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Hydrogen-air deflagrations in open atmosphere: LES analysis of experimental data

First International Conference on HYDROGEN SAFETY. Hydrogen-air deflagrations in open atmosphere: LES analysis of experimental data. V. Molkov*, D. Makarov*, H. Schneider**. * - University of Ulster, UK; ** - Fraunhofer Institut Chemische Technologie, GERMANY.

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Hydrogen-air deflagrations in open atmosphere: LES analysis of experimental data

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  1. First International Conference on HYDROGEN SAFETY Hydrogen-air deflagrations in open atmosphere: LES analysis of experimental data V. Molkov*, D. Makarov*, H. Schneider** * - University of Ulster, UK; ** - Fraunhofer Institut Chemische Technologie, GERMANY 7-10 September 2005, Pisa - Italy

  2. Contents • Experiment in 2094-m3 hemisphere 1 • Theoretical background for modelling 2 • The Large Eddy Simulation model 3 • Theory versus experiment 4

  3. EXPERIMENT 1

  4. Experimental details Experiment: Schneider H., Pförtner H. PNP-Sichcrheitssofortprogramm, Prozebgasfreisetzung-Explosion in der gasfabrik und auswirkungen von Druckwellen auf das Containment, Dezember 1983. 20 meters

  5. Side and top view movies 10 m 20 m

  6. Distributed flame front • Estimate of turbulent flame front (distributed) thickness: • 1. The pocket (“mole”) of size 0.2 m behind a leading edge of the flame front will burn inward during 0.2m:2m/s=0.1s (0.2 m divided by burning velocity 2 m/s); • 2. During this time leading edge will propagate as far as 0.1sx40m/s=4 m! (8 m for “mole” 0.4 m)? 4 m

  7. Experimental results • Flame propagation velocity was independent upon ignition energy in the investigated energy range (10-1000 J or pyrotechnical charge). • The resulting flames propagated in almost hemispherical form with a developed structure. • The maximum visible flame velocity occurs between the original radius of the balloon R0 and radius 1.5R0. • The maximum flame radius reached about 2R0. • No transition to detonation was observed. • The maximum visible flame velocity reached 84 m/s. • At a sufficient distance from the explosion the maximum pressure decayed inversely proportional to the distance. • The positive pressure wave was followed by a negative pressure phase.

  8. THEORETICAL BACKGROUND FOR MODELLING 2

  9. Self-similarity (fractals) Gostintsev et al (1988) analysed about 20 experiments on large-scale unconfined deflagrations and concluded that the hydrodynamic flame instability leads to accelerating, self-similar regime of fully developed turbulent flame propagation. According to this analysis, the flame front surface obeys the fractal theory after self-similar regime is established. The authors found that the transition to the self-similar turbulent regime of flame propagation occurs after the critical value of the flame front radius R* is achieved, which was found to be R*=1.0-1.2 m for near stoichiometric premixed hydrogen-air flames.

  10. Flame generated turbulence The study performed by Karlovits et al (1951) using burner flames led to the conclusion that a flame front itself generates turbulence. The maximum theoretical value of the flame front wrinkling due to flame induced turbulence was found to be: where Ei – combustion products expansion coefficient. LES of large scale problems can not at foreseen future resolve all details of flame front structure and this can be modelled only.

  11. S. Pope (2004): • Physical LES (filter size is ARTIFICIAL parameterD) • Numerical LES (filter size is cell size) The Ulster LES model 3

  12. Ulster LES model (1/3) • Conservation of mass • Conservation of momentum • Conservation of energy

  13. Ulster LES model (2/3) • Premixed flame front propagation (progress variable) • Gradient method for the source term • Yakhot’s RNG like turbulent premixed combustion (inflow) • where u’ – residual SGS velocity • Karlovitz turbulence generated by flame front itself (SGS) • Chemistry is in burning velocity (dependence on T,p, j)

  14. Ulster LES model (3/3) • RNG SGS turbulence model • Dilution of initial H2-air mixture by atmospheric air

  15. Three main “FAQ” • Why gradient method? Decoupling physics and numerics • Integral of source term through numerical flame front is always equal to physical value ruSt (physically correct heat release, given up structure of turbulent flame front) • Why RNG (renormalization group) turbulence model? • No turning. Validated for both laminar and turbulent flows. • No “cut-off” at D but “scaling down” at inertial range. • Why turbulence generated by flame front itself? • LES of large scale accidental combustion can not resolve phenomena at scales comparable with flamelets thickness. • Existence of a theoretical maximum and critical radius:

  16. Domain and grid Structured hexahedral (SHH) 200x200x100 m Characteristic size of control volumes (CV) for 309494 CVs grid: Radius, m CV size, m 0 - 22 0.4 - 1.2 22-30 (UTH zone) 1.2 - 4.0 30-200 (SHH zone) 4.0 (2.0 in direction of pressure gauges) Unstructured tetrahedral

  17. Numerical details • Initial conditions • initial temperature T=283 K;initial pressure p=98.9 kPa • quiescent mixture; progress variable c=0. • hydrogen concentration YH2=0.0287 at R10.0m (Ya=1 for R>10.0 m) • Boundary conditions • no-slip impermeable adiabatic boundary on the ground • non-reflecting boundary conditions in atmosphere • Ignition: 15 ms increase of progress variable in 1 CV • Numerical details • code: FLUENT • explicit linearisationof the governing equations • explicit time marching procedure • second order accurate upwind scheme for convection terms, central-difference scheme for diffusion terms • Courant-Friedrichs-Lewy number CFL=0.8

  18. THEORY versus EXPERIMENT 4

  19. Flame shape 1 EXPERIMENT SIMULATION (averaging c=0.2-0.8)

  20. Flame shape 2 Experiment Simulation

  21. Flame propagation

  22. Flame radius

  23. Burning velocity St Total flame wrinkling factor is about 5,of which RNG SGSis only St/Su=1.2 Balloon rupture at 5 m is a reason for flame acceleration? Ei=7.2

  24. Pressure dynamics 1 Gauge affected by combustion Gauge affected by combustion Gauge affected by combustion Flame zone: 2 m, 5 m, 8 m, 18 m

  25. Pressure dynamics 2 Similar to experiment:the positive pressure wave was followed by a negative pressure phase. Usually the negative pressure wave was somewhat shorter than the positive one providing larger negative pressure peak. Far-field: 35 m, 80 m

  26. Conclusions • The Ulster LES model has been applied to study the dynamics of the largest unconfined deflagration of stoichiometric hydrogen-air mixture. The model has no adjustable parameters and reasonably reproduced the experimental data on dynamics of flame and pressure wave propagation. • Effects of the hydrodynamic flow instabilities and the turbulence induced by turbulent flame front itself on the burning velocity acceleration are accounted separately in the model. It is demonstrated that the main contributor to the turbulent flame propagation is the turbulence generated by flame front itself. • Further studies have to model under resolved fractal structure of large-scale flames to reproduce in more detail the observed monotonous acceleration of the flame front.

  27. THANK YOU

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