Characteristics of unstable detonations near extinction limits

The unstable gaseous detonation of stoichiometric propane/oxygen mixtures, diluted or not with argon or helium, was investigated at near-limit conditi ons. The experiments were performed in a 38-mm-diameter, 24.5-m-long tube; diagnostics included: (i) microwave Doppler velocimetry, using a coaxial wa veguide, for a continuous record of the local wave velocity along the entir e length of the tube, (ii) fast-response piezo-electric pressure transducer s for shock pressure profiles and velocity, and (iii) photodiodes for detec ting the luminosity of the reaction zone and measuring the mean combustion wave velocity over most of the tube length. By varying the initial pressure of a given explosive mixture or by changing the dilution (0, 50, GO, or 70 mol%) and the nature of the diluent (Ar or I-it), four modes of propagatio n could be clearly identified: stable detonation, stuttering mode, gallopin g mode, and fast flame. For each mixture, the limiting initial pressures fo r each mode were determined. In particular, it was found that the galloping mode could not be obtained at dilutions equal to or above 70% argon and 60 % helium molar dilution. It was also found that, for a given dilution, the galloping wave is produced at higher initial pressures for helium as compar ed tr, argon dilution. For the undiluted mixture, it was found that the fea tures of the galloping wave were independent from the initiation conditions , apart from a delay for the reinitiation of the first peak. For the same m ixture, a reduction in initial pressure was found to reduce the period of t he galloping wave. Local measurements have shown that the length of the ind uction zone increases dramatically when the initial pressure is reduced to around 1-2 kPa, which is also the range of limiting upper pressures for the onset of the galloping wave in the mixtures under investigation. These mea surements were in agreement with computed induction times, and the latter c ould be used to explain the influence of dilution and diluent on The onset of the galloping regime. (C) 2000 by The Combustion institute.