Sensitization of hydrocarbon-oxygen mixtures to detonation via cool-flame oxidation

Abstract

The effect of cool flame partial oxidation on the detonation sensitivity of hydrocarbons was experimentally investigated. Sensitivity to detonation was quantified by measuring the detonation cell-size using the smoked-foil technique. A rich pentane oxygen mixture was preheated in a pebble bed before filling a heated glass detonation tube to sub-atmospheric pressure. Cool flame reaction, monitored by a thin K-type thermocouple, occurred in the detonation tube after a known time interval as determined by the tube temperature. The mixture was ignited by a weak spark and onset of detonation was monitored using a streak camera. A smoked foil was inserted in the far end of the tube (opposite to ignition) to permit the measurement of the cell size of a well-developed detonation. The results show that the cell pattern becomes very regular at high temperature but the average cell size practically does not change. However, when the mixture was detonated while undergoing the cool flame reaction, a significant reduction of the cell-size was obtained (as large as 50%). The sensitizing effect was found to occur only in the initial stage of the cool flame reaction. When the mixture was ignited a few hundreds of milliseconds after the beginning of the coolmore » flame, the mixture was desensitized and the cell size increased. The explanation of the sensitizing effect of the cool flame reaction was investigated by using a chemical kinetic model to simulate the cool flame reaction and identify the chemical species that may be responsible for the observed results. By taking snapshots of these chemical concentrations during the simulated cool flame, these species were used as reactants in a zero-dimensional code to compute the induction kinetics for a Chapman Jouguet detonation. The numerically computed induction times closely follow the experimentally observed cell sizes and confirm that the sensitizing effect of the cool flame reaction may be attributed to the presence of free radicals and peroxides associated with the beginning of the cool flame process. However, these radicals are consumed as the cool flame reaction proceeds and the mixture becomes insensitive again.« less