Fundamental Thoughts about Detonation Derek Bradley University of
Fundamental Thoughts about Detonation Derek Bradley University of Leeds UKELG 51st DISCUSSION MEETING Ignition and Explosion Hazards of Industrial Gas and Fuel Mixtures 1st April 2014 Imperial College Autoignition delay times for stoichiometric PRFs at 4 MPa T (K) 1250
1111 1000 909 833 769 714 667 100
100 (iso-octane) 95 90 t i ( ms) 10 80 60 0 (n -heptane) 1 0.1
0.01 0.8 0.9 1 1.1 1.2 1000/T 1.3 1.4 1.5
Requirements for Hot-spot Detonation An autoignition front that propagates close to the acoustic speed: close to unity. A high rate of energy release (excitation time, ) into the acoustic wave, as it propagates through the hot spot in a time of . Rate of energy release indicated by: . Hot-spot autoignition trigger is . Regime Mapping from Hot-spot DNS with CO/H2/air Detailed Kinetics
Limits of the Detonation Peninsula *The bottom thermal explosion boundary has low values of x. As these increase, so does . At A the extent of the detonation regime is limited. *At B this regime is extended by the increase in e, , but this gives a diminishing return. Hot-spot Detonation at Low x (=3, e, =22.7)
Hot-spot Detonation at High x (=10, e, =22.7) Engine Super-knock (Courtesy of Dr P-W Manz, VW, Germany) Increasing Severity of Engine Knock, N2 to E 50 65.2 40 B
xu 33.7 30 x Region of very strong knock is at small values of e, and x. DEVELOPING DETONATION
N2 20 10 K2 48.4 S P 0 0
5 10 E 15 e, xl 20 25 Detonation Transition in a Duct
Conditions for Strong Stable Detonations Radulescu, Shepherd, and Sharpe have proposed that for strong, stable detonations, with minimal dependence on transverse shocks, should be small. For super-knock, (xe, ) should be small. It can be shown that as the autoignition trigger. , with Conclusions Quite a lot is known about transitions to detonations in ducts and engines, but less about transitions in storage depots and refineries.
Small scale events are crucial triggers for transition to detonation and for maintaining them. x, e, , and E/RT, ti/tte , with reactivity gradient, are key parameters. Small product values are associated with strong stable detonations in ducts and engines. They possibly provide more useful criteria for these than does detonation cell size. References
Fieweger, K., Blumenthal, R. and Adomeit, G. 1997. Self-ignition of S.I. engine model fuels: a shock tube investigation at high pressure. Combust. Flame, 109, 599-619. Gu, X.J., Emerson, D.R. and Bradley, D. 2003. Modes of reaction front propagation from hot spots. Combust. Flame 133, 63-74. Bradley, D., Morley, C., Gu, X.J. and Emerson, D.R. 2002. Amplified pressure waves during autoignition: relevance to CAI engines. SAE paper 2002-01-2868. Bradley, D. and Kalghatgi, G.T. 2009. Influence of autoignition delay time characteristics of different fuels on pressure waves and knock in reciprocating engines. Combust. Flame, 156, 2307-2318 Kalghatgi, G.T. and Bradley, D. Pre-ignition and super-knock in turbocharged sparkignition engines, International Journal of Engine Research, 13(4), (2012) 399414. Urtiew, P.A. and Oppenheim, A.K. 1966. Experimental observations of the transition to detonation in an explosive gas. Proc. Roy. Soc. Lond., A295, 13-28.
Bradley, D. Autoignitions and detonations in engines and ducts, Phil Trans. Royal Soc. A 370 (2012) 689-714. Radulescu, M.I., Sharpe, G.J. and Bradley, D. A universal parameter quantifying explosion hazards, detonability and hot spot formation: the number, Proceedings of the Seventh International Seminar on Fire and Explosion Hazards, 2013, pp. 617-626, Research Publishing, Singapore. Eds. D. Bradley, G. Makhviladze, V. Molkov, P. Sunderland, F. Tamanini. The End
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