1. Turbulent flame propagation in large unconfined H2/O2/N2 clouds
Jérôme Daubecha, Christophe Prousta,b, Guillaume Lecocqa
a INERIS, Verneuil en Halatte, France
b UTC, Compiègne, France

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. Explosion in 25 i/s camera

10. 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

11. 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

12. 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

13. 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

14. 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

15. Thank you for your attention