Effect of fuel type on hypergolic ignition of hydrogen peroxide with gelled hydrocarbon fuel

Experimental counterflow and impinging jet studies and modeling analysis of hypergolic hydrogen peroxide and gelled hydrocarbon fuel mixtures were conducted to characterize condensed phase reaction rates and ignition delay times. The gelled hydrocarbon mixtures consisted of nheptane and fumed silica and sodium borohydride particles. The present results were compared with previously obtained results for similar gels, but with dodecane instead of n-heptane. Scanning electron microscopy, x-ray photoelectron spectroscopy, and simultaneous thermogravimetric and differential scanning calorimetry analysis of the sodium borohydride particles were performed to characterize particle size, size distribution, geometry, surface composition, and thermal decomposition. Preliminary rheological characterization of several gelled fuels is also reported. Counterflow experiments, conducted over a range of flow rates, hydrogen peroxide concentrations, and particle loadings, show heat release from the condensed phase reaction increases with sodium borohydride particle loading, hydrogen peroxide concentration, and residence time. These experiments were used to derive a global rate constant for the condensed phase reaction between hydrogen peroxide and sodium borohydride. The derived value was within 21% of the rate constant value obtained with dodecane as the fuel versus n-heptane. Using the condensed phase global reaction and rate constant and a detailed gas phase mechanism for n-heptane oxidation, chemical kinetics calculations were performed to interpret the effect of sodium borohydride loading on ignition delay. Impinging jet experiments indicated ignition delay decreased from 78 to 36 ms as the sodium borohydride weight percent was increased from 3 to 7 weight percent. Model results indicate the same trend. Compared to n-dodecane, ignition delays for n-heptane were longer for a given sodium borohydride particle mass loading, however, when considered on a molar basis, ignition delay times for the two fuels converge. These results imply, for a gelled distillate fuel such as RP-1 (or kerosene), the presence of lighter component hydrocarbons in the liquid fuel does not necessarily result in reduced ignition delay time. Rather, these results suggest, on a mass basis, a higher loading fraction of reactive particles would be required for a lower molecular weight hydrocarbon fuel to achieve ignition delay times comparable to a higher molecular weight fuel.