Jet Initiation of Deflagration and Detonation in Stoichiometric H₂-O₂-N₂ Mixtures

Abstract

New results are reported on the initiation of detonation in hydrogen-oxygen-nitrogen mixtures. These results were obtained at Caltech in the Hyjet facility which consists of an 1180-liter receiver vessel and a 28-liter driver vessel. Orifices of various diameters were used to connect the two vessels. The driver vessel is a 2.2-meterlong tube in which combustion is initiated at the end opposite to the connection to thee receiver vessel. A flame or detonation exits the driver vessel through the orifice and enters the receiver vessel, initiating combustion in the receiver vessel. Previous work carried out in the Hyjet facility used a sensitive driver vessel mixture of 35.7% H_2, 17.9% O_2, and 46.4% N_2. Spark initiation of this mixture resulted in a detonation within the driver. The conditions under which a detonation would be initiated in the receiver vessel were measured as a function of the amount of dilution for stoichiometric hydrogen-oxygen-diluent mixtures in the receiver vessel. In the present work, a less sensitive mixture of 20% H_2 and 80% air was used in the driver, resulting in a flame-jet entering into the receiver vessel instead of the detonation that was observed in previous tests. Critical amounts of nitrogen dilution in the receiver vessel were determined for three jet orifice diameters: 25, 64 and 92 mm and stoichiometric hydrogen-oxygen-nitrogen mixtures. the amount of diluent was measured using a parameter β equal to the molar ratio of N_2 to O_2. The critical value of beta required for prompt initiation of detonation was estimated from a series of experiments for each nozzle size. the critical value of β is estimated to be .4 for the 25-mm-diameter nozzle, 1.3 for the 64-mm-diameter nozzle, and 2.0 for the 92-mm-diameter nozzle. As expected, these values are substantially lower than those measured in the previous experiments that were conducted with the detonating driver mixture. In addition, the range of β for which secondary explosions (DDT) are observed in the present experiments is smaller than in the previous work in Hyjet with the detonating driver mixture.

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