation gives the best result and confirms the past INERIS choice to estimate the flame turbulent velocity

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