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The Crucial Role of the Lewis No. in Jet Ignition

Explore the crucial role of Lewis number in jet ignition of hydrogen, analyzing interplay between diffusion and chemistry, with findings on impact of expansion rate. Discusses physical models and perturbation analysis, highlighting the factors affecting ignition occurrence and mechanisms. Investigates the effects of low Lewis number on ignition time and the relationship between the size of leak and expansion rate in hydrogen safety.

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The Crucial Role of the Lewis No. in Jet Ignition

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  1. The Crucial Role of the Lewis No. in Jet Ignition Nika Rezaeyan, Luc Bauwens University of Calgary Matei Radulescu University of Ottawa Fernando Fachini Filho Instituto Nacional de Pesquisas Espaciais ICHS 2011 San Francisco CA

  2. Outline • Motivation • Jet ignition • Physical Model • Magnitude Analysis and Perturbation • Results • Conclusion

  3. Motivation • Jet ignition: key unresolved issue in hydrogen safety • May hurt or help? • Review by Astbury & Hawksworth (2009) • Original study: Wolanski & Wojicki (1973)

  4. Jet ignition Hydrogen known to ignite in transient jets in leaks from high pressure (Wolanski and Wojcicki, 1973). Formation of high pressure jet, Radulescu & Law (2007)

  5. Issues under focus • Interplay between diffusion and chemistry? • Effect of expansion (Radulescu)? • Lewis number: Mass diffusivity vs. heat diffusivity? • Hydrogen: mass diffusivity > heat –> Low Lewis number • Analysis by Liñan & Crespo (1976) and Liñan & Williams (1993)

  6. Physical Model • One dimensional • frame of reference attached to contact surface initially separating shock-heated air from cold, expanded hydrogen • In that (nearly inertial) frame, low Mach number • Single step Arrhenius chemistry • Negligible cross diffusion • Prescribed expansion rate • Ideal gas, constant specific heat and Lewis number

  7. Shock tube problem

  8. Physical Model • Diffusion problem (heat, fuel, oxidant) with sources: chemistry and expansion • Initial conditions: jump at contact surface • Boundary conditions at infinity consistent with jump

  9. Assumptions/magnitudes • Key physical processes: reaction, diffusion and expansion. • Time short compared with chemical time • High activation energy • Frozen flow regime: chemistry negligible at leading order • Ignition as a perturbation of the order of inverse activation energy.

  10. Frozen Flow Frozen flow: diffusion and expansion (which causes a temperature drop in time) • Mass-weighed coordinate • Self-similar solution:

  11. Frozen Flow

  12. Frozen Flow

  13. Lewis Number • Lewis number: ratio between heat and mass diffusion

  14. Lewis Number • Chemistry peaks close to maximum temperature • Peak larger for smaller fuel Lewis number

  15. Perturbation • Chemistry strongest when departure from maximum temperature is small. So, introduce rescaling • Asymptotic expansion of order of inverse activation energy

  16. Perturbation

  17. Perturbation • Negligible transient and expansion term lead to quasi-steady formulation. • Fuel concentration contains two terms: 1. Mass diffusion 2. Fuel consumption due to chemistry • Then expansion only appears in the Arrhenius term

  18. Le close to unity • Perturbation problem reduced to ODE: • Fuel mass diffusion of same order as fuel consumption • Max value of the perturbation function of ratio initial temperatures difference/ adiabatic flame temperature, times O(1) factor depending upon small difference Le - 1.

  19. Le close to 1,  < 1

  20. Le close to 1,  < 1 • Ignition happens at turning point.

  21. Le close to 1,  < 1 • for uniform pressure (p0'=0) ignition always occurs (Liñan) • If turning point at * < max, ignition occurs. For stronger expansion, no ignition

  22. Le close to 1,  > 1 • Solution (1)() increases monotonically with  so no turning point: so no thermal explosion • Front from warm side toward cold side • Unconditionally quenched by expansion

  23. Le – 1 negative and of O(1) • Fuel supplied by mass diffusion > fuel consumption • Ignition at turning point. • Ignition time shorter for smaller Lewis number. • Similar to Le of O(1),  < 1.

  24. Le > 1 by O(1) • Mass difussion no longer supplies fuel concentration at order e. So, chemistry now limited by fuel. Need to rescale: • Then, problem becomes: • Temperature increase due to chemistry now negligible. • Equilibrium region propagating towards fuel rich region • Eventually expansion quenches ignition • Similar to Le of O(1),  > 1

  25. Le > 1 by O(1)

  26. Conclusions from Analysis • Reaction rate peaks close to hot air side. • For Lewis numbers greater than threshold close to unity, no ignition (jet ignition only observed for hydrogen) • For Lewis numbers below that threshold, ignition occurs at finite time as long as expansion rate < a critical rate • No ignition for expansion rates faster than the critical rate

  27. Conclusions • “Ignition source” in jet ignition: likely interplay between diffusion and reaction • Occurs with hydrogen because hydrogen diffuses easily • Ignition may get killed by expansion • Since there is a clear relationship between leak size and expansion rate, current results consistent with experiments

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