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Simulations and experimental study of DDT behind a single obstacle

Simulations and experimental study of DDT behind a single obstacle. André Vagner Gaathaug Knut Vaagsaether Dag Bjerketvedt Faculty of Technology Telemark University College Norway. Setup of study. 100 x 100 m 2 quadratic cross sectional area, 3000 mm long

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Simulations and experimental study of DDT behind a single obstacle

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  1. Simulations and experimental study of DDT behind a single obstacle André Vagner Gaathaug Knut Vaagsaether Dag Bjerketvedt Faculty of Technology Telemark University College Norway

  2. Setup of study • 100 x 100 m2quadratic cross sectional area, 3000 mm long • One obstacle with variable blockage ratio BR=0.5 to BR=0.9 • Spark ignition at the closed end, open at the other • 5 and 6 pressure trancducers • 15% to 40% Hydrogen in air mixture

  3. Why this work? • Not smooth channel • Not obstructed channel • Not unconfined jet • Earlier work by Vaagsaether and Knudsen • Circular geometry • Various blockage ratio • BR=Blocked area / open area • Investigated where DDT occur, a possibly why. • Challenges related to the problem • Driving section, the first meter • Investigated earlier by the authors

  4. Experimental study Focus

  5. Experimental results High speed frames with sketches of their phenomena. BR=0.84, H2 conc. 30%, 30000 fps

  6. Experimental results High speed film. BR=0.84, H2 conc. 28%, 30000 fps

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  15. Numerical methods • In house code by K. Vaagsaether – FLIC • Flux LImited Centered scheme • 2D TVD method • Details by K. Vaagsaether and E.F. Toro • Euler equation with ideal gas equation of state • Conservation of mass • Conservation of momentum • Conservation of energy • Conservation of turbulent kinetic energy 1. Toro, E.F., Riemann Solvers and Numerical Methods for Fluid Dynamics: A Practical Introduction, 1999, Springer-Verlag, Berlin, Heidelberg. 2. Vaagsaether, K., Modelling of Gas Explosions, PhD thesis, 2010, Telemark University College – NTNU, 2010:221.

  16. Combustion model • Progress variable β is conserved and can represent a concentration. • β =1 are products, while β =0 are reactants • Progress variable α is conserved and represents induction time • α<1 ”not hot enough”, while α=1 auto ignite

  17. Combustion model • The reaction rate is a maximum of two rates. • One turbulent reaction rate and one kinetic reaction rate. • Turbulent burning velocity from Flohr and Pitsch. Original from Zimont (1979), model constant A = 0.52 from Zimont and Lipatnikov (1995). • Flohr, P. and Pitsch, H., Centre for Turbulent Research, Proceedings • of the Summer Program, 2000. • Zimont, V. L. 1979 The theory of turbulent combustion at high • Reynolds numbers. Combust. Expl. and Shock Waves. 15. • Zimont, V. L., & Lipatnikov, A. N. 1995 A numerical model of • premixed turbulent combustion of gases. Chem. Phys. Reports. 14(7).

  18. Combustion model • The kinetic model is given by Korobeinikov et.al. 2002 • Then α is linked to the induction time τ by • Need model for induction time. Korobeinikov, M.S., Levin, V.A., Markov, V.V. and Chernyi, G.G, Propagation of Blast in a Combustible Gas, Astronautica Acta, 17, 1972, pp. 529-537.

  19. Induction time • Sichel et.al. model • Del Alamo et.al. model 1. Sichel, M., Tonello, N.A., Oran, E.S. and Jones, D.A., A Two–Step Kinetics Model for Numerical Simulation of Explosions and Detonations in H2-O2 Mixtures, Proc. R. Soc. Lond. A, 458, 2002, pp. 49-82. 2. Del Alamo, G., Williams, F.A. and Sanchez, A.L., Hydrogen–Oxygen Induction Times Above Crossover Temperatures, Combustion Science and Technology, 176, 2004, pp. 1599–1626.

  20. Reaction rates • Turbulent reaction rate ωT is relevant for deflagrations, where diffusion and mixing is the dominante mechanism. • Kinetic reaction rate ωK is relevant for detonations, where shock compression/heating is the dominante mechanism. 1 0 1. Vaagsaether, K., Modelling of Gas Explosions, PhD thesis, 2010, Telemark University College – NTNU, 2010:221.

  21. Numerical simulations

  22. Numerical results • Focus on the combustion behind the obstacle • Driver section (0 -> 1000 mm) challenge to reproduce • Several small explosions along the walls add up to DDT • Small scale mixing • Pockets of hot reactants • Very dependant on induction time model • Kinetic reaction rate is important

  23. Numerical results Numerical schlieren pictures from the simulation case with BR=0.84 and 35% H2 in air. Frames are not equidistant in time. Induction model: del Alamo.

  24. Numerical results Numerical schlieren pictures from the simulation case with BR=0.84 and 30% H2 in air. Frames are not equidistant in time. Induction model: del Alamo.

  25. Film Case with BR=0.75 and 30% H2 in air. Induction model: del Alamo.

  26. Numerical results Density gradient along top wall • Comparison of one case with two induction time models • One DDT, one without • Need to create large enough volume to explode. • Not too long and not too short induction time • “Draw to bow”

  27. Conclusion • Total run up distance from 1.1 m to 1.6 m in experiments. • Small explosions behind the flame front. • Onset of detonation at the walls, mostly top wall. • Simulations with two step combustion model. • Turbulent reaction rate for deflagrations. • Kinetic reaction rate for detonations. • Several small explosions along the walls. • Dependant on induction time model.

  28. Thank you

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