Detonations in Hydrocarbon Fuel Blends

A study of detonations in high-molecular weight hydrocarbon fuels of interest to pulse detonation engine applications was performed in a 280 mm diameter, 7.3 m long facility. Detonation pressure, wave speed, and cell width measurements were made in JP-10 mixtures and in mixtures representative of the decomposition products of JP-10. Experiments were performed in vapor-phase JP-10 mixtures at 353 K over a range of equivalence ratios (0.7≤ φ ≤1.3), nitrogen dilutions (fuel-oxygen to fuel-air), and initial pressures (20-130 kPa). The cell widths of the JP-10 mixtures are found to be similar to those of propane 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 measured cell width of the fuel blend-air mixture is about half that of JP-10-air. The addition of components of the fuel blend (acetylene, ethylene, and methane) to JP-10 in air at 353 K was characterized. Nitrogen diluted mixtures of stoichiometric hexane-oxygen were studied and the cell widths for hexane-air and JP-10-air are found to be comparable. The addition of lower molecular weight fuels (hydrogen, acetylene, ethylene, 1 and carbon monoxide) to hexane-air was investigated. The measured cell width decreases, 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 produces a small decrease in the cell width, but addition of more than about 75 % (by fuel mass) carbon monoxide results in a significant increase in cell width. Carbon monoxide is a principal intermediate product of hydrocarbon combustion yet there are relatively little cell width data available. Cell width measurements were made in carbon monoxide-air mixtures with the addition of hydrogen or hydrocarbons (acetylene, ethylene, and hexane). A linear relationship is found between the cell width and the reaction zone length when it is defined as the location of the peak in hydroxyl mole fraction.