An Investigation into the Ignition Phenomena of Sprays Using a Shock Tube

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Aiming at elucidating the effects of turbulent mixing on the ignition process of a spray, a technique was developed in which a freely falling droplets column produced by an ultrasonic atomizer was ignited behind a reflected shock. The ignition delays of cetane were determined, and the ignition processes were observed by monitoring ion current, detecting spectroscopically the light emission in UV-, visible- and near IR-bands and by the high-speed photography. The principal results were summarized as follws.(1) The apparent actrivation energy changes radically at 1180K for 0.85MPa; i.e., 170kJ/mol and 31.6kJ/mol, respectively, on the high- and low-temperature sides.(2) On the high-temperature side, the ignition delay is independent of the heating-up and evaporation of mother droplets, and governed by the gas-phase chemical reaction of the micromist generated by the shattering or peeling of droplets behind the incident shock.(3) The gas-phase chemical reaction starts as a preflame reactio … More n accompanied by OH radical emission and ion current, growing into a blue flame accomanied by visible-band emission. Hte light emission therafter synchronizes with the convections due to rarefaction fans.(4) The apparent activation energy of 170kJ/mol on the high-temperature side is close to the data obtained by injecting sprays into a hot stream concurrently or perpendicularly. whereas the one of 31.6kJ/mol on the low-temperature side is close to the data obtained by injecting sprays into a hot stagnant atmosphere or the shock tube data in case the droplets column is disturbed by the secondary flows.(5) If the temperature is lowered, the amount of micromist generated decreased due to the weakened incident shock so that the intensity of light emission decreases considerably. No luminous flame lump, imageable by the high-speed photography, appears below 1280K, and the rate of misfiring increases rapidly below 1060K.(6) It is supposed that turbulent mixing has a considerable effects on the ignition process of droplets column, since a completely different ignition process was observed in a situation that the droplets column is disturbed by the secondary flows from the drain tube or the atomizer installing tube. Less