Detonations in Hydrocarbon Fuel Blends

Abstract

A study of detonations in high molecular weight hydrocarbon fuels was performed in two GALCIT facilities: the 280 mm gaseous detonation tube (GDT) and a 1180 liter vessel (HYJET) with jet initiation capability. In the GDT, detonation pressure, wave speed and cell width measurements were made in hexane-oxygen-nitrogen mixtures with and without the addition of lower molecular weight fuels. Stoichiometric mixtures of hexane-oxygen were studied with nitrogen dilution varying from fuel-oxygen to fuel-air. Hexane-air mixtures were investigated with varying fractions of lower molecular weight fuels (hydrogen, acetylene, ethylene, and carbon monoxide). The measured cell width decreased indicating increased sensitivity to detonation with increasing fraction of hydrogen, acetylene, and ethylene, in order of effectiveness. The addition of a small fraction of carbon monoxide produced a small decrease in the cell width, but addition of more than about 75 % (by fuel mass) carbon monoxide resulted in a rapid increase in cell width. As the oxidation of carbon monoxide is extremely sensitive to the presence of hydrogenous species, cell width measurements were made in carbon monoxide-air mixtures with the addition of hydrogen or hydrocarbons of various H-atom content and structure (acetylene, ethylene, and hexane). A detonation could be initiated in mixtures with very small fraction of hexane (0.07% of the mixture volume). Cell width measurements were compared to calculated ZND reaction zone parameters, including temperature and radical species concentrations. It was determined that for addition of hydrogen, ethylene or hexane, the cell width can be correlated with the product of the peak OH and CO concentrations in the reaction zone. Mixtures containing acetylene also showed the same linear dependance on this parameter, but, for the same peak OH and CO concentration, the cell widths were a factor of two smaller than those of the other mixtures. A fuel blend representative of thermally decomposed JP-10 was studied at 295 K. This blend consisted of hydrogen, carbon monoxide, methane, acetylene, ethylene, and hexane with varying fractions of oxygen and nitrogen. The cell width for stoichiometric fuel blend-oxygen was found to be an order of magnitude smaller than that for fuel blend-air. The cell width for the fuel blend-air mixture was about half that of hexane-air. Further experiments were carried out in the HYJET facility. A hydrogen-oxygen-nitrogen jet was used to initiate detonations in vapor phase mixtures of hexane (at 295 K) and of dodecane (at 380 K) with stoichiometric oxygen. Pressure and wave speed measurements were made. A critical nitrogen dilution limit was determined for each fuel. The critical limit was found to be 2.5 ≤ β ≤ 3.0 for hexane, where β is the ratio of nitrogen to oxygen concentration. This corresponds to a D/λ (nozzle diameter/cell width) ratio of 4, which compares well with the value 4.3 previously determined for this driver. The critical nitrogen dilution limit for dodecane was also found to be 2.5 ≤ β ≤ 3.0. No cell width measurements are currently available for dodecane. An attempt was made to initiate detonation in a dodecane spray. Pressure and velocity measurements were made and clearly show that no detonation could be directly initiated. Different fuel injection systems were tried and the initial temperature of the mixture was varied.

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