Autoignition front

Nonpremixed edge flames (a) 2D mixing layer (From Kioni, P.N., Rogg, B., Bray, K.N.C., and Linan, A., Combust. Flame, 95, 276, 1993. With permission.), (b) laminar jet (From Chung, S.H. and Lee, B.J., Combust. Flame, 86, 62,1991.), (c) flame spread (From Miller, F.J., Easton, J.W., Marchese, A.J., and Ross, H.D., Proc. Combust. Inst., 29, 2561, 2002. With permission.), (d) autoignition front (From Vervisch, L. and Poinsot, T., Annu. Rev. Fluid Mech., 30, 655, 1998. With permission.), and (e) spiral flame in von Karman swirling flow (From Nayagam, V. and Williams, F.A., Combust. Sci. Tech., 176, 2125, 2004. With permission.). (LPF lean premixed flame, RPF rich premixed flame, DF diffusion flame). 

It was recognized by Zeldovich et al. [183,184] and Makhviladze and Rogatykh [185] that, if the conditions are such that the autoignition front moves into the unburned mixture at approximately the acoustic velocity, then the pressure wave generated by the combustion heat release can couple with the autoignition front, with mutual reinforcement of both fronts and very rapid reaction. When the autoignition wave moves much faster or slower than the acoustic velocity such coupling does not occur and the combustion is less intense. 

In the deflagration mode, a larger temperature gradient and consequently larger values of t,, particularly at high values of x, makes (djc/dr,) much smaller. The variation of t, in this mode is illustrated by curve (d). Now dxIdTj is less than a and the acoustic wave runs ahead of the autoignition front. 

So far, explanations have neglected any compression of the unburned gas. Any such compression reduces the effective values of t,. Curve (c) shows values of t,, reduced below those of curve (d) by compression, to increase dxIdTj. A feedback mechanism exists by which an increase in the rate of propagation of the autoignition front further compresses the unburned gas. This reduces the effective values of t, and increases the rate of propagation still further. As a result, with a sufficiently reactive mixture, dxIdTj in the deflagration mode can increase towards the sonic velocity, with an associated increase in knock intensity [114]. 

Occurrence of a local explosion (hot spot) leading to a shock wave The area in front of the reaction zone is ideally prepared for autoignition. Hot spots are being created by focussing of weak waves behind obstacles. Amplification of the shock wave until autoignition and transition to detonation by means of an effective mechanism... 

The temperature and pressure conditions prevailing inside the unburned gas allow it to ignite itself. If this autoignition occurs before the arrival of the flame front, the local pressure increases rapidly and creates vibrations which are expressed as the... 

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