The onset of detonation behind shock waves of moderate intensity in gas phase

The shock-to-detonation transition (SDT) in gaseous n-heptane/oxygen/argon mixtures has been experimentally studied, using a shock tube, at low initial pressure (2-4 kPa) for a better understanding of the deflagration-to-detonation transition process. The detonation is generated by a precursory shock wave (PSW), with a Mach number smaller than that of the self-sustained detonation. Pressure (P²) and temperature (T²) behind incident shock waves have been accurately determined from the PSW velocity. The transition occurs in the measurement zone located between 3.20 m and 3.65 m from the shock tube diaphragm. The auto-ignition of mixture behind PSW is immediately followed by the onset of a combustion wave, which propagates at near Chapman-Jouguet (CJ) detonation velocity in the mixture carried at P², T² conditions. Consequently, the pressure peak can reach 350 times the initial pressure during the transition. The combustion wave merges with the PSW to form an overdriven detonation propagating in the initial mixture at velocity that progressively decreases to a value close to CJ value, of the order of 2 000 m.s^-1. The experimental particles heating times are compared with the computed ignition delay times, τI,², by using a detailed kinetic model of n-heptane oxidation. For stoichiometric n-heptane/oxygen mixtures diluted by 50% Ar, the experimental delay times are compatible with those computed. In this case, τi,² is governed by the so-called "high temperature mechanism" (T² > 1000 K) and varies exponentially with temperature. For undiluted mixtures, the particle heating times is much shorter than τi, ². Then, the chemistry is governed by the "low temperature mechanism" (T² < 950 K). Between 750 K and 950 K, τi, ² decreases or small changes with temperature decrease but is sensitive to pressure. Turbulence in boundary layer could also promote the SDT.